Internal variability in multidecadal trends of surface air temperature over antarctica in austral winter in model simulations

The surface air temperature (SAT) exhibits pronounced warming over West Antarctica in recent decades, especially in austral spring and winter. Using a 30-member ensemble of simulations by Community Earth System Model (CESM), two reanalysis datasets, and observed station data, this study investigates the relative contributions of internally generated low-frequency climate variability and externally forced climate change to the austral winter SAT trend in Antarctica. Although these simulations share the same external forcing, the SAT trends during 1979–2005 show large diversity among the individual members in the CESM ensemble simulations, suggesting that internally generated variability contributes a considerable part to the multidecadal SAT change in Antarctica. Quantitatively, the total forced contribution to the SAT (1979–2005) change is about 0.53 k/27 yr, and the internal variability can be strong enough to double or cancel the externally forced warming trend. A method called “dynamical adjustment” is utilized to further divide the forced response. We find both the forced thermodynamically-induced and the forced dynamically-induced SAT trends are positive over all the regions in Antarctica, with the regional mean values of 0.20 k /27 yr and 0.33 k/27 yr, respectively. The diversity of SAT trends among the simulations is closely linked to a Southern hemisphere Annular Mode (SAM)-like atmospheric circulation multidecadal change in the Southern Hemisphere. When there exists a positive–negative seesaw of pressure trend between Antarctica and the mid-latitudes, the SAT trend is positive over most of Antarctica but negative over the Antarctic Peninsula, and vice versa. The SAM-like atmospheric circulation multidecadal change mainly arises from atmospheric internal variability rather than remote tropical Sea Surface Temperature (SST).

     

Regional variability of Antarctic sea ice extent

Interannual and seasonal variability of regional distribution of Antarctic sea ice extent is studied using monthly mean data on sea ice concentration in 1970-2012. The correlation is estimated between the variations in the area of floating ice in West and East Antarctica as well as in the Atlantic, Pacific, and Indian sectors of the Southern Ocean and the indices of atmospheric circuiation in the Southern Hemisphere.     

Holocene hydrological cycle changes in the Southern Hemisphere documented in East Antarctic deuterium excess records

Four Holocene-long East Antarctic deuterium excess records are used to study past changes of the hydrological cycle in the Southern Hemisphere. We combine simple and complex isotopic models to quantify the relationships between Antarctic deuterium excess fluctuations and the sea surface temperature (SST) integrated over the moisture source areas for Antarctic snow. The common deuterium excess increasing trend during the first half of the Holocene is therefore interpreted in terms of a warming of the average ocean moisture source regions over this time. Available Southern Hemisphere SST records exhibit opposite trends at low latitudes (warming) and at high latitudes (cooling) during the Holocene. The agreement between the Antarctic deuterium excess and low-latitude SST trends supports the idea that the tropics dominate in providing moisture for Antarctic precipitation. The opposite trends in SSTs at low and high latitudes can potentially be explained by the decreasing obliquity during the Holocene inducing opposite trends in the local mean annual insolation between low and high latitudes. It also implies an increased latitudinal insolation gradient that in turn can maintain a stronger atmospheric circulation transporting more tropical moisture to Antarctica. This mechanism is supported by results from a mid-Holocene climate simulation performed using a coupled ocean-atmosphere model. Received: 7 July 1999 / Accepted: 21 July 2000     

南极地区温度和海冰的变化特征及相互关系

对南极地区温度和海冰的时空变化特征及相互关系进行的初步研究结果表明:近30余年来南极地区有显著的变暖趋势，时空差异比较明显。 其中以南极半岛地区的变暖趋势最大，为整个东南极沿岸增温率的2～3倍。近20年来，整个平均的南极海冰和温度的变化趋势相反，年际变化的相关关系不显著。经过聚类分析划分出不同的气候区，能清楚地显示出某些区两者的关系。海冰与同区沿岸温度距平相关信号最强区在南大西洋至西南太平洋海域。     

