1979-2012年北极海冰运动学特征初步分析

利用美国冰雪数据中心(NSIDC)发布的海冰速度和范围数据,本文分析了1979—2012年间北极海冰的运动学特征,以及北极海冰运动与分布范围演变之间的关系。结合欧洲中期天气预报中心(ECMWF)发布的2007和2012年高分辨率的气压场、风场数据,探讨了北极风场和气压场与海冰运动、辐散辐合和海冰面积的关系。结果表明,在1979-2012年间北极海冰平均运动速度呈显著增强的趋势,冬季海冰平均运动速度增加趋势明显强于夏季;北极、波弗特-楚科奇海域和弗拉姆海峡的冬、夏季海冰平均运动速度的增加率分别为2.1%/a和1.7%/a、2.0%/a和1.6%/a以及4.9%/a和2.2%/a。1979-2012年北极海冰平均运动速度和范围的相关性为-0.77,二者存在显著的负相关关系。北极冬季和夏季风场的长期变化趋势与海冰平均运动速度的变化趋势一致,冬季和夏季的相关系数分别为0.50和0.48。风场和气压场对海冰的运动、辐散及重新分布发挥着重要作用。2007年夏季,第234~273天波弗特海域一直被高压系统控制,波弗特涡旋加强,使得波弗特海域海冰聚集在北极中央区;顺时针的风场促使海冰向格陵兰岛和加拿大北极群岛以北聚合。2012年,白令海峡和楚科奇海域处于低压和高压系统的交界处,盛行偏北风,海冰从北极东部往西部输运,加拿大海盆的多年海冰因离岸运动而辐散,向楚科奇海域的海冰输运增加,受太平洋入流暖水影响,移入此区域的海冰加速融化,从而加剧海冰的减少。     

2013年北极最小海冰范围比2012年增加的原因分析

北极海冰范围从1979年有卫星观测资料以来呈现明显下降趋势,尤其是9月份。2012年9月北极海冰范围达到有观测记录以来的最小值,而2013年9月比2012年同期增加了60%。增加的区域主要在东西伯利亚海区、楚科奇海和波弗特海区。本文应用距平和经验模态分解方法,分析了美国国家冰雪数据中心的北极海冰卫星数据、欧洲预报中心的夏季底层大气环流数据和上层海洋的温度,指出2013年北极最小海冰范围比2012年在北冰洋太平洋扇区增加的原因,是由于表面气温(SAT)降低、海平面气压(SLP)升高、气旋式风场异常、表面空气中水汽含量(SH)降低以及海表面温度(SST)降低5个条件形成的冰-SAT、冰-SST和冰-汽(SH)3个正反馈机制共同作用造成的。     

东亚冬季气温的年际变化特征及其与海温和海冰异常的关系

通过谐波分析的方法,对东亚31个冬季(1980—2010年)的气温提取年际变化分量(周期小于8a部分)进行EOF分析。结果发现:在年际变化的时间尺度上,东亚冬季气温表现为高纬模态和低纬模态2个主要模态,它们一起可以解释总方差73%的变化。进一步分析表明,在年际变化尺度上,与气温变化的高纬模态相联系的大气环流表现为显著的北极涛动(AO)负位相分布,海平面气压场上西伯利亚高压和阿留申低压北移,对流层中层东亚大槽西移,高层西风急流向西北方向移动;副热带北太平洋和阿拉斯加湾的海表面温度(SST)变化呈偶极子振荡分布,这种准两年的周期振荡对这一模态的出现有一定的预示意义。而与气温变化的低纬模态相联系的大气环流表现为类AO正位相分布,与之相关的西伯利亚高压和阿留申低压南移,对流层中层东亚大槽东移,高层的西风急流则是向东南方向移动;赤道东太平洋的SST异常可能对这一模态的形成有一定的作用,而东亚近海的SST则更多是被动地改变。此外,海冰异常变化与东亚冬季气温变化的联系主要体现在:在前夏和前秋,东西伯利亚海-波弗特海海冰异常减少(增加)对应着随后东亚冬季气温变化的高纬模态(低纬模态),而冬季东亚气温变化的高纬模态(低纬模态)又与后期春季北极东半球的海冰异常增加(减少)具有较好的相关性,此外白令海和鄂霍次克海的海冰异常变化是伴随东亚冬季气温变化产生的。     

