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Recent Increased Warming of the Alaskan Marine Arctic Due to Midlatitude Linkages

doi: 10.1007/s00376-017-7026-1

  • Alaskan Arctic waters have participated in hemispheric-wide Arctic warming over the last two decades at over two times the rate of global warming. During 2008-13, this relative warming occurred only north of the Bering Strait and the atmospheric Arctic front that forms a north-south thermal barrier. This front separates the southeastern Bering Sea temperatures from Arctic air masses. Model projections show that future temperatures in the Chukchi and Beaufort seas continue to warm at a rate greater than the global rate, reaching a change of +4°C by 2040 relative to the 1981-2010 mean. Offshore at 74°N, climate models project the open water duration season to increase from a current average of three months to five months by 2040. These rates are occasionally enhanced by midlatitude connections. Beginning in August 2014, additional Arctic warming was initiated due to increased SST anomalies in the North Pacific and associated shifts to southerly winds over Alaska, especially in winter 2015-16. While global warming and equatorial teleconnections are implicated in North Pacific SSTs, the ending of the 2014-16 North Pacific warm event demonstrates the importance of internal, chaotic atmospheric natural variability on weather conditions in any given year. Impacts from global warming on Alaskan Arctic temperature increases and sea-ice and snow loss, with occasional North Pacific support, are projected to continue to propagate through the marine ecosystem in the foreseeable future. The ecological and societal consequences of such changes show a radical departure from the current Arctic environment.
    摘要: 过去二十年来, 阿拉斯加北极海域参与了北极半球尺度的增暖, 其增暖速率达到了全球增暖速率的两倍多. 在2008年至2013年期间, 这种相对增暖主要发生在白令海峡和形成南北热力屏障的北极锋区以北. 这一锋区将东南部的白令海温度与北极气团分隔开. 数值模式的预测结果表明, 楚科奇海和博福特海未来将继续以高于全球速率的水平增暖. 到2040年, 该地区的温度相对于1981-2010年的平均值将高出 4°C. 气候模式还预测, 在74°N海面上的开放水域持续时间将从目前的平均3个月增加到2040年的5个月. 这些增速有时会因中纬度的相关而增强. 从2014年8月开始, 北极增暖的进一步加剧始于北太平洋的海面温度(SST)异常增暖, 并伴随着阿拉斯加上方相应转变的南风导致, 特别是2015-16年冬季. 虽然全球增暖和赤道遥相关主要影响北太平洋海温, 但2014-16北太平洋增暖事件的结束表明了在任何一年混沌的大气内部自然变率对天气的重要性. 在可预见的未来, 伴随着北太平洋偶尔的支持, 全球变暖对阿拉斯加的温度增加和北极海冰和积雪减少的影响将继续通过海洋生态系统传播. 这种变化的生态和社会后果将显示出与当前北极环境的根本偏离. (翻译: 陈卫; 校对: Muyin Wang)
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  • Ballinger T. J., S. C. Sheridan, 2014: Associations between circulation pattern frequencies and sea ice minima in the western Arctic.International Journal of Climatology,34,1385-1394, this study, a synoptic climatological approach is employed to assess the relationship between the frequency of circulation patterns (CPs) and the latitude of mid-September sea ice minima in the western Arctic. Fifteen CPs are created via principal component analysis and cluster analysis from daily NCEP/NCAR reanalysis sea-level pressure (SLP) fields across a grid from 50 to 90°N and 150°E–100°W from 1979 to 2011. The frequency of these CPs are statistically compared with the latitude of the sea ice minimum from passive microwave data for each of 11 equally-spaced longitudes (176°W to 126°W) extending into the Chukchi and Beaufort Seas. Monthly frequencies for each of the 15 CPs from March to September, signifying the melt season, for each year are correlated with the ice minimum for that September. These monthly frequencies are then entered into a stepwise multiple linear regression (SMLR) and collectively, CP frequencies explain 40–79% of the total ice retreat variance across the longitudes. The frequency of one cluster, CP 11, representing a broad high pressure area over the Beaufort Sea, is highly correlated with the latitude of the sea ice minima; June and August frequencies of this pattern are the initial predictors at 8 of the 11 longitudes and explain 22–32% of the variance. This pattern has occurred more frequently from 2007 onwards; compared with a June mean occurrence of 965days during 1979–2006, CP 11 occurred 16 times in June 2007, and on average more than 1765days per month during June 2008–2011. The Arctic Dipole (AD), Arctic Oscillation (AO), and Pacific-North American (PNA) pattern indices are significantly correlated with CPs 11–13 frequencies throughout certain summer months, further indicating strong relationships between summer circulation and sea ice minima in the region.
    Ballinger T. J., J. C. Rogers, 2014: Climatic and atmospheric teleconnection indices and western Arctic sea ice variability.Physical Geography,35,459-477, 2014. 949338. study examines the sea ice cover minima in the western Arctic in the context of several climatic mechanisms known to impact its variability. The September latitude of western Arctic sea ice is measured along 11 equally-spaced longitudes extending from 1760265W to 1260265W in the Chukchi and Beaufort Seas, 1953–2010. Indices of seasonal atmospheric and oceanic teleconnections and annual mean Northern Hemisphere temperatures (NHT) and CO2 concentration are orthogonalized using rotated principal component analysis, forming predictors regressed onto the sea ice latitude data at each longitude using stepwise multiple linear regression. Prior to 1998, small amounts of September ice edge variance are explained by teleconnections such as the Arctic Dipole, Arctic Oscillation, and Pacific-North American Pattern. NHTs begin explaining large amounts of ice edge variance starting in 1998. For the 1953–2010 period, up to 68% of the ice edge variance is explained at 161°65W in the Chukchi Sea, mostly by NHTs. With the exception of the three easternmost longitudes (136–126°65W), the teleconnections and NHTs explain over 50% of the regional ice edge variance. Increases in both NHTs and ice retreat since the mid-1990s account for the large explained variances observed in regression analyses extending into recent years.
    Ballinger T. J., S. C. Sheridan, and E. Hanna, 2014: Resolving the Beaufort Sea High using synoptic climatological methods.International Journal of Climatology,34,3312-3319, Melt season frequencies of the Beaufort Sea High (BSH) have a profound effect on western Arctic climate, making the interannual spatial and temporal monitoring of this polar anticyclone important. This manuscript presents two automated synoptic climatological analyses using a two-step cluster procedure to classify daily mean sea level pressure (MSLP) over 180–120°W and 70–85°N with an emphasis on identifying BSH patterns. Separate raw and anomaly MSLP circulation pattern (CP) classifications are compared in order to assess the spatial characteristics of the BSH and its monthly frequency changes during the melt season from 1979 to 2012. Analysis of both classifications shows clear advantages to using the anomaly approach in terms of assessing temporal and spatial changes, particularly in light of the documented recent atmospheric circulation changes that have been observed over the region. Associations between the June anomaly circulation pattern (ACP) 5, a +465hPa BSH pattern situated between 155°W and 135°W, and June indices of atmospheric teleconnections such as the Arctic Dipole and Arctic Oscillation are statistically significant. There is also a statistically significant link to the downstream Greenland Blocking Index; suggesting that the BSH may be intricately related to climatic variability outside the analysed domain as well. Further, ACP 5 occurred nearly 365weeks more often during the melt season in recent massive Arctic Sea ice loss years (2007–2012) compared with the climatology (1979–2006). Recent increases in June ACP 5 frequencies account for a large proportion of this pattern's long-term frequency changes.
    Baxter S., S. Nigam, 2015: Key role of the North Pacific Oscillation-West Pacific Pattern in generating the extreme 2013/14 North American winter.J. Climate,28,8109-8117, Available
    Belleflamme A., X. Fettweis, and M. Erpicum, 2015: Recent summer Arctic atmospheric circulation anomalies in a historical perspective.The Cryosphere,9,53-64, . significant increase in the summertime occurrence of a high pressure area over the Beaufort Sea, the Canadian Arctic Archipelago, and Greenland has been observed since the beginning of the 2000s, and particularly between 2007 and 2012. These circulation anomalies are likely partly responsible for the enhanced Greenland ice sheet melt as well as the Arctic sea ice loss observed since 2007. Therefore, it is interesting to analyse whether similar conditions might have happened since the late 19th century over the Arctic region. We have used an atmospheric circulation type classification based on daily mean sea level pressure and 500 hPa geopotential height data from five reanalysis data sets (ERA-Interim, ERA-40, NCEP/NCAR, ERA-20C, and 20CRv2) to put the recent circulation anomalies in perspective with the atmospheric circulation variability since 1871. We found that circulation conditions similar to 20072012 have occurred in the past, despite a higher uncertainty of the reconstructed circulation before 1940. For example, only ERA-20C shows circulation anomalies that could explain the 19201930 summertime Greenland warming, in contrast to 20CRv2. While the recent anomalies exceed by a factor of 2 the interannual variability of the atmospheric circulation of the Arctic region, their origin (natural variability or global warming) remains debatable.
