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。     

Atmospheric winter response to a projected future Antarctic sea-ice reduction: a dynamical analysis

Several studies have analysed the atmospheric response to sea-ice changes in the Arctic region, but only few have considered the Antarctic. Here, the atmospheric response to sea-ice variability in the Southern Hemisphere is investigated with the atmospheric general circulation model ECHAM5. The model is forced by the present and a projected future seasonal cycle of Antarctic sea ice. In September, the mean atmospheric response exhibits distinct similarities to the structure of the negative phase of the Southern Annular Mode, the leading mode of Southern Hemisphere variability. In the reduced Antarctic sea-ice integration, there is an equatorward shift of the Southern Hemisphere mid-latitude jet and the storm tracks. In contrast to a recent previous study, our findings indicate that a substantial impact of Southern Hemispheric future sea-ice reduction on the mid-latitude circulation cannot be ruled out.     

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。     

北半球夏季南极冰气候效应的数值试验

本文利用9层菱形截断15波的全球大气环流谱模式进行了7月份南极冰正常和无冰的对比试验,进而研究了南极冰的短期气候效应.结果表明:南极冰的移去首先在局地区域加热了低层空气,减弱了南极极涡,然后使中高纬度超长波发生变异尤其是使纬向3波增幅.超长波的异常引起东亚季风区低层越赤道气流加强,从而增强东亚季风环流,增大中南半岛到西太平洋的降雨量和相应的对流潜热释放,结果在北半球激发了一支从东亚到北美的二维Rossby波列,使北半球大气环流发生异常.文中对这一气候效应的动力学机制进行了讨论,并认为南极冰不仅是影响南半球     

Actual and insolation-weighted Northern Hemisphere snow cover and sea-ice between 1973–2002

Actual and insolation-weighted Northern Hemisphere snow cover and sea ice are binned by latitude bands for the years 1973–2002. Antarctic sea-ice is also analyzed for the years 1980–2002. The use of insolation weighting provides an improved estimate of the radiative feedbacks of snow cover and sea-ice into the atmosphere. One conclusion of our assessment is that while a decrease in both areal and insolation-weighted values have occurred, the data does not show a monotonic decrease of either Arctic sea-ice or Northern Hemisphere snow cover. If Arctic perennial sea-ice is decreasing since the total reduction in areal coverage is relatively small, a large portion of it is being replenished each year such that its radiative feedback to the atmosphere is muted. Antarctic sea-ice areal cover shows no significant long-term trend, while there is a slight decrease in the insolation-weighted values for the period 1980–2002. From the early 1990s to 2001, there was a slight increase in both values. The comparison of general circulation model simulations of changes over the last several decades to observed changes in insolation-weighted sea-ice and snow cover should be a priority research topic.     

Fluctuations of snow accumulation in the Antarctic and sea level pressure in the Southern Hemisphere in the last 100 years

This paper summarizes the long-term fluctuations of snow accumulation in the Antarctic and analyzes its correlation with the sea level pressure (SLP) in the middle latitude (40°–50° S) of the Southern Hemisphere. Stratigraphic data which were compiled from studies on ice cores and snow-pits at eight stations in the Antarctic were used in the present study. It was found that the data concerning fluctuations in snow accumulation for East Antarctica showed correlations, whereas no such correlation was observed for the data from West Antarctica.This study shows possible relationships between snow accumulation in the Antarctic and SLP in the middle latitudes. The fluctuations of accumulation at South Pole, Dome C, Wilkes and South Ice Point show correlations with SLP over a large area in the 40°–50° S latitudinal zone. For the long-term fluctuations of SLP in the 40°–50 ° S latitudinal zone, a zonal fluctuation with wave number zero structure and a longitudinal variation of SLP anomalies due to their out-of-phase-fluctuation between the Pacific and the Indian Oceans were observed. The temporal scales for these fluctuations were found to be in the order of 20–30 years and 40–60 years, respectively. The influences of these two modes on the behaviour of snow accumulation in the Antarctic is also discussed.Now at Kitami Institute of Technology, Kitami, Hokkaido, Japan.     