冬季鄂霍次克海海冰对北太平洋风暴轴年际变化的影响

利用北极海冰密集度资料和NCEP＼NCAR再分析资料，运用统计方法讨论了冬季鄂霍次克海及其邻近海区海冰异常与同期北太平洋风暴轴的联系。结果发现，冬季鄂霍次克海西南部海区海冰面积异常与北太平洋海温异常共同作用对北太平洋风暴轴在西北一东南方向的伸缩及强度的年际变化有重要影响，而在海温异常的共同作用下，鄂霍次克海东北部及舍列霍夫海海区海冰面积异常则主要影响风暴轴的南北位移和强弱。     

液态降水与地表气温对北极海冰开始融化时间的影响

海冰最早开始融化时间（EMO）是体现海冰融化的重要指标,也是影响海冰热收支的关键因素。本文使用EMO遥感数据、ERA5再分析资料和海冰密集度数据分析研究了地表气温和液态降水对EMO影响的相对贡献。研究显示,在5个研究海区中,大西洋扇区南区EMO提前最显著,1979-2021年的变化率为-3.3 d/(10 a)。北极各海区的地表气温与EMO有着持续1～2个月的显著相关时段,其中太平洋扇区南区、大西洋扇区北区和南区的地表气温较液态降水与EMO相关的持续时间更长,相关性也更强;而对太平洋扇区北区和北极中央区,只有在EMO发生前的2～3周,液态降水对其EMO有着更高的贡献。对于太平洋扇区北区,大气环流提供的强水汽输送通道伸入该海区,使对流层低层饱和水汽增多,500 hPa位势高度的多年变化趋势具有三波绕极环流加强的结构,也有利于经向的热量交换,使比湿的垂向梯度进一步增加,为该海区EMO的提前起到一定的促进作用。对于北极中央区,在EMO提前的年份,液态降水较常年偏高33%,不仅气候态意义下的太平洋水汽通道的输送加强,欧亚大陆上空的水汽通道也与之汇合,促使北极东部形成气旋式水汽输送模态,为EMO...     

北极海冰的年代际转型与中国冻雨年代际变化的关系

基于1961-2013年HadISST海冰密集度资料,定义了北极海冰的季节性融冰指数,结果显示近几十年来北极季节性融冰范围呈显著的上升趋势,并分别在20世纪70年代末和90年代中期存在显著的年代际转型,相应地,中国冻雨发生频数总体上呈现出显著的减少趋势,但也存在显著的年代际转型。在20世纪70年代末之前,北极季节性融冰范围较小但显著增长,中国冻雨频数年际变化振幅较大,且主要受巴伦支海、喀拉海海冰的影响;20世纪70年代末至90年代中期北极季节性融冰范围维持振荡特征,没有显著的线性趋势,中国冻雨频数变化振幅减小,与北极海冰相关较弱,主要相关因子为北大西洋及北太平洋海表温度变化;而90年代中期以后,北极海冰融化加快,特别是2007年以后,季节性融冰范围大大增加,而中国冻雨频数处于低发时段,其变化与太平洋扇区海冰及堪察加半岛附近海温呈显著负相关,季节性融冰的显著区域也从东西伯利亚海逆时针旋转向波弗特海-加拿大群岛北部扩张,同时向北极中央区扩张。不同年代影响冻雨的海温或海冰关键海区不同,产生特定的大气环流异常响应,进而影响到我国冻雨。     

太平洋夏季水对加拿大海盆海冰的影响

近年来,北极海冰发生了大面积减少,减少的原因仍存在着争议。基于2003-2011年的水文和遥感卫星数据,对北冰洋加拿大海盆的太平洋水和海冰进行研究。通过对比2006年和2007年太平洋水位温与海冰密集度的空间分布,发现太平洋水暖异常于2007年1-3月进入加拿大海盆的中部,并可能导致了2007年夏季海冰大面积的融化。2003-2011年,在加拿大海盆的中部,太平洋水位温与海冰密集度存在着时间上的负相关。选取2007年8月,发现两者在空间上也存在着负相关。这很可能说明太平洋水暖异常在流动的过程中,向上输送了热量,在一定程度上,融化了海冰,从而触发海冰-反照率正反馈,导致海冰的减少。因此,通过白令海峡进入北冰洋的太平洋夏季水,对北极海冰面积的减少有着重要影响。     