    Bezeau P., M. Sharp, and G. Gascon, 2015: Variability in summer anticyclonic circulation over the Canadian Arctic Archipelago and west Greenland in the late 20th/early 21st centuries and its effect on glacier mass balance.International Journal of Climatology,35,540-557, More frequent summer anticyclonic circulation over the Canadian Arctic Archipelago (CAA) between 2007 and 2012 caused more intense and sustained melt of ice caps and glaciers and increased rates of mass loss. To determine the frequency of the occurrence of anticyclonic circulation over the CAA and western Greenland, a self-organizing map (SOM) was used to classify daily 500 hPa geopotential height (GPH) anomalies calculated from National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) Reanalysis 1 (1948–2012) and five Coupled Model Intercomparison Project Phase 5 (CMIP5) models (1950–2025). While the NCEP/NCAR reanalysis indicates that significant summer warming over the CAA is linked to a doubling in the frequency of anticyclonic circulation over the region since 2007, the CMIP5 models were not capable of reproducing the magnitude of the trend in the frequency of anticyclonic circulation over the CAA and western Greenland found in NCEP/NCAR. The variability of the frequency of positive anomalies in summer 500 hPa GPH was found to be related to variability of Arctic sea ice volume/thickness in April, May and June (1979–2012) and to poleward eddy heat flux in June (1979–2012).
    Bond N. A., D. E. Harrison, 2006: ENSO's effect on Alaska during opposite phases of the Arctic Oscillation.International Journal of Climatology,26,1821-1841, NCEP Reanalysis and station data are used to investigate how the winter weather of Alaska during El Ni09o/Southern Oscillation (ENSO) events has varied during different phases of the Arctic Oscillation (AO). Much greater 500-hPa geopotential height, 1000-hPa air temperature, and precipitation anomalies in association with ENSO tend to occur in the negative phase of the AO; these anomalies cannot be attributed to the AO on its own. Analysis of case-to-case variability indicates that the ENSO/AO composite results are robust. It is also shown that much of the variability of the Pacific pole of the AO is associated with those winters with El Ni09o/AO- and La Ni09a/AO+ conditions, suggesting that this pole is much less robust than its counterpart in the North Atlantic. To the extent that winter mean state of the AO can be predicted, our results indicate that incorporation of the state of the AO would provide useful information for improving seasonal weather forecasts in the vicinity of Alaska. Copyright 08 2006 Royal Meteorological Society
    Bond N. A., M. F. Cronin, H. Freeland , and N. Mantua, 2015: Causes and impacts of the 2014 warm anomaly in the NE Pacific.Geophys. Res. Lett.,42,3414-3420, Strongly positive temperature anomalies developed in the NE Pacific Ocean during the boreal winter of 20132014. Based on a mixed layer temperature budget, these anomalies were caused by lower than normal rates of the loss of heat from the ocean to the atmosphere and of relatively weak cold advection in the upper ocean. Both of these mechanisms can be attributed to an unusually strong and persistent weather pattern featuring much higher than normal sea level pressure over the waters of interest. This anomaly was the greatest observed in this region since at least the 1980s. The region of warm sea surface temperature anomalies subsequently expanded and reached coastal waters in spring and summer 2014. Impacts on fisheries and regional weather are discussed. It is found that sea surface temperature anomalies in this region affect air temperatures downwind in Washington state.
    Cassano E. N., J. M. Glisan, J. J. Cassano, W. J. Gutowski Jr., and M. W. Seefeldt, 2015: Self-organizing map analysis of widespread temperature extremes in Alaska and Canada.Climate Research,62,199-218, This paper demonstrates how self-organizing maps (SOMs) can be used to evaluate the large-scale environment, in particular the synoptic circulation associated with widespread temperature extremes. The paper provides details on how SOMs are created, how they can be used to understand extreme events, and lessons learned in applying this methodology for extremes analysis. Using a SOM can be helpful in understanding the underlying physical processes that control extreme events, and how the extremes and the processes that control them may change in time or differ across space. Examples of widespread daily temperature extremes in 4 regions: 2 each in Alaska and in northern Canada during winter (December, January, and February) for 1989-2007 are presented to illustrate the application of the methodology. For the regions studied, the size of the domain over which the synoptic circulation was defined-in particular using a smaller domain focused on particular regions-and a greater number of classes to represent the archetypical synoptic patterns for the regions, give the best relationship between synoptic circulation and extremes. The results are most robust for the Alaskan domains and less so for the Canadian domains, leading to the conclusion that further study is warranted to better understand extremes in the Canadian regions.
    Holland, M. M., C. M. Bitz, 2003: Polar amplification of climate change in coupled models.Climate Dyn.,21,221-232, Northern Hemisphere polar amplification of climate change is documented in models taking part in the Coupled Model Intercomparison Project and in the new version of the Community Climate System Mo
    IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,T.F.,D. Qin,G.-K. Plattner,M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.
    Liu J. P., J. A. Curry, and Y. Y. Hu, 2004: Recent Arctic sea ice variability: Connections to the Arctic Oscillation and the ENSO,Geophys. Res. Lett.,31,L09211, in the satellite-derived Arctic sea ice concentrations (1978-2002) show pronounced decreases in the Barents/Kara Seas, between the Chukchi and Beaufort Seas, the central Sea of Okhotsk and a portion of the Hudson/Baffin Bay by ~2-8% per decade, exceeding the 95% confidence level. Qualitatively speaking, positive phases of the Arctic Oscillation (AO) and El Niño-Southern Oscillation (ENSO) produce similar ice changes in the western Arctic, but opposite ice changes in the eastern Arctic. The manner in which the ice changes are related to the AO and ENSO are demonstrated. Over the last 24 years, the magnitude of the ice changes associated with the positive AO trend and the negative ENSO trend is much smaller than the regional ice trends. Thus, more local or less understood large scale processes should be investigated for explanations.
    Makshtas A. P., I. I. Bolshakova, R. M. Gun, O. L. Jukova, N. E. Ivanov, and S. V. Shutilin, 2011: Climate of the hydrometeorological observatory Tiksi region. Meteorological and Geophysical Investigations, M, Paulsen, Ed., 49- 74.
    Newman, M., Coauthors, 2016: The Pacific decadal oscillation,revisited. J. Climate,29,4399-4427,. its identification in the late 1990's as the dominant pattern of North Pacific sea surface temperature (SST) variability, the Pacific decadal oscillation (PDO) has been connected both to other parts of the climate system and to impacts on natural resources and marine and terrestrial ecosystems. Variability associated with the PDO has often been confused with externally forced climate change including anthropogenic effects. Subsequent research, however, has found that the PDO is not a single physical mode of climate variability but instead largely represents the combination of three groups of processes: (1) changes in ocean surface heat fluxes and Ekman (wind-driven) transport related to the Aleutian low, due to both local, rapidly decorrelating, unpredictable weather noise and to remote forcing from interannual to decadal tropical variability (largely El Nino) via the "atmospheric bridge"; (2) ocean memory, or processes determining oceanic thermal inertia including "re-emergence" and oceanic Rossby waves, that act to integrate this forcing and thus generate added PDO variability on decadal time scales; and (3) decadal changes in the Kuroshio-Oyashio current system forced by the multi-year history of basin-wide Ekman pumping, manifested as SST anomalies along the subarctic front at about 40N in the western Pacific ocean. Thus, the PDO represents the effects of different processes operating on different timescales, with few of its apparent impacts due to extratropical SST anomalies. This talk presents a synthesis of this current view of the PDO, and discusses corresponding implications for climate diagnosis, including of PDO climate impacts and predictability (both oceanographic and atmospheric); potential decadal regime-like behavior; simulations of the PDO in climate models; the interpretation of paleoclimate multicentennial reconstructions of the PDO; and its impacts on marine ecosystems. We conclude with some suggested "best practices" for future PDO diagnosis and forecasts including investigating the potential role of the PDO in the global temperature hiatus.