南极海冰与南半球大气环流关系的初步探讨

本文利用１９７３—１９８２年南极海冰北界资料，分析了南极海冰平均北界（海冰范围）的变化及其与南半球大气环流变化间的联系。１９７６年前，南极海冰平均北界偏北（海冰范围扩大），而１９７７年－１９８０年，南极海冰平均北界偏南（海冰范围缩小）。与此相对应，这两个时期的南半球大气环流具有明显不同的特点。在南极海冰平均北界偏北、海冰范围扩大时期，南极高压和绕极低压带偏弱，南半球中高纬度地区槽脊位置偏西，南印度洋和南大晒洋副热带高压偏弱，南太平洋副热带西凤减弱、中纬度西风加强，而南太平洋副热带高压和印度尼西亚低压带发展，南方涛动处于正位相阶段；在南极海冰平均北界偏南、海冰范围缩小时期，则相反。分析表明，南方涛动与南极海冰之间存在相互联系，并以南极海冰超前南方涛动约２个月时的关系最好，其次是南极海冰落后南方涛动４个月。     

南极海冰涛动对北半球夏季大气环流的影响

南极海冰首要模态呈现偶极子型异常,正负异常中心分别位于别林斯高晋海/阿蒙森海和威德尔海。过去研究表明冬春季节南极海冰涛动异常对后期南极涛动（Antarctic Oscillation,AAO）型大气环流有显著影响,而AAO可以通过经向遥相关等机制影响北半球大气环流和东亚气候。本文中我们利用观测分析发现南极海冰涛动从5～7月（May–July,MJJ）到8～10月（August–October, ASO）有很好的持续性,并进一步分析其对北半球夏季大气环流的可能影响及其物理过程。结果表明,MJJ南极海冰涛动首先通过冰气相互作用在南半球激发持续性的AAO型大气环流异常,使得南半球中纬度和极地及热带之间的气压梯度加大,在MJJ至JAS,纬向平均纬向风呈现显著的正负相间的从南极到北极的经向遥相关型分布。对流层中层位势高度场上,在澳大利亚北部到海洋性大陆区域,出现显著的负异常,在东亚沿岸从低纬到高纬呈现南北走向的“? + ?”太平洋—日本（Pacific–Japan,PJ）遥相关波列,其对应赤道中部太平洋及赤道印度洋存在显著的降水和海温负异常,西北太平洋至我国东部沿海地区存在显著降水正异常和温度负异常;低纬度北美洲到大西洋一带存在的负位势高度异常和北大西洋附近存在的正位势高度异常中心,构成一个类似于西大西洋型遥相关（Western Atlantic,WA）的结构,对应赤道南大西洋降水增加和南撒哈拉地区降水减少。从物理过程来看,南极海冰涛动首先通过局地效应影响Ferrel环流,进而通过经圈环流调整使得海洋性大陆区域和热带大西洋上方的Hadley环流上升支得到增强,海洋性大陆区域特别是菲律宾附近的热带对流活动偏强,激发类似于负位相的PJ波列,影响东亚北太平洋地区的大气环流,而热带大西洋对流增强和北传特征,则通过激发WA遥相关影响大西洋和欧洲地区的大气环流。以上两种通道将持续性MJJ至ASO南极海冰涛动强迫的大气环流信号从南半球中高纬度经热带地区传递到北半球中高纬地区,从而对热带和北半球夏季大气环流产生显著影响。     

Seasonal climate information preserved in West Antarctic ice core water isotopes: relationships to temperature, large-scale circulation, and sea ice

As part of the United States’ contribution to the International Trans-Antarctic Scientific Expedition (ITASE), a network of precisely dated and highly resolved ice cores was retrieved from West Antarctica. The ITASE dataset provides a unique record of spatial and temporal variations of stable water isotopes (δ¹⁸O and δD) across West Antarctica. We demonstrate that, after accounting for water vapor diffusion, seasonal information can be successfully extracted from the ITASE cores. We use meteorological reanalysis, weather station, and sea ice data to assess the role of temperature, sea ice, and the state of the large-scale atmospheric circulation in controlling seasonal average water isotope variations in West Antarctica. The strongest relationships for all variables are found in the cores on and west of the West Antarctic Ice Sheet Divide and during austral fall. During this season positive isotope anomalies in the westernmost ITASE cores are strongly related to a positive pressure anomaly over West Antarctica, low sea ice concentrations in the Ross and Amundsen Seas, and above normal temperatures. Analyses suggest that this seasonally distinct climate signal is due to the pronounced meridional oriented circulation and its linkage to enhanced sea ice variations in the adjacent Southern Ocean during fall, both of which also influence local to regional temperatures.     