Trans-Tasman Sea climate variability since ad 1740 inferred from middle to high latitude tree-ring data

The limited length and spatial coverage of instrumental climate data for many areas of the Southern Hemisphere impedes the study of atmosphere-ocean dynamics prior to the past century. Such analyses are important for understanding interannual to decadal variation of the Southern Hemisphere circulation and whether recent changes are related to anthropogenic effects rather than natural variability. We use a middle- to high-latitude tree-ring width data set (from Tasmania, New Zealand and Tierra del Fuego) to reconstruct sea-level pressure (SLP) variability spanning the Tasman Sea and vicinity since ad 1740. The variables reconstructed are austral summer (November–March) SLP for Hobart, Tasmania (43°S, 147°E) and the Chatham Islands, New Zealand (44°S, 177°E), as well as a meridional circulation index (Hobart-Chatham Islands index) which measures the pressure gradient between these two stations. The three reconstructions are well verified statistically and capture between 40 and 48% of the variance in the SLP data. The instrumental and estimated SLP show similar spatial patterns of correlation with the sea surface temperature (SST) field for the Pacific. Statistically significant (above 95% level) 3–3.5 year spectral peaks are identified in the three reconstructions using multitaper spectral analysis, and a significant 4–5 year peak is found in both the Chatham Islands and Hobart-Chatham Islands SLP reconstructions. These two modes are within the bandwidth of the El Nino-Southern Oscillation. Although very speculative, they may also correspond to a proposed Antarctic circumpolar wave of SLP, SST, wind and sea-ice extent, believed to play a key role in atmosphere-ocean circulation for the Southern Hemisphere. Received: 30 November 1998 / Accepted: 13 December 1999     

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。     

Sea-ice anomalies observed in the Greenland and Labrador seas during 1901–1984 and their relation to an interdecadal Arctic climate cycle

Two independent ice data sets from the Greenland and Labrador Seas have been analyzed for the purpose of characterizing interannual and decadal time scale sea-ice extent anomalies during this century. Sea-ice concentration data for the 1953–1984 period revealed the presence of a large positive anomaly in the Greenland Sea during the 1960s which coincided with the great salinity anomaly, an upper-ocean low-salinity water mass that was observed to travel cyclonically around the northern North Atlantic during 1968–1982. This ice anomaly as well as several smaller ones propagated into the Labrador Sea and then across to the Labrador and east Newfoundland coast, over a period of 3 to 5 years. A complex empirical orthogonal function analysis of the same data also confirmed this propagation phenomenon. An inverse relation between sea-ice and salinity anomalies in the Greenland-Labrador Sea region was also generally found. An analysis of spring and summer ice-limit data obtained from Danish Meteorological Institute charts for the period 1901–1956 indicated the presence of heavy ice conditions (i.e., positive ice anomalies) in the Greenland Sea during 1902–1920 and in the late 1940s, and generally negative ice anomalies during the 1920s and 1930s. Only limited evidence of the propagation of Greenland Sea ice anomalies into the Labrador Sea was observed, however, probably because the data were from the ice-melt seasons. On the other hand, several large ice anomalies in the Greenland Sea occurred 2–3 years after large runoffs (in the early 1930s and the late 1940s) from northern Canada into the western Arctic Ocean. Similarly, a large runoff into the Arctic during 1964–1966 preceded the large Greenland Sea ice anomaly of the 1960s. These facts, together with recent evidence of climatic jumps in the Northern Hemisphere tropospheric circulation, suggest the existence of an interdecadal self-sustained climate cycle in the Arctic. In the Greenland Sea, this cycle is characterized by a state of large sea-ice extent overlying an upper layer of cool, relatively fresh water that does not convectively overturn, which alternates every 10–15 years with a state of small sea-ice extent and relatively warm saline surface water that frequently overturns.Dedicated to Robert W. Stewart on the occasion of his retirement     

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.     

Sensitivity of sea ice to wind-stress and radiative forcing since 1500: a model study of the Little Ice Age and beyond

Three different reconstructed wind-stress fields which take into account variations of the North Atlantic Oscillation, one general circulation model wind-stress field, and three radiative forcings (volcanic activity, insolation changes and greenhouse gas changes) are used with the UVic Earth System Climate Model to simulate the surface air temperature, the sea-ice cover, and the Atlantic meridional overturning circulation (AMOC) since 1500, a period which includes the Little Ice Age (LIA). The simulated Northern Hemisphere surface air temperature, used for model validation, agrees well with several temperature reconstructions. The simulated sea-ice cover in each hemisphere responds quite differently to the forcings. In the Northern Hemisphere, the simulated sea-ice area and volume during the LIA are larger than the present-day area and volume. The wind-driven changes in sea-ice area are about twice as large as those due to thermodynamic (i.e., radiative) forcing. For the sea-ice volume, changes due to wind forcing and thermodynamics are of similar magnitude. Before 1850, the simulations suggest that volcanic activity was mainly responsible for the thermodynamically produced area and volume changes, while after 1900 the slow greenhouse gas increase was the main driver of the sea-ice changes. Changes in insolation have a small effect on the sea ice throughout the integration period. The export of the thicker sea ice during the LIA has no significant effect on the maximum strength of the AMOC. A more important process in altering the maximum strength of the AMOC and the sea-ice thickness is the wind-driven northward ocean heat transport. In the Southern Hemisphere, there are no visible long-term trends in the simulated sea-ice area or volume since 1500. The wind-driven changes are roughly four times larger than those due to radiative forcing. Prior to 1800, all the radiative forcings could have contributed to the thermodynamically driven changes in area and volume. In the 1800s the volcanic forcing was dominant, and during the first part of the 1900s both the insolation changes and the greenhouse gas forcing are responsible for thermodynamically produced changes. Finally, in the latter part of the 1900s the greenhouse gas forcing is the dominant factor in determining the sea-ice changes in the Southern Hemisphere.

Interannual variability of the upper tropospheric circulation

Summary Seventeen years of sea level pressure (SLP), 200-hPa zonal wind and 500-hPa geopotential height data were used to investigate the boreal winter and summer interannual (IA) circulation patterns. The IA patterns for these variables and for their zonally asymmetric (ZA) part were determined by performing empirical orthogonal function (EOF) analyses on the SLP and on ZA SLP. The corresponding patterns for the other variables were obtained by correlating their time series with the amplitude time series of these EOF analyses. For both seasons, the SLP and ZA SLP show a zonal wavenumber one pattern extending from the tropics into the winter hemisphere extratropics, which is consistent with the circulation anomalies related to the El Niño/Southern Oscillation (ENSO) cycles. The zonal wavenumber one pattern observed for the boreal winter describes the SLP and ZA SLP variations related to the mature state of the El Niño and La Niña episodes, and that for the summer, the SLP and ZA SLP variations associated with the initial or decay stages of these phenomena. The 200-hPa zonal wind and 500-hPa geopotential height patterns exhibit strong seasonal dependence, and the ZA parts of these two variables show even more pronounced seasonal differences. These results indicate that the seasonal cycle of the atmospheric circulation, in particular at the upper tropospheric levels, might play an important role in extending the IA wavetrain-like structure into the subtropics as noted for the 200-hPa zonal wind and its ZA part in the Pacific/Americas sector. This wavetrain-like structure shows its Southern Hemisphere (SH) and Northern Hemisphere (NH) branches for the boreal winter, and only its SH branch, for the boreal summer. Thus, the effects of the seasonal cycle of the atmospheric circulation on the IA patterns seem to be stronger for the NH.With 9 Figures     

南极海冰的年际变化对中国东部夏季降水的影响

根据Hadley中心提供的1969—1998年的南极海冰再分析资料和其它多种观测资料，分析了南极海冰的年际和季节变化，指出南极海冰具有显著的年际变化，但与ENSO的关系则较为复杂。南极海冰维持了南半球高纬地区大气环流的季节持续性，因而对短期气候预测有较大帮助。相关分析和时间序列分析均证实中国东部夏季降水与南极海冰的年际变化有关，当北半球春夏季南极海冰增多时，华北降水增多而华南和东北降水减少。研究还表明，此种雨型分布与南极海冰变化引起的东亚夏季风环流变化有关。     

Sea-ice and its response to CO2 forcing as simulated by global climate models

The simulation of sea-ice in global climate models participating in the Coupled Model Intercomparison Project (CMIP1 and CMIP2) is analyzed. CMIP1 simulations are of the unpertubed control climate whereas in CMIP2, all models have been forced with the same 1% yr^–1 increase in CO₂ concentration, starting from a near equilibrium initial condition. These simulations are not intended as forecasts of climate change, but rather provide a means of evaluating the response of current climate models to the same forcing. The difference in modeled response therefore indicates the range (or uncertainty) in model sensitivity to greenhouse gas and other climatic perturbations. The results illustrate a wide range in the ability of climate models to reproduce contemporary sea-ice extent and thickness; however, the errors are not obviously related to the manner in which sea-ice processes are represented in the models (e.g. the inclusion or neglect of sea-ice motion). The implication is that errors in the ocean and atmosphere components of the climate model are at least as important. There is also a large range in the simulated sea-ice response to CO₂ change, again with no obvious stratification in terms of model attributes. In contrast to results obtained earlier with a particular model, the CMIP ensemble yields rather mixed results in terms of the dependence of high-latitude warming on sea-ice initial conditions. There is an indication that, in the Arctic, models that produce thick ice in their control integration exhibit less warming than those with thin ice. The opposite tendency appears in the Antarctic (albeit with low statistical significance). There is a tendency for models with more extensive ice coverage in the Southern Hemisphere to exhibit greater Antarctic warming. Results for the Arctic indicate the opposite tendency (though with low statistical significance).A list of the CMIP modeling groups is included in the Acknowledgements section.     

Analysis of the projected regional sea-ice changes in the Southern Ocean during the twenty-first century

Using the set of simulations performed with atmosphere-ocean general circulation models (AOGCMs) for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4), the projected regional distribution of sea ice for the twenty-first century has been investigated. Averaged over all those model simulations, the current climate is reasonably well reproduced. However, this averaging procedure hides the errors from individual models. Over the twentieth century, the multimodel average simulates a larger sea-ice concentration decrease around the Antarctic Peninsula compared to other regions, which is in qualitative agreement with observations. This is likely related to the positive trend in the Southern Annular Mode (SAM) index over the twentieth century, in both observations and in the multimodel average. Despite the simulated positive future trend in SAM, such a regional feature around the Antarctic Peninsula is absent in the projected sea-ice change for the end of the twenty-first century. The maximum decrease is indeed located over the central Weddell Sea and the Amundsen–Bellingshausen Seas. In most models, changes in the oceanic currents could play a role in the regional distribution of the sea ice, especially in the Ross Sea, where stronger southward currents could be responsible for a smaller sea-ice decrease during the twenty-first century. Finally, changes in the mixed layer depth can be found in some models, inducing locally strong changes in the sea-ice concentration.

Sensitivity of open-water ice growth and ice concentration evolution in a coupled atmosphere-ocean-sea ice model

A coupled atmosphere-ocean-sea ice model is applied to investigate to what degree the area-thickness distribution of new ice formed in open water affects the ice and ocean properties. Two sensitivity experiments are performed which modify the horizontal-to-vertical aspect ratio of open-water ice growth. The resulting changes in the Arctic sea-ice concentration strongly affect the surface albedo, the ocean heat release to the atmosphere, and the sea-ice production. The changes are further amplified through a positive feedback mechanism among the Arctic sea ice, the Atlantic Meridional Overturning Circulation (AMOC), and the surface air temperature in the Arctic, as the Fram Strait sea ice import influences the freshwater budget in the North Atlantic Ocean. Anomalies in sea-ice transport lead to changes in sea surface properties of the North Atlantic and the strength of AMOC. For the Southern Ocean, the most pronounced change is a warming along the Antarctic Circumpolar Current (ACC), owing to the interhemispheric bipolar seasaw linked to AMOC weakening. Another insight of this study lies on the improvement of our climate model. The ocean component FESOM is a newly developed ocean-sea ice model with an unstructured mesh and multi-resolution. We find that the subpolar sea-ice boundary in the Northern Hemisphere can be improved by tuning the process of open-water ice growth, which strongly influences the sea ice concentration in the marginal ice zone, the North Atlantic circulation, salinity and Arctic sea ice volume. Since the distribution of new ice on open water relies on many uncertain parameters and the knowledge of the detailed processes is currently too crude, it is a challenge to implement the processes realistically into models. Based on our sensitivity experiments, we conclude a pronounced uncertainty related to open-water sea ice growth which could significantly affect the climate system sensitivity.     

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).     

On the interannual variability of arctic sea‐level pressure and sea ice

Abstract

Monthly mean sea‐level pressure (SLP) data from the Northern Hemisphere for the period January 1952‐December 1987 are analysed. Fluctuations in this field over the Arctic on interannual time‐scales and their statistical association with fluctuations farther south are determined. The standard deviation of the interannual variability is largest compared with that of the annual cycle along the seaboards of the major land masses. The SLP anomalies are generally in phase over the entire Arctic Basin and extend south over the northern Russia and Canada, but tend to be out of phase with fluctuations at mid‐latitudes. The anomalies are most closely associated with fluctuations over the North Atlantic and Europe except near the Chukchi Sea to the north of Bering Strait. The associations with the North Pacific fluctuations become increasingly more prominent at most Arctic sites (e.g. the Canadian Arctic Archipelago) as the time‐scale increases.

Associations between the SLP fluctuations and atmospheric indices that represent processes affecting sea‐ice drift (wind stress and wind stress curl) are determined. In every case local associations dominate, but some remote ones are also evident. For example, changes in the magnitude of the wind stress curl over the Beaufort Sea are increased if the atmospheric circulation over the North Pacific is intensified; wind stress over the region where sea ice is exchanged between the Beaufort Gyre and the Transpolar Drift Stream is modulated by both the Southern and North Atlantic Oscillations.

Severe sea‐ice conditions in the Greenland Sea (as measured by the Koch Ice Index) coincide with a weakened atmospheric circulation over the North Atlantic.     

Modelling the influence of snow accumulation and snow-ice formation on the seasonal cycle of the Antarctic sea-ice cover

 Recent observational and numerical studies of the maritime snow cover in the Antarctic suggest that snow on top of sea ice plays a major role in shaping the seasonal growth and decay of the ice pack in the Southern Ocean. Here, we make a quantitative assessment of the importance of snow accumulation in controlling the seasonal cycle of the ice cover with a coupled snow–sea-ice–upper-ocean model. The model takes into account snow and ice sublimation and snow deposition by condensation. A parametrisation of the formation of snow ice (ice resulting from the freezing of a mixture of snow and seawater produced by flooding of the ice floes) is also included. Experiments on the sensitivity of the snow–sea-ice system to variations in the sublimation/condensation rate, the precipitation rate, and the amount of snowfall transported by the wind into leads are discussed. Although we focus on the model response in the Southern Hemisphere, results for the Arctic are also discussed in some cases to highlight the relative importance of the processes under study in both hemispheres. It is found that the snow loss by sublimation can account for the removal of 0.45 m of snow per year in the Antarctic and that this loss significantly affects the total volume of snow ice. A precipitation decrease of 50% is conducive to large reductions in the Antarctic snow and snow-ice volumes, but it leads only to an 8% decrease in the annual mean ice volume. The Southern Ocean ice pack is more sensitive to increases in precipitation. For precipitation rates 1.5 times larger than the control ones, the annual mean snow, ice, and snow-ice volumes augment by 30, 20, and 180%, respectively. It is also found that the transfer to the ocean of as much as 50% of the precipitating snow as a result of wind transport has almost negligible effects on the total ice volume. All the experiments exhibit a marked geographical contrast in the ice-cover response, with a much larger sensitivity in the western sector of the Southern Ocean than in the eastern sector. Our results suggest that snow-related processes are of secondary importance for determining the sensitivity of the Arctic sea ice to environmental changes but that these processes could have an important part to play in the response of the Antarctic sea-ice cover to future, or current, climatic changes. Received: 30 June 1997/Accepted: 2 October 1998     

Ice-Core Study of the Link between Sea-Salt Aerosol,Sea-Ice Cover and Climate in the Antarctic Peninsula Area

Three ice cores and a set of snow pit samples collected on James Ross Island, Antarctic Peninsula, in 1979, 1981 and 1991 have been analyzed for water stable isotope content D or ¹⁸O (isotopic temperature) and major chemical species. A reliable and detailed chronological scale has been established first for the upper 24.5 m of water equivalent (1990–1943) where various data sets can be compared, then extended down to 59.5 m of water equivalent (1847) with the aid of seasonal variations and the sulphate peak reflecting the 1883 Krakatoa volcanic eruption. At James Ross Island, sea-salt aerosol is generally produced by ice-free marine surfaces during the summer months, although some winter sea-salt events have been observed. For the upper part of the core (1990–1943), correlations (positive or negative) were calculated between isotopic temperature, chloride content (a sea-salt indicator), sea-ice extent, regional atmospheric temperature changes and atmospheric circulation. The D and chloride content correlation was then extended back to 1847, making it possible to estimate decadal sea-ice cover fluctuations over the study period. Our findings suggest that ice-core records from James Ross Island reflect the recent warming and sea-ice decrease trends observed in the Antarctic Peninsula area from the mid-1940s.