    Overland, J. E., M. Y. Wang, 2005: The third Arctic climate pattern: 1930s and early 2000s,Geophys. Res. Lett.,32,L23808, near-surface warm temperature anomalies have occurred in two localized areas, eastern Siberia/East Siberian Sea and northeastern Canada/Baffin Bay, during winter and spring in recent years (2000-2005) in contrast to previous decades. The proximate cause in winter was a northward displacement and strengthening of the Aleutian Low and a weakening of the Icelandic Low. Spring showed a dipole pattern with higher sea-level pressure (SLP) on the North American side of the Arctic and lower pressures over Eurasia. Phase space trajectories of arctic climate for 1951-2005 based on the first two EOFs of SLP, the Arctic Oscillation and a Pacific North American-like (PNA*) pattern, show multi-annual variability leading to lower SLP and warmer temperatures in the last decades of the 20th century. Recent winters have some projection onto PNA*, but the SLP dipole in recent springs does not strongly project onto either of the two basic climate patterns. The period from 1928-1935 also had a dipole structure in SLP, which contributed to the interdecadal arctic-wide warm temperature anomalies in the first half of the 20th century. Recognition of the recent persistent and somewhat unique Arctic climate pattern is important as it contributes to the ongoing reorganization of arctic ecosystems.
    Overland, J. E., D. B. Percival, H. O. Mofjeld, 2006: Regime shifts and red noise in the North Pacific.Deep Sea Research Part I,53,582-588, 2005. 12. 011. and regime shifts are important concepts for understanding decadal variability in the physical system of the North Pacific because of the potential for an ecosystem to reorganize itself in response to such shifts. There are two prevalent senses in which these concepts are taken in the literature. The first is a formal definition and posits multiple stable states and rapid transitions between these states. The second is more data-oriented and identifies local regimes based on differing average climatic levels over a multi-annual duration, i.e. simply interdecadal fluctuations. This second definition is consistent with realizations from stochastic red noise processes to a degree that depends upon the particular model. Even in 100 year long records for the North Pacific a definition of regimes based solely on distinct multiple stable states is difficult to prove or disprove, while on interdecadal scales there are apparent local step-like features and multi-year intervals where the state remains consistently above or below the long-term mean. The terminologies climatic regime shift, statistical regime shift or climatic event are useful for distinguishing this second definition from the first. To illustrate the difficulty of advocating one definition over the other based upon a relatively short time series, we compare three simple models for the Pacific Decadal Oscillation (PDO). The 104-year PDO record is insufficient to statistically distinguish a single preference between a square wave oscillator consistent with the formal definition for regime shifts, and two red noise models that are compatible with climatic regime shifts. Because of the inability to distinguish between underlying processes based upon data, it is necessary to entertain multiple models and to consider how each model would impact resource management. In particular the persistence in the fitted models implies that certain probabilistic statements can be made regarding climatic regime shifts, but we caution against extrapolation to future states based on curve fitting techniques.
    Overland, J. E., J. A. Francis, E. Hanna, M. Y. Wang, 2012: The recent shift in early summer arctic atmospheric circulation,Geophys. Res. Lett.,39,L19804, last six years (2007-2012) show a persistent change in early summer Arctic wind patterns relative to previous decades. The persistent pattern, which has been previously recognized as the Arctic Dipole (AD), is characterized by relatively low sea-level pressure over the Siberian Arctic with high pressure over the Beaufort Sea, extending across northern North America and over Greenland. Pressure differences peak in June. In a search for a proximate cause for the newly persistent AD pattern, we note that the composite 700 hPa geopotential height field during June 2007-2012 exhibits a positive anomaly only on the North American side of the Arctic, thus creating the enhanced mean meridional flow across the Arctic. Coupled impacts of the new persistent pattern are increased sea ice loss in summer, long-lived positive temperature anomalies and ice sheet loss in west Greenland, and a possible increase in Arctic-subarctic weather linkages through higher-amplitude upper-level flow. The North American location of increased 700 hPa positive anomalies suggests that a regional atmospheric blocking mechanism is responsible for the presence of the AD pattern, consistent with observations of unprecedented high pressure anomalies over Greenland since 2007.
    Overland, J. E., J. Wang, R. S. Pickart, M. Y. Wang, 2014a: Recent and future changes in the meteorology of the Pacific Arctic.The Pacific Arctic Region,J. Grebmeier and W. Maslowski,Eds., Springer, Dordrecht, 17-30, meteorology of the Pacific Arctic (the Bering Sea through the Chukchi and southern Beaufort Seas) represents the transition zone between the moist and relatively warm maritime air mass of the Paci
    Overland, J. E., M. Y. Wang, J. E. Walsh, J. C. Stroeve, 2014b: Future Arctic climate changes: Adaptation and mitigation time scales.Earth's Future,2,68-74, climate in the Arctic is changing faster than in midlatitudes. This is shown by increased temperatures, loss of summer sea ice, earlier snow melt, impacts on ecosystems, and increased economic access. Arctic sea ice volume has decreased by 75% since the 1980s. Long-lasting global anthropogenic forcing from carbon dioxide has increased over the previous decades and is anticipated to increase over the next decades. Temperature increases in response to greenhouse gases are amplified in the Arctic through feedback processes associated with shifts in albedo, ocean and land heat storage, and near-surface longwave radiation fluxes. Thus, for the next few decades out to 2040, continuing environmental changes in the Arctic are very likely, and the appropriate response is to plan for adaptation to these changes. For example, it is very likely that the Arctic Ocean will become seasonally nearly sea ice free before 2050 and possibly within a decade or two, which in turn will further increase Arctic temperatures, economic access, and ecological shifts. Mitigation becomes an important option to reduce potential Arctic impacts in the second half of the 21st century. Using the most recent set of climate model projections (CMIP5), multimodel mean temperature projections show an Arctic-wide end of century increase of +13ºC in late fall and +5ºC in late spring for a business-as-usual emission scenario (RCP8.5) in contrast to +7ºC in late fall and +3ºC in late spring if civilization follows a mitigation scenario (RCP4.5). Such temperature increases demonstrate the heightened sensitivity of the Arctic to greenhouse gas forcing.
    Papineau J. M., 2001: Wintertime temperature anomalies in Alaska correlated with ENSO and PDO.International Journal of Climatology,21,1577-1592, 686. (November-March) surface air temperatures at 14 stations throughout the state of Alaska are correlated with the Southern Oscillation Index and the Pacific Decadal Oscillation index, for the years 1954-2000. On the seasonal and monthly timescales, the principal results are: (i) During El Niño winters, temperatures are near normal in western Alaska but significantly warmer than normal for the eastern two-thirds of the state. (ii) La Niña winters produce significant below normal temperatures statewide. (iii) Temperature patterns produced during El Niño, La Niña, and neutral winters are modified by the concurrent state of the North Pacific sea-surface temperature anomalies, as indicated by the Pacific Decadal Oscillation index.On the sub-monthly timescale, temperatures across Alaska are to the first order correlated with the alternating zonal to meridional Pacific/North American pattern. Analysis of daily winter temperatures at Fairbanks indicates that cold anomalies are more frequent and are longer in duration than warm anomalies, primarily due to radiational cooling of the boundary layer and the subsequent formation of deep temperature inversions. The development of strong inversions over the interior of Alaska limits the response of temperatures to changes in the synoptic-scale flow pattern. Warm anomalies in contrast to cold anomalies, are primarily a function of warm air advection, therefore temperatures during warm anomalies fluctuate in phase with changes in the synoptic-scale flow. Ultimately, air temperatures across Alaska are a function of: synoptic-scale forcings, radiative cooling of the boundary layer as well as local and regional effects such as downslope and drainage winds.
    Petrie R. E., L. C. Shaffrey, and R. T. Sutton, 2015: Atmospheric response in summer linked to recent Arctic sea ice loss.Quart. J. Roy. Meteor. Soc.,141,2070-2076, 2007 a large decline in Arctic sea ice has been observed. The large-scale atmospheric circulation response to this decline is investigated in ERA-Interim reanalyses and HadGEM3 climate model experiments. In winter, post-2007 observed circulation anomalies over the Arctic, North Atlantic and Eurasia are small compared to interannual variability. In summer, the post-2007 observed circulation is dominated by an anticyclonic anomaly over Greenland which has a large signal-to-noise ratio. Climate model experiments driven by observed SST and sea ice anomalies are able to capture the summertime pattern of observed circulation anomalies, although the magnitude is a third of that observed. The experiments suggest high SSTs and reduced sea ice in the Labrador Sea lead to positive temperature anomalies in the lower troposphere which weaken the westerlies over North America through thermal wind balance. The experiments also capture cyclonic anomalies over Northwest Europe, which are consistent with downstream Rossby wave propagation.
    Pithan F., T. Mauritsen, 2014: Arctic amplification dominated by temperature feedbacks in contemporary climate models.Nature Geoscience,7,181-184, change is amplified in the Arctic region. Arctic amplification has been found in past warm and glacial periods, as well as in historical observations and climate model experiments. Feedback effects associated with temperature, water vapour and clouds have been suggested to contribute to amplified warming in the Arctic, but the surface albedo feedback--the increase in surface absorption of solar radiation when snow and ice retreat--is often cited as the main contributor. However, Arctic amplification is also found in models without changes in snow and ice cover. Here we analyse climate model simulations from the Coupled Model Intercomparison Project Phase 5 archive to quantify the contributions of the various feedbacks. We find that in the simulations, the largest contribution to Arctic amplification comes from a temperature feedbacks: as the surface warms, more energy is radiated back to space in low latitudes, compared with the Arctic. This effect can be attributed to both the different vertical structure of the warming in high and low latitudes, and a smaller increase in emitted blackbody radiation per unit warming at colder temperatures. We find that the surface albedo feedback is the second main contributor to Arctic amplification and that other contributions are substantially smaller or even opposeArctic amplification.
    Serreze M. C., R. G. Barry, 2011: Processes and impacts of Arctic amplification: A research synthesis.Global and Planetary Change,77,85-96, 2011. 03. 004. Temperature changes in the Arctic tend to exceed those for the globe as whole. 78 This phenomenon is termed Arctic amplification. 78 Arctic amplification has many causes operating on different time and space scales. 78 Recent Arctic amplification is strongly linked to declining sea ice extent. 78 Arctic amplification is expected to strengthen in coming decades. 78 Impacts of Arctic amplification will extend well beyond the Arctic region.
    Stroeve J. C., M. C. Serreze, M. M. Holland , J. E. Kay, J. Maslanik, and A. P. Barrett, 2012: The Arctic's rapidly shrinking sea ice cover: A research synthesis.Climatic Change,110,1005-1027,
    Walsh J., P. A. Bieniek, B. Brettschneider, E. S. Euskirchen, R. Lader, and R. L. Thoman, 2017: The exceptionally warm winter of 2015/16 in Alaska.J. Climate,30,2069-2088, Alaska experienced record-setting warmth during the 2015/16 cold season (October-April). Statewide average temperatures exceeded the period-of-record mean by more than 4ºC over the 7-month cold season and by more than 6ºC over the 4-month late-winter period, January-April. The record warmth raises two questions: 1) Why was Alaska so warm during the 2015/16 cold season? 2) At what point in the future might this warmth become typical if greenhouse warming continues? On the basis of circulation analogs computed from sea level pressure and 850-hPa geopotential height fields, the atmospheric circulation explains less than half of the anomalous warmth. The warming signal forced by greenhouse gases in climate models accounts for about 1ºC of the anomalous warmth. A factor that is consistent with the seasonal and spatial patterns of the warmth is the anomalous surface state. The surface anomalies include 1) above-normal ocean surface temperatures and below-normal sea ice coverage in the surrounding seas from which air advects into Alaska and 2) the deficient snowpack over Alaska itself. The location of the maximum of anomalous warmth over Alaska and the late-winter-early-spring increase of the anomalous warmth unexplained by the atmospheric circulation implicates snow cover and its albedo effect, which is supported by observational measurements in the boreal forest and tundra biomes. Climate model simulations indicate that warmth of this magnitude will become the norm by the 2050s if greenhouse gas emissions follow their present scenario.
    Wang J., J. L. Zhang, E. Watanabe, M. Ikeda, K. Mizobata, J. E. Walsh, X. Z. Bai, and B. Y. Wu, 2009: Is the Dipole Anomaly a major driver to record lows in Arctic summer sea ice extent? Geophys,Res. Lett.,36,L05706,[1] Recent record lows of Arctic summer sea ice extent are found to be triggered by the Arctic atmospheric Dipole Anomaly (DA) pattern. This local, second-leading mode of sea-level pressure (SLP) anomaly in the Arctic produced a strong meridional wind anomaly that drove more sea ice out of the Arctic Ocean from the western to the eastern Arctic into the northern Atlantic during the summers of 1995, 1999, 2002, 2005, and 2007. In the 2007 summer, the DA also enhanced anomalous oceanic heat flux into the Arctic Ocean via Bering Strait, which accelerated bottom and lateral melting of sea ice and amplified the ice-albedo feedback. A coupled ice-ocean model was used to confirm the historical record lows of summer sea ice extent.
    Wang, J., Coauthors, 2014: Abrupt climate changes and emerging ice-ocean processes in the Pacific Arctic region and the Bering Sea.The Pacific Arctic Region,J. Grebmeier and W. Maslowski,Eds., Springer, 65-99,
    Wang M. Y., J. E. Overland, 2015: Projected future duration of the sea-ice-free season in the Alaskan Arctic.Progress in Oceanography,136,50-59, 2015. 01. 001. warming and continued reduction in sea ice cover will result in longer open water duration in the Arctic, which is important for the shipping industry, marine mammals, and other components of the regional ecosystem. In this study we assess the length of open water duration in the Alaskan Arctic over the next few decades using the set of latest coupled climate models (CMIP5). The Alaskan Arctic, including the Chukchi and the Beaufort Sea, has been a major region of summer sea ice retreat since 2007. Thirty five climate models from CMIP5 are evaluated and twelve are selected for composite projections based on their historical simulation performance. In the regions north of the Bering Strait (north of 70° N), future open-water duration shifts from a current 3–4months to a projected near 5months by 2040 based on the mean of the twelve selected climate models. There is considerable north–south gradient in projected durations. Open water duration is about 1month shorter along the same latitudes in the Beaufort Sea compared with that in the Chukchi Sea. Uncertainty is generally ±1month estimated from the range of model results. Open-water duration in the Alaskan Arctic expands quickly in these models over the next decades which will impact regional economic access and potentially alter ecosystems. Yet the northern Alaskan Arctic from January through May will remain sea ice covered into the second half of the century due to normal lack of sunlight.