Southern hemisphere atmospheric circulation: impacts on Antarctic climate and reconstructions from Antarctic ice core data

The atmospheric circulation patterns in the Southern Hemisphere have had a significant impact on the climate of the Antarctic and there is much evidence that these circulation patterns have changed in the recent past. This change is thought to have contributed to the warming trend observed at the Antarctic Peninsula over the last 50 years—one of the largest trends observed in this period on the planet. The trends associated with the continental Antarctic climate are less clear but are likely to be impacted less directly by atmospheric circulation changes. The circulation changes can be put into the context of longer timescales by considering atmospheric circulation reconstructions that have been performed using data from Antarctic ice cores. In this review paper we look at the main body of work examining: Antarctic climate trends; the understanding and impact of atmospheric circulation of the mid- to high-latitudes of the Southern Hemisphere; and the usefulness and reliability of atmospheric circulation reconstructions from Antarctic ice core data. Finally, beyond several of the more quantitative reconstructions, it is deemed that an assessment of their consistency is not possible due to the variety of circulation characteristics that the various reconstructions consider.     

南极海冰北界涛动指数及其与我国夏季天气气候的关系

南极海冰的变化和全球大气环流关系密切。南极各区海冰的不同变化, 对南北半球大气环流有着不同的影响。文中基于对南极海冰变化的客观分区, 定义了南极海冰北界涛动指数 (ASEOI), 并结合中央气象台提供的南方涛动指数、北半球500 hPa和100 hPa高度场资料以及我国160站降水、温度资料, 利用诊断分析方法, 对ASEOI与我国夏季天气气候的关系进行了研究。研究表明:ASEOI对我国长江中下游降水及全国大部分地区温度具有指示意义。若前一年10月ASEOI偏低, 则当年7月我国长江中下游降水偏多, 引发洪涝灾害的可能性很大; 温度场上, 我国北方气温偏高, 南方气温偏低, 而高温往往伴随着少雨, 这无疑会加剧华北本就严重的旱情。     

近20年来中国极地大气科学研究进展

南极、北极和青藏高原是地球上的 3大气候敏感地区 ,是多个国际计划研究全球变化的关键地区。中国的南极和北极实地考察研究 ,分别始于 2 0世纪 80和 90年代 ,起步较晚 ,但近 2 0余年来有较大的进展。极地大气科学考察与研究是极地科学研究的重要组成部分。讫今为止 ,中国已组织了 2 0次南极考察和 3次北极考察 ,建立了中国南极长城站、中山站和北极黄河站等 3个常年科学考察站 ;进行了常规地面气象、Brewer大气臭氧、近地面物理、高层大气物理、冰雪和大气化学等观测 ,获得了较为系统的极地大气科学第一手资料 ;开展了有关极地与全球变化的研究 ,取得了新的进展。南极地区大气温度、臭氧和海冰的气候变化在时间和空间上都是多样的。南极地区的增暖主要发生在南极半岛地区 ,在南极大陆主体并不明显 ,近 10余年来还有降温趋势。中国南极长城站和中山站的观测资料也证实了这一点。此外 ,还揭示了南极半岛西侧和罗斯海外围的海冰变化具有“翘翘板”特征 ,由此定义的南极涛动指数可用来讨论南极海冰状况和海冰关键区的活动 ;用实地考察资料研究了极地不同下垫面的近地面物理和海 -冰 -气相互作用特征 ,给出了边界层特征参数 ;讨论了极地天气气候和大气环境特征及其对东亚大气环流和中国天气气候的影响 ;利用     

南极海冰涛动与ENSO的关系

对近30年南极海冰密集度资料的EOF和SVD分析，发现南极地区在罗斯海外围和别林斯高晋海的海冰密集度场存在着“翘翘板”的变化特征，并与ENSO有密切联系。由此定义两个海冰关键区的差值为南极海冰涛动指数（ASOI），ASOI超前SOI和Nino3指数2个月时，其正、负相关系数达到最大，并通过α=0.001的信度检验。ASOI高、低指数阶段对应的南半球海平面气温、气压场和风场的合成分析表明，海冰关键区的异常变化可能引起温度、气压、风场的响应而影响南太平洋的洋流，进而对ENSO的发生、发展产生影响。     

