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Simulation by CMIP5 Models of the Atlantic Multidecadal Oscillation and Its Climate Impacts

doi: 10.1007/s00376-016-5270-4

  • This study focuses on the climatic impacts of the Atlantic Multidecadal Oscillation (AMO) as a mode of internal variability. Given the difficulties involved in excluding the effects of external forcing from internal variation, i.e., owing to the short record length of instrumental observations and historical simulations, we assess and compare the AMO and its related climatic impacts both in observations and in the "Pre-industrial" experiments of models participating in CMIP5. First, we evaluate the skill of the 25 CMIP5 models' "Historical" simulations in simulating the observational AMO, and find there is generally a considerable range of skill among them in this regard. Six of the models with higher skill relative to the other models are selected to investigate the AMO-related climate impacts, and it is found that their "Pre-industrial" simulations capture the essential features of the AMO. A positive AMO favors warmer surface temperature around the North Atlantic, and the Atlantic ITCZ shifts northward leading to more rainfall in the Sahel and less rainfall in Brazil. Furthermore, the results confirm the existence of a teleconnection between the AMO and East Asian surface temperature, as well as the late withdrawal of the Indian summer monsoon, during positive AMO phases. These connections could be mainly caused by internal climate variability. Opposite patterns are true for the negative phase of the AMO.
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  • Allen R. J., 2015: A 21st century northward tropical precipitation shift caused by future anthropogenic aerosol reductions. J. Geophys. Res. Atmos.,120, 9087-9102, doi: 10.1002/2015JD 023623.10.1002/;jsessionid=023158567DFCB45AB610B4A213DCFBA3.f04t03Abstract The tropical rain belt is a narrow band of clouds near the equator, where the most intense rainfall on the planet occurs. On seasonal timescales, the rain moves across the equator following the sun, resulting in wet and dry seasons in the tropics. The position of the tropical rain belt also varies on longer time scales. Through the latter half of the 20th century, for example, shifts in tropical rainfall have been associated with severe droughts, including the African Sahel and Amazon droughts. Here, I show that climate models project a northward migration of the tropical rain belt through the 21st century, with future anthropogenic aerosol reductions driving the bulk of the shift. Models that include both aerosol indirect effects yield significantly larger northward shifts than models that lack aerosol indirect effects. Moreover, the rate of the shift corresponds to the rate of the decrease of anthropogenic aerosol emissions across different time periods and future emission scenarios. This response is consistent with relative warming of the Northern Hemisphere, a decrease in northward cross-equatorial moist static energy transport, and a northward shift of the Hadley circulation, including the tropical rain belt. The shift is relatively weak in the Atlantic sector, consistent with both a smaller decrease in aerosol emissions and a larger reduction in northward cross equatorial ocean heat flux. Although aerosol effects remain uncertain, I conclude that future reductions in anthropogenic aerosol emissions may be the dominant driver of a 21st century northward shift of the tropical rain belt.
    Ba, J., Coauthors, 2014: A multi-model comparison of Atlantic multidecadal variability. Climate Dyn.,43, 2333-2348, doi: 10.1007/s00382-014-2056-1.10.1007/ A multi-model analysis of Atlantic multidecadal variability is performed with the following aims: to investigate the similarities to observations; to assess the strength and relative importance of the different elements of the mechanism proposed by Delworth et al. (J Clim 6: 1993-2011, 1993) (hereafter D93) among coupled general circulation models (CGCMs); and to relate model differences to mean systematic error. The analysis is performed with long control simulations from ten CGCMs, with lengths ranging between 500 and 3600 years. In most models the variations of sea surface temperature (SST) averaged over North Atlantic show considerable power on multidecadal time scales, but with different periodicity. The SST variations are largest in the mid-latitude region, consistent with the short instrumental record. Despite large differences in model configurations, we find quite some consistency among the models in terms of processes. In eight of the ten models the mid-latitude SST variations are significantly correlated with fluctuations in the Atlantic meridional overturning circulation (AMOC), suggesting a link to northward heat transport changes. Consistent with this link, the three models with the weakest AMOC have the largest cold SST bias in the North Atlantic. There is no linear relationship on decadal timescales between AMOC and North Atlantic Oscillation in the models. Analysis of the key elements of the D93 mechanisms revealed the following: Most models present strong evidence that highlatitude winter mixing precede AMOC changes. However, the regions of wintertime convection differ among models. In most models salinity-induced density anomalies in the convective region tend to lead AMOC, while temperatureinduced density anomalies lead AMOC only in one model. However, analysis shows that salinity may play an overly important role in most models, because of cold temperature biases in their relevant convective regions. In most models subpolar gyre variations tend to lead AMOC changes, and this relation is strong in more than half of the models.
    Bjerknes J., 1964: Atlantic air-sea interaction. Advances in Geophysics, 10, 1- 82.10.1016/S0065-2687(08) article is concerned with the causes of the variations in the surface temperature of the Atlantic Ocean from year to year and over longer periods. The processes, which influence the ocean temperature, are partly radiative transfers, partly heat exchange at the interface of ocean and atmosphere, and partly advective heat transfers by the ocean currents. The net radiative heat balance of the ocean is influenced by possible variations of the solar radiative output, and by the transmissivity of the atmosphere for short- and long-wave radiation. Variations in cloudiness would be the factor, most likely to influence measurably the annual radiative heat budget of the ocean. The ocean currents provide important contributions to the local heat budget, positive in the warm currents and negative in the cold currents. The changes in intensity of the oceanic circulation are mainly dictated by changes in the atmospheric circulation, and the resulting changes in the temperature field of the ocean surface must in turn, influence the thermodynamics of the atmospheric circulation. A clarification of these relationships is a prerequisite for the understanding of the mechanism of climatic change. This article will present some empirical findings, which have a bearing on those problems. Before proceeding to display the empirical findings on the large-scale oceantmosphere interaction, a brief outline will be given of the theories on the meteorological control of ocean currents and on the interface heat transfers.
    Booth B. B. B., N. J. Dunstone, P. R. Halloran, T. Andrews, and N. Bellouin, 2012: Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature, 484, 228- 232.10.1038/ Systematic climate shifts have been linked to multidecadal variability in observed sea surface temperatures in the North Atlantic Ocean. These links are extensive, influencing a range of climate processes such as hurricane activity and African Sahel and Amazonian droughts. The variability is distinct from historical global-mean temperature changes and is commonly attributed to natural ocean oscillations. A number of studies have provided evidence that aerosols can influence long-term changes in sea surface temperatures, but climate models have so far failed to reproduce these interactions and the role of aerosols in decadal variability remains unclear. Here we use a state-of-the-art Earth system climate model to show that aerosol emissions and periods of volcanic activity explain 76 per cent of the simulated multidecadal variance in detrended 1860-2005 North Atlantic sea surface temperatures. After 1950, simulated variability is within observational estimates; our estimates for 1910-1940 capture twice the warming of previous generation models but do not explain the entire observed trend. Other processes, such as ocean circulation, may also have contributed to variability in the early twentieth century. Mechanistically, we find that inclusion of aerosol-cloud microphysical effects, which were included in few previous multimodel ensembles, dominates the magnitude (80 per cent) and the spatial pattern of the total surface aerosol forcing in the North Atlantic. Our findings suggest that anthropogenic aerosol emissions influenced a range of societally important historical climate events such as peaks in hurricane activity and Sahel drought. Decadal-scale model predictions of regional Atlantic climate will probably be improved by incorporating aerosol-cloud microphysical interactions and estimates of future concentrations of aerosols, emissions of which are directly addressable by policy actions.
    Chen W., R. Y. Lu, and B. W. Dong, 2014: Intensified anticyclonic anomaly over the western North Pacific during El Niño decaying summer under a weakened Atlantic thermohaline circulation. J. Geophys. Res. Atmos.,119, 13 637-13 650, doi: 10.1002/2014JD022199.10.1002/ has been well documented that there is an anticyclonic anomaly over the western North Pacific (WNPAC, hereafter) during El Ni09o decaying summer. This El Ni09o-WNPAC relationship is greatly useful for the seasonal prediction of summer climate in the WNP and East Asia. In this study, we investigate the modification of the El Ni09o-WNPAC relationship induced by a weakened Atlantic thermohaline circulation (THC) in a water-hosing experiment. The results suggest that the WNPAC during the El Ni09o decaying summer, as well as the associated precipitation anomaly over the WNP, is intensified under the weakened THC. On the one hand, this intensification is in response to the increased amplitude and frequency of El Ni09o events in the water-hosing experiment. On the other hand, this intensification is also because of greater climatological humidity over the western to central North Pacific under the weakened THC. We suggest that the increase of climatological humidity over the western to central North Pacific during summer under the weakened THC is favorable for enhanced interannual variability of precipitation, and therefore favorable for the intensification of the WNPAC during El Ni09o decaying summer. This study suggests a possible modulation of the El Ni09o-Southern Oscillation-WNP summer monsoon relationship by the low-frequency fluctuation of Atlantic sea surface temperature. The results offer an explanation for the observed modification of the multidecadal fluctuation of El Ni09o-WNPAC relationship by the Atlantic multidecadal oscillation. It has been well documented that there is an anticyclonic anomaly over the western North Pacific (WNPAC, hereafter) during El Ni09o decaying summer. This El Ni09o-WNPAC relationship is greatly useful for the seasonal prediction of summer climate in the WNP and East Asia. In this study, we investigate the modification of the El Ni09o-WNPAC relationship induced by a weakened Atlantic thermohaline circulation (THC) in a water-hosing experiment. The results suggest that the WNPAC during the El Ni09o decaying summer, as well as the associated precipitation anomaly over the WNP, is intensified under the weakened THC. On the one hand, this intensification is in response to the increased amplitude and frequency of El Ni09o events in the water-hosing experiment. On the other hand, this intensification is also because of greater climatological humidity over the western to central North Pacific under the weakened THC. We suggest that the increase of climatological humidity over the western to central North Pacific during summer under the weakened THC is favorable for enhanced interannual variability of precipitation, and therefore favorable for the intensification of the WNPAC during El Ni09o decaying summer. This study suggests a possible modulation of the El Ni09o-Southern Oscillation-WNP summer monsoon relationship by the low-frequency fluctuation of Atlantic sea surface temperature. The results offer an explanation for the observed modification of the multidecadal fluctuation of El Ni09o-WNPAC relationship by the Atlantic multidecadal oscillation.
    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- 7197.10.1175/ The 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 610.1 to 611.8 Sverdrup (Sv) century 611 ; 1 Sv ≡ 10 6 m 3 s 611 ] over the twentieth century. Additionally, the multimodel ensemble–mean AMOC exhibits multidecadal variability with a ~60-yr periodicity and a peak-to-peak amplitude of ~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.
    Chiang J.-C.-H., C.-Y. Chang, and M.-F. Wehner, 2013: Long-term behavior of the Atlantic interhemispheric SST gradient in the CMIP5 historical simulations. J.Climate, 26, 8628-
    Chylek P., C. K. Folland , G. Lesins, and M. K. Dubey, 2010: Twentieth century bipolar seesaw of the Arctic and Antarctic surface air temperatures. Geophys. Res. Lett., 37,L08703, doi: 10.1029/2010GL042793.10.1029/ the phase relationship between climate changes in the Arctic and Antarctic regions is essential for our understanding of the dynamics of the Earth's climate system. In this paper we show that the 20th century de-trended Arctic and Antarctic temperatures vary in anti-phase seesaw pattern - when the Arctic warms the Antarctica cools and visa versa. This is the first time that a bi-polar seesaw pattern has been identified in the 20th century Arctic and Antarctic temperature records. The Arctic (Antarctic) de-trended temperatures are highly correlated (anti-correlated) with the Atlantic Multi-decadal Oscillation (AMO) index suggesting the Atlantic Ocean as a possible link between the climate variability of the Arctic and Antarctic regions. Recent accelerated warming of the Arctic results from a positive reinforcement of the linear warming trend (due to an increasing concentration of greenhouse gases and other possible forcings) by the warming phase of the multidecadal climate variability (due to fluctuations of the Atlantic Ocean circulation).
    Chylek P., C. K. Folland , H. A. Dijkstra, G. Lesins, and M. K. Dubey, 2011: Ice-core data evidence for a prominent near 20 year time-scale of the Atlantic Multidecadal Oscillation. Geophys. Res. Lett., 38,L13704, doi: 10.1029/2011GL047501.10.1029/ five ice core data sets combined into a single time series, we provide for the first time strong observational evidence for two distinct time scales of Arctic temperature fluctuation that are interpreted as variability associated with the Atlantic Multidecadal Oscillation (AMO). The dominant and the only statistically significant multidecadal signal has a time scale of about 20 years. The longer multidecadal variability of 45-85 years is not well defined and none of the time scales in this band is statistically significant. We compare these observed temperature fluctuations with results of coupled climate model simulations (HadCM3 and GFDL CM2.1). Both the 20-25 year and a variable longer AMO time scale are prominent in the models' long control runs. This periodicity supports our conjecture that the observed ice core fluctuations are a signature of the AMO. The robustness of this short time scale period in both observations and model simulations has implications for understanding the dominant AMO mechanisms in climate.
    Delworth T. L., M. E. Mann, 2000: Observed and simulated multidecadal variability in the Northern Hemisphere. Climate Dyn., 16, 661-
    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( 7499), 209- 212.10.1038/ 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.
    Dong B. W., R. T. Sutton, and A. A. Scaife, 2006: Multidecadal modulation of El Niño-Southern Oscillation (ENSO) variance by Atlantic Ocean sea surface temperatures. Geophys. Res. Lett., 33,L08705, doi: 10.1029/2006GL025766.10.1029/ suggest a possible link between the Atlantic Multidecadal Oscillation (AMO) and El Niño-Southern Oscillation (ENSO) variability, with the warm AMO phase being related to weaker ENSO variability. A coupled ocean-atmosphere model is used to investigate this relationship and to elucidate mechanisms responsible for it. Anomalous sea surface temperatures (SSTs) associated with the positive AMO lead to change in the basic state in the tropical Pacific Ocean. This basic state change is associated with a deepened thermocline and reduced vertical stratification of the equatorial Pacific ocean, which in turn leads to weakened ENSO variability. We suggest a role for an atmospheric bridge that rapidly conveys the influence of the Atlantic Ocean to the tropical Pacific. The results suggest a non-local mechanism for changes in ENSO statistics and imply that anomalous Atlantic ocean SSTs can modulate both mean climate and climate variability over the Pacific.
    Drinkwater K. F., M. Miles, I. Medhaug, O. H. Otter, T. Kristiansen, S. Sundbya, and Y. Q. Gao, 2014: The Atlantic Multidecadal Oscillation: Its manifestations and impacts with special emphasis on the Atlantic region north of 60$^\circ$N. Journal of Marine Systems, 133, 117- 130.10.1016/ paper examines the multidecadal variability in the northern North Atlantic and the Arctic. Observations, modeling and paleo data provide evidence of a strong link between the atmospheric and physical oceanographic variability in these northern regions with Atlantic sea surface temperatures farther south as expressed by the Atlantic Multidecadal Oscillation (AMO). Air and sea temperatures over the past 100–150 years reveal cool periods in the late 1800s to early 1900s and in the 1970s to 1980s with warm periods during the 1920s to 1960s and from the 1990s through to the present, similar to the variability in the AMO index where a positive (negative) AMO index represents warm (cold) periods. Sea-ice extent in the north has also varied at multidecadal scales with the ice retreating during the warm periods and expanding during the cold periods. The presence of multidecadal variability is also suggested from marine sediment paleo data as well as ice-core oxygen isotope data. Observations of biological impacts of the multidecadal variability in the northern regions include a general increase in plankton and fish productivity, as well as expansion of the species distributions northward, in conjunction with the AMO warm periods and the opposite during AMO cold periods. In addition, a review of the mechanisms responsible for the AMO and a brief discussion of the linkages between the multidecadal variability in the northern and southern hemispheres, including between the Arctic and Antarctic, are presented.
    Dunstone N. J., D. M. Smith, B. B. B. Booth, L. Hermanson, and R. Eade, 2013: Anthropogenic aerosol forcing of Atlantic tropical storms. Nature Geoscience, 6( 7), 534- 539.10.1038/ in vitro estrogen receptor (ER) agonist and androgen receptor (AR) antagonist potencies of offshore produced water effluents collected from the Norwegian Sector were determined using recombinant yeast estrogen and androgen screens. Solid phase extraction (SPE) concentrates of the effluents showed E2 agonist activities similar to those previously reported for the United Kingdom (UK) Continental Shelf (<0.1–402ng02E202L 611 ). No activity was detected in the filtered oil droplets suggesting that produced water ER activity is primarily associated with the dissolved phase. Targeted analysis for methyl- to nonyl-substituted alkylphenol isomers show the occurrence of known ER agonists in the analysed samples. For the first time, AR antagonists were detected in both the dissolved and oil associated phase at concentrations of between 20 and 800002μg of flutamide equivalents L 611 . The identity of the AR antagonists is unknown, however this represents a significant input into the marine environment of unknown compounds that exert a known biological effect. It is recommended that further analysis using techniques such as bioassay-directed analysis is performed to identify the compounds/groups of compounds that are responsible in order to improve the assessment of the risk posed by produced water discharges to the marine environment.
    Enfield D. B., A. M. Mestas-Nu\nez, and P. J. Trimble, 2001: The Atlantic Multidecadal Oscillation and its relation to rainfall and river flows in the continental U.S. Geophys. Res. Lett., 28( 10), 2077-
    Feng S., Q. Hu, 2008: How the North Atlantic Multidecadal Oscillation may have influenced the Indian summer monsoon during the past two millennia. Geophys. Res. Lett., 35,L01707, doi: 10.1029/
    Frankignoul C., N. Sennèchael, 2007: Observed influence of North Pacific SST anomalies on the atmospheric circulation. J. Climate, 20, 592-606, doi: 10.1175/JCLI4021.1.10.1175/ A lagged maximum covariance analysis (MCA) of monthly anomaly data from the NCEP–NCAR reanalysis shows significant relations between the large-scale atmospheric circulation in two seasons and prior North Pacific sea surface temperature (SST) anomalies, independent from the teleconnections associated with the ENSO phenomenon. Regression analysis based on the SST anomaly centers of action confirms these findings. In late summer, a hemispheric atmospheric signal that is primarily equivalent barotropic, except over the western subtropical Pacific, is significantly correlated with an SST anomaly mode up to at least 5 months earlier. Although the relation is most significant in the upper troposphere, significant temperature anomalies are found in the lower troposphere over North America, the North Atlantic, Europe, and Asia. The SST anomaly is largest in the Kuroshio Extension region and along the subtropical frontal zone, resembling the main mode of North Pacific SST anomaly variability in late winter and spring, and it is itself driven by the atmosphere. The predictability of the atmospheric signal, as estimated from cross-validated correlation, is highest when SST leads by 4 months because the SST anomaly pattern is more dominant in the spring than in the summer. In late fall and early winter, a signal resembling the Pacific–North American (PNA) pattern is found to be correlated with a quadripolar SST anomaly during summer, up to 4 months earlier, with comparable statistical significance throughout the troposphere. The SST anomaly changes shape and propagates eastward, and by early winter it resembles the SST anomaly that is generated by the PNA pattern. It is argued that this results via heat flux forcing and meridional Ekman advection from an active coupling between the SST and the PNA pattern that takes place throughout the fall. Correspondingly, the predictability of the PNA-like signal is highest when SST leads by 2 months. In late summer, the maximum atmospheric perturbation at 250 mb reaches 35 m K 611 in the MCA and 20 m K 611 in the regressions. In early winter, the maximum atmospheric perturbation at 250 mb ranges between 70 m K 611 in the MCA and about 35 m K 611 in the regressions. This suggests that North Pacific SST anomalies have a substantial impact on the Northern Hemisphere climate. The back interaction of the atmospheric response onto the ocean is also discussed.
    Gao Y.Q., Coauthors, 2015: Arctic sea ice and Eurasian climate: A review. Adv. Atmos. Sci.,32(1), 92-114, doi: 10.1007/s00376-014-0009-6.10.1007/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。
    Goswami B. N., M. S. Madhusoodanan, C. P. Neema, and D. Sengupta, 2006: A physical mechanism for North Atlantic SST influence on the Indian summer monsoon. Geophys. Res. Lett., 33,L02706, doi: 10.1029/2005GL024803.10.1029/[1] A link between the Atlantic Multidecadal Oscillation (AMO) and multidecadal variability of the Indian summer monsoon rainfall is unraveled and a long sought physical mechanism linking Atlantic climate and monsoon has been identified. The AMO produces persistent weakening (strengthening) of the meridional gradient of tropospheric temperature (TT) by setting up negative (positive) TT anomaly over Eurasia during northern late summer/autumn resulting in early (late) withdrawal of the south west monsoon and persistent decrease (increase) of seasonal monsoon rainfall. On internnual time scales, strong North Atlantic Oscillation (NAO) or North Annular mode (NAM) influences the monsoon by producing similar TT anomaly over Eurasia. The AMO achieves the interdecadal modulation of the monsoon by modulating the frequency of occurrence of strong NAO/NAM events. This mechanism also provides a basis for explaining the observed teleconnection between North Atlantic temperature and the Asian monsoon in paleoclimatic proxies.
    Gray S. T., L. J. Graumlich, J. L. Betancourt, and G. T. Pederson, 2004: A tree-ring based reconstruction of the Atlantic Multidecadal Oscillation since 1567 A.D. Geophys. Res. Lett., 31,L12205, doi: 10.1029/
    Gulev S. K., M. Latif, N. Keenlyside, W. Park, and K. P. Koltermann, 2013: North Atlantic Ocean control on surface heat flux on multidecadal timescales. Nature, 499( 7459), 464- 467.10.1038/ 50 years ago Bjerknes suggested that the character of large-scale air-sea interaction over the mid-latitude North Atlantic Ocean differs with timescales: the atmosphere was thought to drive directly most short-term--interannual--sea surface temperature () variability, and the ocean to contribute significantly to long-term--multidecadal--and potentially atmospheric variability. Although the conjecture for short timescales is well accepted, understanding Atlantic multidecadal variability () of remains a challenge as a result of limited ocean observations. is nonetheless of major socio-economic importance because it is linked to important climate phenomena such as Atlantic hurricane activity and Sahel rainfall, and it hinders the detection of anthropogenic signals in the North Atlantic sector. Direct evidence of the oceanic influence of can only be provided by surface heat fluxes, the language of ocean-atmosphere communication. Here we provide observational evidence that in the mid-latitude North Atlantic and on timescales longer than 10 years, surface turbulent heat fluxes are indeed driven by the ocean and may force the atmosphere, whereas on shorter timescales the converse is true, thereby confirming the Bjerknes conjecture. This result, although strongest in boreal winter, is found in all seasons. Our findings suggest that the predictability of mid-latitude North Atlantic air-sea interaction could extend beyond the ocean to the climate of surrounding continents.
    Hansen J., M. Sato, R. Ruedy, K. Lo, D. Lea, and M. Medina-Elizade, 2006: Global temperature change. Proceedings of the National Academy of Sciences of the United States of America,103, 14 288-14 293, doi: 10.1073/pnas. 0606291103.10.1073/ surface temperature has increased approximately 0.2 degrees C per decade in the past 30 years, similar to the warming rate predicted in the 1980s in initial global climate model simulations with transient greenhouse gas changes. Warming is larger in the Western Equatorial Pacific than in the Eastern Equatorial Pacific over the past century, and we suggest that the increased West-East temperature gradient may have increased the likelihood of strong El Nios, such as those of 1983 and 1998. Comparison of measured sea surface temperatures in the Western Pacific with paleoclimate data suggests that this critical ocean region, and probably the planet as a whole, is approximately as warm now as at the Holocene maximum and within approximately 1 degrees C of the maximum temperature of the past million years. We conclude that global warming of more than approximately 1 degrees C, relative to 2000, will constitute "dangerous" climate change as judged from likely effects on sea level and extermination of species.
    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, doi: 10.1029/2008GL037079.10.1029/ 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., Coauthors, 2009: Decadal climate prediction: Opportunities and challenges. Proc. OceanObs'09: Sustained Ocean Observations and Information for Society, ESA Publication, Venice, 521- 533.10.5270/
    Kavvada A., A. Ruiz-Barradas, and S. Nigam, 2013: AMO's structure and climate footprint in observations and IPCC AR5 climate simulations. Climate Dyn., 41( 5-6), 1345- 1364.10.1007/ study aims to characterize the spatiotemporal features of the low frequency Atlantic Multidecadal Oscillation (AMO), its oceanic and atmospheric footprint and its associated hydroclimate impact. To accomplish this, we compare and evaluate the representation of AMO-related features both in observations and in historical simulations of the twentieth century climate from models participating in the IPCC CMIP5 project. Climate models from international leading research institutions are chosen: CCSM4, GFDL-CM3, UKMO-HadCM3 and ECHAM6/MPI-ESM-LR. Each model employed includes at least three and as many as nine ensemble members. Our analysis suggests that the four models underestimate the characteristic period of the AMO, as well as its temporal variability; this is associated with an underestimation/overestimation of spectral peaks in the 70-80 year/10-20 year range. The four models manifest the mid-latitude focus of the AMO-related SST anomalies, as well as certain features of its subsurface heat content signal. However, they are limited when it comes to simulating some of the key oceanic and atmospheric footprints of the phenomenon, such as its signature on subsurface salinity, oceanic heat content and geopotential height anomalies. Thus, it is not surprising that the models are unable to capture the majority of the associated hydroclimate impact on the neighboring continents, including underestimation of the surface warming that is linked to the positive phase of the AMO and is critical for the models to be trusted on projections of future climate and decadal predictions.
    Keenlyside N. S., M. Latif, J. Jungclaus, L. Kornblueh, and E. Roeckner, 2008: Advancing decadal-scale climate prediction in the North Atlantic sector. Nature, 453: 84- 88.10.1038/ climate of the North Atlantic region exhibits fluctuations on decadal timescales that have large societal consequences. Prominent examples include hurricane activity in the Atlantic, and surface-temperature and rainfall variations over North America, Europe and northern Africa. Although these multidecadal variations are potentially predictable if the current state of the ocean is known, the lack of subsurface ocean observations that constrain this state has been a limiting factor for realizing the full skill potential of such predictions. Here we apply a simple approach09”that uses only sea surface temperature (SST) observations09”to partly overcome this difficulty and perform retrospective decadal predictions with a climate model. Skill is improved significantly relative to predictions made with incomplete knowledge of the ocean state, particularly in the North Atlantic and tropical Pacific oceans. Thus these results point towards the possibility of routine decadal climate predictions. Using this method, and by considering both internal natural climate variations and projected future anthropogenic forcing, we make the following forecast: over the next decade, the current Atlantic meridional overturning circulation will weaken to its long-term mean; moreover, North Atlantic SST and European and North American surface temperatures will cool slightly, whereas tropical Pacific SST will remain almost unchanged. Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.
    Knight J. R., C. K. Folland , and A. A. Scaife, 2006: Climate impacts of the Atlantic Multidecadal Oscillation. Geophys. Res. Lett., 33,L17706, doi: 10.1029/2006GL026242.10.1029/ Atlantic Multidecadal Oscillation (AMO) is a near-global scale mode of observed multidecadal climate variability with alternating warm and cool phases over large parts of the Northern Hemisphere. Many prominent examples of regional multidecadal climate variability have been related to the AMO, such as North Eastern Brazilian and African Sahel rainfall, Atlantic hurricanes and North American and European summer climate. The relative shortness of the instrumental climate record, however, limits confidence in these observationally derived relationships. Here, we seek evidence of these links in the 1400 year control simulation of the HadCM3 climate model, which produces a realistic long-lived AMO as part of its internal climate variability. By permitting the analysis of more AMO cycles than are present in observations, we find that the model confirms the association of the AMO with almost all of the above phenomena. This has implications for the predictability of regional climate.
    Li F., H. J. Wang, and Y. Q. Gao, 2015: Extratropical ocean warming and winter Arctic sea ice cover since the 1990s. J. Climate,28, 5510-5522, doi: 10.1175/JCLI-D-14-00629.1.10.1175/ Despite the fact that the Arctic Oscillation (AO) has reached a more neutral state and a global-warming hiatus has occurred in winter since the late 1990s, the Arctic sea ice cover (ASIC) still shows a pronounced decrease. This study reveals a close connection ( R = 0.5) between the extratropical sea surface temperature (ET-SST) and ASIC in winter from 1994 to 2013. In response to one positive (negative) unit of deviation in the ET-SST pattern, the ASIC decreases (increases) in the Barentsara Seas and Hudson Bay (the Baffin Bay and Bering Sea) by 100-400 km 2 . This relationship might be maintained because of the impact of warming extratropical oceans on the polar vortex. Positive SST anomalies in the midlatitudes of the North Pacific and Atlantic (around 40N) strengthen the equatorward planetary wave propagation, whereas negative SST anomalies in the high latitudes weaken the upward planetary wave propagation from the lower troposphere to the stratosphere. The former indicates a strengthening of the poleward meridional eddy momentum flux, and the latter implies a weakening of the poleward eddy heat flux, which favors an intensified upper-level polar night jet and a colder polar vortex, implying a stronger-than-normal polar vortex. Consequently, an anomalous cyclone emerges over the eastern Arctic, limiting or encouraging the ASIC by modulating the mean meridional heat flux. A possible reason for the long-term changes in the relationship between the ET-SST and ASIC is also discussed.
    Li S. L., G. T. Bates, 2007: Influence of the Atlantic multidecadal oscillation on the winter climate of East China. Adv. Atmos. Sci.,24(1), 126-135, doi: 10.1007/s00376-007-0126-6.10.1007/ Atlantic Multidecadal Oscillation (AMO), the multidecadal variation of North Atlantic sea surface temperature (SST), exhibits an oscillation with a period of 65-80 years and an amplitude of 0.4C. Observational composite analyses reveal that the warm phase AMO is linked to warmer winters in East China, with enhanced precipitation in the north of this region and reduced precipitation in the south, on multidecaclal time scales. The pattern is reversed during the cold phase AMO. Whether the AMO acts as a forcing of the multidecadal winter climate of East China is explored by investigating the atmospheric response to warm AMO SST anomalies in a large ensemble of atmospheric general circulation model (AGCM) experiments.The results from three AGCMs are consistent and suggest that the AMO warmth favors warmer winters in East China. This influence is realized through inducing negative surface air pressure anomalies in the hemispheric-wide domain extending from the midlatitude North Atlantic to midlatitude Eurasia. These negative surface anomalies favor the weakening of the Mongolian Cold High, and thus induce a weaker East Asian Winter Monsoon.
    Li S. L., J. Perlwitz, X. W. Quan, and M. P. Hoerling, 2008: Modelling the influence of North Atlantic multidecadal warmth on the Indian summer rainfall. Geophys. Res. Lett., 35,L05804, doi: 10.1029/2007GL032901.10.1029/ experiments with an atmospheric general circulation model reveal that a positive (warm) ocean phase of the Atlantic Multidecadal Oscillation (AMO) increases Indian summer rainfall. The intensification is driven by extratropical North Atlantic warmth, with some cancellation associated with monsoon weakening in response to tropical North Atlantic warmth. Mechanistically, warm extratropical North Atlantic SSTs increase local rainfall, inducing an arching extratropical wavetrain response. The latter leads to intensified northern subsidence of monsoon mean meridional streamflow as well as widespread low surface pressure over North Africa, the Middle East and the western Indian Ocean contributing to a strengthened Indian monsoon trough and increased monsoon rainfall. Warm tropical North Atlantic SSTs primarily increase local tropical Atlantic rainfall that induces a tropically-confined response consisting of low level easterly wind anomalies over the Indian Ocean and dynamically induced subsident drying over India.
    Lu R. Y., B. W. Dong, and H. Ding, 2006: Impact of the Atlantic Multidecadal Oscillation on the Asian summer monsoon. Geophys. Res. Lett., 33,L24701, doi: 10.1029/2006GL 027655.10.1029/[1] The impact of the Atlantic Multidecadal Oscillation (AMO) on the Asian summer monsoon is investigated using a coupled atmosphere-ocean global general circulation model by imposing the AMO-associated sea surface temperature anomalies in the Atlantic as boundary forcing, and allowing atmosphere-ocean interactions outside the Atlantic. The positive AMO phase, characterized by anomalous warm North Atlantic and cold South Atlantic, leads to strong Southeast and east Asian summer monsoons, and late withdrawal of the Indian summer monsoon. These changes of monsoons are mainly through coupled atmosphere-ocean feedbacks in the western Pacific and Indian Oceans and tropospheric temperature changes over Eurasia in response to the imposed forcing in the Atlantic. The results are in agreement with the observed climate changes in China corresponded to the AMO phases. They suggest a non-local mechanism for the Asian summer monsoon variability and provide an alternative view to understanding its interdecadal variation during the twentieth century.
    Luo F. F., S. L. Li, 2014: Joint statistical-dynamical approach to decadal prediction of East Asian surface air temperature. Science China Earth Sciences,57, 3062-3072, doi: 10.1007/s11430-014-4984-3.10.1007/ joint statistical-dynamical method addressing both the internal decadal variability and effect of anthropogenic forcing was developed to predict the decadal components of East Asian surface air temperature(EATs)for three decades(2010-2040).As previous studies have revealed that the internal variability of EATs(EATs_int)is influenced mainly by the ocean,we first analyzed the lead-lag connections between EATs_int and three sea surface temperature(SST)multidecadal modes using instrumental records from 1901 to 1999.Based on the lead-lag connections,a multiple linear regression was constructed with the three SST modes as predictors.The hindcast for the years from 2000 to 2005 indicated the regression model had high skill in simulating the observational EATs_int.Therefore,the prediction for EATs_int(Re_EATs_int)was obtained by the regression model based on quasi-periods of the decadal oceanic modes.External forcing from greenhouse gases is likely associated with global warming.Using monthly global land surface air temperature from historical and projection simulations under the Representative Concentration Pathway(RCP)4.5 scenario of 19 Coupled General Circulation Models participating in the fifth phase of the Coupled Model Intercomparison Project(CMIP5),we predicted the curve of EATs(EATs_trend)relative to1970-1999 by a second-order fit.EATs_int and EATs_trend were combined to form the reconstructed EATs(Re_EATs).It was expected that a fluctuating evolution of Re_EATs would decrease slightly from 2015 to 2030 and increase gradually thereafter.Compared with the decadal prediction in CMIP5 models,Re_EATs was qualitatively in agreement with the predictions of most of the models and the multi-model ensemble mean,indicating that the joint statistical-dynamical approach for EAT is rational.
    Luo F. F., S. L. Li, and T. Furevik, 2011: The connection between the Atlantic Multidecadal Oscillation and the Indian summer monsoon in Bergen Climate Model Version 2.0. J. Geophys. Res., 116,D19117, doi: 10.1029/2011JD015848.10.1029/ pre-industrial multicentury simulation with Bergen Climate Model Version 2 (BCM in brief) is used to investigate the linkage between the Atlantic Multidecadal Oscillation (AMO) and the Indian Summer Monsoon (ISM). The results suggest that the model reproduces the general characters of the observed linkage between AMO and ISM, and that a positive AMO favors more rainfall over India from July to October. The ISM is intensified and the seasonal withdrawal of ISM delayed with one month, in agreement with previous observational and model's results. Further diagnoses indicate that this impact is achieved through an atmospheric teleconnection pattern. A propagating Rossby wave train from the North Atlantic across South Asia leads to enhanced South Asia high and consequently a strengthening of the ISM.
    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- 876.10.1175/1520-0442(2004)0172.0.CO; 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.
    Martin E. R., C. Thorncroft, and B. B. B. Booth, 2014: The Multidecadal Atlantic SST-Sahel rainfall teleconnection in CMIP5 simulations. J.Climate, 27, 784- 806.10.1175/ Available
    Medhaug I., T. Furevik, 2011: North Atlantic 20th century multidecadal variability in coupled climate models: Sea surface temperature and ocean overturning circulation. Ocean Science Discussions, 8, 353-;link_type=DOI
    Meehl, G. A., Coauthors, 2009: Decadal prediction: Can it be skillful? Bull. Amer. Meteor. Soc., 90, 1467- 1485.10.1175/ new field of study, "decadal prediction." is emerging in climate science. Decadal prediction lies between seasonal/interannual forecasting and longer-term climate change projections, and focuses on time-evolving regional climate conditions over the next 10-30 yr. Numerous assessments of climate information user needs have identified this time scale as being important to infrastructure planners, water resource managers, and many others. It is central to the information portfolio required to adapt effectively to and through climatic changes. At least three factors influence time-evolving regional climate at the decadal time scale: 1) climate change commitment (further warming as the coupled climate system comes into adjustment with increases of greenhouse gases that have already occurred), 2) external forcing, particularly from future increases of greenhouse gases and recovery of the ozone hole, and 3) internally generated variability. Some decadal prediction skill has been demonstrated to arise from the first two of these factors, and there is evidence that initialized coupled climate models can capture mechanisms of internally generated decadal climate variations, thus increasing predictive skill globally and particularly regionally. Several methods have been proposed for initializing global coupled climate models for decadal predictions, all of which involve global time-evolving three dimensional ocean data, including temperature and salinity. An experimental framework to address decadal predictability/prediction is described in this paper and has been incorporated into the coordinated Coupled Model Intercomparison Model, phase 5 (CMIP5) experiments, some of which will he assessed for the IPCC Fifth Assessment Report (AR5). These experiments will likely guide work in this emerging field over the next 5 yr.
    Mitchell T. D., P. D. Jones, 2005: An improved method of constructing a database of monthly climate observations and associated high-resolution grids. International Journal of Climatology,25, 693-712, doi: 10.1002/joc.1181.10.1002/ A database of monthly climate observations from meteorological stations is constructed. The database includes six climate elements and extends over the global land surface. The database is checked for inhomogeneities in the station records using an automated method that refines previous methods by using incomplete and partially overlapping records and by detecting inhomogeneities with opposite signs in different seasons. The method includes the development of reference series using neighbouring stations. Information from different sources about a single station may be combined, even without an overlapping period, using a reference series. Thus, a longer station record may be obtained and fragmentation of records reduced. The reference series also enables 1961–90 normals to be calculated for a larger proportion of stations. The station anomalies are interpolated onto a 0.5° grid covering the global land surface (excluding Antarctica) and combined with a published normal from 1961–90. Thus, climate grids are constructed for nine climate variables (temperature, diurnal temperature range, daily minimum and maximum temperatures, precipitation, wet-day frequency, frost-day frequency, vapour pressure, and cloud cover) for the period 1901–2002. This dataset is known as CRU TS 2.1 and is publicly available ( TODO: clickthrough URL ). Copyright 08 2005 Royal Meteorological Society
    Msadek R., C. Frankignoul, and L. Z. X. Li, 2011: Mechanisms of the atmospheric response to North Atlantic multidecadal variability: A model study. Climate Dyn.,36, 1255-1276, doi: 10.1007/s00382-010-0958-0.10.1007/ atmospheric circulation response to decadal fluctuations of the Atlantic meridional overturning circulation (MOC) in the IPSL climate model is investigated using the associated sea surface temperature signature. A SST anomaly is prescribed in sensitivity experiments with the atmospheric component of the IPSL model coupled to a slab ocean. The prescribed SST anomaly in the North Atlantic is the surface signature of the MOC influence on the atmosphere detected in the coupled simulation. It follows a maximum of the MOC by a few years and resembles the model Atlantic multidecadal oscillation. It is mainly characterized by a warming of the North Atlantic south of Iceland, and a cooling of the Nordic Seas. There are substantial seasonal variations in the geopotential height response to the prescribed SST anomaly, with an East Atlantic Pattern-like response in summer and a North Atlantic oscillation-like signal in winter. In summer, the response of the atmosphere is global in scale, resembling the climatic impact detected in the coupled simulation, albeit with a weaker amplitude. The zonally asymmetric or eddy part of the response is characterized by a trough over warm SST associated with changes in the stationary waves. A diagnostic analysis with daily data emphasizes the role of transient-eddy forcing in shaping and maintaining the equilibrium response. We show that in response to an intensified MOC, the North Atlantic storm tracks are enhanced and shifted northward during summer, consistent with a strengthening of the westerlies. However the anomalous response is weak, which suggests a statistically significant but rather modest influence of the extratropical SST on the atmosphere. The winter response to the MOC-induced North Atlantic warming is an intensification of the subtropical jet and a southward shift of the Atlantic storm track activity, resulting in an equatorward shift of the polar jet. Although the SST anomaly is only prescribed in the Atlantic ocean, significant impacts are found globally, indicating that the Atlantic ocean can drive a large scale atmospheric variability at decadal timescales. The atmospheric response is highly non-linear in both seasons and is consistent with the strong interaction between transient eddies and the mean flow. This study emphasizes that decadal fluctuations of the MOC can affect the storm tracks in both seasons and lead to weak but significant dynamical changes in the atmosphere.
    Otter, O. H., M. Bentsen, H. Drange, L. L. Suo, 2010: External forcing as a metronome for Atlantic multidecadal variability. Nature Geoscience, 3, 688- 694.10.1038/ records, proxy data and climate modelling show that multidecadal variability is a dominant feature of North Atlantic sea-surface temperature variations, with potential impacts on regional climate. To understand the observed variability and to gauge any potential for climate predictions it is essential to identify the physical mechanisms that lead to this variability, and to explore the spatial and temporal characteristics of multidecadal variability modes. Here we use a coupled ocean-atmosphere general circulation model to show that the phasing of the multidecadal fluctuations in the North Atlantic during the past 600 years is, to a large degree, governed by changes in the external solar and volcanic forcings. We find that volcanoes play a particularly important part in the phasing of the multidecadal variability through their direct influence on tropical sea-surface temperatures, on the leading mode of northern-hemisphere atmosphere circulation and on the Atlantic thermohaline circulation. We suggest that the implications of our findings for decadal climate prediction are twofold: because volcanic eruptions cannot be predicted a decade in advance, longer-term climate predictability may prove challenging, whereas the systematic post-eruption changes in ocean and atmosphere may hold promise for shorter-term climate prediction.
    Rayner N. A., D. E. Parker, E. B. Horton, C. K. Folland , L. V. Alexand er, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108(D14),4407, doi: 10.1029/2002JD 002670.10.1029/[1] We present the Met Office Hadley Centre's sea ice and sea surface temperature (SST) data set, HadISST1, and the nighttime marine air temperature (NMAT) data set, HadMAT1. HadISST1 replaces the global sea ice and sea surface temperature (GISST) data sets and is a unique combination of monthly globally complete fields of SST and sea ice concentration on a 1 latitude-longitude grid from 1871. The companion HadMAT1 runs monthly from 1856 on a 5 latitude-longitude grid and incorporates new corrections for the effect on NMAT of increasing deck (and hence measurement) heights. HadISST1 and HadMAT1 temperatures are reconstructed using a two-stage reduced-space optimal interpolation procedure, followed by superposition of quality-improved gridded observations onto the reconstructions to restore local detail. The
    Steinman B. A., M. E. Mann, and S. K. Miller, 2015: Atlantic and Pacific multidecadal oscillations and Northern Hemisphere temperatures. Science, 347( 6225), 988- 991.10.1126/ recent slowdown in global warming has brought into question the reliability of climate model projections of future temperature change and has led to a vigorous debate over whether this slowdown is the result of naturally occurring, internal variability or forcing external to Earth's climate system. To address these issues, we applied a semi-empirical approach that combines climate observations and model simulations to estimate Atlantic- and Pacific-based internal multidecadal variability (termed "" and "PMO," respectively). Using this method, the and PMO are found to explain a large proportion of internal variability in Northern Hemisphere mean temperatures. Competition between a modest positive peak in the and a substantially negative-trending PMO are seen to produce a slowdown or "false pause" in warming of the past decade.
    Suo L. L., O. H. Otter, M. Bentsen, Y. Q. Gao, and O. M. Johannessen, 2013: External forcing of the early 20th century arctic warming. Tellus A, 65, 20578.10.3402/ observed Arctic warming during the early 20th century was comparable to present-day warming in terms of magnitude. The causes and mechanisms for the early 20th century Arctic warming are less clear and need to be better understood when considering projections of future climate change in the Arctic. The simulations using the Bergen Climate Model (BCM) can reproduce the surface air temperature (SAT) fluctuations in the Arctic during the 20th century reasonably well. The results presented here, based on the model simulations and observations, indicate that intensified solar radiation and a lull in volcanic activity during the 1920s-1950s can explain much of the early 20th century Arctic warming. The anthropogenic forcing could play a role in getting the timing of the peak warming correct. According to the model the local solar irradiation changes play a crucial role in driving the Arctic early 20th century warming. The SAT co-varied closely with local solar irradiation changes when natural external forcings are included in the model either alone or in combination with anthropogenic external forcings. The increased Barents Sea warm inflow and the anomalous atmosphere circulation patterns in the northern Europe and north Atlantic can also contribute to the warming. In summary, the early 20th century warming was largely externally forced.
    Sutton R. T., D. L. R. Hodson, 2005: Atlantic Ocean forcing of North American and European summer climate. Science, 309, 115- 118.10.1126/ Recent extreme events such as the devastating 2003 European summer heat wave raise important questions about the possible causes of any underlying trends, or low-frequency variations, in regional climates. Here, we present new evidence that basin-scale changes in the Atlantic Ocean, probably related to the thermohaline circulation, have been an important driver of multidecadal variations in the summertime climate of both North America and western Europe. Our findings advance understanding of past climate changes and also have implications for decadal climate predictions.
    Sutton R. T., D. L. R. Hodson, 2007: Climate response to basin-scale warming and cooling of the North Atlantic Ocean. J.Climate, 20, 891- 907.10.1175/ experiments with an atmospheric general circulation model, the climate impacts of a basin-scale warming or cooling of the North Atlantic Ocean are investigated. Multidecadal fluctuations with this pattern were observed during the twentieth century, and similar variations09”but with larger amplitude09”are believed to have occurred in the more distant past. It is found that in all seasons the response to warming the North Atlantic is strongest, in the sense of highest signal-to-noise ratio, in the Tropics. However there is a large seasonal cycle in the climate impacts. The strongest response is found in boreal summer and is associated with suppressed precipitation and elevated temperatures over the lower-latitude parts of North and South America. In August09“September09“October there is a significant reduction in the vertical shear in the main development region for Atlantic hurricanes. In winter and spring, temperature anomalies over land in the extratropics are governed by dynamical changes in circulation rather than simply reflecting a thermodynamic response to the warming or cooling of the ocean. The tropical climate response is primarily forced by the tropical SST anomalies, and the major features are in line with simple models of the tropical circulation response to diabatic heating anomalies. The extratropical climate response is influenced both by tropical and higher-latitude SST anomalies and exhibits nonlinear sensitivity to the sign of the SST forcing. Comparisons with multidecadal changes in sea level pressure observed in the twentieth century support the conclusion that the impact of North Atlantic SST change is most important in summer, but also suggest a significant influence in lower latitudes in autumn and winter. Significant climate impacts are not restricted to the Atlantic basin, implying that the Atlantic Ocean could be an important driver of global decadal variability. The strongest remote impacts are found to occur in the tropical Pacific region in June09“August and September09“November. Surface anomalies in this region have the potential to excite coupled ocean09“atmosphere feedbacks, which are likely to play an important role in shaping the ultimate climate response.
    Taylor K. E., 2001: Summarizing multiple aspects of model performance in a single diagram. J. Geophys.Res, 106, 7183-
    Taylor K. E., R. J. Stouffer, G. A. Meehl, 2012: An overview of CMIP5 and the experiment design. Bull. Amer. Meteor. Soc., 93, 485-
    Ting M. F., Y. Kushnir, R. Seager, and C. H.Li, 2009: Forced and internal twentieth-century SST trends in the North Atlantic. J.Climate, 22, 1469- 1481.10.1175/ recent years, two alarming trends in North Atlantic climate have been noted: an increase in the intensity and frequency of Atlantic hurricanes and a rapid decrease in Greenland ice sheet volume. Both of these phenomena occurred while a significant warming took place in North Atlantic sea surface temperatures (SSTs), thus sparking a debate on whether the warming is a consequence of natural climate variations, anthropogenic forcing, or both; and if both, what their relative roles are. Here models and observations are used to detect and attribute long-term (multidecadal) twentieth-century North Atlantic (NA) SST changes to their anthropogenic and natural causes. A suite of Intergovernmental Panel on Climate Change (IPCC) twentieth-century (C20C) coupled model simulations with multiple ensemble members and a signal-to-noise maximizing empirical orthogonal function analysis are used to identify a model-based estimate of the forced, anthropogenic component in NA SST variability. Comparing the results to observations, it is argued that the long-term, observed, North Atlantic basin-averaged SSTs combine a forced global warming trend with a distinct, local multidecadal 0904oscillation0909 that is outside of the range of the model-simulated, forced component and most likely arose from internal variability. This internal variability produced a cold interval between 1900 and 1930, followed by 30 yr of relative warmth and another cold phase from 1960 to 1990, and a warming since then. This natural variation, referred to previously as the Atlantic Multidecadal Oscillation (AMO), thus played a significant role in the twentieth-century NA SST variability and should be considered in future, near-term climate projections as a mechanism that, depending on its behavior, can act either constructively or destructively with the region's response to anthropogenic influence, temporarily amplifying or mitigating regional climate change.
    Ting M. F., Y. Kushnir, R. Seager, and C. H. Li, 2011: Robust features of Atlantic multi-decadal variability and its climate impacts. Geophys. Res. Lett., 38,L17705, doi: 10.1029/2011GL 048712.10.1029/ Multi-decadal Variability (AMV), also known as the Atlantic Multi-decadal Oscillation (AMO), is characterized by a sharp rise and fall of the North Atlantic basin-wide sea surface temperatures (SST) on multi-decadal time scales. Widespread consequences of these rapid temperature swings were noted in many previous studies. Among these are the drying of Sahel in the 1960-70s and change in the frequency and intensity of Atlantic hurricanes on multi-decadal time scales. Given the short instrumental data records (about a century long) the central question is whether these climate fluctuations are robustly linked with the AMV and to what extent are these connections subject to changes in a changing climate. Here we address this issue by using the CMIP3 simulations for the 20th, 21st, and pre-industrial eras with 23 IPCC models. While models tend to produce AMV of shorter time scales and less periodic than suggested by the observations, the spatial structures of the SST anomaly patterns, and their association with worldwide precipitation, are surprisingly similar between models (with differing external forcing) and observations. Our results confirm the strong link between AMV and Sahel rainfall and suggest a clear physical mechanism for the linkage in terms of meridional shifts of the Atlantic ITCZ. The results also help clarify influences that may not be robust, such as the impacts over North America, India, and Australia.
    Wang Y. M., S. L. Li, and D. H. Luo, 2009: Seasonal response of Asian monsoonal climate to the Atlantic Multidecadal Oscillation. J. Geophys. Res., 114,D02112, doi: 10.1029/2008JD 010929.10.1029/ influence of the Atlantic Multidecadal Oscillation (AMO) on Asian monsoonal climate in all four seasons is investigated by comprehensive observational analyses and ensemble experiments with atmospheric general circulation models (AGCMs). Three AGCMs are forced by prescribed climatological seasonal cycle of sea surface temperature (SST) or with additional SST anomalies representing the warmth phase of the AMO. The results in both the observations and the models consistently suggest that the warm AMO phase gives rise to elevated air temperatures in East Asia and northern India but decreased air temperatures in much of central-southern India in all four seasons. This positive AMO anomaly also causes more rainfall in central and southern India in every season, particularly in summer and fall. In contrast, the sign of AMO influences on East Asian rainfall is season-dependent: in southeastern China, it induces increased rainfall in summer but suppressed rainfall in autumn. It is suggested that these AMO influences are realized by warming Eurasian middle and upper troposphere in all four seasons, resulting in weakened Asian winter monsoons but enhanced summer monsoons. Furthermore, the formation of the troposphere heating anomaly may be related to the wave guidance mechanism associated with the Asian upper jet.
    Wilcox L. J., E. J. Highwood, and N. J. Dunstone, 2013: The influence of anthropogenic aerosol on multi-decadal variations of historical global climate. Environmental Research Letters, 8( 2), 024033.10.1088/1748-9326/8/2/ of single forcing runs from CMIP5 (the fifth Coupled Model Intercomparison Project) simulations shows that the mid-twentieth century temperature hiatus, and the coincident decrease in precipitation, is likely to have been influenced strongly by anthropogenic aerosol forcing. Models that include a representation of the indirect effect of aerosol better reproduce inter-decadal variability in historical global-mean near-surface temperatures, particularly the cooling in the 1950s and 1960s, compared to models with representation of the aerosol direct effect only. Models with the indirect effect also show a more pronounced decrease in precipitation during this period, which is in better agreement with observations, and greater inter-decadal variability in the inter-hemispheric temperature difference. This study demonstrates the importance of representing aerosols, and their indirect effects, in general circulation models, and suggests that inter-model diversity in aerosol burden and representation of aerosol-cloud interaction can produce substantial variation in simulations of climate variability on multi-decadal timescales.
    Wu B. Y., R. H. Zhang, R. D'Arrigo, and J. Z. Su, 2013: On the Relationship between winter sea ice and summer atmospheric circulation over Eurasia. J. Climate, 26, 5523- 5536.10.1175/ NCEP-NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) data, this paper investigates the association between winter sea ice concentration (SIC) in Baffin Bay southward to the eastern coast of Newfoundland, and the ensuing summer atmospheric circulation over the mid- to high latitudes of Eurasia. It is found that winter SIC anomalies are significantly correlated with the ensuing summer 500-hPa height anomalies that dynamically correspond to the Eurasian pattern of 850-hPa wind variability and significantly influence summer rainfall variability over northern Eurasia. Spring atmospheric circulation anomalies south of Newfoundland, associated with persistent winter-spring SIC and a horseshoe-like pattern of sea surface temperature (SST) anomalies in the North Atlantic, act as a bridge linking winter SIC and the ensuing summer atmospheric circulation anomalies over northern Eurasia. Indeed, this study only reveals the association based on observations and simple simulation experiments with SIC forcing. The more precise mechanism for this linkage needs to be addressed in future work using numerical simulations with SIC and SST as the external forcings. The results herein have the following implication: Winter SIC west of Greenland is a possible precursor for summer atmospheric circulation and rainfall anomalies over northern Eurasia. 2013 American Meteorological Society.
    Yu L., Y. Q. Gao, H. J. Wang, D. Guo, and S. L. Li, 2009: The responses of East Asian Summer monsoon to the North Atlantic Meridional Overturning Circulation in an enhanced freshwater input simulation. Chinese Science Bulletin,54, 4724-4732, doi: 10.1007/s11434-009-0720-3.10.1007/ investigated the response of the East Asian Summer Monsoon (EASM) to a weakened Atlantic Meridional Overturning Circulation (AMOC) and its mechanism in an enhanced freshwater input experiment (FW) by using a fully-coupled climate model. The response was a weakened EASM and the mechanisms can be explained as follows. The simulated weakened AMOC resulted in a drop in sea surface temperature (SST) in the North Atlantic (NA) and, correspondingly, an anomalous high sea level pressure (SLP) over the North American regions, which in turn increased the northeast surface winds across the equator in the eastern tropical Pacific (ETP). The anomalous northeast winds then induced further upwelling in the ETP and stronger air/sea heat exchange, therefore leading to an anomalous cooling of the eastern tropical sea surface. As a result, the climatologic Hadley Circulation (HC) was weakened due to an anomalous stronger sinking of air in the ETP north of the equator, whereas the Walker Circulation (WC) in the western tropical Pacific (WTP) north of the equator was strengthened with an eastward-shifted upwelling branch. This feature was in agreement with the anomalous convergent winds in the WTP, and led to a weakened EASM and less East Asian summer precipitation (EASP). Furthermore, comparison with previous freshwater experiments indicates that the strength of EASP could be influenced by the magnitude of the added freshwater.
    Zhang L. P., C. Z. Wang, 2013: Multidecadal North Atlantic sea surface temperature and Atlantic meridional overturning circulation variability in CMIP5 historical simulations. J. Geophys. Res. Oceans,118, 5772-5791, doi: 10.1002/jgrc. 20390.10.1002/ this paper, simulated variability of the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC) and their relationship has been investigated. For the first time, climate models of the Coupled Model Intercomparison Project phase 5 (CMIP5) provided to the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC-R5) in historical simulations have been used for this purpose. The models show the most energetic variability on the multidecadal timescale band both with respect to the AMO and AMOC, but with a large model spread in both amplitude and frequency. The relationship between the AMO and AMOC in most of the models resembles the delayed advective oscillation proposed for the AMOC on multidecadal timescales. A speed up (slow down) of the AMOC is in favor of generating a warm (cold) phase of the AMO by the anomalous northward (southward) heat transport in the upper ocean, which reversely leads to a weakening (strengthening) of the AMOC through changes in the meridional density gradient after a delayed time of ocean adjustment. This suggests that on multidecadal timescales the AMO and AMOC are related and interact with each other.
    Zhang R., T. L. Delworth, 2005: Simulated tropical response to a substantial weakening of the Atlantic thermohaline circulation. J.Climate, 18, 1853-;link_type=DOI
    Zhang R., T. L. Delworth, 2006: Impact of Atlantic multidecadal oscillations on India/Sahel rainfall and Atlantic hurricanes. Geophys. Res. Lett., 33,L17712, doi: 10.1029/2006GL
    Zhou X. M., S. L. Li, F. F. Luo, Y. Q. Gao, and T. Furevik, 2015: Air-sea coupling enhances the East Asian winter climate response to the Atlantic Multidecadal Oscillation. Adv. Atmos. Sci.,32(12), 1647-1659, doi: 10.1007/s00376-015-5030-x.10.1007/ simple air–sea coupled model, the atmospheric general circulation model (AGCM) of the National Centers for Environmental Prediction coupled to a mixed-layer slab ocean model, is employed to investigate the impact of air–sea coupling on the signals of the Atlantic Multidecadal Oscillation (AMO). A regional coupling strategy is applied, in which coupling is switched off in the extratropical North Atlantic Ocean but switched on in the open oceans elsewhere. The coupled model is forced with warm-phase AMO SST anomalies, and the modeled responses are compared with those from parallel uncoupled AGCM experiments with the same SST forcing. The results suggest that the regionally coupled responses not only resemble the AGCM simulation, but also have a stronger intensity. In comparison, the coupled responses bear greater similarity to the observational composite anomaly. Thus, air–sea coupling enhances the responses of the East Asian winter climate to the AMO. To determine the mechanism responsible for the coupling amplification, an additional set of AGCM experiments, forced with the AMO-induced tropical SST anomalies, is conducted. The SST anomalies are extracted from the simulated AMO-induced SST response in the regionally coupled model. The results suggest that the SST anomalies contribute to the coupling amplification. Thus, tropical air–sea coupling feedback tends to enhance the responses of the East Asian winter climate to the AMO.
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Manuscript History

Manuscript received: 15 December 2015
Manuscript revised: 18 June 2016
Manuscript accepted: 08 July 2016
通讯作者: 陈斌,
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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Simulation by CMIP5 Models of the Atlantic Multidecadal Oscillation and Its Climate Impacts

  • 1. Key Laboratory of Regional Climate-Environment for Temperate East Asia, Chinese Academy of Sciences, Beijing 100029, China
  • 2. Nansen-Zhu International Research Centre and Climate Change Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 3. Nansen Environmental and Remote Sensing Center and Bjerknes Centre for Climate Research, Bergen 5006, Norway
  • 4. Geophysical Institute, University of Bergen and BjerknesCentre for Climate Research, Bergen 5007, Norway

Abstract: This study focuses on the climatic impacts of the Atlantic Multidecadal Oscillation (AMO) as a mode of internal variability. Given the difficulties involved in excluding the effects of external forcing from internal variation, i.e., owing to the short record length of instrumental observations and historical simulations, we assess and compare the AMO and its related climatic impacts both in observations and in the "Pre-industrial" experiments of models participating in CMIP5. First, we evaluate the skill of the 25 CMIP5 models' "Historical" simulations in simulating the observational AMO, and find there is generally a considerable range of skill among them in this regard. Six of the models with higher skill relative to the other models are selected to investigate the AMO-related climate impacts, and it is found that their "Pre-industrial" simulations capture the essential features of the AMO. A positive AMO favors warmer surface temperature around the North Atlantic, and the Atlantic ITCZ shifts northward leading to more rainfall in the Sahel and less rainfall in Brazil. Furthermore, the results confirm the existence of a teleconnection between the AMO and East Asian surface temperature, as well as the late withdrawal of the Indian summer monsoon, during positive AMO phases. These connections could be mainly caused by internal climate variability. Opposite patterns are true for the negative phase of the AMO.

1. Introduction
2. Models and data
  • The modeled monthly SST, surface temperature and precipitation are utilized in this study, based on the two types of simulations in CMIP5: (1) "Historical" simulations for 1850-2005, with observed forcing agents, including emissions or concentrations of well-mixed greenhouse gases, natural and anthropogenic aerosols, solar forcing and land use change (Taylor et al., 2012); (2)"Pre-industrial Control" simulations, with non-evolving and pre-industrial conditions, including prescribed well-mixed gases, natural aerosols or their precursors, and some short-lived species (Taylor et al., 2012). In the "Pre-industrial Control" simulations, solar forcing is kept constant and there are no volcanoes. Output is downloaded from the PCMDI CMIP5 website ( The "Historical" simulations are used for the validation and selection of CMIP5 models, and the "Pre-industrial" simulations focus on the AMO's characteristics and impacts on temperature and precipitation, which indicates the internal variability of the AMO. Table 1 lists the modeling group, model name, the horizontal resolution of the oceanic and atmospheric components, and ensemble members, for each of the 25 chosen models. Additionally, the time spans of the "Pre-industrial" simulations for the six models chosen from these 25 for further analysis are also listed. Ten models are earth system models, including ocean and atmospheric chemistry and interactive land surface processes. More detailed information on the CMIP5 models and experiments can be found at, and in related papers (e.g., Taylor et al., 2012).

    Figure 1.  Power spectrum of the AMO index for the (a) observation (HadISST) and (1-25) the models. The power spectrum is given by the black line, significant above the red line at the 5% level.

    The observational SST dataset is HadISST (Rayner et al., 2003), spanning the years 1870-2010 and gridded to 1.0° latitude by 1.0° longitude. The monthly global land temperature and precipitation for 1901-2009, on a 0.5°× 0.5° grid, is obtained from the CRU TS 3.1 dataset (Mitchell and Jones, 2005). Given the sparse records of HadISST and CRU over the polar regions, the monthly surface temperature anomalies compiled by NASA's GISS are applied, covering the years 1880-2014 and on a 2.0°× 2.0° grid (Hansen et al., 2006).

    Before analysis, all of the model output, together with the observational data, are interpolated onto a 2.0°× 2.0° grid using a linear interpolation scheme. To reduce the possible impacts of greenhouse gases, all of the "Historical" simulation, observation and reanalysis datasets are first detrended linearly. Then, the detrended time series are low-pass filtered with a nine-point running-mean filter to obtain low-frequency components. The degrees of freedom for the t-tests used throughout the study are n/9-1, where n stands for the number of samples. The AMO index is defined as the yearly averaged low-frequency SST anomaly in the North Atlantic basin (0°-60°N, 75°-7.5°W) (Enfield et al., 2001; Wang et al., 2009).

3. Results
  • First, we evaluate (by comparing with observation) the skill of the 25 CMIP5 models in terms of their "Historical" simulations of the AMO. Spectral analysis shows that the observed AMO index exhibits one dominant period of around 50-70 years (Fig. 1a), consistent with earlier studies (e.g., Enfield et al., 2001; Kavvada et al., 2013). As for the models [Figs. 1(b)-(z)], most showdominant periods of around 10-70 years, with more than one significant peak. FGOALS-g2 and GFDL-ESM2G are the exceptions, showing no significant period on the decadal-to-multidecadal timescale. Figure 2 displays the similarities between the modeled and observed AMO index via a Taylor diagram (Taylor, 2001). It is clear that the models differ substantially in their representation of the evolution of the AMO index, exhibiting a wide range of temporal correlation coefficients from -0.3 to 0.6. This is expected, considering there is no initialization of the oceanic conditions, and the intrinsic stochastic forcing in the models. The majority of the models have weaker amplitudes, less than or equal to one standard deviation of the observation.

    The spatial structures of the AMO in the models are compared with those in the observation (Fig. 3). Similar to many previous studies (e.g., Delworth and Mann, 2000; Zhang and Delworth, 2005; Kavvada et al., 2013; Zhang and Wang, 2013; Ba et al., 2014), the observational pattern is characterized by a horseshoe-like pattern, with a maximum center south of Greenland and east of Newfoundland in the midlatitude Atlantic (the domain represented by the black square in Fig. 3a), and with relatively weak anomalies along the coast of Northwest Africa and extending westward into the tropics. Figure 4 shows the similarities between the modeled AMO patterns and that observed via a Taylor diagram (the better the model, the shorter the distance between the model and "OBS"). The skill of the different models varies greatly, represented by the wide range of spatial correlation coefficients (SCCs) (from -0.3 to 0.6) and normalized standard deviations (from 0.2 to 7.1) (Fig. 4). These diversities between the models and the observation are mainly manifested in three aspects. First, although the maximum anomalies in the models are in the mid-high latitudes, the centers depart from the observation, locating to the north or east. Second, negative anomalies can be found to the southeast of Newfoundland in some of the models. And third, the subtropical/tropical warming in most of the models is not presented in the same way as in the observation, with some of the warming situated to the north relative to that observed. These differences not only exist between the observation and the models, but also from model to model. The normalized standard deviations of some models are greater than one standard deviation of the observation, which is likely due to the strong warming around Greenland in these models (Fig. 3).

    The above-mentioned analyses suggest a wide range of skill among the models in simulating the observational AMO.

    Figure 2.  Taylor diagram showing the temporal features of the AMO index for the period 1874-2001. The x-axis shows the normalized standard deviation and the arc shows the correlation values between observations and the ensemble mean for each model. The red numbers correspond to the model reference numbers listed in Table 1.

    Model selection is based on the following criteria: (1) a significant multidecadal period; (2) a temporal normalized standard deviation within the range of 0.5 standard deviations of the observation; (3) spatial correlations above 0.3; (4) spatial normalized standard deviations within the range of 1 standard deviations of the observation. Table 2 summarizes the selection criteria and shows that six models qualify for selection. These six models[BCC_CSM1.1(m), CNRM-CM5, GFDLCM3, INM-CM4.0, IPSL-CM5A-MR, and MPI-ESM-P] are therefore employed to investigate the AMO-related climatic patterns in the internal natural system.

    Figure 3.  Regressions of SST onto the standardized AMO index in the (a) observation and (1-25) models. Black dots indicate statistical significance at the >95% confidence level, based on the t-test. The black frames represent the maximum center in the observation. Units: °C.

  • The six models' "Pre-industrial" simulations show one dominant period of 20-70 years (Fig. 5). Compared to the period of the "Historical" simulation for each model, the period of the "Pre-industrial" simulations shows some obvious differences. For example, in the "Historical" simulation, MPI-ESM-P has only one peak period of around 70 years, but in the "Pre-industrial" simulation it has multiple significant multidecadal periods. The difference may be due to the different forcing conditions between the two simulations. Table 3 displays the standard deviations of the AMO index results. Both in the "Pre-industrial" and "Historical" simulations, the six modeled amplitudes are weaker than the observed amplitude (0.15°C).

    Figure 4.  Taylor diagram showing the spatial features of the regressions displayed by Fig. 3. Note that GFDL-ESM2G has a normalized standard deviation larger than 4.0 and is not shown.

    Figure 5.  Power spectrum of the AMO index for the six selected models' "Pre-industrial" simulations. The power spectrum is given by the black line, significant above the red line at the 5% level.

    Figure 6.  Regressions of SST onto the standardized AMO index in the six selected models' "Pre-industrial" simulations. Black dots indicate statistical significance at the >95% confidence level, based on the t-test. Units: °C.

    Figure 6 shows the spatial patterns of SST variations associated with the positive phase of the AMO in the "Pre-industrial" simulations. The differences in the spatial patterns between the "Historical" simulation and observation also exist in the "Pre-industrial" simulation. However, the "Pre-industrial" simulations still capture the essential features of the observed AMO, as indicated by the high SCCs from 0.41 to 0.67 (Fig. 6). It is interesting that the patterns in four of the six models' "Pre-industrial" simulations are closer to the observation than they are for their "Historical" simulations. These analyses indicate the existence of the AMO as an internal mode in the six models, and imply that it is rational to investigate the AMO-related climatic patterns based on these six models' "Pre-industrial" simulations.

  • 3.3.1. Surface temperature

    Figure 7.  Regressions of surface temperature onto the standardized AMO index: (a-d) CRU (for land temperature) and HadISST (for SST); (e-h) GISS. Black dots indicate statistical significance at the >95% confidence level, based on the t-test. Units: °C.

    Figure 8.  Regression coefficients of surface temperature onto the AMO in each of the six selected models, where the sign of the regression agrees. Red dots indicate positive coefficients.

    Figure 7 displays the AMO-related spatial patterns of surface temperature in the observations in all four seasons. On decadal-to-multidecadal timescales, oceanic thermal conditions play a key role in influencing climate variability (Bjerknes, 1964; Gulev et al., 2013). Hence, we first examine the AMO-related patterns of surface temperature in the ocean. The observations exhibit analogous spatial patterns in all seasons (Fig. 7). During positive AMO phases, the SSTs show a basin-scale cooling in the South Atlantic, and a maximum close to the Weddell Sea. For the Pacific, the observations show a tripolar pattern with positive temperature anomalies in the North and South Pacific, and slightly negative temperature anomalies in the equatorial Pacific (Dong et al., 2006). For the Indian Ocean, there is a warm signal in both HadISST (Figs. 7a-d) and GISS (Figs. 7e-h), though the area of significance is much smaller in the former than the latter.

    As for the AMO-related surface temperature over land, we focus on the Northern Hemisphere, considering the larger impact on the Northern Hemisphere than the Southern Hemisphere and the lower reliability of the data for the Southern Hemisphere compared with the Northern Hemisphere for the first half of the 1900s. Observations show increased temperatures over Greenland in all four seasons, maximum anomalies in winter, and minimum anomalies in summer (Fig. 7). Warming appears over the whole of the North American continent in winter, summer and autumn, while there is an east-west dipolar pattern with cooling over eastern North America and warming over western North America in spring (Fig. 7). Warming can be found over North Africa in all seasons except summer (Fig. 7). In winter, a tripolar pattern is apparent, characterized by warm-cold-warm anomalies from north to south over Eurasia (Figs. 7a and e). In spring, the anomalies exhibit an east-west pattern, with cooling over Europe and warming over most of Asia (Figs. 7b and f). In summer and autumn, Europe and eastern Asia show positive anomalies, except for a slight cooling over central Asia (Figs. 7c-h). In addition, a dipolar seesaw of the surface temperature over the Arctic and Antarctic is apparent in GISS in winter and spring (Chylek et al., 2010).

    To illustrate the regions that have signals that agree among the six models(Figs. S1-S4 in the supporting information), we mark red/blue dots to represent the matching positive/negative regression coefficients in all of the models (Fig. 8). As can be seen, there is little similarity in the Southern Hemisphere among the models, and matchingpositive signals are present over most of the Northern Hemisphere. The AMO-related SST patterns bear no obvious seasonality and show warm anomalies over the equatorial Pacific, most of the North Pacific and Indian oceans, in addition to the North Atlantic. For the AMO-related surface temperature over land, it can be seen that there is a warming over Greenland, with a maximum extent in winter and spring and a minimum in summer. Over eastern North America and North Africa, the warming exists in all four seasons. For Eurasia, warming can be seen in all four seasons over the Scandinavian Peninsula, central Asia and eastern Asia. In particular, there is a band of warming from the Scandinavian Peninsula to East Asia in autumn.

    Figure 9.  Regressions of precipitation (based on CRU data) onto the standardized AMO index. Black dots indicate statistical significance at the >95% confidence level, based on the t-test. Units: mm d-1.

    Figure 10.  Regression coefficients of precipitation onto the AMO in each of the six selected models, where the sign of the regression agrees. Red dots indicate positive coefficients, and blue dots negative.

    Compared to the observations, a number of differences can be found in the models. First, the models are unable to capture the signals of the Southern Hemisphere and the negative surface temperature anomalies in the observations, such as those over western North America in spring and over Eurasia in winter and spring. Second, there is an opposite signal in the equatorial Pacific, in which the models portray positive anomalies but the observations show weakly negative anomalies. However, besides the warming in the North Atlantic, a number of similarities can be seen between the observations and models, such as the warming in the North Pacific, Greenland, Scandinavian Peninsula, most of North America, North Africa and East Asia.

    3.3.2. Precipitation

    Given that the impact of the AMO on precipitation mainly occurs in summer and autumn (e.g., Sutton and Hodson, 2005, 2007; Zhang and Delworth, 2006; Goswami et al., 2006; Wang et al., 2009; Kavvada et al., 2013), we focus here on discussing these two seasons. Figure 9 shows the spatial pattern of precipitation associated with positive phases of the AMO in the observation. Increased rainfall can be seen over the Sahel and north of Brazil, but decreased rainfall over Brazil, which is a result of the northward shift of the Atlantic ITCZ (Ting et al., 2011). The amplitude of rainfall over the Sahel is stronger in summer than in autumn. Less rainfall is apparent over North America in these two seasons. Europe features a dipolar pattern, with more rainfall over western Europe and less rainfall over eastern Europe in summer. There is enhanced rainfall over Siberia in summer. For the East Asian summer monsoon, a positive AMO favors intensified rainfall over East Asia, but less rainfall in autumn. In addition, there is more rainfall over India in the two seasons, which indicates a late withdrawal of the Indian summer monsoon (Lu et al., 2006; Luo et al., 2011).

    Similar to Fig. 8, Fig. 10 displays signals of precipitation in the six models (Figs. S5-S6 in the supplementary materials) that agree. Relative to surface temperature, however, the precipitation signals are less distinct. Enhanced precipitation is situated north of the equator over the Atlantic, implying a northward shift of the Atlantic ITCZ. More rainfall can be seen over the North Atlantic, Caribbean Sea and tropical eastern Pacific, which is related to the warming in these regions. Moreover, the models produce precipitation patterns that resemble the observed patterns of more rainfall over the Sahel and less rainfall over Brazil, and the late withdrawal of the Indian summer monsoon. However, more rainfall is apparent over Europe in autumn in the models, but there is no such significant rainfall in the observation. Additionally, a number of the signals in the observation cannot be found in the models, such as the enhanced rainfall over Siberia in summer. It should also be mentioned that less rainfall over South China in summer is apparent, which is contrary to the observational results and previous studies (Lu et al., 2006; Wang et al., 2009; Yu et al., 2009). The reason is unclear and needs to be investigated further.

4. Summary and discussion
  • Given the difficulties involved in removing the impacts of external forcing from the AMO in instrumental records and historical simulations, the "Historical" and "Pre-industrial Control" simulations of 25 CMIP5 models are used to investigate the AMO as a mode ofinternal variability, and its associated climatic impacts. First, we assess the skill of the 25 models in simulating the observed AMO, based on their "Historical" simulations. The results suggest a wide range of skill among the models. Six models that demonstrate better skill relative to the other models are selected via certain selection criteria. The "Pre-industrial" simulations of these six models also capture the essential features of the AMO, including the multidecadal variability and basin-scale warming in the North Atlantic.

    The modeled AMO-related surface temperatures show the AMO may have a larger impact on the Northern than the Southern Hemisphere. Compared to observations, the distinct difference is the opposite signals in the eastern tropical Pacific, where SSTs play an important role in AMO-related non-local impacts (Zhang and Delworth, 2005; Chen et al., 2014). As for the surface temperature over land, the main difference is that the negative anomalies over Eurasia and North America in the observations are not reproduced in the models during positive AMO phases. Nonetheless, two key similarities are found: (1) warming in the North Pacific; and (2) warming over Greenland, the Scandinavian Peninsula, North Africa, eastern North America, and East Asia.

    Based on the results of the "Pre-industrial" simulations, two hypotheses are proposed. (Ding et al., 2014) suggested that the recent surface warming in winter over eastern Canada and Greenland is likely caused by the natural climate variability. Here, we also show that the warming over Greenland is natural climate variability, but related to the AMO instead of LaNiña-like SST over the eastern equatorial Pacific. The recent Eurasian cooling has been suggested to be related to the Arctic sea-ice decline (e.g., Honda et al., 2009; Wu et al., 2013; Gao et al., 2015) and warming SSTs in the North Atlantic (Magnusdottir et al., 2004). However, our results imply that this recent cooling may not be relatedto the warming SSTs in the North Atlantic.

    The modeled AMO-related rainfall displays a meridional shift northward of the Atlantic ITCZ, subsequently leading to more rainfall over the Caribbean Sea and the Sahel, but less rainfall over Brazil. In addition, more rainfall can be found over the North Atlantic corresponding to the magnitude of the warming there. This variability of rainfall over land as well as the increased rainfall over India in autumn in the models is consistent with that observed.

    In summary, there is good agreement between the observation and models in terms of the AMO-related signals around the North Atlantic. However, a number of differences should nevertheless be noted. The reason why the negative surface temperatures related to positive phases of the AMO observed over Eurasia and North America are not reproduced by the models remains unclear, although three possibilities are suggested as follows: The first is the model bias in simulating the backgroud atmospheric circulation. (Kavvada et al., 2013) pointed out that models successfully capturing the observed features over the ocean do not necessarily also capture the observed atmospheric pattern. The second is the large difference between the models and observations in terms of the North Pacific SST anomalies,which could influence the surface temperature over North America and Europe through the atmospheric circulation (Frankignoul and Sennèchael, 2007). And the third is the limitations of the observational data due to short instrumental records, or impacts from external forcing (Suo et al., 2013).

    Regarding AMO remote connections, the same signals are mainly shown for the warming in the North Pacific and East Asia, as well as the late withdrawal of the Indian summer monsoon during positive AMO phases. This is consistenet with many previous studies (Zhang and Delworth, 2005; Lu et al., 2006; Wang et al., 2009; Zhou et al., 2015). The oceanic SST anomalies may be the primary cause inducing temperature/rainfall change over East Asia/India (Lu et al., 2006; Li et al., 2008; Zhou et al., 2015). But how the AMO links to the SST variations of the North Pacific is unclear. Besides, the impact of the AMO on East Asia may be associated with the surface temperature changes from the Greenland Sea to the Kara Sea,linked to sea ice (Li et al., 2015).

    The AMO-related surface temperature and precipitation patterns can be found in "Pre-industrial" simulations, which means that these may be related to internal climate variability. The AMO is considered as a vital decadal-scalepredictor because of its low-frequency nature (Keenlyside et al., 2008; Hurrell et al., 2009; Meehl et al., 2009; Kavvada et al., 2013). This study provides clues for decadal prediction in regions mainly influenced by internal climate variability, on the basis of the view that the real climate consists of natural internal variability and external forcing (Luo and Li, 2014; Steinman et al., 2015).




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