    Wang M., Q. Yang, J. E. Overland , and P. Stabeno, 2017: Sea-ice Evolution in the Pacific Arctic: the present to mid-century by selected CMIP5 models. Deep Sea Research Part II, (in press).
    Wassmann P., 2015: Overarching perspectives of contemporary and future ecosystems in the Arctic Ocean.Progress in Oceanography,139,1-12, 2015. 08. 004. Arctic region has a number of specific characteristics that provide the region an exceptional global position. It comprises 5% of the earth surface, 1% of world ocean volume, 3% of world ocean area, 25% of world continental shelf, 35% of world coastline, 11% of global river runoff and 20 of worlds 100 longest rivers. The Arctic region encompasses only 0.05% of the global population, but 22% undiscovered petroleum, 15% of global petroleum production, many metals and non-metals resources and support major global fisheries (60 and 80ºN). In times of increasing resource demand and limitation the world focuses increasingly onto the Arctic Ocean (AO) and adjacent regions. This development is emphasised by the recent awareness of rapid climate change in the AO, the most significant on the globe, and has resulted in increased attention to the oceanography of the high north. The loss of Arctic sea ice has emerged as a leading signal of global warming. It is taking place at a rate 2-3 times faster than global rates and sea-ice cover has decreased more than 10% per decade, while sea-ice volume may have been reduced by minimum 40% over the last 30 years (Meier et al., 2014). The reduction of ice cover and thickness makes the region available for commercial interest. The region drives also critical effects on the biophysical, political and economic system of the Northern Hemisphere (e.g., Grambling, 2015). These striking changes in physical forcing have left marine ecological footprints of climate change in the Arctic ecosystem (Wassmann et al., 2011). However, predicting the future of the pan-Arctic ecosystem remains a challenge not only because of the ever-accelerating nature of both physical and biological alterations, but also because of lack of marine ecological knowledge, that is staggering for the majority of regions (except the Barents, Chukchi and Beaufort seas).
    Wood K. R., J. E. Overland , S. A. Salo, N. A. Bond, W. J. Williams, and X. Q. Dong, 2013: Is there a "new normal" climate in the Beaufort Sea? Polar Research,32, 19552, . Since 2007, environmental conditions in the Beaufort Sea have appeared to be consistently different from those in the past. Is a ‘‘new normal’’ climate emerging in the region? Sea-surface temperatures (SSTs) have been notably warm during the summer, leading to delayed freeze-up in the fall along with large surface air temperature (SAT) anomalies due to the release of stored ocean heat to the atmosphere. In the autumn of 2011 and 2012, SST and SAT anomalies in Arctic marginal seas were the largest observed in the Northern Hemisphere. Since 2007, there has been an increase in easterly winds, which has helped set the stage for Arctic amplification by advecting sea ice out of the region and enhancing surface stratification due to the offshore transport of fresh water from the large Mackenzie River discharge plume. These winds are linked to an intensification of the Beaufort High and are evident throughout the troposphere. Their occurrence has undoubtedly contributed to the acceleration of sea-ice loss and surface warming in the Beaufort Sea, with additional impacts likely throughout the ecosystem. Keywords : Arctic change; sea ice; Beaufort Sea; Mackenzie River; Arctic amplification; atmospheric circulation (Published: 17 October 2013) To access the supplementary material for this article, please see Supplementary files in the column to the right (under Article Tools). Citation : Polar Research 2013, 32 , 19552,
    Wu B. Y., J. Wang, and J. E. Walsh, 2006: Dipole anomaly in the winter arctic atmosphere and its association with sea ice motion.J. Climate,19,210-225, . paper identified an atmospheric circulation anomaly dipole structure anomaly in the Arctic atmosphere and its relationship with winter sea ice motion, based on the International Arctic Buoy Program (IABP) dataset (1979 98) and datasets from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) for the period 1960 2002. The dipole anomaly corresponds to the second-leading mode of EOF of monthly mean sea level pressure (SLP) north of 70ºN during the winter season (October March) and accounts for 13% of the variance. One of its two anomalous centers is stably occupied between the Kara Sea and Laptev Sea; the other is situated from the Canadian Archipelago through Greenland extending southeastward to the Nordic seas. The dipole anomaly differs from one described in other papers that can be attributed to an eastward shift of the center of action of the North Atlantic Oscillation. The finding shows that the dipole anomaly also differs from the 淏arents Oscillation revealed in a study by Skeie. Since the dipole anomaly shows a strong meridionality, it becomes an important mechanism to drive both anomalous sea ice exports out of the Arctic Basin and cold air outbreaks into the Barents Sea, the Nordic seas, and northern Europe.When the dipole anomaly remains in its positive phase, that is, negative SLP anomalies appear between the Kara Sea and the Laptev Sea with concurrent positive SLP over from the Canadian Archipelago extending southeastward to Greenland, there are large-scale changes in the intensity and character of sea ice transport in the Arctic basin. The significant changes include a weakening of the Beaufort gyre, an increase in sea ice export out of the Arctic basin through Fram Strait and the northern Barents Sea, and enhanced sea ice import from the Laptev Sea and the East Siberian Sea into the Arctic basin. Consequently, more sea ice appears in the Greenland and the Barents Seas during the positive phase of the dipole anomaly. During the negative phase of the dipole anomaly, SLP anomalies show an opposite scenario in the Arctic Ocean and its marginal seas when compared to the positive phase, with the center of negative SLP anomalies over the Nordic seas. Correspondingly, sea ice exports decrease from the Arctic basin flowing into the Nordic seas and the northern Barents Sea because of the strengthened Beaufort gyre.The finding indicates that influences of the dipole anomaly on winter sea ice motion are greater than that of the winter AO, particularly in the central Arctic basin and northward to Fram Strait, implying that effects of the dipole anomaly on sea ice export out of the Arctic basin become robust. The dipole anomaly is closely related to atmosphere ice ocean interactions that influence the Barents Sea sector.
    Zhang L. P., T. L. Delworth, 2015: Analysis of the characteristics and mechanisms of the pacific decadal oscillation in a suite of coupled models from the geophysical fluid dynamics laboratory.J. Climate,28,7678-7701, Pacific decadal oceanic and atmospheric variability is examined in a suite of coupled climate models developed at the Geophysical Fluid Dynamics Laboratory (GFDL). The models have ocean horizontal resolutions ranging from 1º to 0.1º and atmospheric horizontal resolutions ranging from 200 to 50 km. In all simulations the dominant pattern of decadal-scale sea surface temperature (SST) variability over the North Pacific is similar to the observed Pacific decadal oscillation (PDO). Simulated SST anomalies in the Kuroshio-Oyashio Extension (KOE) region exhibit a significant spectral peak at approximately 20 yr. Sensitivity experiments are used to show that (i) the simulated PDO mechanism involves extratropical air-sea interaction and oceanic Rossby wave propagation; (ii) the oscillation can exist independent of interactions with the tropics, but such interactions can enhance the PDO; and (iii) ocean-atmosphere feedback in the extratropics is critical for establishing the approximately 20-yr time scale of the PDO. The spatial pattern of the PDO can be generated from atmospheric variability that occurs independently of ocean-atmosphere feedback, but the existence of a spectral peak depends on active air-sea coupling. The specific interdecadal time scale is strongly influenced by the propagation speed of oceanic Rossby waves in the subtropical and subpolar gyres, as they provide a delayed feedback to the atmosphere. The simulated PDO has a realistic association with precipitation variations over North America, with a warm phase of the PDO generally associated with positive precipitation anomalies over regions of the western United States. The seasonal dependence of this relationship is also reproduced by the model.
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Manuscript History