南极地区气候系统变化: 过去、现在和将来

 南极科学委员会（SCAR）下属的"南极与全球气候系统（AGCS）计划"专家委员会发布了"南极与南大洋气候系统（SASOCS）"白皮书,重点评估了过去50 a南极地区气候系统的变化并预估了未来100 a情景。白皮书总体认为,过去50 a南极气候系统变化表现出很强的区域特征。南极半岛地区升温明显,半岛及亚南极岛屿上的冰川均处于退缩状态;南半球环状模（SAM）转为正位相,西南极上空的暖湿气团入侵加强,南极冬季对流层有升温趋势,平流层变冷,极涡消退日期推迟;东南极外围的南极底层水变淡,Weddell海区的底层水有变暖趋势。虽有上述区域变化,整个南极地区在过去50 a中近地面气温并无明显升高,降水亦无明显增加。自20世纪80年代以来海冰面积也无明显变化,只在某些扇区变化强烈。模式预估结果为：到21世纪末南极内陆地区将增暖（3.4±1.0）℃, 海冰面积将缩小约30%。现有的冰盖模式尚不足以回答未来气候变暖情景下冰盖融化与海平面变化之间的定量关系,有待更深入研究。     

南极地区气候系统变化: 过去、现在和将来

南极科学委员会（SCAR）下属的"南极与全球气候系统（AGCS）计划"专家委员会发布了"南极与南大洋气候系统（SASOCS）"白皮书,重点评估了过去50 a南极地区气候系统的变化并预估了未来100 a情景。白皮书总体认为,过去50 a南极气候系统变化表现出很强的区域特征。南极半岛地区升温明显,半岛及亚南极岛屿上的冰川均处于退缩状态;南半球环状模（SAM）转为正位相,西南极上空的暖湿气团入侵加强,南极冬季对流层有升温趋势,平流层变冷,极涡消退日期推迟;东南极外围的南极底层水变淡,Weddell海区的底层水有变暖趋势。虽有上述区域变化,整个南极地区在过去50 a中近地面气温并无明显升高,降水亦无明显增加。自20世纪80年代以来海冰面积也无明显变化,只在某些扇区变化强烈。模式预估结果为：到21世纪末南极内陆地区将增暖（3.4±1.0）℃, 海冰面积将缩小约30%。现有的冰盖模式尚不足以回答未来气候变暖情景下冰盖融化与海平面变化之间的定量关系,有待更深入研究。     

Stratospheric Ozone-induced Cloud Radiative Effects on Antarctic Sea Ice

Recent studies demonstrate that the Antarctic Ozone Hole has important influences on Antarctic sea ice.While most of these works have focused on effects associated with atmospheric and oceanic dynamic processes caused by stratospheric ozone changes,here we show that stratospheric ozone-induced cloud radiative effects also play important roles in causing changes in Antarctic sea ice.Our simulations demonstrate that the recovery of the Antarctic Ozone Hole causes decreases in clouds over Southern Hemisphere(SH)high latitudes and increases in clouds over the SH extratropics.The decrease in clouds leads to a reduction in downward infrared radiation,especially in austral autumn.This results in cooling of the Southern Ocean surface and increasing Antarctic sea ice.Surface cooling also involves ice-albedo feedback.Increasing sea ice reflects solar radiation and causes further cooling and more increases in Antarctic sea ice.     

Teleconnections and predictive characteristics of Australian seasonal rainfall

A new methodology is proposed that allows patterns of interannual covariability, or teleconnections, between the intraseasonal and slow components of seasonal mean Australian rainfall and the corresponding components in the Southern Hemisphere atmospheric circulation to be estimated. In all seasons, the dominant rainfall–circulation teleconnections in the intraseasonal component are shown to have the characteristic features associated with well-known intraseasonal dynamical and statistical atmospheric modes and their relationship with rainfall. Thus, for example, there are patterns of interannual covariability that reflect rainfall relationships with the intraseasonal Southern Annular Mode, the Madden-Julian Oscillation and wavenumber 3 and 4 intraseasonal modes of variability. The predictive characteristics of the atmospheric circulation–rainfall relationship are shown to reside with the slow components. In all seasons, we find rainfall–circulation teleconnections in the slow components related to the El Niño-Southern Oscillation. Each season also has a coupled mode, with a statistically significant trend in the time series of the atmospheric component that appears to be related to recent observed trends in rainfall. The slow Southern Annular Mode also features in association with southern Australian rainfall, especially during austral winter and spring. There is also evidence of an influence of Indian Ocean sea surface temperature variability on rainfall in southeast Australia during austral winter and spring.     

