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Remarkable Link between Projected Uncertainties of Arctic Sea-Ice Decline and Winter Eurasian Climate


doi: 10.1007/s00376-017-7156-5

  • We identify that the projected uncertainty of the pan-Arctic sea-ice concentration (SIC) is strongly coupled with the Eurasian circulation in the boreal winter (December-March; DJFM), based on a singular value decomposition (SVD) analysis of the forced response of 11 CMIP5 models. In the models showing a stronger sea-ice decline, the Polar cell becomes weaker and there is an anomalous increase in the sea level pressure (SLP) along 60°N, including the Urals-Siberia region and the Iceland low region. There is an accompanying weakening of both the midlatitude westerly winds and the Ferrell cell, where the SVD signals are also related to anomalous sea surface temperature warming in the midlatitude North Atlantic. In the Mediterranean region, the anomalous circulation response shows a decreasing SLP and increasing precipitation. The anomalous SLP responses over the Euro-Atlantic region project on to the negative North Atlantic Oscillation-like pattern. Altogether, pan-Arctic SIC decline could strongly impact the winter Eurasian climate, but we should be cautious about the causality of their linkage.
    摘要: 本研究分析了CMIP5 11个模式对冬季(12月至翌年3月)北极海冰面积在本世纪末的预估的不确定性及其与欧亚环流的关系. 我们通过奇异值分解 (SVD)得出两者强耦合的主模态, 当中反映了北极海冰覆盖范围的预估. 当北极海冰范围减少的预估值比模式集合更大时, 极地环流相对更弱, 其南侧(约北纬60度)出现异常的下沉气流, 乌拉尔山至西伯利亚地区及冰岛一带的海平面气压相对更高. 与此同时, 中纬度的西风带和费雷尔环流 (Ferrell Cell) 相对更弱, 北大西洋海温相对更暖. 在地中海地区, 海平面气压相对偏低而降水相对较多. 此情形下北大西洋气压的差异类似北大西洋涛动的负位相. 总体而言, 北极海冰未来预估的不确定性或会影响到欧亚冬季气候的预估, 不过我们须谨慎分析它们的因果关系.
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  • Allen R. J., C. S. Zender, 2011: Forcing of the Arctic Oscillation by Eurasian snow cover.J. Climate,24,6528-6539, https://doi.org/10.1175/2011JCLI4157.1.10.1175/2011JCLI4157.1b84784337e34795bce05f0506a6c8835http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011jcli...24.6528ahttp://journals.ametsoc.org/doi/abs/10.1175/2011JCLI4157.1Throughout much of the latter half of the twentieth century, the dominant mode of Northern Hemisphere (NH) extratropical wintertime circulation variability-the Arctic Oscillation (AO)-exhibited a positive trend, with decreasing high-latitude sea level pressure (SLP) and increasing midlatitude SLP. General circulation models (GCMs) show that this trend is related to several factors, including North Atlantic SSTs, greenhouse gas/ozone-induced stratospheric cooling, and warming of the Indo-Pacific warm pool. Over the last approximately two decades, however, the AO has been decreasing, with 2009/10 featuring the most negative AO since 1900. Observational and idealized modeling studies suggest that snow cover, particularly over Eurasia, may be important. An observed snow-AO mechanism also exists, involving the vertical propagation of a Rossby wave train into the stratosphere, which induces a negative AO response that couples to the troposphere. Similar to other GCMs, the authors show that transient simulations with the Community Atmosphere Model, version 3 (CAM3) yield a snow-AO relationship inconsistent with observations and dissimilar AO trends. However, Eurasian snow cover and its interannual variability are significantly underestimated. When the albedo effects of snow cover are prescribed in CAM3 (CAM PS) using satellite-based snow cover fraction data, a snow-AO relationship similar to observations develops. Furthermore, the late-twentieth-century increase in the AO, and particularly the recent decrease, is reproduced by CAM PS. The authors therefore conclude that snow cover has helped force the observed AO trends and that it may play an important role in future AO trends.
    閼存笧thun, M., T. Eldevik, L. H. Smedsrud, 閼达拷. Skagseth, R. B. Ingvaldsen, 2012: Quantifying the influence of Atlantic heat on Barents Sea ice variability and retreat.J. Climate,25,4736-4743, https://doi.org/10.1175/JCLI-D-11-00466.1.10.1175/JCLI-D-11-00466.1b21267d2e1794c34065f3b1b75f70557http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012JCli...25.4736Ahttp://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-11-00466.1The recent Arctic winter sea ice retreat is most pronounced in the Barents Sea. Using available observations of the Atlantic inflow to the Barents Sea and results from a regional ice-ocean model the authors assess and quantify the role of inflowing heat anomalies on sea ice variability. The interannual variability and longer-term decrease in sea ice area reflect the variability of the Atlantic inflow, both in observations and model simulations. During the last decade (1998-2008) the reduction in annual (July―June) sea ice area was 218 × 10
    Ayarzagüena, B., J. A. Screen, 2016: Future Arctic sea ice loss reduces severity of cold air outbreaks in midlatitudes.Geophys. Res. Lett.,43,2801-2809, https://doi.org/10.1002/2016GL068092.10.1002/2016GL068092438b3f1c3b6ec03db8a2cb18261ab806http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2016GL068092%2Ffullhttp://doi.wiley.com/10.1002/2016GL068092The effects of Arctic sea-ice loss on cold air outbreaks (CAOs) in midlatitudes remains unclear. Previous studies have defined CAOs relative to present-day climate, but changes in CAOs, defined in such a way, may reflect changes in mean climate and not in weather variability, and society is more sensitive to the latter. Here we revisit this topic but applying changing temperature thresholds relating to climate conditions of the time. CAOs do not change in frequency or duration in response to projected sea-ice loss. However, they become less severe, mainly due to advection of warmed polar air, since the dynamics associated with the occurrence of CAOs are largely not affected. CAOs weaken even in midlatitude regions where the winter-mean temperature decreases in response to Arctic sea-ice loss. These results are robustly simulated by two atmospheric models prescribed with differing future sea ice states and in transient runs where external forcings are included.
    Barnes E. A., L. M. Polvani, 2015: CMIP5 projections of Arctic amplification,of the North American/North Atlantic circulation,and of their relationship. J. Climate,28, 5254-5271,.https://doi.org/10.1175/JCLI-D-14-00589.110.1175/JCLI-D-14-00589.166b0c1656ebd2e209930fd1a652221bchttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.5254Bhttp://journals.ametsoc.org/doi/10.1175/JCLI-D-14-00589.1AbstractRecent studies have hypothesized that Arctic amplification, the enhanced warming of the Arctic region compared to the rest of the globe, will cause changes in midlatitude weather over the 21st Century. In this study we exploit the recently completed Phase 5 of the Coupled Model Intercomparison Project (CMIP5), and examine 27 state-of-the-art climate models to determine if their projected changes in the midlatitude circulation are consistent with the hypothesized impact of Arctic amplification over North America and the North Atlantic.Under the largest future greenhouse forcing (RCP8.5), we find that every model, in every season, exhibits Arctic amplification by 2100. At the same time, we find that the projected circulation responses are either opposite in sign to those hypothesized, or too widely spread among the models to discern any robust change. However, in a few seasons and for some of the circulation metrics we have examined, we find correlations between the model spread in Arctic amplificat...
    Barnes E. A., J. A. Screen, 2015: The impact of Arctic warming on the midlatitude jet-stream: Can it? Has it? Will it? WIREs Climate Change,6, 277-286, 337.https://doi.org/10.1002/wcc.10.1002/wcc.33752ebe04260204b0275c4a31be646b49chttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F273578310_the_impact_of_arctic_warming_on_the_midlatitude_jet-stream_can_it_has_it_will_ithttp://www.researchgate.net/publication/273578310_the_impact_of_arctic_warming_on_the_midlatitude_jet-stream_can_it_has_it_will_itThe Arctic lower atmosphere has warmed more rapidly than that of the globe as a whole, and this has been accompanied by unprecedented sea ice melt. Such large environmental changes are already having profound impacts on the flora, fauna, and inhabitants of the Arctic region. An open question, however, is whether these Arctic changes have an effect on the jet恠tream and thereby influence weather patterns farther south. This broad question has recently received a lot of scientific and media attention, but conclusions appear contradictory rather than consensual. We argue that one point of confusion has arisen due to ambiguities in the exact question being posed. In this study, we frame our inquiries around three distinct questions: We argue that framing the discussion around the three questions: provides insight into the common themes emerging in the literature as well as highlights the challenges ahead.
    Bintanja R., F. M. Selten, 2014: Future increases in Arctic precipitation linked to local evaporation and sea-ice retreat.Nature,509,479-482, https://doi.org/10.1038/nature13259.10.1038/nature1325924805239b32c7c068de3281b8100a9c6600b2fbehttp%3A%2F%2Fwww.nature.com%2Fnature%2Fjournal%2Fvaop%2Fncurrent%2Ffull%2Fnature13259.html%3FWT.ec_id%3DNATURE-20140508http://www.nature.com/doifinder/10.1038/nature13259Abstract Precipitation changes projected for the end of the twenty-first century show an increase of more than 50 per cent in the Arctic regions. This marked increase, which is among the highest globally, has previously been attributed primarily to enhanced poleward moisture transport from lower latitudes. Here we use state-of-the-art global climate models to show that the projected increases in Arctic precipitation over the twenty-first century, which peak in late autumn and winter, are instead due mainly to strongly intensified local surface evaporation (maximum in winter), and only to a lesser degree due to enhanced moisture inflow from lower latitudes (maximum in late summer and autumn). Moreover, we show that the enhanced surface evaporation results mainly from retreating winter sea ice, signalling an amplified Arctic hydrological cycle. This demonstrates that increases in Arctic precipitation are firmly linked to Arctic warming and sea-ice decline. As a result, the Arctic mean precipitation sensitivity (4.5 per cent increase per degree of temperature warming) is much larger than the global value (1.6 to 1.9 per cent per kelvin). The associated seasonally varying increase in Arctic precipitation is likely to increase river discharge and snowfall over ice sheets (thereby affecting global sea level), and could even affect global climate through freshening of the Arctic Ocean and subsequent modulations of the Atlantic meridional overturning circulation.
    Blackport R., P. J. Kushner, 2017: Isolating the atmospheric circulation response to Arctic sea ice loss in the coupled climate system.J. Climate,30,2163-2185, https://doi.org/10.1175/JCLI-D-16-0257.1.10.1175/JCLI-D-16-0257.1e9113aa8260329a8aacfb91c57a86f27http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2017EGUGA..1916837Khttp://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0257.1In the coupled climate system, projected global warming drives extensive sea-ice loss, but sea-ice loss drives warming that amplifies and can be confounded with the global warming process. This makes it challenging to cleanly attribute the atmospheric circulation response to sea-ice loss within coupled earth-system model (ESM) simulations of greenhouse warming. In this study, many centuries of output from coupled ocean/atmosphere/land/sea-ice ESM simulations driven separately by sea-ice albedo reduction and by projected greenhouse-dominated radiative forcing are combined to cleanly isolate the hemispheric scale response of the circulation to sea-ice loss. To isolate the sea-ice loss signal, a pattern scaling approach is proposed in which the local multidecadal mean atmospheric response is assumed to be separately proportional to the total sea-ice loss and to the total low latitude ocean surface warming. The proposed approach estimates the response to Arctic sea-ice loss with low latitude ocean temperatures fixed and vice versa. The sea-ice response includes a high northern latitude easterly zonal wind response, an equatorward shift of the eddy driven jet, a weakening of the stratospheric polar vortex, an anticyclonic sea level pressure anomaly over coastal Eurasia, a cyclonic sea level pressure anomaly over the North Pacific, and increased wintertime precipitation over the west coast of North America. Many of these responses are opposed by the response to low-latitude surface warming with sea ice fixed. However, both sea-ice loss and low latitude surface warming act in concert to reduce storm track strength throughout the mid and high latitudes. The responses are similar in two related versions of the National Center for Atmospheric Research earth system models, apart from the stratospheric polar vortex response. Evidence is presented that internal variability can easily contaminate the estimates if not enough independent climate states are used to construct them. References: Blackport, R. and P. Kushner, 2017: Isolating the atmospheric circulation response to Arctic sea-ice loss in the coupled climate system. J. Climate, in press. Blackport, R. and P. Kushner, 2016: The Transient and Equilibrium Climate Response to Rapid Summertime Sea Ice Loss in CCSM4. J. Climate, 29, 401-417, doi: 10.1175/JCLI-D-15-0284.1.
    Bretherton C. S., C. Smith, and J. M. Wallace, 1992: An intercomparison of methods for finding coupled patterns in climate data. J. Climate, 5, 541-560, https://doi.org/10.1175/1520-0442(1992)005<0541:AIOMFF>2.0,CO;2.10.1175/1520-0442(1992)005<0541:AIOMFF>2.0.CO;2f0eb2fb48c2a11fdfe6e6af74cce0f65http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1992JCli....5..541Bhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281992%29005%3C0541%3AAIOMFF%3E2.0.CO%3B2This paper introduces a conceptual framework for comparing methods that isolate important coupled modes of variability between time series of two fields. Four specific methods are compared: principal component analysis with the fields combined (CPCA), canonical correlation analysis (CCA) and a variant of CCA proposed by Barnett and Preisendorfer (BP), principal component analysis of one single field followed by correlation of its component amplitudes with the second field (SFPCA), and singular value decomposition of the covariance matrix between the two fields (SVD). SVD and CPCA are easier to implement than BP, and do not involve user-chosen parameters. All methods are applied to a simple but geophysically relevant model system and their ability to detect a coupled signal is compared as parameters such as the number of points in each field, the number of samples in the time domain, and the signal-to-noise ratio are varied.In datasets involving geophysical fields, the number of sampling times may not be much larger than the number of observing locations or grid points for each field. In a model system with these characteristics, CPCA usually extracted the coupled pattern somewhat more accurately than SVD, BP, and SFPCA, since the patterns it yielded exhibit smaller sampling variability; SVD and BP gave quite similar results; and CCA was uncompetitive due to a high sampling variability unless the coupled signal was highly localized. The coupled modes derived from CPCA and SFPCA exhibit an undesirable mean bias toward the leading EOFs of the individual fields; in fact, for small signal-to-noise ratios these methods may identify a coupled signal that is similar to a dominant EOF of one of the fields but is completely orthogonal to the true coupled signal within that field. For longer time series, or in situations where the coupled signal does not resemble the EOFs of the individual fields, these biases can make SVD and BP substantially superior to CPCA.
    Chang C.-P., Z. Wang, and H. Hendon, 2006: The Asian winter monsoon. The Asian Monsoon, B. Wang, Ed., Springer, 89- 127.
    Chen H. W., F. Q. Zhang, and R. B. Alley, 2016: The robustness of midlatitude weather pattern changes due to Arctic sea ice loss.J. Climate,29,7831-7849, https://doi.org/10.1175/JCLI-D-16-0167.1.10.1175/JCLI-D-16-0167.17835c455b72af401c9bccc92e6d1a61chttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F306086712_The_robustness_of_mid-latitude_weather_pattern_changes_due_to_Arctic_sea-ice_losshttp://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0167.1Abstract The significance and robustness of the link between Arctic sea ice loss and changes in midlatitude weather patterns is investigated through a series of model simulations from the Community Atmosphere Model, version 5.3, with systematically perturbed sea ice cover in the Arctic. Using a large ensemble of 10 sea ice scenarios and 550 simulations, it is found that prescribed Arctic sea ice anomalies produce statistically significant changes for certain metrics of the midlatitude circulation but not for others. Furthermore, the significant midlatitude circulation changes do not scale linearly with the sea ice anomalies and are not present in all scenarios, indicating that the remote atmospheric response to reduced Arctic sea ice can be statistically significant under certain conditions but is generally nonrobust. Shifts in the Northern Hemisphere polar jet stream and changes in the meridional extent of upper-level large-scale waves due to the sea ice perturbations are generally small and not clearly distinguished from intrinsic variability. Reduced Arctic sea ice may favor a circulation pattern that resembles the negative phase of the Arctic Oscillation and may increase the risk of cold outbreaks in eastern Asia by almost 50%, but this response is found in only half of the scenarios with negative sea ice anomalies. In eastern North America the frequency of extreme cold events decreases almost linearly with decreasing sea ice cover. This study's finding of frequent significant anomalies without a robust linear response suggests interactions between variability and persistence in the coupled system, which may contribute to the lack of convergence among studies of Arctic influences on midlatitude circulation.
    Cheng W., J. C. H. Chiang, and D. X. Zhang, 2013: Atlantic Meridional Overturning Circulation (AMOC) in CMIP5 models: RCP and historical simulations.J. Climate,26,7187-7197, https://doi.org/10.1175/JCLI-D-12-00496.1.10.1175/JCLI-D-12-00496.16d7cf85f42b57a7e64417b87ccce926ehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013JCli...26.7187Chttp://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00496.1The Atlantic meridional overturning circulation (AMOC) simulated by 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the historical (1850-2005) and future climate is examined. The historical simulations of the AMOC mean state are more closely matched to observations than those of phase 3 of the Coupled Model Intercomparison Project (CMIP3). Similarly to CMIP3, all models predict a weakening of the AMOC in the twenty-first century, though the degree of weakening varies considerably among the models. Under the representative concentration pathway 4.5 (RCP4.5) scenario, the weakening by year 2100 is 5%-40% of the individual model's historical mean state; under RCP8.5, the weakening increases to 15%-60% over the same period. RCP4.5 leads to the stabilization of the AMOC in the second half of the twenty-first century and a slower (then weakening rate) but steady recovery thereafter, while RCP8.5 gives rise to a continuous weakening of the AMOC throughout the twenty-first century. In the CMIP5 historical simulations, all but one model exhibit a weak downward trend [ranging from -0.1 to -1.8 Sverdrup (Sv) century(-1); 1 Sv 10(6) m(3) s(-1)] over the twentieth century. Additionally, the multimodel ensemble-mean AMOC exhibits multidecadal variability with a similar to 60-yr periodicity and a peak-to-peak amplitude of similar to 1 Sv; all individual models project consistently onto this multidecadal mode. This multidecadal variability is significantly correlated with similar variations in the net surface shortwave radiative flux in the North Atlantic and with surface freshwater flux variations in the subpolar latitudes. Potential drivers for the twentieth-century multimodel AMOC variability, including external climate forcing and the North Atlantic Oscillation (NAO), and the implication of these results on the North Atlantic SST variability are discussed.
    Cohen J. L., J. C. Furtado, M. A. Barlow, V. A. Alexeev, and J. E. Cherry, 2012: Arctic warming,increasing snow cover and widespread boreal winter cooling.Environmental Research Letters,7,014007, .https://doi.org/10.1088/1748-9326/7/1/01400710.1088/1748-9326/7/1/014007577d901c7bfb946e00b6b6dd7d8543fbhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012ERL.....7a4007Chttp://stacks.iop.org/1748-9326/7/i=1/a=014007?key=crossref.b774aafc53389f43a9e3492e8a36e8b1The most up to date consensus from global climate models predicts warming in the Northern Hemisphere (NH) high latitudes to middle latitudes during boreal winter. However, recent trends in observed NH winter surface temperatures diverge from these projections. For the last two decades, large-scale cooling trends have existed instead across large stretches of eastern North America and northern Eurasia. We argue that this unforeseen trend is probably not due to internal variability alone. Instead, evidence suggests that summer and autumn warming trends are concurrent with increases in high-latitude moisture and an increase in Eurasian snow cover, which dynamically induces large-scale wintertime cooling. Understanding this counterintuitive response to radiative warming of the climate system has the potential for improving climate predictions at seasonal and longer timescales.
    Cohen, J. L., Coauthors, 2014: Recent Arctic amplification and extreme mid-latitude weather.Nature Geoscience,7,627-637, https://doi.org/10.1038/NGEO2234.10.1038/ngeo22344cd471caba2fba3502432bd1eab5ae32http%3A%2F%2Fwww.nature.com%2Fabstractpagefinder%2F10.1038%2Fngeo2234http://www.nature.com/doifinder/10.1038/ngeo2234The Arctic region has warmed more than twice as fast as the global average a phenomenon known as Arctic amplification. The rapid Arctic warming has contributed to dramatic melting of Arctic sea ice and spring snow cover, at a pace greater than that simulated by climate models. These profound changes to the Arctic system have coincided with a period of ostensibly more frequent extreme weather events across the Northern Hemisphere mid-latitudes, including severe winters. The possibility of a link between Arctic change and mid-latitude weather has spurred research activities that reveal three potential dynamical pathways linking Arctic amplification to mid-latitude weather: changes in storm tracks, the jet stream, and planetary waves and their associated energy propagation. Through changes in these key atmospheric features, it is possible, in principle, for sea ice and snow cover to jointly influence mid-latitude weather. However, because of incomplete knowledge of how high-latitude climate change influences these phenomena, combined with sparse and short data records, and imperfect models, large uncertainties regarding the magnitude of such an influence remain. We conclude that improved process understanding, sustained and additional Arctic observations, and better coordinated modelling studies will be needed to advance our understanding of the influences on mid-latitude weather and extreme events.
    Collins, M., Coauthors, 2013: Long-term climate change: Projections, commitments and irreversibility. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, T. F. Stocker et al., Eds., Cambridge University Press, 1029- 1136.
    Deser C., R. Tomas, M. Alexand er, and D. Lawrence, 2010: The seasonal atmospheric response to projected Arctic sea ice loss in the late twenty-first century.J. Climate,23,333-351, https://doi.org/10.1175/2009JCLI3053.1.10.1002/9781444328509.ch971304202c94a33449757dc7f829413cfhttp%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103053329.htmlhttp://journals.ametsoc.org/doi/abs/10.1175/2009JCLI3053.1The authors investigate the atmospheric response to projected Arctic sea ice loss at the end of the twenty-first century using an atmospheric general circulation model (GCM) coupled to a land surface model. The response was obtained from two 60-yr integrations: one with a repeating seasonal cycle of specified sea ice conditions for the late twentieth century (1980–99) and one with that of sea ice conditions for the late twenty-first century (2080–99). In both integrations, a repeating seasonal cycle of SSTs for 1980–99 was prescribed to isolate the impact of projected future sea ice loss. Note that greenhouse gas concentrations remained fixed at 1980–99 levels in both sets of experiments. The twentieth- and twenty-first-century sea ice (and SST) conditions were obtained from ensemble mean integrations of a coupled GCM under historical forcing and Special Report on Emissions Scenarios (SRES) A1B scenario forcing, respectively. The loss of Arctic sea ice is greatest in summer and fall, yet the response of the net surface energy budget over the Arctic Ocean is largest in winter. Air temperature and precipitation responses also maximize in winter, both over the Arctic Ocean and over the adjacent high-latitude continents. Snow depths increase over Siberia and northern Canada because of the enhanced winter precipitation. Atmospheric warming over the high-latitude continents is mainly confined to the boundary layer (below 850 hPa) and to regions with a strong low-level temperature inversion. Enhanced warm air advection by submonthly transient motions is the primary mechanism for the terrestrial warming. A significant large-scale atmospheric circulation response is found during winter, with a baroclinic (equivalent barotropic) vertical structure over the Arctic in November–December (January–March). This response resembles the negative phase of the North Atlantic Oscillation in February only. Comparison with the fully coupled model reveals that Arctic sea ice loss accounts for most of the seasonal, spatial, and vertical structure of the high-latitude warming response to greenhouse gas forcing at the end of the twenty-first century.
    Deser C., L. T. Sun, R. A. Tomas, and J. Screen, 2016: Does ocean coupling matter for the northern extratropical response to projected Arctic sea ice loss? Geophys.Res. Lett.,43,2149-2157, https://doi.org/10.1002/2016GL067792.10.1002/2016GL067792b2c5b28d74611e0e77bd30d1465de4f0http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2016GL067792%2Ffullhttp://doi.wiley.com/10.1002/2016GL067792The question of whether ocean coupling matters for the extratropical Northern Hemisphere atmospheric response to projected late 21st century Arctic sea ice loss is addressed using a series of experiments with Community Climate System Model version 4 at 1º spatial resolution under different configurations of the ocean model component: no interactive ocean, thermodynamic slab ocean, and full-depth (dynamic plus thermodynamic) ocean. Ocean-atmosphere coupling magnifies the response to Arctic sea ice loss but does not change its overall structure; however, a slab ocean is inadequate for inferring the role of oceanic feedbacks. The westerly winds along the poleward flank of the eddy-driven jet weaken in response to Arctic sea ice loss, accompanied by a smaller-magnitude strengthening on the equatorward side, with largest amplitudes in winter. Dynamical and thermodynamic oceanic feedbacks amplify this response by approximately 50%. Air temperature, precipitation, and sea level pressure responses also show sensitivity to the degree of ocean coupling.
    Ding Q. H., J. M. Wallace, D. S. Battisti, E. J. Steig, A. J. E. Gallant, H.-J. Kim, and L. Geng, 2014: Tropical forcing of the recent rapid Arctic warming in northeastern Canada and Greenland.Nature,509,209-212, https://doi.org/10.1038/nature13260.10.1038/nature1326024805345dcbdccd6121537cf78f27f7cc233dd7ehttp%3A%2F%2Fwww.nature.com%2Fnature%2Fjournal%2Fv509%2Fn7499%2Fnature13260%2Fmetricshttp://www.nature.com/doifinder/10.1038/nature13260Rapid Arctic warming and sea-ice reduction in the Arctic Ocean are widely attributed to anthropogenic climate change. The Arctic warming exceeds the global average warming because of feedbacks that include sea-ice reduction and other dynamical and radiative feedbacks. We find that the most prominent annual mean surface and tropospheric warming in the Arctic since 1979 has occurred in northeastern Canada and Greenland. In this region, much of the year-to-year temperature variability is associated with the leading mode of large-scale circulation variability in the North Atlantic, namely, the North Atlantic Oscillation. Here we show that the recent warming in this region is strongly associated with a negative trend in the North Atlantic Oscillation, which is a response to anomalous Rossby wave-train activity originating in the tropical Pacific. Atmospheric model experiments forced by prescribed tropical sea surface temperatures simulate the observed circulation changes and associated tropospheric and surface warming over northeastern Canada and Greenland. Experiments from the Coupled Model Intercomparison Project Phase 5 (ref. 16) models with prescribed anthropogenic forcing show no similar circulation changes related to the North Atlantic Oscillation or associated tropospheric warming. This suggests that a substantial portion of recent warming in the northeastern Canada and Greenland sector of the Arctic arises from unforced natural variability.
    Ding Y. H., 1994: Monsoons over China. Kluwer Academic Publishers, 420 pp.
    Gao, Y. Q., Coauthors, 2015: Arctic sea ice and Eurasian climate: A review.Adv. Atmos. Sci.,32,92-114, https://doi.org/10.1007/s003946-014-0009-6.10.1007/s00376-014-0009-6d871921aaa624189bf66d49c90050b24http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00376-014-0009-6http://link.springer.com/10.1007/s00376-014-0009-6北极在气候系统起一个基本作用,包括温暖的北极和北极海冰程度和厚度的衰落并且在最近的十年显示出重要气候变化。与温暖的北极和北极海冰的减小相对照,欧洲,东亚和北美洲经历了反常地冷的条件,与在最近的年期间的记录降雪。在这篇论文,我们在欧亚的气候上考察海冰影响的当前的理解。Paleo,观察并且建模研究被盖住总结几个主要主题,包括:北极海冰和它的控制的可变性;可能的原因和北极海冰的明显的影响在卫星时代,以及过去和投射未来影响和趋势期间衰退;在北极海冰和北极摆动 / 北方大西洋摆动之间的连接和反馈机制,最近的欧亚的冷却,大气的循环,在东亚的夏天降水,在欧亚大陆上的春天降雪,东方亚洲冬季季风,和 midlatitude 极端捱过的冬季;并且遥远的气候反应(例如,大气的循环,空气温度) 到在北极海冰的变化。我们为未来研究与一篇简短和建议得出结论。
    Graversen R. G., T. Mauritsen, M. Tjernström E. Källèn, and G. Svensson, 2008: Vertical structure of recent Arctic warming.Nature,451,53-56, https://doi.org/10.1038/nature06502.10.1038/nature06502181724959bd01a7bdc8d49b30efaef8debb6f5a8http%3A%2F%2Fwww.europepmc.org%2Fabstract%2FMED%2F18172495http://www.nature.com/doifinder/10.1038/nature06502Near-surface warming in the Arctic has been almost twice as large as the global average over recent decades-a phenomenon that is known as the 'Arctic amplification'. The underlying causes of this temperature amplification remain uncertain. The reduction in snow and ice cover that has occurred over recent decades may have played a role. Climate model experiments indicate that when global temperature rises, Arctic snow and ice cover retreats, causing excessive polar warming. Reduction of the snow and ice cover causes albedo changes, and increased refreezing of sea ice during the cold season and decreases in sea-ice thickness both increase heat flux from the ocean to the atmosphere. Changes in oceanic and atmospheric circulation, as well as cloud cover, have also been proposed to cause Arctic temperature amplification. Here we examine the vertical structure of temperature change in the Arctic during the late twentieth century using reanalysis data. We find evidence for temperature amplification well above the surface. Snow and ice feedbacks cannot be the main cause of the warming aloft during the greater part of the year, because these feedbacks are expected to primarily affect temperatures in the lowermost part of the atmosphere, resulting in a pattern of warming that we only observe in spring. A significant proportion of the observed temperature amplification must therefore be explained by mechanisms that induce warming above the lowermost part of the atmosphere. We regress the Arctic temperature field on the atmospheric energy transport into the Arctic and find that, in the summer half-year, a significant proportion of the vertical structure of warming can be explained by changes in this variable. We conclude that changes in atmospheric heat transport may be an important cause of the recent Arctic temperature amplification.
    Harvey B. J., L. C. Shaffrey, and T. J. Woollings, 2015: Deconstructing the climate change response of the Northern Hemisphere wintertime storm tracks.Climate Dyn.,45,2847-2860, https://doi.org/10.1007/s00382-015-2510-8.10.1007/s00382-015-2510-880ced53b0dfd87ab2dd2528165d64a2fhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-015-2510-8http://link.springer.com/10.1007/s00382-015-2510-8There are large uncertainties in the circulation response of the atmosphere to climate change. One manifestation of this is the substantial spread in projections for the extratropical storm tracks made by different state-of-the-art climate models. In this study we perform a series of sensitivity experiments, with the atmosphere component of a single climate model, in order to identify the causes of the differences between storm track responses in different models. In particular, the Northern Hemisphere wintertime storm tracks in the CMIP3 multi-model ensemble are considered. A number of potential physical drivers of storm track change are identified and their influence on the storm tracks is assessed. The experimental design aims to perturb the different physical drivers independently, by magnitudes representative of the range of values present in the CMIP3 model runs, and this is achieved via perturbations to the sea surface temperature and the sea-ice concentration forcing fields. We ask the question: can the spread of projections for the extratropical storm tracks present in the CMIP3 models be accounted for in a simple way by any of the identified drivers? The results suggest that, whilst the changes in the upper-tropospheric equator-to-pole temperature difference have an influence on the storm track response to climate change, the large spread of projections for the extratropical storm track present in the northern North Atlantic in particular is more strongly associated with changes in the lower-tropospheric equator-to-pole temperature difference.
    Hodson D. L. R., S. P. E. Keeley, A. West, J. Ridley, E. Hawkins, and H. T. Hewitt, 2013: Identifying uncertainties in Arctic climate change projections.Climate Dyn.,40,2849-2865, https://doi.org/10.1007/s00382-012-1512-z.10.1007/s00382-012-1512-z128f05964f9cc92bc2c4470644137060http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-012-1512-zhttp://link.springer.com/10.1007/s00382-012-1512-zWide ranging climate changes are expected in the Arctic by the end of the 21st century, but projections of the size of these changes vary widely across current global climate models. This variation represents a large source of uncertainty in our understanding of the evolution of Arctic climate. Here we systematically quantify and assess the model uncertainty in Arctic climate changes in two COdoubling experiments: a multimodel ensemble (CMIP3) and an ensemble constructed using a single model (HadCM3) with multiple parameter perturbations (THC-QUMP). These two ensembles allow us to assess the contribution that both structural and parameter variations across models make to the total uncertainty and to begin to attribute sources of uncertainty in projected changes. We find that parameter uncertainty is an major source of uncertainty in certain aspects of Arctic climate. But also that uncertainties in the mean climate state in the 20th century, most notably in the northward Atlantic ocean heat transport and Arctic sea ice volume, are a significant source of uncertainty for projections of future Arctic change. We suggest that better observational constraints on these quantities will lead to significant improvements in the precision of projections of future Arctic climate change.
    Honda M., J. Inoue, and S. Yamane, 2009: Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters,Geophys. Res. Lett.,36,L08707, https://doi.org/10.1029/2008GL037079.10.1029/2008GL03707911ff7459b32da24cee92554351efd9cbhttp%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103055291.htmlhttp://www.cabdirect.org/abstracts/20103055291.htmlInfluence of low Arctic sea-ice minima in early autumn on the wintertime climate over Eurasia is investigated. Observational evidence shows that significant cold anomalies over the Far East in early winter and zonally elongated cold anomalies from Europe to Far East in late winter are associated with the decrease of the Arctic sea-ice cover in the preceding summer-to-autumn seasons. Results fro...
    Hoskins B. J., D. J. Karoly, 1981: The steady linear response of a spherical atmosphere to thermal and orographic forcing. J. Atmos. Sci., 38, 1179-1196, https://doi.org/10.1175/1520-0469(1981)038<1179:TSLROA>2.0,CO;2.10.1175/1520-0469(1981)038<1179:TSLROA>2.0.CO;279fff4e3f8ece1da0529adaf44d4ea5dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1981JAtS...38.1179Hhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281981%29038%3C1179%3ATSLROA%3E2.0.CO%3B2Motivated by some results from barotropic models, a linearized steady-state five-layer baroclinic model is used to study the response of a spherical atmosphere to thermal and orographic forcing. At low levels the significant perturbations are confined to the neighborhood of the source and for midlatitude thermal forcing these perturbations are crucially dependent on the vertical distribution of the source. In the upper troposphere the sources generate wavetrains which are very similar to those given by barotropic models. For a low-latitude source, long wavelengths propagate strongly polewards as well as eastwards. Shorter wavelengths are trapped equatorward of the poleward flank of the jet, resulting in a split of the wave-trains at this latitude. Using reasonable dissipation magnitudes, the easiest way to produce an appreciable response in middle and high latitudes is by subtropical forcing. These results suggest an explanation for the shapes of patterns described in observational studies.The theory for waves propagating in a slowly varying medium is applied to Rossby waves propagating in a barotropic atmosphere. The slow variation of the medium is associated with the sphericity of the domain and the latitudinal structure of the zonal wind. Rays along which wave activity propagates, the speeds of propagation, and the amplitudes and phases along these rays are determined for a constant angular velocity basic flow as well as a more realistic jet flow. They agree well with the observational and numerical model results and give a simple interpretation of them.
    Jaiser R., K. Dethloff, D. Hand orf, A. Rinke, and J. Cohen, 2012: Impact of sea ice cover changes on the Northern Hemisphere atmospheric winter circulation,Tellus A,64,11595, v64i0. 11595.https://doi.org/10.3402/tellusa.10.3402/tellusa.v64i0.1159573cf21d64de757276aa85a42a608612chttp%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Fabs%2F10.3402%2Ftellusa.v64i0.11595https://www.tandfonline.com/doi/full/10.3402/tellusa.v64i0.11595The response of the Arctic atmosphere to low and high sea ice concentration phases based on European Center for Medium-Range Weather Forecast (ECMWF) Re-Analysis Interim (ERA-Interim) atmospheric data and Hadley Centre's sea ice dataset (HadISST1) from 1989 until 2010 has been studied. Time slices of winter atmospheric circulation with high (1990-2000) and low (2001-2010) sea ice concentration in the preceding August/September have been analysed with respect to tropospheric interactions between planetary and baroclinic waves. It is shown that a changed sea ice concentration over the Arctic Ocean impacts differently the development of synoptic and planetary atmospheric circulation systems. During the low ice phase, stronger heat release to the atmosphere over the Arctic Ocean reduces the atmospheric vertical static stability. This leads to an earlier onset of baroclinic instability that further modulates the non-linear interactions between baroclinic wave energy fluxes on time scales of 2.56 d and planetary scales of 10-90 d. Our analysis suggests that Arctic sea ice concentration changes exert a remote impact on the large-scale atmospheric circulation during winter, exhibiting a barotropic structure with similar patterns of pressure anomalies at the surface and in the mid-troposphere. These are connected to pronounced planetary wave train changes notably over the North Pacific.
    Jung O., M.-K. Sung, K. Sato, Y.-K. Lim, S.-J. Kim, E.-H. Baek, and B.-M. Kim, 2017: How does the SST variability over the western North Atlantic Ocean control Arctic warming over the Barents-Kara Seas? Environmental Research Letters,12, 034021, https://doi.org/10.1088/1748-9326/aa5f3b.10.1088/1748-9326/aa5f3b8e30ddc74b89cd98baa3f646a308cbc8http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2017ERL....12c4021Jhttp://stacks.iop.org/1748-9326/12/i=3/a=034021?key=crossref.b91985c596b94ded1c3e94f48ec75181Arctic warming over the Barents–Kara Seas and its impacts on the mid-latitude circulations have been widely discussed. However, the specific mechanism that brings the warming still remains unclear. In this study, a possible cause of the regional Arctic warming over the Barents–Kara Seas during early winter (October–December) is suggested. We found that warmer sea surface temperature anomalies over the western North Atlantic Ocean (WNAO) modulate the transient eddies overlying the oceanic frontal region. The altered transient eddy vorticity flux acts as a source for the Rossby wave straddling the western North Atlantic and the Barents–Kara Seas (Scandinavian pattern), and induces a significant warm advection, increasing surface and lower-level temperature over the Eurasian sector of the Arctic Ocean. The importance of the sea surface temperature anomalies over the WNAO and subsequent transient eddy forcing over the WNAO was also supported by both specially designed simple model experiments and general circulation model experiments.
    Kang S. M., I. M. Held, D. M. W. Frierson, and M. Zhao, 2008: The response of the ITCZ to extratropical thermal forcing: Idealized slab-ocean experiments with a GCM.J. Climate,21,3521-3532, https://doi.org/10.1175/2007JCLI2146.1.10.1175/2007JCLI2146.158d648b32270dc64c7b9232584fda93ahttp%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2008JCli...21.3521K%26amp%3Bdb_key%3DPHY%26amp%3Blink_type%3DABSTRACT%26amp%3Bhigh%3D31320http://journals.ametsoc.org/doi/abs/10.1175/2007JCLI2146.1
    Kim, B.-M., Coauthors, 2014: Weakening of the stratospheric polar vortex by Arctic sea-ice loss,Nature Communications,5,4646, https://doi.org/10.1038/ncomms5646.10.1038/ncomms5646251813900c34446785d2f1fe02375ca8641143f2http%3A%2F%2Fwww.nature.com%2Fncomms%2F2014%2F140902%2Fncomms5646%2Fabs%2Fncomms5646.htmlhttp://www.nature.com/doifinder/10.1038/ncomms5646Successive cold winters of severely low temperatures in recent years have had critical social and economic impacts on the mid-latitude continents in the Northern Hemisphere. Although these cold winters are thought to be partly driven by dramatic losses of Arctic sea-ice, the mechanism that links sea-ice loss to cold winters remains a subject of debate. Here, by conducting observational analyses and model experiments, we show how Arctic sea-ice loss and cold winters in extra-polar regions are dynamically connected through the polar stratosphere. We find that decreased sea-ice cover during early winter months (November-December), especially over the Barents-Kara seas, enhances the upward propagation of planetary-scale waves with wavenumbers of 1 and 2, subsequently weakening the stratospheric polar vortex in mid-winter (January-February). The weakened polar vortex preferentially induces a negative phase of Arctic Oscillation at the surface, resulting in low temperatures in mid-latitudes.
    King M. P., M. Hell, and N. Keenlyside, 2016: Investigation of the atmospheric mechanisms related to the autumn sea ice and winter circulation link in the Northern Hemisphere.Climate Dyn.,46,1185-1195, https://doi.org/10.1007/s00382-015-2639-5.10.1007/s00382-015-2639-56794a0d3a32425a56ae1088ed8bbac49http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-015-2639-5http://link.springer.com/10.1007/s00382-015-2639-5The relationship of Barents–Kara sea ice concentration in October and November with atmospheric circulation in the subsequent winter is examined using reanalysis and observational data. The analyses are performed on data with the 5-year running means removed to reduce the potential effects of slowly-varying external driving factors, such as global warming. We show that positive (negative) Barents–Kara sea ice concentration anomaly in autumn is associated with a positive (negative) North Atlantic Oscillation-like (NAO) pattern with lags of up to 3 months. The month-to-month variations in the lag relationships of the atmospheric anomalies related to November sea ice concentration are presented. Further analysis shows that the stratosphere-troposphere interaction may provide the memory in the system: positive (negative) sea ice concentration anomaly in November is associated with a strengthened (weakened) stratospheric polar vortex and these anomalies propagate downward leading to the positive (negative) NAO-like pattern in the late December to early January. This stratosphere mechanism may also play a role for Barents–Kara sea ice anomaly in December, but not for September and October. Consistently, Eliassen-Palm, eddy heat and momentum fluxes suggest that there is strong forcing of the zonal winds in November.
    Kug J.-S., J.-H. Jeong, Y.-S. Jang, B.-M. Kim, C. K. Folland , S.-K. Min, and S.-W. Son, 2015: Two distinct influences of Arctic warming on cold winters over North America and East Asia.Nature Geoscience,8,759-762, https://doi.org/10.1038/ngeo2517.10.1038/NGEO251776adf9a50b5913a10d5735fde3c115c3http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv8%2Fn10%2Fngeo2517%2Fmetricshttp://www.nature.com/doifinder/10.1038/ngeo2517Arctic warming has sparked a growing interest because of its possible impacts on mid-latitude climate. A number of unusually harsh cold winters have occurred in many parts of East Asia and North America in the past few years, and observational and modelling studies have suggested that atmospheric variability linked to Arctic warming might have played a central role. Here we identify two distinct influences of Arctic warming which may lead to cold winters over East Asia or North America, based on observational analyses and extensive climate model results. We find that severe winters across East Asia are associated with anomalous warmth in the Barents-Kara Sea region, whereas severe winters over North America are related to anomalous warmth in the East Siberian-Chukchi Sea region. Each regional warming over the Arctic Ocean is accompanied by the local development of an anomalous anticyclone and the downstream development of a mid-latitude trough. The resulting northerly flow of cold air provides favourable conditions for severe winters in East Asia or North America. These links between Arctic and mid-latitude weather are also robustly found in idealized climate model experiments and CMIP5 multi-model simulations. We suggest that our results may help improve seasonal prediction of winter weather and extreme events in these regions.
    Magnusdottir G., C. Deser, and R. Saravanan, 2004: The effects of North Atlantic SST and sea ice anomalies on the winter circulation in CCM3. Part I: Main features and storm track characteristics of the response. J. Climate, 17, 857-876, https://doi.org/10.1175/1520-0442(2004)017 <0857:TEONAS>2.0,CO;2.
    Mahlstein I., R. Knutti, 2011: Ocean heat transport as a cause for model uncertainty in projected Arctic warming.J. Climate,24,1451-1460, https://doi.org/10.1175/2010JCLI3713.1.10.1175/2010JCLI3713.1703bacde7b50d9aab86bce92e04bbadchttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011JCli...24.1451Mhttp://journals.ametsoc.org/doi/abs/10.1175/2010JCLI3713.1ABSTRACT The Arctic climate is governed by complex interactions and feedback mechanisms between the atmosphere, ocean, and solar radiation. One of its characteristic features, the Arctic sea ice, is very vulnerable to anthropogenically caused warming. Production and melting of sea ice is influenced by several physical processes. The authors show that the northward ocean heat transport is an important factor in the simulation of the sea ice extent in the current general circulation models. Those models that transport more energy to the Arctic show a stronger future warming, in the Arctic as well as globally. Larger heat transport to the Arctic, in particular in the Barents Sea, reduces the sea ice cover in this area. More radiation is then absorbed during summer months and is radiated back to the atmosphere in winter months. This process leads to an increase in the surface temperature and therefore to a stronger polar amplification. The models that show a larger global warming agree better with the observed sea ice extent in the Arctic. In general, these models also have a higher spatial resolution. These results suggest that higher resolution and greater complexity are beneficial in simulating the processes relevant in the Arctic and that future warming in the high northern latitudes is likely to be near the upper range of model projections, consistent with recent evidence that many climate models underestimate Arctic sea ice decline.
    Manzini, E., Coauthors, 2014: Northern winter climate change: Assessment of uncertainty in CMIP5 projections related to stratosphere-troposphere coupling.J. Geophys. Res.,119,7979-7998, https://doi.org/10.1002/2013JD021403.10.1002/2014JA020445http://doi.wiley.com/10.1002/2014JA020445
    McCusker K. E., J. C. Fyfe, and M. Sigmond, 2016: Twenty-five winters of unexpected Eurasian cooling unlikely due to Arctic sea-ice loss.Nature Geoscience,9,838-843, https://doi.org/10.1038/ngeo2820.10.1038/ngeo282086ebb2cb4509993b8b6992484cd04e67http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv9%2Fn11%2Fngeo2820%2Fmetricshttp://www.nature.com/doifinder/10.1038/ngeo2820Winter cooling over Eurasia has been suggested to be linked to Arctic sea-ice loss. Climate model simulations reveal no evidence for such a link and instead suggest that a persistent atmospheric circulation pattern is responsible.
    Meleshko V. P., O. M. Johannessen, A. V. Baidin, T. V. Pavlova, and V. A. Govorkova, 2016: Arctic amplification: Does it impact the polar jet stream? Tellus A,68, 32330, . 32330.https://doi.org/10.3402/tellusa.v6810.3402/tellusa.v68.32330be8d23e19c02eec67b977d002ca501fahttp%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Ffull%2F10.3402%2Ftellusa.v68.32330https://www.tandfonline.com/doi/full/10.3402/tellusa.v68.32330It has been hypothesised that the Arctic amplification of temperature changes causes a decrease in the northward temperature gradient in the troposphere, thereby enhancing the oscillation of planetary waves leading to extreme weather in mid-latitudes. To test this hypothesis, we study the response of the atmosphere to Arctic amplification for a projected summer sea-ice-free period using an atmospheric model with prescribed surface boundary conditions from a state-of-the-art Earth system model. Besides a standard global warming simulation, we also conducted a sensitivity experiment with sea ice and sea surface temperature anomalies in the Arctic. We show that when global climate warms, enhancement of the northward heat transport provides the major contribution to decrease the northward temperature gradient in the polar troposphere in cold seasons, causing more oscillation of the planetary waves. However, while Arctic amplification significantly enhances near-surface air temperature in the polar region, it is not large enough to invoke an increased oscillation of the planetary waves. Keywords: arctic amplification, modelling, jet stream, surface air temperature (Published: 4 October 2016) Citation: Tellus A 2016, 68, 32330, http://dx.doi.org/10.3402/tellusa.v68.32330
    Mori M., M. Watanabe, H. Shiogama, J. Inoue, and M. Kimoto, 2014: Robust Arctic sea-ice influence on the frequent Eurasian cold winters in past decades.Nature Geoscience,7,869-873, https://doi.org/10.1038/ngeo2277.10.1038/ngeo2277cdef8a86f56c39f6052fde6e5d1dd7bbhttp%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv7%2Fn12%2Fabs%2Fngeo2277.htmlhttp://www.nature.com/doifinder/10.1038/ngeo2277Over the past decade, severe winters occurred frequently in mid-latitude Eurasia, despite increasing global- and annual-mean surface air temperatures. Observations suggest that these cold Eurasian winters could have been instigated by Arctic sea-ice decline, through excitation of circulation anomalies similar to the Arctic Oscillation. In climate simulations, however, a robust atmospheric response to sea-ice decline has not been found, perhaps owing to energetic internal fluctuations in the atmospheric circulation. Here we use a 100-member ensemble of simulations with an atmospheric general circulation model driven by observation-based sea-ice concentration anomalies to show that as a result of sea-ice reduction in the Barents-Kara Sea, the probability of severe winters has more than doubled in central Eurasia. In our simulations, the atmospheric response to sea-ice decline is approximately independent of the Arctic Oscillation. Both reanalysis data and our simulations suggest that sea-ice decline leads to more frequent Eurasian blocking situations, which in turn favour cold-air advection to Eurasia and hence severe winters. Based on a further analysis of simulations from 22 climate models we conclude that the sea-ice-driven cold winters are unlikely to dominate in a warming future climate, although uncertainty remains, due in part to an insufficient ensemble size.
    Nakamura T., K. Yamazaki, K. Iwamoto, M. Honda, Y. Miyoshi, Y. Ogawa, and J. Ukita, 2015: A negative phase shift of the winter AO/NAO due to the recent Arctic sea-ice reduction in late autumn.J. Geophys. Res.,120,3209-3227, org/10.1002/2014JD022848.https://doi.10.1002/2014JD022848d1e2e6ebc5048d2580b8cab05197bd59http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014JD022848%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1002/2014JD022848/pdfAbstract This paper examines the possible linkage between the recent reduction in Arctic sea-ice extent and the wintertime Arctic Oscillation (AO)/North Atlantic Oscillation (NAO). Observational analyses using the ERA interim reanalysis and merged Hadley/Optimum Interpolation Sea Surface Temperature data reveal that a reduced (increased) sea-ice area in November leads to more negative (positive) phases of the AO and NAO in early and late winter, respectively. We simulate the atmospheric response to observed sea-ice anomalies using a high-top atmospheric general circulation model (AGCM for Earth Simulator, AFES version 4.1). The results from the simulation reveal that the recent Arctic sea-ice reduction results in cold winters in mid-latitude continental regions, which are linked to an anomalous circulation pattern similar to the negative phase of AO/NAO with an increased frequency of large negative AO events by a factor of over two. Associated with this negative AO/NAO phase, cold air advection from the Arctic to the mid-latitudes increases. We found that the stationary Rossby wave response to the sea-ice reduction in the Barents Sea region induces this anomalous circulation. We also found a positive feedback mechanism resulting from the anomalous meridional circulation that cools the mid-latitudes and warms the Arctic, which adds an extra heating to the Arctic air column equivalent to about 60% of the direct surface heat release from the sea-ice reduction. The results from this high-top model experiment also suggested a critical role of the stratosphere in deepening the tropospheric annular mode and modulation of the NAO in mid to late winter through stratosphere-troposphere coupling.
    Nummelin A., C. Li, and P. J. Hezel, 2017: Connecting ocean heat transport changes from the midlatitudes to the Arctic Ocean.Geophys. Res. Lett.,44,1899-1908, https://doi.org/10.1002/2016GL071333.10.1002/2016GL071333bc70eb7b63aa4bd7dadf91a0f8df338chttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2016GL071333%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/2016GL071333/fullAbstract Under greenhouse warming, climate models simulate a weakening of the Atlantic Meridional Overturning Circulation and the associated ocean heat transport at mid-latitudes, but an increase in the ocean heat transport to the Arctic Ocean. These opposing trends lead to what could appear to be a discrepancy in the reported ocean contribution to Arctic amplification. This study clarifies how ocean heat transport affects Arctic climate under strong greenhouse warming using a set of 21st century simulations performed within the Coupled Model Intercomparison Project (CMIP5). The results suggest that a future reduction in subpolar ocean heat loss enhances ocean heat transport to the Arctic Ocean, driving an increase in Arctic Ocean heat content and contributing to the intermodel spread in AA. The results caution against extrapolating the forced oceanic signal from the mid-latitudes to the Arctic.
    Omrani N.-E., J. Bader, N. S. Keenlyside, and E. Manzini, 2016: Troposphere-stratosphere response to large-scale North Atlantic Ocean variability in an atmosphere/ocean coupled model.Climate Dyn.,46,1397-1415, https://doi.org/10.1007/s00382-015-2654-6.10.1007/s00382-015-2654-62fb1f7fde882d3edc93709b6c0b911dfhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-015-2654-6http://link.springer.com/10.1007/s00382-015-2654-6The instrumental records indicate that the basin-wide wintertime North Atlantic warm conditions are accompanied by a pattern resembling negative North Atlantic oscillation (NAO), and cold conditions with pattern resembling the positive NAO. This relation is well reproduced in a control simulation by the stratosphere resolving atmosphere cean coupled Max-Planck-Institute Earth System Model (MPI-ESM). Further analyses of the MPI-ESM model simulation shows that the large-scale warm North Atlantic conditions are associated with a stratospheric precursory signal that propagates down into the troposphere, preceding the wintertime negative NAO. Additional experiments using only the atmospheric component of MPI-ESM (ECHAM6) indicate that these stratospheric and tropospheric changes are forced by the warm North Atlantic conditions. The basin-wide warming excites a wave-induced stratospheric vortex weakening, stratosphere/troposphere coupling and a high-latitude tropospheric warming. The induced high-latitude tropospheric warming is associated with reduction of the growth rate of low-level baroclinic waves over the North Atlantic region, contributing to the negative NAO pattern. For the cold North Atlantic conditions, the strengthening of the westerlies in the coupled model is confined to the troposphere and lower stratosphere. Comparing the coupled and uncoupled model shows that in the cold phase the tropospheric changes seen in the coupled model are not well reproduced by the standalone atmospheric configuration. Our experiments provide further evidence that North Atlantic Ocean variability (NAV) impacts the coupled stratosphere/troposphere system. As NAV has been shown to be predictable on seasonal-to-decadal timescales, these results have important implications for the predictability of the extra-tropical atmospheric circulation on these time-scales
    Omrani N.-E., N. S. Keenlyside, J. Bader, and E. Manzini, 2014: Stratosphere key for wintertime atmospheric response to warm Atlantic decadal conditions.Climate Dyn.,42,649-663, https://doi.org/10.1007/S00382-013-1860-3.10.1007/s00382-013-1860-39687bf74387cfd236248754b7d3d2abbhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-013-1860-3http://link.springer.com/10.1007/s00382-013-1860-3There is evidence that the observed changes in winter North Atlantic Oscillation (NAO) drive a significant portion of Atlantic Multi Decadal Variability (AMV). However, whether the observed decadal NAO changes can be forced by the ocean is controversial. There is also evidence that artificially imposed multi-decadal stratospheric changes can impact the troposphere in winter. But the origins of such stratospheric changes are still unclear, especially in early to mid winter, where the radiative ozone-impact is negligible. Here we show, through observational analysis and atmospheric model experiments, that large-scale Atlantic warming associated with AMV drives high-latitude precursory stratospheric warming in early to mid winter that propagates downward resulting in a negative tropospheric NAO in late winter. The mechanism involves stratosphere/troposphere dynamical coupling, and can be simulated to a large extent, but only with a stratosphere resolving model (i.e., high-top). Further analysis shows that this precursory stratospheric response can be explained by the shift of the daily extremes toward more major stratospheric warming events. This shift cannot be simulated with the atmospheric (low-top) model configuration that poorly resolves the stratosphere and implements a sponge layer in upper model levels. While the potential role of the stratosphere in multi-decadal NAO and Atlantic meridional overturning circulation changes has been recognised, our results show that the stratosphere is an essential element of extra-tropical atmospheric response to ocean variability. Our findings suggest that the use of stratosphere resolving models should improve the simulation, prediction, and projection of extra-tropical climate, and lead to a better understanding of natural and anthropogenic climate change.
    Overland, J. E., M. Y. Wang, 2010: Large-scale atmospheric circulation changes are associated with the recent loss of Arctic sea ice.Tellus A,62,1-9, https://doi.org/10.1111/j.1600-0870.2009.00421.x.10.1111/j.1600-0870.2009.00421.xb0ed8ee2685a605c97c5df18b284831chttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1111%2Fj.1600-0870.2009.00421.x%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1111/j.1600-0870.2009.00421.x/citedbyRecent loss of summer sea ice in the Arctic is directly connected to shifts in northern wind patterns in the following autumn, which has the potential of altering the heat budget at the cold end of the global heat engine. With continuing loss of summer sea ice to less than 20% of its climatological mean over the next decades, we anticipate increased modification of atmospheric circulation patterns. While a shift to a more meridional atmospheric climate pattern, the Arctic Dipole (AD), over the last decade contributed to recent reductions in summer Arctic sea ice extent, the increase in late summer open water area is, in turn, directly contributing to a modification of large scale atmospheric circulation patterns through the additional heat stored in the Arctic Ocean and released to the atmosphere during the autumn season. Extensive regions in the Arctic during late autumn beginning in 2002 have surface air temperature anomalies of greater than 3 °C and temperature anomalies above 850 hPa of 1 °C. These temperatures contribute to an increase in the 1000–500 hPa thickness field in every recent year with reduced sea ice cover. While gradients in this thickness field can be considered a baroclinic contribution to the flow field from loss of sea ice, atmospheric circulation also has a more variable barotropic contribution. Thus, reduction in sea ice has a direct connection to increased thickness fields in every year, but not necessarily to the sea level pressure (SLP) fields. Compositing wind fields for late autumn 2002–2008 helps to highlight the baroclinic contribution; for the years with diminished sea ice cover there were composite anomalous tropospheric easterly winds of 651.4 m s–1, relative to climatological easterly winds near the surface and upper tropospheric westerlies of 653 m s–1. Loss of summer sea ice is supported by decadal shifts in atmospheric climate patterns. A persistent positive Arctic Oscillation pattern in late autumn (OND) during 1988–1994 and in winter (JFM) during 1989–1997 shifted to more interannual variability in the following years. An anomalous meridional wind pattern with high SLP on the North American side of the Arctic—the AD pattern, shifted from primarily small interannual variability to a persistent phase during spring (AMJ) beginning in 1997 (except for 2006) and extending to summer (JAS) beginning in 2005.
    Overland, J., J. A. Francis, R. Hall, E. Hanna, S.-J. Kim, T. Vihma, 2015: The melting Arctic and midlatitude weather patterns: Are they connected? J.Climate,28,7917-7932, https://doi.org/10.1175/JCLI-D-14-00822.1.10.1175/JCLI-D-14-00822.10eff56c315ae22f8ed4dcb680ca380fchttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.7917Ohttp://journals.ametsoc.org/doi/10.1175/JCLI-D-14-00822.1The potential of recent Arctic changes to influence hemispheric weather is a complex and controversial topic with considerable uncertainty, as time series of potential linkages are short (<10 yr) and understanding involves the relative contribution of direct forcing by Arctic changes on a chaotic climatic system. A way forward is through further investigation of atmospheric dynamic mechanisms. During several exceptionally warm Arctic winters since 2007, sea ice loss in the Barents and Kara Seas initiated eastward-propagating wave trains of high and low pressure. Anomalous high pressure east of the Ural Mountains advected Arctic air over central and eastern Asia, resulting in persistent cold spells. Blocking near Greenland related to low-level temperature anomalies led to northerly flow into eastern North America, inducing persistent cold periods. Potential Arctic connections in Europe are less clear. Variability in the North Pacific can reinforce downstream Arctic changes, and Arctic amplification can accentuate the impact of Pacific variability. The authors emphasize multiple linkage mechanisms that are regional, episodic, and based on amplification of existing jet stream wave patterns, which are the result of a combination of internal variability, lower-tropospheric temperature anomalies, and midlatitude teleconnections. The quantitative impact of Arctic change on midlatitude weather may not be resolved within the foreseeable future, yet new studies of the changing Arctic and subarctic low-frequency dynamics, together with additional Arctic observations, can contribute to improved skill in extended-range forecasts, as planned by the WMO Polar Prediction Project (PPP). 2015 American Meteorological Society.
    Panagiotopoulos F., M. Shahgedanova, A. Hannachi, and D. B. Stephenson, 2005: Observed trends and teleconnections of the Siberian high: A recently declining center of action.J. Climate,18,1411-1422, https://doi.org/10.1175/JCLI3352.1.10.1175/JCLI3352.1a08ea06bd0546ba9f473bae12036e5cbhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005JCli...18.1411Phttp://journals.ametsoc.org/doi/abs/10.1175/JCLI3352.1This study investigates variability in the intensity of the wintertime Siberian high (SH) by defining a robust SH index (SHI) and correlating it with selected meteorological fields and teleconnection indices. A dramatic trend of 芒鈧2.5 hPa decade-1 has been found in the SHI between 1978 and 2001 with unprecedented (since 1871) low values of the SHI. The weakening of the SH has been confirmed by analyzing different historical gridded analyses and individual station observations of sea level pressure (SLP) and excluding possible effects from the conversion of surface pressure to SLP. SHI correlation maps with various meteorological fields show that SH impacts on circulation and temperature patterns extend far outside the SH source area extending from the Arctic to the tropical Pacific. Advection of warm air from eastern Europe has been identified as the main mechanism causing milder than normal conditions over the Kara and Laptev Seas in association with a strong SH. Despite the strong impacts of the variability in the SH on climatic variability across the Northern Hemisphere, correlations between the SHI and the main teleconnection indices of the Northern Hemisphere are weak. Regression analysis has shown that teleconnection indices are not able to reproduce the interannual variability and trends in the SH. The inclusion of regional surface temperature in the regression model provides closer agreement between the original and reconstructed SHI.
    Peings Y., G. Magnusdottir, 2014: Response of the wintertime Northern Hemisphere atmospheric circulation to current and projected Arctic sea ice decline: A numerical study with CAM5.J. Climate,27,244-264, https://doi.org/10.1175/JCLI-D-13-00272.1.10.1175/JCLI-D-13-00272.1e9732dd6358dbd0d3b6b5b82193b2c9chttp%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1175%2FJCLI-D-13-00272.1http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00272.1The wintertime Northern Hemisphere (NH) atmospheric circulation response to current (2007-12) and projected (2080-99) Arctic sea ice decline is examined with the latest version of the Community Atmospheric Model (CAM5). The numerical experiments suggest that the current sea ice conditions force a remote atmospheric response in late winter that favors cold land surface temperatures over midlatitudes, as has been observed in recent years. Anomalous Rossby waves forced by the sea ice anomalies penetrate into the stratosphere in February and weaken the stratospheric polar vortex, resulting in negative anomalies of the northern annular mode (NAM) that propagate downward during the following weeks, especially over the North Pacific. The seasonality of the response is attributed to timing of the phasing between the forced and climatological waves. When sea ice concentration taken from projections of conditions at the end of the twenty-first century is prescribed to the model, negative anomalies of theNAMare visible in the troposphere, both in early and late winter. This response is mainly driven by the large warming of the lower troposphere over the Arctic, as little impact is found in the stratosphere in this experiment. As a result of the thermal expansion of the polar troposphere, the westerly flow is decelerated and a weak but statistically significant increase of the midlatitude meanders is identified. However, the thermodynamical response extends beyond the Arctic and offsets the dynamical effect, such that the stronger sea ice forcing has limited impact on the intensity of cold extremes over midlatitudes. 2014 American Meteorological Society.
    Petoukhov V., V. A. Semenov, 2010: A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents,J. Geophys. Res.,115,D21111, https://doi.org/10.1029/2009JD013568.10.1029/2009JD013568dc1ac9e62c94b87f316ae99122829c96http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009JD013568%2Fpdfhttp://doi.wiley.com/10.1029/2009JD013568The recent overall Northern Hemisphere warming was accompanied by several severe northern continental winters, as for example, extremely cold winter 2005-2006 in Europe and northern Asia. Here we show that anomalous decrease of wintertime sea ice concentration in the Barents-Kara (B-K) seas could bring about extreme cold events like winter 2005-2006. Our simulations with the ECHAM5 general circulation model demonstrate that lower-troposphere heating over the B-K seas in the Eastern Arctic caused by the sea ice reduction may result in strong anticyclonic anomaly over the Polar Ocean and anomalous easterly advection over northern continents. This causes a continental-scale winter cooling reaching -1.5ºC, with more than 3 times increased probability of cold winter extremes over large areas including Europe. Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it, being in qualitative agreement with the simulated large-scale atmospheric circulation realignment. Furthermore, our results suggest that high-latitude atmospheric circulation response to the B-K sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then back again to cyclonic type of circulation as the B-K sea ice concentration gradually reduces from 100% to ice free conditions. We present a conceptual model that may explain the nonlinear local atmospheric response in the B-K seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area.
    Reintges A., T. Martin, M. Latif, and N. S. Keenlyside, 2017: Uncertainty in twenty-first century projections of the Atlantic Meridional Overturning Circulation in CMIP3 and CMIP5 models.Climate Dyn.,49,1495-1511, https://doi.org/10.1007/s00382-016-3180-x.10.1007/s00382-016-3180-x149131d6febc728c56d9f122d96c1a8fhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00382-016-3180-xhttp://link.springer.com/10.1007/s00382-016-3180-xUncertainty in the strength of the Atlantic Meridional Overturning Circulation (AMOC) is analyzed in the Coupled Model Intercomparison Project Phase 3 (CMIP3) and Phase 5 (CMIP5) projections for the twenty-first century; and the different sources of uncertainty (scenario, internal and model) are quantified. Although the uncertainty in future projections of the AMOC index at 30ºN is larger in CMIP5 than in CMIP3, the signal-to-noise ratio is comparable during the second half of the century and even larger in CMIP5 during the first half. This is due to a stronger AMOC reduction in CMIP5. At lead times longer than a few decades, model uncertainty dominates uncertainty in future projections of AMOC strength in both the CMIP3 and CMIP5 model ensembles. Internal variability significantly contributes only during the first few decades, while scenario uncertainty is relatively small at all lead times. Model uncertainty in future changes in AMOC strength arises mostly from uncertainty in density, as uncertainty arising from wind stress (Ekman transport) is negligible. Finally, the uncertainty in changes in the density originates mostly from the simulation of salinity, rather than temperature. High-latitude freshwater flux and the subpolar gyre projections were also analyzed, because these quantities are thought to play an important role for the future AMOC changes. The freshwater input in high latitudes is projected to increase and the subpolar gyre is projected to weaken. Both the freshening and the gyre weakening likely influence the AMOC by causing anomalous salinity advection into the regions of deep water formation. While the high model uncertainty in both parameters may explain the uncertainty in the AMOC projection, deeper insight into the mechanisms for AMOC is required to reach a more quantitative conclusion.
    Sato K., J. Inoue, and M. Watanabe, 2014: Influence of the Gulf Stream on the Barents Sea ice retreat and Eurasian coldness during early winter,Environmental Research Letters,9,084009, https://doi.org/10.1088/1748-9326/9/8/084009.10.1088/1748-9326/9/8/0840099c5d56ad9ccb94480ed2f47502d57e80http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014AGUFM.A33D3218Shttp://stacks.iop.org/1748-9326/9/i=8/a=084009?key=crossref.6f28bcabaff7bcbdba0494c16983ca82Abnormal sea-ice retreat over the Barents Sea during early winter has been considered a leading driver of recent midlatitude severe winters over Eurasia. However, causal relationships between such retreat and the atmospheric circulation anomalies remains uncertain. Using a reanalysis dataset, we found that poleward shift of a sea surface temperature front over the Gulf Stream likely induces warm southerly advection and consequent sea-ice decline over the Barents Sea sector, and a cold anomaly over Eurasia via planetary waves triggered over the Gulf Stream region. The above mechanism is supported by the steady atmospheric response to the diabatic heating anomalies over the Gulf Stream region obtained with a linear baroclinic model. The remote atmospheric response from the Gulf Stream would be amplified over the Barents Sea region via interacting with sea-ice anomaly, promoting the warm Arctic and cold Eurasian pattern. (letter)
    Rogers J. C., 1997: North Atlantic storm track variability and its association to the North Atlantic Oscillation and climate variability of Northern Europe. J. Climate, 10, 1635-1647, https://doi.org/10.1175/1520-0442(1997)010<1635:NASTVA>2.0,CO;2.10.1175/1520-0442(1997)0102.0.CO;21f72561e5887b0dbbbab8e788e07d618http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1997JCli...10.1635Rhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281997%29010%3C1635%3ANASTVA%3E2.0.CO%3B2The primary mode of North Atlantic track variability is identified using rotated principal component analysis (RPCA) on monthly fields of root-mean-squares of daily high-pass filtered (2-8-day periods) sea level pressures (SLP) for winters (December-February) 1900-92. It is examined in terms of its association with (1) monthly mean SLP fields, (2) regional low-frequency teleconnections, and (3) the seesaw in winter temperatures between Greenland and northern Europe. 32 refs., 9 figs.
    Screen J. A., 2014: Arctic amplification decreases temperature variance in northern mid- to high-latitudes.Nat. Clim. Change,4,577-582, https://doi.org/10.1038/nclimate2268.10.1038/nclimate22680981f852a5d3ddb77b2a2925d26f94b0http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv4%2Fn7%2Ffig_tab%2Fnclimate2268_f3.htmlhttp://www.nature.com/doifinder/10.1038/nclimate2268Changes in climate variability are arguably more important for society and ecosystems than changes in mean climate, especially if they translate into altered extremes [1, 2, 3]. There is a common perception and growing concern that human-induced climate change will lead to more volatile and extreme weather [4]. Certain types of extreme weather have increased in frequency and/or severity [5, 6, 7], in part because of a shift in mean climate but also because of changing variability [1, 2, 3, 8, 9, 10]. In spite of mean climate warming, an ostensibly large number of high-impact cold extremes have occurred in the Northern Hemisphere mid-latitudes over the past decade [11]. One explanation is that Arctic amplificationhe greater warming of the Arctic compared with lower latitudes [12] associated with diminishing sea ice and snow covers altering the polar jet stream and increasing temperature variability [13, 14, 15, 16]. This study shows, however, that subseasonal cold-season temperature variability has significantly decreased over the mid- to high-latitude Northern Hemisphere in recent decades. This is partly because northerly winds and associated cold days are warming more rapidly than southerly winds and warm days, and so Arctic amplification acts to reduce subseasonal temperature variance. Previous hypotheses linking Arctic amplification to increased weather extremes invoke dynamical changes in atmospheric circulation [11, 13, 14, 15, 16], which are hard to detect in present observations [17, 18] and highly uncertain in the future [19, 20]. In contrast, decreases in subseasonal cold-season temperature variability, in accordance with the mechanism proposed here, are detectable in the observational record and are highly robust in twenty-first-century climate model simulations.
    Screen J. A., 2017: Simulated atmospheric response to regional and pan-Arctic sea ice loss.J. Climate,30,3945-3962, https://doi.org/10.1175/JCLI-D-16-0197.1.10.1175/JCLI-D-16-0197.15a55386fa74a2b1cde0e04944b938f1bhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F312642732_Simulated_Atmospheric_Response_to_Regional_and_Pan-Arctic_Sea-Ice_Losshttp://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0197.1Theloss ofArctic sea-iceis already having profound environmental, societal and ecologicalimpactslocally.A highlyuncertainareaofscientificresearch,however,iswhethersuchArcticchange has a tangible effect on weather and climate at lower latitudes. There is emergingevidencethat thegeographicallocation of sea-icelossis criticallyimportantin determiningthe large-scale atmospheric circulation response and associated mid-latitude impacts.However,such regionaldependencieshavenotbeenexploredinathorough andsystematicmanner.Tomake progress on thisissue, this study analyses ensemble simulations with anatmosphericgeneralcirculationmodel prescribedwithsea-icelossseparatelyinnine regionsof theArctic, toelucidate thedistinctresponses toregionalsea-iceloss. Theresultssuggestthatin some regions sea-iceloss triggerslarge-scale dynamical responseswhereasin otherregionssea-icelossinducesonlylocalthermodynamicalchanges.Sea-icelossintheBarents-KaraSeaisuniqueindrivingaweakeningofthestratosphericpolarvortex,followedintimeby a tropospheric circulation response that resembles the North Atlantic Oscillation. ForOctober-to-March, the largest spatial-scale responses are driven by sea-ice loss in theBarents-KaraSeaandSeaofOkhotsk;however,differentregionsassumegreaterimportanceinotherseasons.Theatmosphererespondsverydifferentlytoregionalsea-icelossesthantopan-Arcticsea-iceloss,andthelattercannotbeobtainedbylinearadditionoftheresponsesto regional sea-ice losses. The results imply that diversity in past studies of the simulatedresponse toArcticsea-icelosscanbepartlyexplainedbythedifferentspatialpatternsofsea-iceloss imposed.
    Screen J. A., I. Simmonds, 2010: The central role of diminishing sea ice in recent Arctic temperature amplification.Nature,464,1334-1337, https://doi.org/10.1038/nature09051.10.1038/nature0905120428168563313f31bedb9f26c0837efeb54868ehttp%3A%2F%2Fwww.nature.com%2Fnature%2Fjournal%2Fv464%2Fn7293%2Fabs%2Fnature09051.htmlhttp://www.nature.com/doifinder/10.1038/nature09051The rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades-a feature known as 'Arctic amplification'. Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming; however, the underlying causes of Arctic amplification remain uncertain. The roles of reductions in snow and sea ice cover and changes in atmospheric and oceanic circulation, cloud cover and water vapour are still matters of debate. A better understanding of the processes responsible for the recent amplified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss. Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature amplification. The findings reinforce suggestions that strong positive ice-temperature feedbacks have emerged in the Arctic, increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic.
    Screen J. A., J. A. Francis, 2016: Contribution of sea-ice loss to Arctic amplification is regulated by Pacific Ocean decadal variability.Nat. Clim. Change,6,856-860, https://doi.org/10.1038/nclimate3011.10.1038/nclimate3011ade758bb9e2b74adffaf0c5ceebe4078http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv6%2Fn9%2Fnclimate3011%2Fmetricshttp://www.nature.com/doifinder/10.1038/nclimate3011The pace of Arctic warming is about double that at lower latitudes a robust phenomenon known as Arctic amplification (AA)1. Many diverse climate processes and feedbacks cause AA2-7, including positive feedbacks associated with diminished sea ice6,7. However, the precise contribution of sea-ice loss to AA remains uncertain7,8. Through analyses of both observations and model simulations, we show that the contribution of sea-ice loss to wintertime AA appears dependent on the phase of the Pacific Decadal Oscillation (PDO). Our results suggest that for the same pattern and amount of sea-ice loss, consequent Arctic warming is larger during the negative PDO phase, relative to the positive phase, leading to larger reductions in the poleward gradient of tropospheric thickness and to more pronounced reductions in the upper-level westerlies. Given the oscillatory nature of the PDO, this relationship has the potential to increase skill in decadal-scale predictability of Arctic and sub-Arctic climate. Our results indicate that Arctic warming in response to the ongoing long-term sea-ice decline9,10 is greater (reduced) during periods of negative (positive) PDO phase. We speculate that the observed recent shift to the positive PDO phase, if maintained and all other factors being equal, could act to temporarily reduce the pace of wintertime Arctic warming in the near future.
    Seidel D. J., Q. Fu, W. J. Rand el, and T. J. Reichler, 2008: Widening of the tropical belt in a changing climate.Nature Geoscience,1,21-24, 2007. 38.https://doi.org/10.1038/ngeo.10.1038/ngeo.2007.38dc9ef90c954febaa814bc42aab1634bdhttp%3A%2F%2Fwww.nature.com%2Farticles%2Fngeo.2007.38http://www.nature.com/articles/ngeo.2007.38Some of the earliest unequivocal signs of climate change have been the warming of the air and ocean, thawing of land and melting of ice in the Arctic. But recent studies are showing that the tropics are also changing. Several lines of evidence show that over the past few decades the tropical belt has expanded. This expansion has potentially important implications for subtropical societies and may lead to profound changes in the global climate system. Most importantly, poleward movement of large-scale atmospheric circulation systems, such as jet streams and storm tracks, could result in shifts in precipitation patterns affecting natural ecosystems, agriculture, and water resources. The implications of the expansion for stratospheric circulation and the distribution of ozone in the atmosphere are as yet poorly understood. The observed recent rate of expansion is greater than climate model projections of expansion over the twenty-first century, which suggests that there is still much to be learned about this aspect of global climate change.
    Sokolova E., K. Dethloff, A. Rinke, and A. Benkel, 2007: Planetary and synoptic scale adjustment of the Arctic atmosphere to sea ice cover changes,Geophys. Res. Lett.,34,L17816, https://doi.org/10.1029/2007GL030218.10.1029/2007GL030218c143d87f14f62176f11ac260f5f07a0ehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2007GL030218%2Ffullhttp://doi.wiley.com/10.1029/2007GL030218By means of unforced simulations with a global coupled circulation model it is shown that naturally occurring changes between high and low sea ice cover phases of the Arctic Ocean exert a strong influence on the Northern Hemisphere storm tracks. This work emphasizes the nonlinear dynamical feedback between Arctic sea ice cover and the Arctic Oscillation (AO) such as atmospheric response depending upon the wintertime sea ice distribution. Two seven year long time slices, with high and low sea ice cover, were analyzed with respect to the feedbacks between the time-mean flow, the quasi-stationary planetary and the baroclinic waves. The wave energy fluxes on time scales of 2 to 6 days increase in the middle troposphere between 30 and 60ºN during the high sea ice phase and increase the zonal wind. This increase is compensated by a strong reduction in the Eliassen-Palm fluxes on time scales from 10 to 90 days between 60 and 70ºN during high sea ice phases, accompanied by reduced zonal winds. High sea ice cover phases are related to the zonal wind changes during the positive phases of the AO, especially over the northern part of the Atlantic Ocean.
    Sorokina S. A., C. Li, J. J. Wettstein, and N. G. Kvamst闁帮拷, 2016: Observed atmospheric coupling between Barents sea ice and the warm-Arctic cold-Siberian anomaly pattern.J. Climate,29,495-511, https://doi.org/10.1175/JCLI-D-15-0046.1.10.1175/JCLI-D-15-0046.1b29ffa8838c909be8dfad5ed9ad398ffhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2016JCli...29..495Shttp://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0046.1The decline in Barents Sea ice has been implicated in forcing the “warm-Arctic cold-Siberian” (WACS) anomaly pattern via enhanced turbulent heat flux (THF). This study investigates interannual variability in winter [December–February (DJF)] Barents Sea THF and its relationship to Barents Sea ice and the large-scale atmospheric flow. ERA-Interim and observational data from 1979/80 to 2011/12 are used. The leading pattern (EOF1: 33%) of winter Barents Sea THF variability is relatively weakly correlated (r = 0.30) with Barents Sea ice and appears to be driven primarily by atmospheric variability. The sea ice–related THF variability manifests itself as EOF2 (20%, r = 0.60). THF EOF2 is robust over the entire winter season, but its link to the WACS pattern is not. However, the WACS pattern emerges consistently as the second EOF (20%) of Eurasian surface air temperature (SAT) variability in all winter months. When Eurasia is cold, there are indeed weak reductions in Barents Sea ice, but the associated THF anomalies are on average negative, which is inconsistent with the proposed direct atmospheric response to sea ice variability. Lead–lag correlation analyses on shorter time scales support this conclusion and indicate that atmospheric variability plays an important role in driving observed variability in Barents Sea THF and ice cover, as well as the WACS pattern.
    Thompson D. W. J., J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and temperature field.Geophys. Res. Lett.,25,1297-1300, https://doi.org/10.1029/98GL00950.10.1029/98GL00950adf244c1165dc0c5b3e8ecc1d4c5e7fehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F98GL00950%2Fpdfhttp://doi.wiley.com/10.1029/98GL00950The leading empirical orthogonal function of the wintertime sea-level pressure field is more strongly coupled to surface air temperature fluctuations over the Eurasian continent than the North Atlantic Oscillation (NAO). It resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance. Coupled to strong fluctuations at the 50-hPa level on the intraseasonal, interannual, and interdecadal time scales, this rctic Oscillation (AO) can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft. It is proposed that the zonally asymmetric surface air temperature and mid-tropospheric circulation anomalies observed in association with the AO may be secondary baroclinic features induced by the land-sea contrasts. The same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature, sea-level pressure, and 50-hPa height over the past 30 years: parts of Eurasia have warmed by as much as several K, sea-level pressure over parts of the Arctic has fallen by 4 hPa, and the core of the lower stratospheric polar vortex has cooled by several K. These trends can be interpreted as the development of a systematic bias in one of the atmosphere's dominant, naturally occurring modes of variability.
    Tokinaga H., S.-P. Xie, and H. Mukougawa, 2017: Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability.Proc. Nat. Acad. Sci,114,6227-6232, https://doi.org/10.1073/pnas.1615880114.10.1073/pnas.1615880114285593411ad6b0552db23ac6eb5831c3302ebc36http%3A%2F%2Feuropepmc.org%2Fabstract%2FMED%2F28559341http://www.pnas.org/lookup/doi/10.1073/pnas.1615880114Abstract With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean-atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region.
    Trenberth K. E., J. T. Fasullo, G. Branstator, and A. S. Phillips, 2014: Seasonal aspects of the recent pause in surface warming.Nat. Clim. Change,4,911-916, https://doi.org/10.1038/nclimate2341.10.1038/nclimate2341554955ca35f59c7488dc6adc881a1670http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv4%2Fn10%2Fnclimate2341%2Fmetricshttp://www.nature.com/doifinder/10.1038/nclimate2341Factors involved in the recent pause in the rise of global mean temperatures are examined seasonally. For 1999 to 2012, the hiatus in surface warming is mainly evident in the central and eastern Pacific. It is manifested as strong anomalous easterly trade winds, distinctive sea-level pressure patterns, and large rainfall anomalies in the Pacific, which resemble the Pacific Decadal Oscillation (PDO). These features are accompanied by upper tropospheric teleconnection wave patterns that extend throughout the Pacific, to polar regions, and into the Atlantic. The extratropical features are particularly strong during winter. By using an idealized heating to force a comprehensive atmospheric model, the large negative anomalous latent heating associated with the observed deficit in central tropical Pacific rainfall is shown to be mainly responsible for the global quasi-stationary waves in the upper troposphere. The wave patterns in turn created persistent regional climate anomalies, increasing the odds of cold winters in Europe. Hence, tropical Pacific forcing of the atmosphere such as that associated with a negative phase of the PDO produces many of the pronounced atmospheric circulation anomalies observed globally during the hiatus.
    Vihma T., 2014: Effects of Arctic sea ice decline on weather and climate: A review.Surveys in Geophysics,35,1175-1214, https://doi.org/10.1007/s10712-014-92824-0.10.1007/s10712-014-9284-077da958a10209d7918a429ac5f8df01chttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs10712-014-9284-0http://link.springer.com/10.1007/s10712-014-9284-0The areal extent, concentration and thickness of sea ice in the Arctic Ocean and adjacent seas have strongly decreased during the recent decades, but cold, snow-rich winters have been common over mid-latitude land areas since 2005. A review is presented on studies addressing the local and remote effects of the sea ice decline on weather and climate. It is evident that the reduction in sea ice cover has increased the heat flux from the ocean to atmosphere in autumn and early winter. This has locally increased air temperature, moisture, and cloud cover and reduced the static stability in the lower troposphere. Several studies based on observations, atmospheric reanalyses, and model experiments suggest that the sea ice decline, together with increased snow cover in Eurasia, favours circulation patterns resembling the negative phase of the North Atlantic Oscillation and Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia, which favours cold winters in Europe and northeastern Eurasia. A high over the western and a low over the eastern North America have also been suggested, favouring advection of Arctic air masses to North America. Mid-latitude winter weather is, however, affected by several other factors, which generate a large inter-annual variability and often mask the effects of sea ice decline. In addition, the small sample of years with a large sea ice loss makes it difficult to distinguish the effects directly attributable to sea ice conditions. Several studies suggest that, with advancing global warming, cold winters in mid-latitude continents will no longer be common during the second half of the twenty-first century. Recent studies have also suggested causal links between the sea ice decline and summer precipitation in Europe, the Mediterranean, and East Asia.
    Wallace J. M., C. Smith, and C. S. Bretherton, 1992: Singular value decomposition of wintertime sea surface temperature and 500-mb height anomalies. J. Climate, 5, 561-576, https://doi.org/10.1175/1520-0442(1992)005<0561:SVDOWS>2.0,CO;2.10.1175/1520-0442(1992)005<0561:SVDOWS>2.0.CO;29f5087e15d696642ee7f23563be45d4ehttp%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di1551-5036-68-sp1-65-bibr058%26amp%3Bdbid%3D16%26amp%3Bdoi%3D10.2112%252FSI68-009.1%26amp%3Bkey%3D10.1175%252F1520-0442%281992%29005%26lt%3B0561%253ASVDOWS%26gt%3B2.0.CO%253B2http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281992%29005%3C0561%3ASVDOWS%3E2.0.CO%3B2
    Wang C. Z., L. P. Zhang, S.-K. Lee, L. X. Wu, and C. R. Mechoso, 2014: A global perspective on CMIP5 climate model biases.Nat. Clim. Change,4,201-205, https://doi.org/10.1038/NCLIMATE2118.10.1038/nclimate2118ed494b0c2af3f90e35a681e89e80ac66http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv4%2Fn3%2Ffull%2Fnclimate2118.htmlhttp://www.nature.com/doifinder/10.1038/nclimate2118The Intergovernmental Panel on Climate Change's Fifth Assessment Report largely depends on simulations, predictions and projections by climate models. Most models, however, have deficiencies and biases that raise large uncertainties in their products. Over the past several decades, a tremendous effort has been made to improve model performance in the simulation of special regions and aspects of the climate system. Here we show that biases or errors in special regions can be linked with others at far away locations. We find in 22 climate models that regional sea surface temperature (SST) biases are commonly linked with the Atlantic meridional overturning circulation (AMOC), which is characterized by the northward flow in the upper ocean and returning southward flow in the deep ocean. A simulated weak AMOC is associated with cold biases in the entire Northern Hemisphere with an atmospheric pattern that resembles the Northern Hemisphere annular mode. The AMOC weakening is also associated with a strengthening of Antarctic Bottom Water formation and warm SST biases in the Southern Ocean. It is also shown that cold biases in the tropical North Atlantic and West African/Indian monsoon regions during the warm season in the Northern Hemisphere have interhemispheric links with warm SST biases in the tropical southeastern Pacific and Atlantic, respectively. The results suggest that improving the simulation of regional processes may not suffice for overall better model performance, as the effects of remote biases may override them.
    Wang M. Y., J. E. Overland, 2012: A sea ice free summer Arctic within 30 years: An update from CMIP5 models,Geophys. Res. Lett.,39,L18501, https://doi.org/10.1029/2012GL052868.10.1029/2012GL052868d7d10d1cac4b9a72e6650d3aec8618e5http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2012GL052868%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2012GL052868/pdfSeptember 2008 followed 2007 as the second sequential year with an extreme summer Arctic sea ice extent minimum. Although such a sea ice loss was not indicated until much later in the century in the Intergovernmental Panel on Climate Change 4th Assessment Report, many models show an accelerating decline in the summer minimum sea ice extent during the 21st century. Using the observed 2007/2008 September sea ice extents as a starting point, we predict an expected value for a nearly sea ice free Arctic in September by the year 2037. The first quartile of the distribution for the timing of September sea ice loss will be reached by 2028. Our analysis is based on projections from six IPCC models, selected subject to an observational constraints. Uncertainty in the timing of a sea ice free Arctic in September is determined based on both within-model contributions from natural variability and between-model differences.
    Woollings T., J. M. Gregory, J. G. Pinto, M. Reyers, and D. J. Brayshaw, 2012: Response of the North Atlantic storm track to climate change shaped by ocean-atmosphere coupling.Nature Geoscience,5,313-317, https://doi.org/10.1038/ngeo1438.10.1038/ngeo1438547f3159c9ec9a1396d18aadd49cddbfhttp%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv5%2Fn5%2Fabs%2Fngeo1438.htmlhttp://www.nature.com/doifinder/10.1038/ngeo1438A poleward shift of the mid-latitude storm tracks in response to anthropogenic greenhouse-gas forcing has been diagnosed in climate model simulations1, 2. Explanations of this effect have focused on atmospheric dynamics3, 4, 5, 6, 7. However, in contrast to storm tracks in other regions, the North Atlantic storm track responds by strengthening and extending farther east, in particular on its southern flank8. These adjustments are associated with an intensification and extension of the eddy-driven jet towards western Europe9 and are expected to have considerable societal impacts related to a rise in storminess in Europe10, 11, 12. Here, we apply a regression analysis to an ensemble of coupled climate model simulations to show that the coupling between ocean and atmosphere shapes the distinct storm-track response to greenhouse-gas forcing in the North Atlantic region. In the ensemble of simulations we analyse, at least half of the differences between the storm-track responses of different models are associated with uncertainties in ocean circulation changes. We compare the fully coupled simulations with both the associated slab model simulations and an ocean-forced experiment with one climate model to establish causality. We conclude that uncertainties in the response of the North Atlantic storm track to anthropogenic emissions could be reduced through tighter constraints on the future ocean circulation.
    Yang S. T., J. H. Christensen, 2012: Arctic sea ice reduction and European cold winters in CMIP5 climate change experiments,Geophys. Res. Lett.,39,L20707, https://doi.org/10.1029/2012GL053338.10.1029/2012GL05333890d092cea023cd4dccecad2859795f93http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2012GL053338%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2012GL053338/pdfEuropean winter climate and its possible relationship with the Arctic sea ice reduction in the recent past and future as simulated by the models of the Climate Model Intercomparison Project phase 5 (CMIP5) is investigated, with focus on the cold winters. While Europe will warm overall in the future, we find that episodes of cold months will continue to occur and there remains substantial probability for the occurrence of cold winters in Europe linked with sea ice reduction in the Barents and Kara Sea sector. A pattern of cold-European warm-Arctic anomaly is typical for the cold events in the future, which is associated with the negative phase of the Arctic Oscillation. These patterns, however, differ from the corresponding patterns in the historical period, and underline the connection between European cold winter events and Arctic sea ice reduction.
    Zhang P. F., Y. T. Wu, and K. L. Smith, 2017: Prolonged effect of the stratospheric pathway in linking Barents-Kara Sea sea ice variability to the midlatitude circulation in a simplified model. Climate Dyn., https://doi.org/10.1007/s00382-017-3624-y, (in press)10.1007/s00382-017-3624-yed6354730cb40f92c62bdc17a9e29a3dhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-017-3624-yhttp://adsabs.harvard.edu/abs/2017EGUGA..1919203ZObservations show a delayed midlatitude circulation response during late winter following early winter Barents-Kara Sea (BKS) sea ice variability. To better understand the dynamical mechanism that accounts for the observed lead-lag correlation, a series of numerical experiments are conducted using a simplified atmospheric general circulation model (AGCM) with a prescribed idealized near-surface heating over the BKS region. A prolonged effect is found in the idealized experiments following the near-surface heating and can be explicitly attributed to the stratospheric pathway and the long time scale in the stratosphere. The analysis of the Eliassen-Palm (EP) flux shows that, as a result of the imposed heating and linear constructive interference, anomalous upward propagating planetary-scale waves are excited and weaken the stratospheric polar vortex. This stratospheric response persists for approximately 1-2 months accompanied by downward migration to the troposphere and the surface. This downward migration largely amplifies and extends the low-level jet deceleration in the midlatitudes and cold air advection over central Asia. The idealized model experiments also suggest that the BKS region is the most effective in affecting the midlatitude circulation than other regions over the Arctic.
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Manuscript History

Manuscript received: 22 June 2017
Manuscript revised: 14 September 2017
Manuscript accepted: 26 September 2017
通讯作者: 陈斌, bchen63@163.com
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Remarkable Link between Projected Uncertainties of Arctic Sea-Ice Decline and Winter Eurasian Climate

  • 1. Geophysical Institute, University of Bergen, Bergen 5007, Norway
  • 2. Bjerknes Centre for Climate Research, University of Bergen, Bergen 5007, Norway
  • 3. Nansen Environmental and Remote Sensing Center, Bergen 5006, Norway
  • 4. Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong, China
  • 5. City University of Hong Kong Shenzhen Research Institute, Shenzhen 518057, China

Abstract: We identify that the projected uncertainty of the pan-Arctic sea-ice concentration (SIC) is strongly coupled with the Eurasian circulation in the boreal winter (December-March; DJFM), based on a singular value decomposition (SVD) analysis of the forced response of 11 CMIP5 models. In the models showing a stronger sea-ice decline, the Polar cell becomes weaker and there is an anomalous increase in the sea level pressure (SLP) along 60°N, including the Urals-Siberia region and the Iceland low region. There is an accompanying weakening of both the midlatitude westerly winds and the Ferrell cell, where the SVD signals are also related to anomalous sea surface temperature warming in the midlatitude North Atlantic. In the Mediterranean region, the anomalous circulation response shows a decreasing SLP and increasing precipitation. The anomalous SLP responses over the Euro-Atlantic region project on to the negative North Atlantic Oscillation-like pattern. Altogether, pan-Arctic SIC decline could strongly impact the winter Eurasian climate, but we should be cautious about the causality of their linkage.

摘要: 本研究分析了CMIP5 11个模式对冬季(12月至翌年3月)北极海冰面积在本世纪末的预估的不确定性及其与欧亚环流的关系. 我们通过奇异值分解 (SVD)得出两者强耦合的主模态, 当中反映了北极海冰覆盖范围的预估. 当北极海冰范围减少的预估值比模式集合更大时, 极地环流相对更弱, 其南侧(约北纬60度)出现异常的下沉气流, 乌拉尔山至西伯利亚地区及冰岛一带的海平面气压相对更高. 与此同时, 中纬度的西风带和费雷尔环流 (Ferrell Cell) 相对更弱, 北大西洋海温相对更暖. 在地中海地区, 海平面气压相对偏低而降水相对较多. 此情形下北大西洋气压的差异类似北大西洋涛动的负位相. 总体而言, 北极海冰未来预估的不确定性或会影响到欧亚冬季气候的预估, 不过我们须谨慎分析它们的因果关系.

1. Introduction
  • Over Eurasia, the wintertime large-scale climatological circulation has two distinct characteristics. First, a dipole pressure pattern, consisting of the Icelandic low and the Azores high, extends zonally over the Euro-Atlantic region. This is strongly linked to the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO; Thompson and Wallace, 1998). Second, a cold-core surface high is centered over the Siberian-Mongolian region. The Siberian high is the key circulation feature of the East Asian winter monsoon that brings cold air masses equatorward via cold surges (Ding, 1994; Chang et al., 2006). In recent decades, one of the most distinct wintertime circulation features is the warm-Arctic-cold-midlatitude temperature pattern (Cohen et al., 2012; Kug et al., 2015; Sorokina et al., 2016). These temperature changes have motivated more research to study whether and how the Arctic changes and the frequency of extreme weather are connected in the present and the future climate [e.g., see the reviews of (Cohen et al., 2014), (Vihma, 2014), (Barnes and Polvani, 2015) and (Gao et al., 2015)]. Specifically, the strengthened Siberian high and the negative NAO/AO contributed to the cold extremes in the Eurasian continent (Honda et al., 2009; Cohen et al., 2012; Kim et al., 2014; Mori et al., 2014; King et al., 2016). However, these circulation changes could be due to the internal climate variability instead of the sea-ice loss (McCusker et al., 2016; Ogawa et al., 2017 a ).

    In recent decades, the sea-ice cover has dramatically decreased and this directly affects local heat fluxes and atmospheric circulation (Deser et al., 2010; Screen and Simmonds, 2010). Meanwhile, the Arctic warms faster than other regions and this is called Arctic Amplification (AA; e.g., Graversen et al., 2008). In the late 21st century, when the radiative forcing becomes much stronger than the present climate, the Arctic is expected to become ice-free in summer (Wang and Overland, 2012) and the AA will be stronger. In response to the sea-ice decline and AA, some numerical studies have shown a negative AO-like/NAO-like circulation (e.g., Magnusdottir et al., 2004; Sokolova et al., 2007; Peings and Magnusdottir, 2014; Blackport and Kushner, 2017), and a higher SLP over northern Eurasia (Deser et al., 2016). These circulation changes potentially advect more cold polar air equatorward. However, due to the warmer cold polar air, the AA-related cold-air outbreaks would be weaker than the present climate (Ayarzagüena and Screen, 2016). The thermodynamic effect due to increasing sea surface temperature (SST) would also outweigh the dynamic cooling caused by AA (Deser et al., 2016). The temperature over extratropical Eurasia would generally increase (Deser et al., 2010, 2016) and the probability of cold winters would decrease (Yang and Christensen, 2012), unlike the increasing tendency for the warm-Arctic-cold-midlatitude temperature pattern during the recent AA period (e.g., Cohen et al., 2014).

    Although there is high confidence that sea-ice cover will rapidly diminish under future global warming, its rate of retreat has a large intermodel spread across the CMIP5 models (Wang and Overland, 2012). Meanwhile, the midlatitude circulation changes could result from the competing effect of AA and other drivers (Vihma, 2014; Chen et al., 2016; Deser et al., 2016). Thus, the intermodel spread in sea-ice projection could be related to uncertainties in midlatitude atmospheric circulation change. For instance, over North America and the North Atlantic, models disagree on the sign of change of the wintertime midlatitude westerly wind and speed, but the intermodel spread of these quantities is significantly correlated to that of AA (Barnes and Polvani, 2015). Through analyzing model outputs from the CMIP5 archive, we address the following questions that have not been analyzed thoroughly by previous studies: (1) What are the projected uncertainties of Arctic sea-ice cover and the Eurasian climate? (2) What is the relationship between these projected uncertainties? (3) Do the models showing a stronger sea-ice decline correspond to cooling or less pronounced warming in any parts of Eurasia?

    To answer these questions, we focus on the intermodel spread of the forced response to the Arctic sea-ice decline. To minimize the internal atmospheric variability, we only analyze the 11 CMIP5 models (https://esgf-node.llnl.gov/projects/esgf-llnl/) that include at least three ensemble members for both the historical and RCP8.5 simulations (Table 1). The forced response is regarded as the climatological difference between the period 2069-98 in the RCP8.5 run and 1971-2000 in the historical run; below, we refer to this difference simply as "the response". In each model, the climatology is the unweighted average of all ensemble members listed in Table 1. The multimodel ensemble mean (MME) response is defined by averaging the individual model means. The reason for choosing 2069-98 instead of 2071-2100 is due to some missing outputs in 2099 and 2100. All atmospheric and oceanic variables are interpolated to a horizontal resolution of 2.5°× 2.5° and 1.0°× 1.0°, respectively.

2. Coherent model uncertainties in the sea-ice-atmosphere response
  • Before analyzing the coupled linkage between the projected uncertainty of sea-ice cover and the Eurasian circulation, we present the MME and the intermodel standard deviation of the forced response of the sea-ice concentration (SIC) over the Arctic and SLP over Eurasia during boreal winter [December-March (DJFM)] (Fig. 1). Compared to the present climate, the SIC has a robust decline over most of the Arctic in the late 21st century, and this is strongest over the Barents-Kara Sea (>60%; Fig. 1a). However, the intermodel standard deviation is also large over the Barents-Kara Sea (∼40%), and its magnitude is comparable to the MME response over large parts of the Arctic (Fig. 1b). This indicates a large uncertainty of SIC in boreal winter, agreeing with the CMIP3 results from (Hodson et al., 2013) and the CMIP5 results from (Wang and Overland, 2012).

    Figure 1.  (a, c) MME mean global warming response (2069-98 relative to 1971-2000) in DJFM and (b, d) the corresponding intermodel standard deviation of the response. (a, b) SIC (%), and (c, d) mean SLP (hPa), where the green box indicates the domain of the SVD analysis in Fig. 2. In (a) and (c), white and black dotted regions indicate at least 7 (∼ 65%) and 10 (∼ 90%) out of 11 models agreeing on the sign of change. In (b) and (d), contours indicate the MME mean response in (a) and (c).

    Associated with a robust decline in SIC, the SLP consistently decreases over the Arctic in the MME (Fig. 1c). In other polar regions, the strongest SLP decline is over the North Pacific and this is associated with a stronger Aleutian low (Fig. 1c). In contrast, the SLP over the North Atlantic and Greenland increases, but there is not a large agreement among models on the sign of response (<90%; Fig. 1c). These regions also have a large intermodel standard deviation, suggesting a large projected uncertainty of the Icelandic low and the NAO.

    There is also a robust response in SLP outside the polar region. This includes an increase in SLP near the Mediterranean Sea and a decrease in SLP over western Africa (Fig. 1c), suggesting a northeastward extension of the Azores high. In addition, the SLP increases robustly over Southeast Asia and south of Japan (Fig. 1c), reflecting an expansion of the subtropical high over the western North Pacific and a northward shift of the East Asian trough. In other parts of Eurasia, the MME response of the SLP is comparable and even smaller than the intermodel standard deviation, and most models do not agree on the sign of the response (Fig. 1d). As cold air originating from the polar region strongly influences the Eurasian climate, it is important to assess the potential links between the projected uncertainties in the Arctic SIC and the Eurasian climate.

    During the late 21st century, most models agree in simulating an ice-free Arctic in boreal autumn, whereas this agreement has a large spread in boreal winter. As the polar air strongly affects the lower-latitude regions, we hypothesize a simultaneous linkage between the intermodel spread of the forced response of the SIC (SIC) and the Eurasian circulation in boreal winter. To test our hypothesis, we use singular value decomposition (SVD) to identify the spatial pattern accounting for the largest fraction of their covariability.

    In the SVD analysis, the left-hand vector is the DJFM-mean response in (SIC) over the entire Arctic, whereas the right-hand vector is the DJFM-mean SLP response (SLP) over (0°-90°N, 60°W-180°), which is a domain able to capture the large-scale circulation features in Eurasia, of the 11 models. Note that the series of our SVD analysis is a time-invariant model-dependent parameter (i.e., the forced response of different models) instead of a time-varying parameter in a conventional SVD analysis, as mentioned in (Bretherton et al., 1992) and (Wallace et al., 1992). Therefore, the expansion coefficient of our SVD analysis is a series of 11 models instead of time steps. This approach was adopted in (Wang et al., 2014), who analyzed the covariability between the biases of the global SST and the meridional overturning circulation across CMIP5 models. Note that the power of the SVD analysis here is to identify the spatial patterns accounting for the largest covariability between the model uncertainties of SIC and SLP, and to quantify their covariability. The coupling between these uncertainties can be due to different physical processes. This is not trivial using simpler composite or correlation analysis, which requires an index (say, the sea-ice change in the Arctic) to be first defined. Such a composite analysis cannot measure how strongly the uncertainties of SIC are coupled to SLP.

    The first three SVD modes (SVD1-3) explain 70.5%, 16.4% and 6.63% of the total squared covariance, and hence we focus only on the first mode (SVD1). The correlation between the expansion coefficients of SIC from the SVD1 and SIC (the homogeneous correlation map) is shown in Fig. 2a. The spatial pattern of SIC represents a strong decline in SIC over most of the Arctic, except the Barents Sea openings. Consistently, the expansion coefficient of SIC is almost perfectly correlated (across the 11 models) to the DJFM-mean Arctic total sea-ice extent (the total area of grid points with SIC >0.15) for the period 2069-98, and the difference between 2069-98 and 1971-2000 (r=-0.97 in both cases).

    Figure 2.  The dominant relation between uncertainties in the global warming response in winter of sea ice and SLP over Eurasia, explaining 70% of intermodel covariability. Intermodel correlation between the expansion coefficients of the DJFM SIC response from the SVD1 and the response in DJFM-mean (a) SIC, (b) SLP, and (c) SLP (shading), 250-hPa wind (magenta arrows) and 850-hPa vector wind (black arrows); the left-hand vector and the right-hand vector of the SVD1 is the DJFM-mean change of sea-ice cover and the SLP, respectively. Thick gray lines denote p=0.1; white dotted regions and vectors indicate p<0.1.

    The correlation between the expansion coefficients of SIC of the SVD1 and SLP in Eurasia (the heterogeneous correlation map) is shown in Fig. 2b. The spatial pattern of SLP consists of a north-south-oriented dipole over the Euro-Atlantic region and a strong anticyclone over the Eurasian continent (Fig. 2b). First, the positive SLP over the North Atlantic suggests a weaker Icelandic low; here, models with stronger sea-ice loss reinforce the MME response (Fig. 1b). Second, the negative SLP over the Mediterranean region suggests a weaker northeastward extension of the Azores high; here, models with increased sea-ice loss suppress the MME response (Fig. 1b). Such a dipole SLP pattern suggests a linkage between the SVD1 and the projected difference of the NAO. Third, the signal over Asia represents a positive SLP anomaly at the northwestern flank of the Siberian high, where the climatological center is located at (40°-65°N, 80°-120°E) (Panagiotopoulos et al., 2005; Fig. 2b). This is opposite in sign to the MME response.

    The linkage between SIC and the large-scale circulation response (Fig. 2b) could be associated with thermal interaction or a large-scale response to an eddy-mean flow interaction. To assess the relative importance of these two effects, we correlate the SIC of the SVD1 to the wind response at 250 hPa (the upper troposphere) and at 850 hPa (the lower troposphere). When the linkage is related to an eddy-mean flow interaction, the atmospheric response has an equivalent barotropic structure. In contrast, the atmospheric response due to a thermal forcing has a baroclinic structure (Hoskins and Karoly, 1981; Overland and Wang, 2010; Jaiser et al., 2012). As shown in Fig. 2c, models with a stronger SIC decline exhibit lower SLP over the entire Arctic. In the lower troposphere, the associated wind response is an anticyclonic flow over the Asian side, suggesting a baroclinic response. On the other hand, the associated wind response is a cyclonic flow over the Euro-Atlantic side, suggesting an equivalent barotropic response associated with an eddy-mean flow interaction. Next, in section 3, we investigate if the atmospheric response related to the SIC of the SVD1 is accompanied by forcing originating from outside of the Arctic.

3. Linkage to large-scale circulation
  • We further depict the linkage between the SIC of the SVD1 and the DJFM-mean large-scale circulation features using intermodel regression, where the forced response of other variables is regressed against the standardized expansion coefficient of the left-hand vector of SVD1 (i.e., SIC). All statistical analyses apply the two-tailed Student's t-test with the 90% confidence level.

    Note that the objective of this study is to analyze the significant link between the projected uncertainties of the Arctic sea-ice decline and the Eurasian circulation. However, one may be interested to know if the intermodel response to the SIC of the SVD1 has the same or opposite sign to the MME response. One may also be interested to know if the sign of these responses is in large agreement across the models. Accordingly, we also show the MME response of the large-scale parameters and highlight the regions that the models agree on the sign of the response by at least 65% and 90%, i.e., the same as Figs. 1a and b.

  • During the recent AA period, one of the potential causes of the SIC decline is the remote signals of SST originating from the tropical Pacific (Ding et al., 2014; Trenberth et al., 2014). In particular, model studies suggest the Pacific Decadal Oscillation can contribute to AA (Svendsen et al., 2017; Tokinaga et al., 2017), and could modulate the response to sea-ice loss (Screen and Francis, 2016). Because part of the projected uncertainties of SIC is probably linked to the forcing outside the Arctic, it is interesting to see if the SIC of the SVD1 shows a strong linkage with the simultaneous response of the SST (SST) and the associated turbulent heat fluxes anywhere.

    As shown in Fig. 3, only the Barents-Kara Sea and the midlatitude North Atlantic have pronounced differences in DJFM-mean SST associated with a stronger SIC decline. In the former region, the models robustly simulate an increase in SST and turbulent heat fluxes (Figs. 3c and d), which is related to the SIC decline. Associated with a stronger SIC decline of the SVD1, both the SST and turbulent heat fluxes have a stronger increase (Figs. 3a and b). For the second region, the majority of models simulate a weakened Atlantic meridional overturning circulation in the 21st century, although with large uncertainties in strength (Cheng et al., 2013; Collins et al., 2013; Reintges et al., 2017). Whereas the models robustly simulate a reduction of turbulent heat fluxes (Fig. 3d), they have a small agreement for the SST projection in this region (Fig. 3c). Because a stronger SIC decline of the SVD1 accompanies an anomalous SST warming in this region (Fig. 3b), the projected uncertainty of SIC may be related to the Atlantic meridional overturning circulation, either through an oceanic pathway (Årthun et al., 2012) or an atmospheric connection (Sato et al., 2014). Specifically, models with stronger SST warming coincide with stronger turbulent heat fluxes locally (Fig. 3b). This is associated with a decrease in the low-level baroclinicity (figure not shown) and weaker westerly winds in the lower and upper troposphere (Fig. 2c). Therefore, the midlatitude circulation response uncertainties associated with the SIC of the SVD1 could be due to both the projected uncertainties of the SIC decline and the SST warming in the North Atlantic (Woollings et al., 2012). The tropical SSTs seem to play an insignificant role in the dominant linkage between the uncertainties of sea-ice-Northern Hemisphere atmospheric responses in winter.

    Figure 3.  (a, b) Intermodel regression of the forced response against the standardized expansion coefficient of SVD1 in boreal winter: (a) SST (K); (b) turbulent heat fluxes (shading; W m-2; positive upwards) and SLP (contours; hPa). Thick gray lines denote p=0.1 and dotted regions indicate p<0.1 for the shaded variable. (c, d) As in (a, b) but for the MME response of the shaded terms in (a, b), where white and black dotted regions indicate at least 7 (∼ 65%) and 10 (∼ 90%) out of 11 models agreeing on the sign of change.

  • The spatial pattern of both the SIC of the SVD1 and its associated turbulent heat fluxes in the polar region exhibit strong zonal wave number-0 components (Fig. 2a and Fig. 3b). Thus, we explore the linkage between the SIC of the SVD1 and the DJFM zonal-mean circulation changes at different altitudes. Among the 11 models, only three are high-top models with a model top above the stratopause (Table 1). Assuming that the low-top models do not resolve the stratospheric dynamics well, we only show the composite differences up to the 100-hPa level (the lower stratosphere).

    A stronger SIC decline associated with SVD1 is linked to an increased zonal-mean Arctic warming confined to the lower troposphere (Fig. 4a). Compared to the MME response, models with a stronger SIC decline (Fig. 4a) do not contribute significantly to the intermodel spread in the pronounced upper-tropospheric warming aloft in the Arctic and outside of the Arctic (Fig. 4d). This is consistent with other studies (e.g., Screen and Simmonds, 2010; Manzini et al., 2014; Blackport and Kushner, 2017; Ogawa et al., 2017 a). Models with more pronounced lower-tropospheric warming in the Arctic than in the low-latitude region exhibit weakening of the equator-to-pole temperature gradient and midlatitude westerlies (Fig. 4b). These tropospheric circulation features are the first-order response of AA (Cohen et al., 2014; Vihma, 2014). The SVD analysis suggests the uncertainties in the MME response seen in the midlatitude westerlies (Fig. 4e) are related to pan-Arctic sea-ice decline.