Manuscript received: 29 January 2017
Manuscript revised: 15 July 2017
Manuscript accepted: 03 August 2017
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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Recent Increased Warming of the Alaskan Marine Arctic Due to Midlatitude Linkages

  • 1. NOAA/Pacific Marine Environmental Laboratory, Seattle WA 98115, USA
  • 2. Joint Institute for the Study of Atmosphere and Oceans/University of Washington, Seattle WA 98115, USA
  • 3. Department of Geography, Texas State University, San Marcos TX 78666, USA

Abstract: Alaskan Arctic waters have participated in hemispheric-wide Arctic warming over the last two decades at over two times the rate of global warming. During 2008-13, this relative warming occurred only north of the Bering Strait and the atmospheric Arctic front that forms a north-south thermal barrier. This front separates the southeastern Bering Sea temperatures from Arctic air masses. Model projections show that future temperatures in the Chukchi and Beaufort seas continue to warm at a rate greater than the global rate, reaching a change of +4°C by 2040 relative to the 1981-2010 mean. Offshore at 74°N, climate models project the open water duration season to increase from a current average of three months to five months by 2040. These rates are occasionally enhanced by midlatitude connections. Beginning in August 2014, additional Arctic warming was initiated due to increased SST anomalies in the North Pacific and associated shifts to southerly winds over Alaska, especially in winter 2015-16. While global warming and equatorial teleconnections are implicated in North Pacific SSTs, the ending of the 2014-16 North Pacific warm event demonstrates the importance of internal, chaotic atmospheric natural variability on weather conditions in any given year. Impacts from global warming on Alaskan Arctic temperature increases and sea-ice and snow loss, with occasional North Pacific support, are projected to continue to propagate through the marine ecosystem in the foreseeable future. The ecological and societal consequences of such changes show a radical departure from the current Arctic environment.

摘要: 过去二十年来, 阿拉斯加北极海域参与了北极半球尺度的增暖, 其增暖速率达到了全球增暖速率的两倍多. 在2008年至2013年期间, 这种相对增暖主要发生在白令海峡和形成南北热力屏障的北极锋区以北. 这一锋区将东南部的白令海温度与北极气团分隔开. 数值模式的预测结果表明, 楚科奇海和博福特海未来将继续以高于全球速率的水平增暖. 到2040年, 该地区的温度相对于1981-2010年的平均值将高出 4°C. 气候模式还预测, 在74°N海面上的开放水域持续时间将从目前的平均3个月增加到2040年的5个月. 这些增速有时会因中纬度的相关而增强. 从2014年8月开始, 北极增暖的进一步加剧始于北太平洋的海面温度(SST)异常增暖, 并伴随着阿拉斯加上方相应转变的南风导致, 特别是2015-16年冬季. 虽然全球增暖和赤道遥相关主要影响北太平洋海温, 但2014-16北太平洋增暖事件的结束表明了在任何一年混沌的大气内部自然变率对天气的重要性. 在可预见的未来, 伴随着北太平洋偶尔的支持, 全球变暖对阿拉斯加的温度增加和北极海冰和积雪减少的影响将继续通过海洋生态系统传播. 这种变化的生态和社会后果将显示出与当前北极环境的根本偏离. (翻译: 陈卫; 校对: Muyin Wang)

1. Introduction
  • This paper provides an update on the major climate changes in the marine Alaskan Arctic over the past decade (Wood et al., 2013; Ballinger and Sheridan, 2014; Overland et al., 2014a; Cassano et al., 2015). At the decadal/regional scale of climate change, Fig. 1 shows the 13-month running mean surface air temperature (SAT) anomalies for Barrow, Alaska (red), and areal averages for the Northern Hemisphere land area (blue), relative to a baseline period of 1981-2010. Temperature anomalies at Barrow are predominantly positive since 2003, and in winter 2015-16 are roughly four times the magnitude of the Northern Hemisphere mean temperature increase. Alaskan Arctic waters participate in the hemispheric-wide warming of the Arctic over the last two decades. Additional warming of Alaskan Arctic coastal waters from autumn 2014 through autumn 2016 can be attributed to the shift to warm ocean temperature anomalies in the North Pacific and associated shifts to southerly winds over Alaska. Such North Pacific contributions subside after autumn 2016, but greenhouse gas (GHG) and regional Arctic feedback contributions remain.

    Figure 1.  13-month running mean SAT anomalies for Barrow and Northern Hemisphere lands relative to their respective 1981-2010 means (from CRUTEM4; available at

    Arctic air and ocean surface temperature increases coincide with the expansion of sea-ice-free areas, increases in the mobility of sea ice, shifts in ocean currents, and biological impacts at all trophic levels from primary productivity increases through loss of walrus habitat (Wassmann, 2015). In the following sections, we track atmospheric changes in the Alaskan Arctic, compare them relative to the Bering Sea, assess future climate projections, and address Alaskan Arctic temperature increases since August 2014 due to warming in the North Pacific.

    Figure 2.  Mean (1961-2010) near-surface temperature (units: °C) for the four seasons over the western Arctic. Data are from the NCEP-NCAR Reanalysis via NOAA/ESRL, generated online at Figure is similar to Fig. 2.2 in Overland et al. (2014).

    Figure 3.  Mean (1961-2010) SLP (units: hPa) for the four seasons over the western Arctic. Data are from the NCEP-NCAR Reanalysis via NOAA/ESRL, generated online at Figure is similar to Fig.2.3 in Overland et al. (2014).

2. Regional climate of the maritime Alaskan Arctic
  • The Pacific Arctic discussed here is defined as regions north of 66°N, which covers the area north of the Bering Strait and the southern Chukchi and Beaufort Seas. Climatologically, this region is on the northern side of the transition zone between the relatively warm and moist storm tracks of the Aleutian low weather system reaching into the Bering Sea, and the colder, drier, and higher-pressure Arctic air mass to the north. As summarized in (Overland et al., 2014a): "Located in the southern part of the Pacific Arctic is a region of large north-south gradients in atmospheric properties such as near-surface air temperature (Fig. 2) and atmospheric sea level pressure (SLP, Fig. 3). This region of strongest gradients moves north and south with the seasonal cycle. Maximum temperature gradients in winter are located over the central Bering Sea with sub-freezing temperatures and extensive sea-ice coverage. In summer, the greatest air temperature gradients are found across the southern Chukchi Sea and seaward of the coast of Alaska with SAT above-freezing. Large north-south gradients in SLP produce a vast east-west trending region of strong climatological winds from the east across a relatively narrow band of latitudes in all seasons but summer." The Aleutian low center to the south of the Alaskan Arctic is a dominant feature shown in the climatology of SLP plots (Fig. 3) in all seasons except summer.

    From 2007 through mid-2014, the Pacific air mass to the south and the Arctic air mass to the north are on different trajectories. To the north, the Chukchi/Beaufort Sea region is part of the decadal change of Arctic warming where recent sea-ice and snow losses are allowing extensive areas to absorb more late-spring and summer solar radiation than in the past, and are changing the atmospheric climatology of the region with positive temperature anomalies extending throughout the year (i.e. Arctic amplification, AA). Figure 4 (top) shows monthly SAT at Barrow, in which it is notable that there is an observed shift to positive temperature anomalies beginning in 1995. To the south, the Bering Sea, represented by Saint Paul SAT (Fig. 4, bottom), turns colder with extensive seasonal sea-ice cover in 2007-13, which has not been observed since the mid-1970s. This period contrasts an earlier warmer-than-normal Bering Sea temperature anomaly period for the southern Bering Sea from 2000 through 2006. Beginning in 2014, the Bering Sea returned to consistent warm anomalies, tied to ocean temperature changes in the greater North Pacific. While this short-term warming event ends in autumn 2016, lower tropospheric air temperatures continue to remain above-normal in the Alaskan Arctic. We return to discussing this latest North Pacific Ocean impact on the Pacific Arctic in section 5.

    Figure 4.  Monthly SAT anomalies for Barrow and Saint Paul, Alaska, compared to their respective 1981-2010 mean values. Anomalies are based on NWS weather station data.

3. Arctic change
  • Arctic-wide average surface temperatures have increased at double the rate of global mean temperatures——a well-documented phenomenon referred to as AA (Holland and Bitz, 2003; Serreze and Barry, 2011). Figure 5 shows the difference in mean annual Northern Hemisphere lower tropospheric air temperatures for 2010-14 relative to the end of the 20th century (1971-2000). Although the entire Northern Hemisphere polewards of 40°N has witnessed positive changes in annual mean temperatures in recent years, much of central Arctic shows increases of at least +2°C. Note that the southeastern Bering Sea does not show a change in recent temperatures relative to the late 20th century. The spatial pattern of AA (Fig. 5) does not resemble the temperature spatial pattern of major atmospheric circulation variability indices such as the Arctic Oscillation (AO), suggesting that radiative forcing is a primary forcing for AA. Mechanisms for AA include reduced summer albedo due to sea-ice and snow-cover loss, decreased total cloudiness in summer and increased cloud cover in winter, additional atmospheric heating generated by newly sea-ice-free ocean areas that are maintained later into the autumn, increased longwave radiation due to local and advected atmospheric moisture sources, and the decreased rate of heat loss to space in the Arctic relative to the subtropics due to lower mean temperatures (Makshtas et al., 2011; Pithan and Mauritsen, 2014).

    Figure 5.  Annual Arctic 925-hPa air temperature increases for 2010-14 relative to the end of the 20th century (1971-2000). Figure created through the NOAA/ESRL website.

    Upward trends in GHGs and resulting AA significantly influence multiple changes throughout the Arctic environment (Stroeve et al., 2012). This is evident in the Alaskan Arctic where robust lagged relationships are found between Northern Hemisphere SAT and September Beaufort Sea ice extent, particularly since the early 1990s (Ballinger and Rogers, 2014). This lag is explained by ongoing increases in GHGs causing global warming that contribute to AA temperature increases and resultant thinning of summer sea-ice cover in the Alaskan Arctic over the course of several years, resulting in a series of recent, anomalous September sea-ice losses.

    Figure 6.  (a) Composite of June SLP (units: hPa) for 2007-15, illustrating the SLP distribution for the negative phase of the AD pattern. Data are from the NCEP-NCAR Reanalysis through the NOAA/ESRL. (b) The AO Index, an Arctic wide index low pressure in its positive phase, and the AD, during early summer months. The negative phase of the AD pattern is often associated with higher pressure in the Beaufort Sea. Note the recent presence of negative AD values in June and July 2015 (b and c) (updated from Overland and Wang 2005). Note also that some authors define the dipole with the opposite sign (e.g., Wu et al., 2006).

    Warm temperatures in the Alaskan Arctic have persisted since 2007. Extended periods of sea-ice-free conditions have a role in the pronounced temperature departures from the norm. In particular, Beaufort Sea ice formation occurs progressively later during this era, especially during autumn 2012 when ice formed approximately five weeks later relative to the 1981-2010 climatology (Fig. 6). Since 2007, there has been an increase in easterly winds, which has helped set the stage for AA by advecting sea ice out of the Alaskan region and enhancing ocean surface stratification due to the offshore transport of fresh water from the large Mackenzie River discharge plume (Wood et al., 2013).

    Figure 7.  Beaufort Sea ice freeze date anomalies, 1979-2016, compared to the 1981-2010 mean freeze date (data obtained from Jeffrey Miller, Cryospheric Sciences Laboratory, NASA Goddard Space Flight Center and KBRwyle).

    Recent decades are also associated with the predominance of a large spatial-scale climate pattern referred to as the Arctic Dipole (AD; Figs. 7a, b and c), which is characterized by low SLP on the Siberian side of the Arctic and high SLP on the North American side in its negative phase (Fig. 7a; Overland and Wang, 2005; Wang et al., 2009, 2014). Note, however, some authors define the dipole with the opposite sign (e.g., Wu et al., 2006). One can also interpret this decadal change as the increased presence of a summer Beaufort high region located north of Alaska continuing from its springtime climatology (Ballinger and Sheridan, 2014; Ballinger et al., 2014). These anomalous Beaufort high patterns have occurred more often since 2007, aligned with an era of abrupt sea-ice decline, as compared to previous years dating to the late 1970s. This summer Beaufort high is a major change for the Alaskan Arctic, as the previous summer climatology often consisted of weak pressure gradients and monthly-averaged low pressure in the central Arctic basin. As summarized by (Overland et al., 2014a): "While the negative AD pattern was present in spring as early as 1997, its recent occurrence began in summer 2007 when it was present in all months and contributed to 2007 record minimum summer sea ice extent (Wang et al., 2009). Most years after 2007 have seen the AD pattern persist for at least part of the summer. For example, in 2010, the AD pattern was present in May and June, but then the Arctic reverted to the more traditional climatological summer SLP pattern involving a weak central-Arctic low-pressure center. But by August 2010 the AD pattern had returned." The AD pattern was absent in the summer 2013 and 2017, but reappeared in 2014 and 2015 with Beaufort/Chukchi sea-ice extents below normal.

    The increased Beaufort high and AD patterns since 2007 are also connected with unprecedented higher pressure systems across Greenland and the North Atlantic Arctic sector in one large positive hemispheric SLP anomaly pattern (Overland et al., 2012; Belleflamme et al., 2015; Bezeau et al., 2015; Petrie et al., 2015). Whether this shift in atmospheric pressure and wind patterns is tied to AA is unknown, but its persistence is noted relative to more strictly interannual variability before 2007 (Fig. 7b and c). This provides further evidence that Alaskan Arctic changes are tied to large-scale Arctic-centric changes.

    In summary, the Alaskan Arctic has participated in the Arctic-wide AA driven by increases in GHGs and amplified by regional, Arctic-specific feedback processes. Persistent higher than normal surface pressures in the Pacific Arctic influenced by the anomalous occurrence of the summer Beaufort high pressure system have increased easterly winds in the region and contributed to ocean circulation changes and sea-ice loss.

4. Decadal future projections
  • The AA of SAT is projected to continue through the 21st century (Fig. 8) according to CMIP5 climate models, which formed the basis for IPCC AR5 (Overland et al., 2014b). Because CO2 remains in the atmosphere for many decades, the GHG contribution from the previous decades and projected emissions for the next two decades lead to a model-estimated, Arctic-wide October-March SAT increase of +4°C by 2040. Beyond 2050, the SAT depends on which GHG emissions scenario is chosen for the projection. The red curves for summer and winter periods give the high CO2 business-as-usual emissions scenario, referred to as RCP8.5, according to IPCC AR5 (IPCC, 2013). The blue curves provide the change in SAT based on the aggressive but not extreme GHG mitigation scenario, RCP4.5.

    According to the CMIP5 models, sea-ice loss for the Alaskan Arctic is projected to continue over the next decades. The important change is the increase in the number of sea-ice-free months (Fig. 9). The duration of months with open-water conditions generally decreases with northerly latitude. A rough change estimate at 74° N is from three months of open water in 2010 to five months by 2040 (Wang and Overland, 2015, updated). These average changes are based on GHG increases; actual sea-ice loss is projected to continue to have a large year-to-year component due to variations in weather patterns. It will be difficult for the open-water duration to extend much later than November due to seasonal darkness, or occur earlier in spring due to solar reflection off of snow and sea-ice cover; thus, the future duration of Alaskan sea-ice-free duration will be limited by the winter atmospheric climatology.

    Figure 8.  Future Arctic-wide SAT increases for a business-as-usual increase in CO2 emissions (RCP8.5, red) and for a modest (RCP4.5, blue) CO2 mitigation scenario.

    Figure 9.  Annual duration of sea-ice cover averaged over the period 1990-2014 (left) based on satellite data. Right: change (relative to 1990-2014) in annual sea-ice duration by the middle of the century (2030-44) based on seven CMIP5 model means under the RCP8.5 emissions scenario. Figures are adapted from Wang et al. (2017) with modification. The subset of CIMP5 models were selected by matching the monthly sea-ice extent and magnitude of the seasonal cycle. See Wang and Overland (2015) and Wang et al. (2017) for more information.

5. Recent changes in the North Pacific
  • After more than a decade-and-a-half of both positive and negative SSTs and associated weather patterns in the central and eastern North Pacific Ocean, there is evidence of multi-month persistent positive ocean temperature anomaly patterns since 2013. These consist of near-record positive SST anomalies centered near (45°N, 145°W) during 2013, labeled the "blob" by (Bond et al., 2015), and the return of the positive Pacific Decadal Oscillation (PDO) climate pattern in 2015, with SST maxima near the northeastern North Pacific coast. Persistent, above-average geopotential heights in the mid-level atmosphere during 2012-15 associated with warmer air temperatures, which steer the prevailing wind direction more from the south and transport heat towards the north over the central and eastern North Pacific, have resulted in what has been referred to as the Ridiculously Resistant Ridge (RRR) of high pressure (e.g. Fig. 10b).

    Figure 10.  September 2014 through July 2015 925-hPa air temperature anomalies over western North America (a), and corresponding anomalies in 700-hPa geopotential height (b); anomalous winds follow the contours with a southerly wind component over the Gulf of Alaska. Anomaly maps are presented with respect to the 1981-2010 climatological values. Data are from the NCEP-NCAR Reanalysis through NOAA/ESRL.

    Figure 11.  The PDO index time series from 1900-2016. Positive values correlate with elevated SST in the Gulf of Alaska. The PDO index is obtained from

    Beginning in late autumn 2014, Alaska experienced record positive temperature anomalies associated with the RRR orientation of mid-tropospheric geopotential heights over the west coast of North America, and a positive PDO with above-average lower tropospheric air temperatures, situated polewards from the southern Alaskan coast (Fig. 10a and b). Winds flow clockwise around high geopotential height centers (parallel to contours), thus directing the air flow from the North Pacific northwards across Alaska to the Alaskan Arctic region. The PDO index supports this wind pattern and is strongly positive (>+1.0) beginning September 2014 and decreased into 2017 (Fig. 11).

    Winter 2015-16 continued the warm pattern, with widespread Alaskan temperature anomalies of +5°C (Walsh et al., 2017). The 700-hPa geopotential height pattern is similar to 2015, as shown in Fig. 10, but the low geopotential height Aleutian low center is more dominant than the coastal ridge feature. Loss of snow cover and decreased land surface albedo in southern Alaska added to the persistence of positive near-surface air temperature anomalies (Walsh et al., 2017). El Niño conditions, as well as warm North Pacific SSTs, continue for winter 2015-16. Previous research suggests warm temperature anomalies in the Alaskan marine Arctic during El Niño often result in diminished Beaufort and Chukchi ice cover (Papineau, 2001; Liu et al., 2004; Bond and Harrison, 2006). (Walsh et al., 2017) also estimated that about 20% of the 2015-16 Alaska warm temperature anomalies (about +1°C) was due to global warming, as projected by CMIP5 models.

    Autumn 2016 marked the end of warm northeast Pacific SSTs, with a return to more zonal 700-hPa wind flow and with the Aleutian low feature moving northwest spanning northeastern Siberia and the Sea of Okhotsk with above normal temperatures confined to the Chukchi Sea and the Alaskan Arctic (Figs. 12a and b).

    Figure 12.  925-hPa air temperature anomalies over western North America (a), and corresponding 700-hPa geopotential height (b) for autumn 2016. Anomaly maps are presented with respect to the 1981-2010 climatological values. Data are from the NCEP-NCAR Reanalysis through NOAA/ESRL.

    (Newman et al., 2016) discusses causal contributions to North Pacific SSTs and the PDO and concludes there is a combination of tropical forcing, North Pacific Ocean memory, and interannual chaotic atmospheric variability. North Pacific atmospheric processes have a long-memory stochastic (random) character (Overland et al., 2006), rejecting purely cyclic predictions.

    Despite a shift towards a weak La Niña, autumn 2016 showed some evidence that the PDO might continue to be neutral or weakly positive based on persistence, and there is some evidence for warm subsurface ocean temperature anomalies (Zhang and Delworth, 2015). Yet, strong zonal atmospheric flow (Fig. 12b) is the primary reason for the termination of the North Pacific contribution to Alaskan Arctic warming. (Baxter and Nigam, 2015) show that notable climate anomalies in the Pacific-North American sector can be caused by such internal variability of regional atmospheric patterns, and need not originate from the tropics or local surface forcing. The future for the Alaskan marine Arctic primarily involves continued warm temperatures based on AA with occasional midlatitude support.

6. Summary
  • One should note that future air temperature increases are likely to manifest as considerable year-to-year extremes based on internal random variability of the atmosphere added to long-term GHG-induced trends, rather than the smooth projections shown in Fig. 8. Extreme Arctic temperature events, as a combination of anthropogenically forced temperature increases combined with natural variability, will become common, exceeding previous thresholds. Such an event occurred with +4°C temperature anomalies for Alaska in November-December 2014 and +5°C January-April 2016, related to recent warm Pacific SSTs. Breaking the string of cold, southern Bering Sea temperature anomalies and mostly negative PDO years from 2006-13, recent years show interaction of the Beaufort and Chukchi Seas with the subarctic. Regional warm temperature anomalies associated with loss of sea ice and snow for the Alaskan Arctic have been supplemented by southerly air flow in addition to the monotonic AA signal. This North Pacific SST connection broke down in autumn 2016 due to internal atmospheric variability that manifested as strong zonal winds.

    For the foreseeable future (out to 2040), continuing rapid environmental changes in Alaskan Arctic seas, land, atmosphere and sea ice are likely, and the appropriate response is to plan for adaptation to meet these mean and extreme-event changes. Arctic and global climate changes will continue to propagate throughout the biological ecosystem through shifts in winds and air temperatures, sea-ice loss, ocean circulation and stratification changes, and permafrost melt, with impacts on societal systems.




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