The effect of Antarctic sea ice on the Southern Hemisphere atmosphere during the southern summer

This study examines the influence of Antarctic sea ice distribution on the large scale circulation of the Southern Hemisphere using a fully coupled GCM where the sea ice submodel is replaced by a climatology of observed extremes in sea ice concentration. Three 150-year simulations were completed for maximum, minimum and average sea ice concentrations and the results for the austral summer (January?CMarch) were compared using the surface temperatures forced by the sea ice distributions as a filter for creating the composite differences. The results indicate that in the austral summer the polar cell expands (contracts) under minimum (maximum) sea ice conditions with corresponding shifts in the midlatitude Ferrell cell. We suggest that this response occurs because sea ice lies in the margin between the polar and midlatitude cells. The polarity of the Southern Hemisphere Annular (SAM) mode is also influenced such that when sea ice is at a minimum (maximum) the polarity of the SAM tends to be negative (positive).     

Recent Strengthening of the Regional Hadley Circulation over the Western Pacific during Boreal Spring

Previous studies suggested that there are large discrepancies in the intensity trend of the zonally averaged Hadley circulation (ZAHC) among different reanalyses. As the land, ocean, and topography are not evenly distributed, the ZAHC may mask the regional variability. Changes in the regional HC have important implications for regional climate change. Here, we detect the long-term trend of the boreal spring regional Hadley circulation intensity over the western Pacific (WPHC) since 1979 in both hemispheres using six reanalysis datasets. Unlike the ZAHC, we find that the trend of the spring WPHC intensity is consistent among various reanalysis datasets. All reanalyses show pronounced strengthening trends for the WPHC in both the Northern and Southern Hemisphere, which may be partly attributable to the robust warming trends of sea surface temperature in the tropical western Pacific. The result could improve our understanding of Hadley circulation variability at the regional scale and has implications for regional climate changes.     

南、北极海冰的长期变化趋势及其与大气环流的联系

采用南、北极海冰面积指数 1°× 1°经纬度格点资料及海平面气压资料 ,运用多种统计方法 ,研究了南、北极海冰的长期变化趋势、突变特征及其与大气环流的联系 ,发现近年来南极冬、春、秋季海冰逐渐减少 ,夏季海冰逐渐增加 ;北极春、夏、秋季海冰均不同程度地减少 ,冬季海冰变化趋势不明显 ;南、北极各季海冰的年际变化均存在一定的突发性 ,大气环流在海冰突变年前后有显著的差异     

Influence of Antarctic ice sheet lowering on the Southern Hemisphere climate: modeling experiments mimicking the mid-Miocene

A coupled global atmosphere-ocean model is used to study the influence of the Antarctica ice sheet in a configuration that mimics that of the early Miocene on the atmospheric and oceanic circulations. Based on different climate simulations of the present day (CTR) and conducted with distinct Antarctic ice sheet topography (AIS-EXP), it is found that the reduction of the Antarctic ice sheet topography (AIS) induces warming of the Southern Hemisphere and reduces the meridional thermal gradient. Consequently, the atmospheric transient low level eddy heat flux $[(\overline{v^{\prime}T^{\prime}})]$ and the eddy momentum flux $[(\overline{u^{\prime}v^{\prime}})]$ are reduced causing the reduced transport of heat from the mid-latitudes to the pole. The stationary flow and transient wave anomalies generate changes in the SSTs which modify the rate of deep water formation, strengthening the formation of the Antarctic Bottom Water. Substantial changes are predicted to occur in the atmospheric and oceanic heat transport and a comparison between the total heat transport of the atmosphere-ocean system, as simulated by the AIS-EXP and the CTR runs, shows that the reduction of the AIS height leads to reduced Southern Hemisphere poleward and increased equatorward heat transport. These results are in agreement with reduced storm track activities and baroclinicity.     

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter.