doi:10.1007/s00376-017-6294-0

Alexand er, M. A., U. S. Bhatt, J. E. Walsh, M. S. Timlin, J. S. Miller, J. D. Scott, 2004: The atmospheric response to realistic Arctic sea ice anomalies in an AGCM during winter. J. Climate, 17, 890-905, .https://doi.org/10.1175/1520-0442(2004)017<0890:TARTRA>2.0,CO;2

10.1175/1520-0442(2004)017<0890:TARTRA>2.0.CO;2

fc0ae199b9c2388844c40377f6b33268

http%3A%2F%2Fadsabs.harvard.edu%2Fcgi-bin%2Fnph-data_query%3Fbibcode%3D2004JCli...17..890A%26amp%3Bdb_key%3DPHY%26amp%3Blink_type%3DABSTRACT%26amp%3Bhigh%3D06304

http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282004%29017%3C0890%3ATARTRA%3E2.0.CO%3B2

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.

10.1002/2016GL068092

438b3f1c3b6ec03db8a2cb18261ab806

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2016GL068092%2Ffull

http://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., 2013: Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes.Geophys. Res. Lett.,40,4734-4739, https://doi.org/10.1002/grl.50880.

10.1002/grl.50880

4eac873e26e3495cfdaa4b0a5b97ec51

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fgrl.50880%2Ffull

http://doi.wiley.com/10.1002/grl.50880Previous studies have suggested that Arctic amplification has caused planetary-scale waves to elongate meridionally and slow down, resulting in more frequent blocking patterns and extreme weather. Here trends in the meridional extent of atmospheric waves over North America and the North Atlantic are investigated in three reanalyses, and it is demonstrated that previously reported positive trends are likely an artifact of the methodology. No significant decrease in planetary-scale wave phase speeds are found except in October-November-December, but this trend is sensitive to the analysis parameters. Moreover, the frequency of blocking occurrence exhibits no significant increase in any season in any of the three reanalyses, further supporting the lack of trends in wave speed and meridional extent. This work highlights that observed trends in midlatitude weather patterns are complex and likely not simply understood in terms of Arctic amplification alone.

Barnes E. A., E. Dunn-Sigouin G. Masato, and T. Woollings, 2014: Exploring recent trends in Northern Hemisphere blocking.Geophys. Res. Lett.,41,638-644, https://doi.org/10.1002/2013GL058745.

10.1002/2013GL058745

ced64c628dd9b61b16dc6f2091d45ed0

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2013GL058745%2Fpdf

http://doi.wiley.com/10.1002/2013GL058745Observed blocking trends are diagnosed to test the hypothesis that recent Arctic warming and sea ice loss has increased the likelihood of blocking over the Northern Hemisphere. To ensure robust results, we diagnose blocking using three unique blocking identification methods from the literature, each applied to four different reanalyses. No clear hemispheric increase in blocking is found for any blocking index, and while seasonal increases and decreases are found for specific isolated regions and time periods, there is no instance where all three methods agree on a robust trend. Blocking is shown to exhibit large interannual and decadal variability, highlighting the difficulty in separating any potentially forced response from natural variability.

Chen M.Y., W. Q. Wang, and A. Kumar, 2013: Lagged ensembles,forecast configuration,and seasonal predictions. Mon. Wea. Rev.,141, 3477-3497,.https://doi.org/10.1175/MWR-D-12-00184.1

Cohen J. L., J. C. Furtado, M. Barlow, V. A. Alexeev, and J. E. Cherry, 2012: Asymmetric seasonal temperature trends,Geophys. Res. Lett.,39,L04705, https://doi.org/10.1029/2011GL050582.

Cohen, J., Coauthors, 2014: Recent Arctic amplification and extreme mid-latitude weather.Nature Geoscience,7,627-637, https://doi.org/10.1038/NGEO2234.

10.1038/ngeo2234

4cd471caba2fba3502432bd1eab5ae32

http%3A%2F%2Fwww.nature.com%2Fabstractpagefinder%2F10.1038%2Fngeo2234

http://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.

Comiso J. C., C. L. Parkinson, R. Gersten, and L. Stock, 2008: Accelerated decline in the arctic sea ice cover,Geophys. Res. Lett.,35,L01703, https://doi.org/10.1029/2007GL031972.

10.1029/2007GL031972

343feb606e7415f45d25b40e917085b6

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2007GL031972%2Fpdf

http://onlinelibrary.wiley.com/doi/10.1029/2007GL031972/pdfSatellite data reveal unusually low Arctic sea ice coverage during the summer of 2007, caused in part by anomalously high temperatures and southerly winds. The extent and area of the ice cover reached minima on 14 September 2007 at 4.1 10kmand 3.6 10km, respectively. These are 24% and 27% lower than the previous record lows, both reached on 21 September 2005, and 37% and 38% less than the climatological averages. Acceleration in the decline is evident as the extent and area trends of the entire ice cover (seasonal and perennial ice) have shifted from about -2.2 and -3.0% per decade in 1979-1996 to about -10.1 and -10.7% per decade in the last 10 years. The latter trends are now comparable to the high negative trends of -10.2 and -11.4% per decade for the perennial ice extent and area, 1979-2007.

Dee, D. P., Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system.Quart. J. Roy. Meteor. Soc.,137,553-597, https://doi.org/10.1002/qj.828.

10.1002/qj.828

5b3115ec8b338ee97111270a1831c4b2

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.828%2Fpdf

http://doi.wiley.com/10.1002/qj.v137.656ERA-Interim is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA-Interim project was conducted in part to prepare for a new atmospheric reanalysis to replace ERA-40, which will extend back to the early part of the twentieth century. This article describes the forecast model, data assimilation method, and input datasets used to produce ERA-Interim, and discusses the performance of the system. Special emphasis is placed on various difficulties encountered in the production of ERA-40, including the representation of the hydrological cycle, the quality of the stratospheric circulation, and the consistency in time of the reanalysed fields. We provide evidence for substantial improvements in each of these aspects. We also identify areas where further work is needed and describe opportunities and objectives for future reanalysis projects at ECMWF. Copyright 2011 Royal Meteorological Society

Deser C., G. Magnusdottir, R. Saravanan, and A. Phillips, 2004: The effects of North Atlantic SST and sea ice anomalies on the winter circulation in CCM3. Part II: Direct and indirect components of the response. J. Climate, 17, 877-889, https://doi.org/10.1175/1520-0442(2004)017 <0877:TEONAS>2.0,CO;2.10.1175/1520-0442(2004)0172.0.CO;2

cb7f9af6c5c735dc8ecff23c70090363

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2004JCli...17..877D

http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282004%29017%3C0877%3ATEONAS%3E2.0.CO%3B2Observed multidecadal trends in extratropical atmospheric flow, such as the positive trend in the North Atlantic Oscillation (NAO) index, may be attributable to a number of causes. This study addresses the question of whether the atmospheric trends may be caused by observed trends in oceanic boundary forcing. Experiments were carried out using the NCAR atmospheric general circulation model with specified sea surface temperature (SST) and sea ice anomalies confined to the North Atlantic sector. The spatial pattern of the anomalous forcing was chosen to be realistic in that it corresponds to the recent 40-yr trend in SST and sea ice, but the anomaly amplitude was exaggerated in order to make the response statistically more robust. The wintertime response to both types of forcing resembles the NAO to first order. Even for an exaggerated amplitude, the atmospheric response to the SST anomaly is quite weak compared to the observed positive trend in the NAO, but has the same sign, indicative of a weak positive feedback. The anomalies in sea ice extent are more efficient than SST anomalies at exciting an atmospheric response comparable in amplitude to the observed NAO trend. However, this atmospheric response has the opposite sign to the observed trend, indicative of a significant negative feedback associated with the sea ice forcing. Additional experiments using SST anomalies with opposite sign to the observed trend indicate that there are significant nonlinearities associated with the atmospheric response. The transient eddy response to the observed SST trend is consistent with the positive NAO response, with the North Atlantic storm track amplifying downstream and developing a more pronounced meridional tilt. In contrast, the storm track response to the observed sea ice trend corresponds to a weaker, southward-shifted, more zonal storm track, which is consistent with the negative NAO response. 1.

Ding, Q. H., Coauthors, 2017: Influence of high-latitude atmospheric circulation changes on summertime Arctic sea ice.Nat. Clim. Change,7,289-295, https://doi.org/10.1038/nclimate3241.

10.1038/nclimate3241

1da74620564d58c7ba8872dcc685e144

http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv7%2Fn4%2Fnclimate3241%2Fmetrics

http://www.nature.com/doifinder/10.1038/nclimate3241The Arctic is warming and sea ice is declining, but how the two link is unclear. This study shows changes in summertime atmospheric circulation and internal variability may have caused up to 60% of September sea-ice decline since 1979.

Francis J. A., S. J. Vavrus, 2012: Evidence linking Arctic amplification to extreme weather in mid-latitudes,Geophys. Res. Lett.,39,L06801, https://doi.org/10.1029/2012GL051000.

10.1029/2012GL051000

c6c56c6a021edd43d82f7e1fc8bad5d9

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2012GL051000%2Ffull

http://onlinelibrary.wiley.com/doi/10.1029/2012GL051000/fullIn this presentation, we will build on analysis presented in Francis and Vavrus (GRL, 2012) in which mechanisms were proposed and demonstrated that link enhanced warming in the Arctic during recent decades with changes in the trajectory of the upper-level flow in mid-latitudes. Evidence was presented that suggests Arctic Amplification may have contributed to an increase in large-scale wave amplitude and slower zonal winds, both of which favor more persistent weather patterns in mid-latitudes. Prolonged weather conditions are often associated with extreme weather -- such as droughts, cold spells, heat waves, and some flooding events, -- which have become more frequent in recent years. New analysis of fields from reanalyses will be presented that illustrates the response of mid-latitude upper-level flow characteristics to Arctic Amplification, in particular the regional and seasonal variability in large-scale wave propagation speed and wave amplitude that may favor an increase in extreme weather events.

Francis J. A., S. J. Vavrus, 2015: Evidence for a wavier jet stream in response to rapid Arctic warming,Environmental Research Letters,10,014005, https://doi.org/10.1088/1748-9326/10/1/014005.

10.1088/1748-9326/10/1/014005

642468448ecf3fa179cb674a410f211e

http%3A%2F%2Fwww.ingentaconnect.com%2Fcontent%2Fiop%2Ferl%2F2015%2F00000010%2F00000001%2Fart014005

http://stacks.iop.org/1748-9326/10/i=1/a=014005?key=crossref.74581076f734b2377ec8042d3aebe25dNew metrics and evidence are presented that support a linkage between rapid Arctic warming, relative to Northern hemisphere mid-latitudes, and more frequent high-amplitude (wavy) jet-stream configurations that favor persistent weather patterns. We find robust relationships among seasonal and regional patterns of weaker poleward thickness gradients, weaker zonal upper-level winds, and a more meridional flow direction. These results suggest that as the Arctic continues to warm faster than elsewhere in response to rising greenhouse-gas concentrations, the frequency of extreme weather events caused by persistent jet-stream patterns will increase.

Furtado J. C., J. L. Cohen, A. H. Butler, E. E. Riddle and A. Kumar, 2015: Eurasian snow cover variability and links to winter climate in the CMIP5 models.Climate Dyn.,45,2591-2605, https://doi.org/10.1007/s00382-015-2494-4.

10.1007/s00382-015-2494-4

e4c5b6e8b26fd8f6a5b570abb999193f

http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-015-2494-4

http://link.springer.com/10.1007/s00382-015-2494-4Observational studies and modeling experiments illustrate that variability in October Eurasian snow cover extent impacts boreal wintertime conditions over the Northern Hemisphere (NH) through a dynamical pathway involving the stratosphere and changes in the surface-based Arctic Oscillation (AO). In this paper, we conduct a comprehensive study of the Eurasian snow-AO relationship in twenty coupled climate models run under pre-industrial conditions from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our analyses indicate that the coupled climate models, individually and collectively, do not capture well the observed snow-AO relationship. The models lack a robust lagged response between October Eurasian snow cover and several NH wintertime variables (e.g., vertically propagating waves and geopotential heights). Additionally, the CMIP5 models do not simulate the observed spatial distribution and statistics of boreal fall snow cover across the NH including Eurasia. However, when analyzing individual 40-year time slices of the models, there are periods of time in select models when the observed snow-AO relationship emerges. This finding suggests that internal variability may play a significant role in the observed relationship. Further analysis demonstrates that the models poorly capture the downward propagation of stratospheric anomalies into the troposphere, a key facet of NH wintertime climate variability irrespective of the influence of Eurasian snow cover. A weak downward propagation signal may be related to several factors including too few stratospheric vortex disruptions and weaker-than-observed tropospheric wave driving. The analyses presented can be used as a roadmap for model evaluations in future studies involving NH wintertime climate variability, including those considering future climate change.

Gao, Y. Q., Coauthors, 2015: Arctic sea ice and Eurasian climate: A review.Adv. Atmos. Sci.,32,92-114, https://doi.org/10.1007/s00376-014-0009-6.

10.1007/s00376-014-0009-6

d871921aaa624189bf66d49c90050b24

http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00376-014-0009-6

http://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。

Griffies S. M., M. J. Harrison, R. C. Pacanowski, and A. Rosati, 2003: Technical guide to MOM4. GFDL Ocean Group Technical Report No. 5 NOAA/Geophysical Fluid Dynamics Laboratory, 337 pp. [Available online at .]https://www.gfdl.noaa.gov/\simfms

94ea74fdf82b7f26c2f9c7d8a96f1d86

http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F245105251_A_Technical_Guide_to_MOM4

http://www.researchgate.net/publication/245105251_A_Technical_Guide_to_MOM4Available online at www.gfdl.noaa.gov Information about how to download and run MOM4 can be found at the GFDL Flexible Modeling System (FMS) web site accessible from www.gfdl.noaa.gov. This document was prepared using L ATEX as described by Lamport (1994) and Goosens et al. (1994).CONTENTS I Basics of MOM4 11

Hand orf, D., R. Jaiser, K. Dethloff, A. Rinke, J. Cohen, 2015: Impacts of Arctic sea ice and continental snow cover changes on atmospheric winter teleconnections.Geophys. Res. Lett.,42,2367-2377, https://doi.org/10.1002/2015GL063203.

10.1002/2015GL063203

9f31473f829920b4fd2ac9efde054976

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2015GL063203%2Fpdf

http://doi.wiley.com/10.1002/2015GL063203Abstract Extreme winters in Northern Hemisphere midlatitudes in recent years have been connected to declining Arctic sea ice and continental snow cover changes in autumn following modified planetary waves in the coupled troposphere-stratosphere system. Through analyses of reanalysis data and model simulations with a state-of-the-art atmospheric general circulation model, we investigate the mechanisms between Arctic Ocean sea ice and Northern Hemisphere land snow cover changes in autumn and atmospheric teleconnections in the following winter. The observed negative Arctic Oscillation in response to sea ice cover changes is too weakly reproduced by the model. The planetary wave train structures over the Pacific and North America regions are well simulated. The strengthening and westward shift of the Siberian high-pressure system in response to sea ice and snow cover changes is underestimated compared to ERA-Interim data due to deficits in the simulated changes in planetary wave propagation characteristics.

Hartmann, D. L., Coauthors, 2013: Observations: Atmosphere and Surface. 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, Cambridge, United Kingdom, New York, NY, USA, 159- 254.

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.

10.1029/2008GL037079

11ff7459b32da24cee92554351efd9cb

http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103055291.html

http://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...

Hurrell J. W., J. J. Hack, D. Shea, J. M. Caron, and J. Rosinski, 2008: A new sea surface temperature and sea ice boundary dataset for the community atmosphere model.J. Climate,21,5145-5153, https://doi.org/10.1175/2008JCLI2292.1.

10.1175/2008JCLI2292.1

0ff603cbc3bfb3d5101e80534d7908a6

http%3A%2F%2Ficesjms.oxfordjournals.org%2Fexternal-ref%3Faccess_num%3D10.1175%2F2008JCLI2292.1%26amp%3Blink_type%3DDOI

http://journals.ametsoc.org/doi/abs/10.1175/2008JCLI2292.1

Inoue J., M. E. Hori, and K. Takaya, 2012: The role of Barents Sea ice in the wintertime cyclone track and emergence of a warm-arctic cold-Siberian anomaly.J. Climate,25,2561-2568, https://doi.org/10.1175/JCLI-D-11-00449.1.

10.1175/JCLI-D-11-00449.1

88b1d18f3d346383e29f96c85749d9df

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012JCli...25.2561I

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-11-00449.1Abstract Sea ice variability over the Barents Sea with its resultant atmospheric response has been considered one of the triggers of unexpected downstream climate change. For example, East Asia has experienced several major cold events while the underlying temperature over the Arctic has risen steadily. To understand the influence of sea ice in the Barents Sea on atmospheric circulation during winter from a synoptic perspective, this study evaluated the downstream response in cyclone activities with respect to the underlying sea ice variability. The composite analysis, including all cyclone events over the Nordic seas, revealed that an anticyclonic anomaly prevailed along the Siberian coast during light ice years over the Barents Sea. This likely caused anomalous warm advection over the Barents Sea and cold advection over eastern Siberia. The difference in cyclone paths between heavy and light ice years was expressed as a warm-Arctic cold-Siberian (WACS) anomaly. The lower baroclinicity over the Barents Sea during the light ice years, which resulted from a weak gradient in sea surface temperature, prevented cyclones from traveling eastward. This could lead to fewer cyclones and hence to an anticyclonic anomaly over the Siberian coast.

Johansson A., 2007: Prediction skill of the NAO and PNA from daily to seasonal time scales.J. Climate,20,1957-1975, https://doi.org/10.1175/JCLI4072.1.

10.1175/JCLI4072.1

8aa9ec741768986e267b247dd92d1a20

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2006AGUSM.A42A..06J

http://journals.ametsoc.org/doi/abs/10.1175/JCLI4072.1The skill of state-of-the-art operational dynamical models in predicting the two most important modes of variability in the Northern Hemisphere extratropical atmosphere, the North Atlantic Oscillation (NAO) and Pacific09“North American (PNA) teleconnection patterns, is investigated at time scales ranging from daily to seasonal. Two uncoupled atmospheric models used for deterministic forecasting in the short to medium range as well as eight fully coupled atmosphere09“land09“ocean forecast models used for monthly and seasonal forecasting are examined and compared. For the short to medium range, the level of forecast skill for the two indices is higher than that for the entire Northern Hemisphere extratropical flow. The forecast skill of the PNA is higher than that of the NAO. The forecast skill increases with the magnitude of the NAO and PNA indices, but the relationship is not pronounced. The probability density function (PDF) of the NAO and PNA indices is negatively skewed, in agreement with the distribution of skewness of the geopotential field. The models maintain approximately the observed PDF, including the negative skewness, for the first week. Extreme negative NAO/PNA events have larger absolute values than positive extremes in agreement with the negative skewness of the two indices. Recent large extreme events are generally well forecasted by the models. On the intraseasonal time scale it is found that both NAO and PNA have lingering forecast skill, in contrast to the Northern Hemisphere extratropical flow as a whole. This fact offers some hope for extended range forecasting, even though the skill is quite low. No conclusive positive benefit is seen from using higher horizontal resolution or coupling to the oceans. On the monthly and seasonal time scales, the level of forecast skill for the two indices is generally quite low, with the exception of winter predictions at short lead times.

Kumar, A., Coauthors, 2010: Contribution of sea ice loss to Arctic amplification,Geophys. Res. Lett.,37,L21701, https://doi.org/10.1029/2010GL045022.

10.1029/2010GL045022

839e22a6dd0175cb443a8e41b6517af4

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2010GL045022%2Fpdf

http://onlinelibrary.wiley.com/doi/10.1029/2010GL045022/pdfAtmospheric climate models are subjected to the observed sea ice conditions during 2007 to estimate the regionality, seasonality, and vertical pattern of temperature responses to recent Arctic sea ice loss. It is shown that anomalous sea ice conditions accounted for virtually all of the estimated Arctic amplification in surface-based warming over the Arctic Ocean, and furthermore they accounted for a large fraction of Arctic amplification occurring over the high-latitude land between 60ºN and the Arctic Ocean. Sea ice loss did not appreciably contribute to observed 2007 land temperature warmth equatorward of 60ºN. Likewise, the observed warming of the free atmosphere attributable to sea ice loss is confined to Arctic latitudes, and is vertically confined to the lowest 1000 m. The results further highlight a strong seasonality of the temperature response to the 2007 sea ice loss. A weak signal of Arctic amplification in surface based warming is found during boreal summer, whereas a dramatically stronger signal is shown to develop during early autumn that persisted through December even as sea ice coverage approached its climatological values in response to the polar night.

Liu J. P., J. A. Curry, H. J. Wang, M. R. Song, and R. M. Horton, 2012: Impact of declining Arctic sea ice on winter snowfall.Proceedings of the National Academy of Sciences of the United States of America,109(11),4074-4079, https://doi.org/10.1073/pnas.1114910109.

10.1073/pnas.1114910109

22371563

e869b196cae446b30762b978476557d4

http%3A%2F%2Fwww.jstor.org%2Fstable%2F41507098

http://www.pnas.org/cgi/doi/10.1073/pnas.1114910109While the Arctic region has been warming strongly in recent decades, anomalously large snowfall in recent winters has affected large parts of North America, Europe, and east Asia. Here we demonstrate that the decrease in autumn Arctic sea ice area is linked to changes in the winter Northern Hemisphere atmospheric circulation that have some resemblance to the negative phase of the winter Arctic oscillation. However, the atmospheric circulation change linked to the reduction of sea ice shows much broader meridional meanders in midlatitudes and clearly different interannual variability than the classical Arctic oscillation. This circulation change results in more frequent episodes of blocking patterns that lead to increased cold surges over large parts of northern continents. Moreover, the increase in atmospheric water vapor content in the Arctic region during late autumn and winter driven locally by the reduction of sea ice provides enhanced moisture sources, supporting increased heavy snowfall in Europe during early winter and the northeastern and midwestern United States during winter. We conclude that the recent decline of Arctic sea ice has played a critical role in recent cold and snowy winters.

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,9,838-842, https://doi.org/10.1038/NGEO2820.

10.1038/ngeo2820

86ebb2cb4509993b8b6992484cd04e67

http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv9%2Fn11%2Fngeo2820%2Fmetrics

http://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.

Moorthi S., H.-L. Pan, and P. Caplan, 2001: Changes to the 2001 NCEP operational MRF/AVN global analysis/forecast system,NOAA Technical Bulletin No. 484,National Centers for Environmental Prediction,Washington, DC, 14 pp.08872ea833cfe1bce97dbde8442a7f39

http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F284698001_Changes_to_the_2001_NCEP_operational_MRFAVN_global_analysisforecast_system

http://www.researchgate.net/publication/284698001_Changes_to_the_2001_NCEP_operational_MRFAVN_global_analysisforecast_systemCiteSeerX - Scientific documents that cite the following paper: 2001: Changes to the 2001 NCEP operational MRF/AVN global analysis/forecast systemhttp://www.researchgate.net/publication/284698001_Changes_to_the_2001_NCEP_operational_MRFAVN_global_analysisforecast_system

Mori M., M. Wananabe, 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/ngeo2277

cdef8a86f56c39f6052fde6e5d1dd7bb

http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv7%2Fn12%2Fabs%2Fngeo2277.html

http://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, https://doi.org/10.1002/2014JD022848.

10.1002/2014JD022848

d1e2e6ebc5048d2580b8cab05197bd59

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014JD022848%2Fpdf

http://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.

Overland, J. E., 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-14-00822.1

0eff56c315ae22f8ed4dcb680ca380fc

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.7917O

http://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.

Overland, J. E., 2016: A difficult Arctic science issue: Midlatitude weather linkages.Polar Science,10,210-216, 2016. 04. 011.https://doi.org/10.1016/j.polar.

10.1016/j.polar.2016.04.011

d47184414b704542138e770ad96c7174

http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS187396521630024X

http://linkinghub.elsevier.com/retrieve/pii/S187396521630024XThere is at present unresolved uncertainty whether Arctic amplification (increased air temperatures and loss of sea ice) impacts the location and intensities of recent major weather events in midlatitudes. There are three major impediments. The first is the null hypothesis where the shortness of time series since major amplification (15 years) is dominated by the variance of the physical process in the attribution calculation. This makes it impossible to robustly distinguish the influence of Arctic forcing of regional circulation from random events. The second is the large chaotic jet stream variability at midlatitudes producing a small Arctic forcing signal-to-noise ratio. Third, there are other potential external forcings of hemispheric circulation, such as teleconnections driven by tropical and midlatitude sea surface temperature anomalies. It is, however, important to note and understand recent emerging case studies. There is evidence for a causal connection of Barents-Kara sea ice loss, a stronger Siberian High, and cold air outbreaks into eastern Asia. Recent cold air penetrating into the southeastern United States was related to a shift in the long-wave atmospheric wind pattern and reinforced by warmer temperatures west of Greenland. Arctic Linkages is a major research challenge that benefits from an international focus on the topic.

Perlwitz J., M. Hoerling, and R. Dole, 2015: Arctic tropospheric warming: causes and linkages to lower latitudes.J. Climate,28,2154-2167, https://doi.org/10.1175/JCLI-D-14-00095.1.

10.1175/JCLI-D-14-00095.1

e1f15b352e80b6968f6ca4e5cee932b6

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.2154P

http://journals.ametsoc.org/doi/10.1175/JCLI-D-14-00095.1Arctic temperatures have risen dramatically relative to those of lower latitudes in recent decades, with a common supposition being that sea ice declines are primarily responsible for amplified Arctic tropospheric warming. This conjecture is central to a hypothesis in which Arctic sea ice loss forms the beginning link of a causal chain that includes weaker westerlies in midlatitudes, more persistent and amplified midlatitude waves, and more extreme weather. Through model experimentation, the first step in this chain is examined by quantifying contributions of various physical factors to October揇ecember (OND) mean Arctic tropospheric warming since 1979. The results indicate that the main factors responsible for Arctic tropospheric warming are recent decadal fluctuations and long-term changes in sea surface temperatures (SSTs), both located outside the Arctic. Arctic sea ice decline is the largest contributor to near-surface Arctic temperature increases, but it accounts for only about 20% of the magnitude of 1000500-hPa warming. These findings thus disconfirm the hypothesis that deep tropospheric warming in the Arctic during OND has resulted substantially from sea ice loss. Contributions of the same factors to recent midlatitude climate trends are then examined. It is found that pronounced circulation changes over the North Atlantic and North Pacific result mainly from recent decadal ocean fluctuations and internal atmospheric variability, while the effects of sea ice declines are very small. Therefore, a hypothesized causal chain of hemisphere-wide connections originating from Arctic sea ice loss is not supported.

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.

10.1029/2009JD013568

dc1ac9e62c94b87f316ae99122829c96

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009JD013568%2Fpdf

http://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.

Riddle E. E., A. H. Butler, J. C. Furtado, J. L. Cohen, and A. Kumar, 2013: CFSv2 ensemble prediction of the wintertime Arctic Oscillation.Climate Dyn.,41,1099-1116, https://doi.org/10.1007/s00382-013-1850-5.

10.1007/s00382-013-1850-5

3fc1c67d31d4bbf2b4383b206e63fd56

http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1007%2Fs00382-013-1850-5

http://link.springer.com/10.1007/s00382-013-1850-5Lagged ensembles from the operational Climate Forecast System version 2 (CFSv2) seasonal hindcast dataset are used to assess skill in forecasting interannual variability of the December-February Arctic Oscillation (AO). We find that a small but statistically significant portion of the interannual variance (>20 %) of the wintertime AO can be predicted at leads up to 2 months using lagged ensemble averages. As far as we are aware, this is the first study to demonstrate that an operational model has discernible skill in predicting AO variability on seasonal timescales. We find that the CFS forecast skill is slightly higher when a weighted ensemble is used that rewards forecast runs with the most accurate representations of October Eurasian snow cover extent (SCE), hinting that a stratospheric pathway linking October Eurasian SCE with the AO may be responsible for the model skill. However, further analysis reveals that the CFS is unable to capture many important aspects of this stratospheric mechanism. Model deficiencies identified include: (1) the CFS significantly underestimates the observed variance in October Eurasian SCE, (2) the CFS fails to translate surface pressure anomalies associated with SCE anomalies into vertically propagating waves, and (3) stratospheric AO patterns in the CFS fail to propagate downward through the tropopause to the surface. Thus, alternate boundary forcings are likely contributing to model skill. Improving model deficiencies identified in this study may lead to even more skillful predictions of wintertime AO variability in future versions of the CFS.

Saha, S., Coauthors, 2010: The NCEP climate forecast system reanalysis.Bull. Amer. Meteor. Soc.,91,1015-1057, https://doi.org/10.1175/2010BAMS3001.1.

10.1175/2010BAMS3001.1

http://journals.ametsoc.org/doi/10.1175/2010BAMS3001.1

Saha, S., Coauthors, 2014: The NCEP climate forecast system version 2.J. Climate,27,2185-2208, https://doi.org/10.1175/JCLI-D-12-00823.1.

10.1175/JCLI-D-12-00823.1

39ad083f324a6b82519f0f80e2a8a0a5

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014JCli...27.2185S

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-12-00823.1The second version of the NCEP Climate Forecast System (CFSv2) was made operational at NCEP in March 2011. This version has upgrades to nearly all aspects of the data assimilation and forecast model components of the system. A coupled reanalysis was made over a 32-yr period (1979-2010), which provided the initial conditions to carry out a comprehensive reforecast over 29 years (1982-2010). This was done to obtain consistent and stable calibrations, as well as skill estimates for the operational subseasonal and seasonal predictions at NCEP with CFSv2. The operational implementation of the full system ensures a continuity of the climate record and provides a valuable up-to-date dataset to study many aspects of predictability on the seasonal and subseasonal scales. Evaluation of the reforecasts show that the CFSv2 increases the length of skillful MJO forecasts from 6 to 17 days (dramatically improving subseasonal forecasts), nearly doubles the skill of seasonal forecasts of 2-m temperatures over the United States, and significantly improves global SST forecasts over its predecessor. The CFSv2 not only provides greatly improved guidance at these time scales but also creates many more products for subseasonal and seasonal forecasting with an extensive set of retrospective forecasts for users to calibrate their forecast products. These retrospective and real-time operational forecasts will be used by a wide community of users in their decision making processes in areas such as water management for rivers and agriculture, transportation, energy use by utilities, wind and other sustainable energy, and seasonal prediction of the hurricane season.

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.

10.1088/1748-9326/9/8/084009

9c5d56ad9ccb94480ed2f47502d57e80

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014AGUFM.A33D3218S

http://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)

Screen J. A., C. Deser, and I. Simmonds, 2012: Local and remote controls on observed Arctic warming,Geophys. Res. Lett.,39,L10709, https://doi.org/10.1029/2012GL051598.

10.1029/2012GL051598

0d2bcba75b9feef142830ce668ae7653

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2012GL051598%2Fpdf

http://onlinelibrary.wiley.com/doi/10.1029/2012GL051598/pdfThe Arctic is warming two to four times faster than the global average. Debate continues on the relative roles of local factors, such as sea ice reductions, versus remote factors in driving, or amplifying, Arctic warming. This study examines the vertical profile and seasonality of observed tropospheric warming, and addresses its causes using atmospheric general circulation model simulations. The simulations enable the isolation and quantification of the role of three controlling factors of Arctic warming: 1) observed Arctic sea ice concentration (SIC) and sea surface temperature (SST) changes; 2) observed remote SST changes; and 3) direct radiative forcing (DRF) due to observed changes in greenhouse gases, ozone, aerosols, and solar output. Local SIC and SST changes explain a large portion of the observed Arctic near-surface warming, whereas remote SST changes explain the majority of observed warming aloft. DRF has primarily contributed to Arctic tropospheric warming in summer.

Screen J. A., C. Deser, I. Simmonds, and R. Tomas, 2014: Atmospheric impacts of Arctic sea-ice loss,1979-2009: Separating forced change from atmospheric internal variability. Climate Dyn.,43,333-344,.https://doi.org/10.1007/s00382-013-1830.9

10.1007/s00382-013-1830-9

78a94d276b2c35549fb7c59cae5959d7

http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1007%2Fs00382-013-1830-9

http://link.springer.com/10.1007/s00382-013-1830-9The ongoing loss of Arctic sea-ice cover has implications for the wider climate system. The detection and importance of the atmospheric impacts of sea-ice loss depends, in part, on the relative magnitudes of the sea-ice forced change compared to natural atmospheric internal variability (AIV). This study analyses large ensembles of two independent atmospheric general circulation models in order to separate the forced response to historical Arctic sea-ice loss (19792009) from AIV, and to quantify signal-to-noise ratios. We also present results from a simulation with the sea-ice forcing roughly doubled in magnitude. In proximity to regions of sea-ice loss, we identify statistically significant near-surface atmospheric warming and precipitation increases, in autumn and winter in both models. In winter, both models exhibit a significant lowering of sea level pressure and geopotential height over the Arctic. All of these responses are broadly similar, but strengthened and/or more geographically extensive, when the sea-ice forcing is doubled in magnitude. Signal-to-noise ratios differ considerably between variables and locations. The temperature and precipitation responses are significantly easier to detect (higher signal-to-noise ratio) than the sea level pressure or geopotential height responses. Equally, the local response (i.e., in the vicinity of sea-ice loss) is easier to detect than the mid-latitude or upper-level responses. Based on our estimates of signal-to-noise, we conjecture that the local near-surface temperature and precipitation responses to past Arctic sea-ice loss exceed AIV and are detectable in observed records, but that the potential atmospheric circulation, upper-level and remote responses may be partially or wholly masked by AIV.

Screen J. A., I. Simmonds, 2013: Exploring links between Arctic amplification and mid-latitude weather.Geophys. Res. Lett.,40,959-964, https://doi.org/10.1002/GRL.50174.

10.1002/grl.50174

0cd6e8a4c2dcc0572533baa4173acb28

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fgrl.50174%2Ffull

http://doi.wiley.com/10.1002/grl.50174This study examines observed changes (19792011) in atmospheric planetary-wave amplitude over northern mid-latitudes, which have been proposed as a possible mechanism linking Arctic amplification and mid-latitude weather extremes. We use two distinct but equally-valid definitions of planetary-wave amplitude, termed meridional amplitude, a measure of north-south meandering, and zonal amplitude, a measure of the intensity of atmospheric ridges and troughs at 45 degrees N. Statistically significant changes in either metric are limited to few seasons, wavelengths, and longitudinal sectors. However in summer, we identify significant increases in meridional amplitude over Europe, but significant decreases in zonal amplitude hemispherically, and also individually over Europe and Asia. Therefore, we argue that possible connections between Arctic amplification and planetary waves, and implications of these, are sensitive to how waves are conceptualized. The contrasting meridional and zonal amplitude trends have different and complex possible implications for midlatitude weather, and we encourage further work to better understand these.

Screen J. A., 2014: Arctic amplification decreases temperature variance in northern mid- to high-latitudes.Nat. Clim. Change,4,577-582, 1038/NCLIMATE 2268.https://doi.org/10.

10.1038/nclimate2268

0981f852a5d3ddb77b2a2925d26f94b0

http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv4%2Fn7%2Ffig_tab%2Fnclimate2268_f3.html

http://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., C. Deser, and L. T. Sun, 2015: Reduced risk of North American cold extremes due to continued Arctic sea ice loss.Bull. Amer. Meteor. Soc.,96,1489-1503, https://doi.org/10.1175/BAMS-D-14-00185.1.

10.1175/BAMS-D-14-00185.1

1abe8e08ba8f26d31c3662db25ad62aa

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015BAMS...96.1489S

http://journals.ametsoc.org/doi/10.1175/BAMS-D-14-00185.1Contrary to recent claims, North American cold extremes are expected to become less frequent as a result of continuing Arctic sea ice loss.

Serreze M. C., M. M. Holland , and J. Stroeve, 2007: Perspectives on the Arctic's shrinking sea-ice cover.Science,315,1533-1536, https://doi.org/10.1126/science.1139426.

10.1126/science.1139426

17363664

7c4c5ff088ffbe067cf10e6490eb6cde

http%3A%2F%2Feuropepmc.org%2Fabstract%2FMED%2F17363664

http://www.sciencemag.org/cgi/doi/10.1126/science.1139426Linear trends in arctic sea-ice extent over the period 1979 to 2006 are negative in every month. This ice loss is best viewed as a combination of strong natural variability in the coupled ice-ocean-atmosphere system and a growing radiative forcing associated with rising concentrations of atmospheric greenhouse gases, the latter supported by evidence of qualitative consistency between observed trends and those simulated by climate models over the same period. Although the large scatter between individual model simulations leads to much uncertainty as to when a seasonally ice-free Arctic Ocean might be realized, this transition to a new arctic state may be rapid once the ice thins to a more vulnerable state. Loss of the ice cover is expected to affect the Arctic's freshwater system and surface energy budget and could be manifested in middle latitudes as altered patterns of atmospheric circulation and precipitation.

Serreze M. C., R. G. Barry, 2011: Processes and impacts of Arctic amplification: A research synthesis.Global and Planetary Change,77,85-96, 2011. 03. 004.https://doi.org/10.1016/j.gloplacha.

10.1016/j.gloplacha.2011.03.004

e13513b501cd20f1448d7b3a5223b0d8

http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0921818111000397

http://linkinghub.elsevier.com/retrieve/pii/S092181811100039778 Temperature changes in the Arctic tend to exceed those for the globe as whole. 78 This phenomenon is termed Arctic amplification. 78 Arctic amplification has many causes operating on different time and space scales. 78 Recent Arctic amplification is strongly linked to declining sea ice extent. 78 Arctic amplification is expected to strengthen in coming decades. 78 Impacts of Arctic amplification will extend well beyond the Arctic region.

Sillmann J., M. G. Donat, J. C. Fyfe, and F. W, Zwiers, 2014: Observed and simulated temperature extremes during the recent warming hiatus,Environmental Research Letters,9,064023, https://doi.org/10.1088/1748-9326/9/6/064023.

10.1088/1748-9326/9/6/064023

96c7a7def7fa2ea0d431155b458e6dd0

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014ERL.....9f4023S

http://stacks.iop.org/1748-9326/9/i=6/a=064023?key=crossref.d1187cdc6b9128d413b4f529b6d3fe1fThe discrepancy between recent observed and simulated trends in global mean surface temperature has provoked a debate about possible causes and implications for future climate change projections. However, little has been said in this discussion about observed and simulated trends in global temperature extremes. Here we assess trend patterns in temperature extremes and evaluate the consistency between observed and simulated temperature extremes over the past four decades (1971-2010) in comparison to the recent 15 years (1996-2010). We consider the coldest night and warmest day in a year in the observational dataset HadEX2 and in the current generation of global climate models (CMIP5). In general, the observed trends fall within the simulated range of trends, with better consistency for the longer period. Spatial trend patterns differ for the warm and cold extremes, with the warm extremes showing continuous positive trends across the globe and the cold extremes exhibiting a coherent cooling pattern across the Northern Hemisphere mid-latitudes that has emerged in the recent 15 years and is not reproduced by the models. This regional inconsistency between models and observations might be a key to understanding the recent hiatus in global mean temperature warming.

Sun L. T., J. Perlwitz, and M. Hoerling, 2016: What caused the recent "Warm Arctic,Cold Continents" trend pattern in winter temperatures? Geophys. Res. Lett.,43,5345-5352,https://doi.org/10.1002/2016GL069024.

Tang Q. H., X. J. Zhang, X. H. Yang, and J. A. Francis, 2013: Cold winter extremes in northern continents linked to arctic sea ice loss,Environmental Research Letters,8,014036, https://doi.org/10.1088/1748-9326/8/1/014036.

10.1088/1748-9326/8/1/014036

4a2689c416c3793196f6b65170ec673d

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013ERL.....8a4036T

http://stacks.iop.org/1748-9326/8/i=1/a=014036?key=crossref.3b07e12952c5b61acb8cedc9b207e051中国科学院机构知识库(中国科学院机构知识库网格（CAS IR GRID）)以发展机构知识能力和知识管理能力为目标，快速实现对本机构知识资产的收集、长期保存、合理传播利用，积极建设对知识内容进行捕获、转化、传播、利用和审计的能力，逐步建设包括知识内容分析、关系分析和能力审计在内的知识服务能力，开展综合知识管理。

Wallace J. M., I. M. Held, D. W. J. Thompson, K. E. Trenberth, and J. E. Walsh, 2014: Global warming and winter weather.Science,343,729-730, .http://doi.org/10.1126/science/343/6172.729

10.1126/science.343.6172.729

24531953

82fe21d5fa572c5ffb9cfd325d2ac105

http%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpubmed%2F24531953

http://www.sciencemag.org/cgi/doi/10.1126/science.343.6172.729Authors: John M. Wallace, Isaac M. Held, David W. J. Thompson, Kevin E. Trenberth, John E. Walsh

Zhang X. D., C. H. Lu, and Z. Y. Guan, 2012: Weakened cyclones,intensified anticyclones and recent extreme cold winter weather events in Eurasia.Environmental Research Letters,7,044044, https://doi.org/10.1088/1748-9326/7/4/044044.

10.1088/1748-9326/7/4/044044

434a3ad75227708aaa996fd09e7521b2

http%3A%2F%2Fwww.ingentaconnect.com%2Fcontent%2Fiop%2Ferl%2F2012%2F00000007%2F00000004%2Fart044044

http://stacks.iop.org/1748-9326/7/i=4/a=044044?key=crossref.98b6b9a08772130a16c83249896492d1Extreme cold winter weather events over Eurasia have occurred more frequently in recent years in spite of a warming global climate. To gain further insight into this regional mismatch with the global mean warming trend, we analyzed winter cyclone and anticyclone activities, and their interplay with the regional atmospheric circulation pattern characterized by the semi-permanent Siberian high. We found a persistent weakening of both cyclones and anticyclones between the 1990s and early 2000s, and a pronounced intensification of anticyclone activity afterwards. It is suggested that this intensified anticyclone activity drives the substantially strengthening and northwestward shifting/expanding Siberian high, and explains the decreased midlatitude Eurasian surface air temperature and the increased frequency of cold weather events. The weakened tropospheric midlatitude westerlies in the context of the intensified anticyclones would reduce the eastward propagation speed of Rossby waves, favoring persistence and further intensification of surface anticyclone systems. (letter)