    The dynamical response corresponding to a stronger SIC decline of the SVD1 can be approximated by weaker tropospheric Polar and Ferrell cells, where the mass stream function response is opposite in sign to the climatology, and the boundary between these two cells shifts southward (i.e., the zero-line shifts southward; Figs. 4b and c). When less cold polar air sinks near the surface, the SLP becomes lower across the polar region (Fig. 2b). This is associated with an anomalous upward motion in the poleward branch of the Polar cell, and an anomalous downward motion in the equatorward branch of the Polar cell and the poleward branch of the Ferrell cell (Figs. 4b and c). Due to the linkage between the vertical velocity and the surface divergence, there is a stronger increase in SLP around 60°N (Fig. 2b), where the anomalous zonal-mean downward motion is strongest (Fig. 4b). At the southern flank of the positive SLP response linked to a stronger Arctic warming response (Fig. 2b and Fig. 4a), the deceleration of westerly winds is strongest (∼50°N; Fig. 4b). This anomalous zonal-mean zonal wind response has a barotropic structure, with pronounced easterly anomalies in the upper troposphere and the lower stratosphere (Fig. 4b).

    Figure 4.  (a-c) Latitude-height cross sections showing the intermodel regression of the forced response of the zonal-mean fields against the standardized expansion coefficient of SVD1: (a) air temperature (K); (b) zonal-mean zonal wind (shading; m s-1) and meridional wind together with the vertical velocity (vectors; m s-1 in the meridional direction and 0.01 Pa s-1 in the vertical direction); (c) mass stream function (109 kg s-1), where the black contours represent the 2069-98 climatology (109 kg s-1). Thick green lines denote p=0.1 and dotted regions have p<0.1. (d-f) As in (a-c), but for the MME response of the shaded terms in (a-c), where white and black dotted regions indicate at least 7 (∼65%) and 10 (∼90%) out of 11 models agreeing on the sign of change. In (e), the lines represent the intermodel standard deviation (interval: 0.25 m s-1) of the zonal-mean zonal wind change.

    It should be noted that the models do not robustly simulate a weaker Polar cell in the lower troposphere by the end of the century (Fig. 4f), although the SIC decline is a robust signal (Fig. 1a). This suggests that the MME response (not its uncertainties) of surface circulation changes in the Arctic are also influenced by the forcing other than the sea ice, such as tropical SST forcing (e.g., Ding et al., 2014). Moreover, the models tend to simulate a strong Polar cell in the upper troposphere (Fig. 4f). Similarly, whereas the models robustly simulate weakening of the upper-tropospheric zonal wind aloft in the Arctic (Fig. 4e), the zonal-mean zonal wind here is slightly weakened by a stronger SIC decline of the SVD1 (Fig. 4b). Although the regressed anomalies are statistically significant, the magnitude is small compared to the MME response (Figs. 4b and e). These again suggest that the strong sea-ice decline of the SVD1 is not associated with strong upper-tropospheric circulation changes aloft in the Arctic.

    In the midlatitudes, the zonal-mean zonal wind generally strengthens and this MME response is most robust near the tropopause and in the lower stratosphere (Fig. 4e). This is due to an intensification and a northward shift of the subtropical jet in response to global warming (Seidel et al., 2008). Because the zonal-mean zonal wind response that is linked to a stronger SIC decline of the SVD1 is opposite in sign to the MME response (Figs 4b and e), the SIC-related forcing appears to weaken the global warming response. This contrast can also be seen in the mass stream function of the Ferrell cell, where the anomalous response to a stronger SIC decline of the SVD1 is positive in sign (Fig. 4c) and the MME response is negative in sign (Fig. 4f). The positive anomalous response suggests an anomalously weaker Ferrell cell (Fig. 4c), which accompanies less poleward transport of eddy momentum and heat fluxes. In addition, the intermodel spread of the zonal-mean zonal wind is largest in the stratosphere (above 100 hPa; not shown) and it extends downward into the lower troposphere (Fig. 4e). The strengthening of the midlatitude zonal-mean zonal wind in the MME response appears to be linked to the stratospheric signals, whereas the weakening of the zonal-mean zonal wind in the SVD1 is due to SIC-related signals. The former is consistent with (Manzini et al., 2014), who highlighted the importance of stratospheric forcing in future surface circulation changes.

    In addition to the linkage with anomalously weaker Polar and Ferrell cells, the stronger SIC decline of the SVD1 is linked to an overall weaker Hadley cell (Fig. 4c). Similar to the MME response, the anomalous response to a stronger SIC decline of the SVD1 suggests a stronger Hadley cell at its northern edge and in the upper troposphere (Fig. 4f). This represents a northward shift and a deeper Hadley cell. In short, the SIC of the SVD1 is linked to the hemispheric-scale circulation in boreal winter, where the classical three-cell meridional circulations are weakened, consistent with weaker poleward heat transport (Kang et al., 2008).

  • Whereas the SIC of the SVD1 has a strong linkage with the projected difference of the zonal-mean circulation, it also has a zonal asymmetric component (Fig. 2a). But how strongly does it affect the intermodel agreement of the large-scale circulation features in Eurasia, including the heterogeneous SLP pattern as shown in Fig. 2b? To demonstrate these linkages, we show the intermodel regression of different large-scale atmospheric variables against the expansion coefficient of the SVD1 for the DJFM period in Fig. 5. Because the SIC of the SVD1 is almost perfectly correlated to the response of the total sea-ice extent, we also define several large-scale circulation indices (Table 2) and show their scatterplot against the response of the total Arctic sea-ice extent for the DJFM period in Fig. 6.

    3.3.1. Central and East Asia

    Recall that the MME of SIC shows the largest decrease in SIC around the sea-ice edge, where the primary center is located at the Barents-Kara Sea (>60%) and the secondary center is located at the Bering Strait (>40%; Fig. 1a). The intermodel regression shows that the largest decrease north of the Kara Sea (>50%) and the difference over the Barents Sea opening is insignificantly small (<10%; figure not shown). The local response to stronger pan-Arctic sea-ice decline exhibits the largest increase in surface air temperature near the Kara Sea (Fig. 5a). Meanwhile, the stronger sea-ice decline leads to an increase in the water vapor content in the air column (Bintanja and Selten, 2014). This also enhances the precipitation (Fig. 5b) and decreases the vertical stability (Fig. 5d) locally. These changes reinforce the MME response (Figs. 5e-g). As the Arctic warming extends upward in the lower troposphere, the 1000-500 hPa thickness height increases and attains a maximum over the Barents-Kara Sea (∼75°N, 50°E; Fig. 5a). This is associated with a stronger surface anticyclone over the Urals-Siberia region (∼60°-110°E) and stronger southerly winds near the Barents Sea (Fig. 5b).

    Figure 5.  Intermodel regression against the standardized expansion coefficient of SVD1 in DJFM: (a) surface air temperature (shading; K) and thickness height between 1000 and 500 hPa; (b) precipitation (shading; mm month-1) and 850-hPa wind (black vectors; m s-1), (c) meridional surface air temperature gradient (10-5 K m-1); (d) vertical stability at 925 hPa (K hPa-1). Thick white lines denote p=0.1 and dotted regions and vectors have p<0.1. (e-h) As in (a-d) but for the MME response of the shaded terms in (a-d), where white and black dotted regions indicate at least 7 (∼ 65%) and 10 (∼ 90%) out of 11 models agreeing on the sign of change.

    Figure 6.  Scatterplots of the forced response of large-scale circulation indices against the decrease in sea-ice extent in DJFM: (a) Urals-Siberia SLP; (b) Icelandic low index; (c) Mediterranean SLP; (d) NAO index. In each plot, the number denotes the response of individual models listed in Table 1, whereas the open circle represents the MME response. The correlation of the intermodel regression line (thick solid line) and the corresponding level of significance are shown at the top.

    The intermodel correlation between the SLP response over the Urals-Siberia region and the pan-Arctic sea-ice decline is -0.752 (∼ 57% of the total variance; Fig. 6a). The SIC signals of the SVD1 appear to modulate instead of dominate the SLP response, as most models (9 out of 11) simulate a negative SLP response over this region (Fig. 6a). The anticyclone related to the increased SIC decline extends across the whole of northern Asia. Whereas the stronger anticyclone likely strengthens the northerly cold-air advection, the meridional temperature gradient over the high-latitude region sharply decreases and this weakens the northerly cold-air advection (Fig. 5c). Hence, it is unclear if the seasonal-mean cold-air advection is strengthened by a larger sea-ice decline of SVD1. Note that correlation analysis does not imply any causality of the linkage (i.e., increased Arctic SIC decline could instead be driven by the Eurasian SLP changes, or both the sea ice and SLP might be independently affected by a third factor).

    The anomalous surface air temperature response of the SVD1 shows a more pronounced warming spread across the high-latitude region of Asia (Fig. 5e), whereas part of the Siberian-Mongolian region (∼ 40°-55°N, 90°-120°E) has a slight and insignificant "cooling" associated with the SVD1 [note that this "cooling" means the warming is less pronounced, as the magnitude of the intermodel regression is much smaller than the MME response (Figs. 5a and e). The stronger increase in temperature over northern Asia (Fig. 5a) is mainly due to the stronger reduction in the meridional temperature gradient (Fig. 5c). Part of the stronger warming over Northeast Asia (∼ 100°-140°E) is related to the increase in vertical stability (Fig. 5d). The change in the downwelling shortwave radiation and the turbulent heat fluxes play an insignificant role (figure not shown).

    3.3.2. Euro-Atlantic region

    Over the Euro-Atlantic region, the intermodel regression against the SVD1 projects on to a negative NAO-like dipole pattern, with an anomalous high near Iceland, weak anomalies over the subtropical Atlantic, and an anomalous low near the Mediterranean Sea (Fig. 2b). On the one hand, the majority of models (9 out of 11) simulate a weaker Icelandic low that is intensified in models simulating a stronger sea-ice decline of the SVD1 (Fig. 6b). A stronger sea-ice decline is associated with a weaker Polar cell and anomalous downward motion near 60°N (Fig. 4b), which is close to the center of action of the Icelandic low. Moreover, a stronger sea-ice decline of the SVD1 is accompanied by a stronger Arctic warming and a smaller equator-to-pole temperature gradient. According to (Harvey et al., 2015), this is related to the lower tropospheric baroclinicity and is hence crucial for reducing the storm tracks in the northern North Atlantic (see their Fig. 5c). As the sea-ice decline is a robust feature in the future climate, the increase in SLP near the Icelandic low region appears to be linked to the storm track changes. Under a stronger SIC decline of the SVD1, the meridional surface temperature gradient becomes weaker along the Gulf Stream (Fig. 5c). As can also be seen in Fig. 5b, this accompanies an anomalous anticyclonic flow and negative precipitation anomalies extending northeastward from Iceland toward Scandinavia. All the aforementioned features suggest a further reduction in the Northeastern Atlantic storm tracks (Rogers, 1997), which needs to be investigated in future studies.

    On the other hand, all but one of the models simulate an increase in SLP in Mediterranean Europe (Fig. 6c), and this response is strongly suppressed by a stronger SIC decline of the SVD1 (Fig. 2b). As can be inferred from Fig. 5b, a stronger sea-ice decline is associated with an anomalous cyclonic flow over the tropical and subtropical North Atlantic. The Azores high might have a smaller northeastward extension toward Mediterranean Europe, where the SLP robustly increases (Fig. 1c). This anomalous response can be regarded as a weaker Hadley cell (Fig. 4c), where the intermodel correlation between the zonal-mean mass stream function averaged over 10°-20°N in the 850-500 hPa levels and the SLP over Mediterranean Europe is +0.872. The anomalous low over Mediterranean Europe is associated with a stronger southerly advection of the warm subtropical air toward southeastern Europe. This accompanies an anomalous increase in surface air temperature and precipitation over part of Central Europe, Mediterranean Europe and the Middle East (Figs. 5a and b).

    Because a stronger pan-Arctic sea-ice decline is linked to weakening of the Icelandic low but little change to the intensity of the Azores high, it has a significant negative correlation with the NAO response (Fig. 6d). However, it is noticeable that the NAO response does not robustly show a negative tendency. The spread is consistent with the inconsistency of the NAO response among previous studies (Vihma, 2014), suggesting other factors also affecting the NAO change. Moreover, the stronger negative NAO response does not correspond to a colder and even a less warm climate over Europe (Fig. 5a). Altogether, a stronger pan-Arctic sea-ice decline in boreal winter might significantly modulate the key circulation features over Eurasia, where the anomalous SLP and precipitation responses (Fig. 2b and Fig. 5b) are often opposite in sign to the MME response (Fig. 1c and Fig. 5f). However, an anomalous high does not correspond to anomalous cooling, unlike the warm-Arctic-cold-Eurasia temperature pattern during the recent AA period (e.g., Cohen et al., 2014).

4. Summary and discussion
  • We have demonstrated strong linkages between the intermodel spread of the pan-Arctic sea-ice decline and the Eurasian climate. The linkages explain 70.5% of the total variance, when represented by the joint SVD1 mode of the Arctic SIC (as the left-hand vector) and the Eurasian SLP (as the right-hand vector). The intermodel spread of the Arctic SIC is significantly linked to the MME response of the Eurasian climate, including (1) the SLP over the Eurasian continent, (2) the Icelandic low and possibly the northeastern Atlantic storm tracks, (3) the SLP over Mediterranean Europe, and (4) the eastward shift of the NAO-like response.

    Our results suggest that a stronger Arctic sea-ice decline of the SVD1 is associated with an anomalous increase in SLP over high-latitude Eurasia, including the Urals-Siberia region and the Icelandic low region. However, we did not find significantly stronger northerly winds over Eurasia. Nor did we find any cooling or even less pronounced warming in any part of Eurasia. This is different from the present climate, where a stronger high pressure and persistent snow cover over Eurasia might enhance the upward propagation of the planetary waves from the troposphere to the stratosphere (e.g., Allen and Zender, 2011; Cohen et al., 2012; Kim et al., 2014). One possible reason is that only 3 out of the 11 models have a well-resolved stratosphere, which is crucial for simulating the midlatitude atmospheric response associated with the troposphere-stratosphere interaction (Omrani et al., 2014, 2016; Nakamura et al., 2015; Zhang et al., 2017).

    Other possible reasons are that the impact of the pan-Arctic sea-ice loss is different from the regional sea-ice loss (Screen, 2017), or the recent changes are dominated by internal atmospheric variability (McCusker et al., 2016; Ogawa et al., 2017 a). Regarding the former factor, a stronger anticyclone over high-latitude Eurasia driven by a regional sea-ice decline over the Barents-Kara Sea could enhance the downstream cold-air advection, as the sea ice is still present downstream. The resultant dynamic effect of regional sea-ice loss could cause cooling in the mid and high latitudes (Mori et al., 2014; Kug et al., 2015; Overland et al., 2015).

    Conversely, in response to the pan-Arctic sea-ice loss, the cold-air intensity over the entire Arctic becomes weaker. The meridional temperature gradient in the high latitudes decreases and the midlatitude westerlies weaken. Unless the northerly winds become much stronger (such as a higher amplitude flow), the northerly cold-air advection would be weaker. (Screen, 2014) also suggested that the northerly wind makes a larger contribution to the warming trend in the high-latitude region than the southerly wind during boreal winter. Recently, (Meleshko et al., 2016) showed that the ocean heat transport is more important for the higher amplitude planetary wave in the midlatitudes. Based on these findings, it is unlikely that the pan-Arctic sea-ice loss causes any cooling effect in the extratropical region on seasonal timescales, which agrees with the results of (Deser et al., 2016) and (Screen, 2017).

    A stronger Arctic sea-ice decline of the SVD1 is also linked to weakening of the three-cell circulations and a warmer SST in the midlatitude North Atlantic. In the midlatitudes, a weaker Ferrell cell is characterized by a higher SLP in the midlatitudes and weaker zonal-mean zonal winds. However, we cannot assess the causality of these linkages. Indeed, the stronger SST warming over the North Atlantic could enhance the poleward ocean heat transport and could then melt more sea ice (Mahlstein and Knutti, 2011; Jung et al., 2017; Nummelin et al., 2017). The basin-wide Atlantic warming is also crucial for the negative tendency of the NAO, via an atmospheric wave train (Sato et al., 2014) and troposphere-stratosphere interaction (Omrani et al., 2016). The atmospheric response to the sea-ice decline might also be highly nonlinear (Petoukhov and Semenov, 2010). In a future study, we intend to design several sensitivity experiments based on the intermodel spread of SST and SIC, in order to assess the relative importance of these projected uncertainties in the future climate change of the Northern Hemisphere.

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