北极秋季海冰减少与亚洲大陆冬季温度异常

本文使用SVD等诊断分析方法探讨北极秋季海冰密集度与亚洲冬季温度异常之间的关系。结果表明,近30余年来,北极秋季海冰减少伴随着亚洲大陆冬季温度降低,但青藏高原地区、北冰洋和北太平洋沿岸除外。北极秋季海冰密集度减小激发欧亚大陆和北冰洋北部两个区域位势高度的改变,这种异常的变化模态从秋季持续到冬季。位势高度异常的负值中心位于巴伦支海和喀拉海。位势高度异常的正值中心位于蒙古区域。与重力位势高度异常伴随的风场异常为亚洲冬季温度降低提供自北向南的冷气流。随着北极海冰的不断减少,其与亚洲大陆冬季温度降低之间的关系将为气候长期预测提供参考。     

巴芬湾西部和格陵兰岛东部大气经向热量输送协同变化对夏季北极海冰的影响

利用欧洲中心气候再分析资料和美国国家冰雪数据中心北极海冰面积资料，分析了夏季北极海冰面积与前期大气经向热量输送年际变化的联系。结果表明：6月北半球中高纬大气的经向热量输送以瞬变热量形式为主，其中巴芬湾西部（B区）和格陵兰岛东部（G区）是瞬变热量向极区传输的两个通道，二者之间存在反位相的协同变化，且这种协同变化与夏季北极海冰面积变化密切相关。可能的机制为：6月，AD、AO和NAO三种北极大气环流型能够引起巴芬湾西部和格陵兰岛东部瞬变热量输送的协同变化，这种协同变化通过涡旋动力作用激发夏季极区大气表现为AD异常，同时影响途经区域的气温，从而通过热动力作用影响夏季北极海冰。将向极区输送的热量称为暖输送，从极区输出的热量为冷输送，则上述两个区域的瞬变热量协同输送可分为三种情况：B暖G冷、B冷G暖、B和G均冷，而B和G均暖的情况十分罕见。当B区向极区输入、G区输出热量时，有利于太平洋扇区和喀拉海的海冰偏少；当G区输入、B区输出热量时，利于喀拉海和拉普捷夫海海冰偏少；当B区和G区均输出热量时，利于波佛特海南部、喀拉海和拉普捷夫海海冰偏多，反之则相反。     

北半球中高纬度大气环流主模态的季节演变及其与北极海冰变化的联系

利用31a(1979—2009)气候月平均的海平面气压(SLP)资料,提出1种与北半球中高纬度环流转变相适应的分季法。并根据这个客观分季方法,通过SEOF分析,发现大气环流主模态的季节演变有着典型的北极涛动(AO)空间结构,其时间系数在1990年代中期发生转型。500hPa上纬向波的涡度有着南北反位相的分布特征,冬季正涡度的区域对应着气旋性环流,其覆盖范围广,而夏季正涡度区域更偏北,可见AO在冬季增强,夏季减弱。北半球SLP异常的EOF分解第一模态为北极涛动(AO),第二模态是偶极异常(DA);将这2个模态称之为北半球中高纬度大气环流异常的优势模态。通过计算优势模态与海冰面积的超前滞后相关性,发现AO依然是控制海冰变化的前期大气环流异常的模态,而DA则可能是海冰快速变化后期大气环流的主导模态。     

北极各海域海冰覆盖范围的变化特征

Sea ice in the Arctic has been reducing rapidly in the past half century due to global warming.This study analyzes the variations of sea ice extent in the entire Arctic Ocean and its sub regions.The results indicate that sea ice extent reduction during 1979–2013 is most significant in summer,following by that in autumn,winter and spring.In years with rich sea ice,sea ice extent anomaly with seasonal cycle removed changes with a period of 4–6 years.The year of 2003–2006 is the ice-rich period with diverse regional difference in this century.In years with poor sea ice,sea ice margin retreats further north in the Arctic.Sea ice in the Fram Strait changes in an opposite way to that in the entire Arctic.Sea ice coverage index in melting-freezing period is an critical indicator for sea ice changes,which shows an coincident change in the Arctic and sub regions.Since 2002,Region C2 in north of the Pacific sector contributes most to sea ice changes in the central Aarctic,followed by C1 and C3.Sea ice changes in different regions show three relationships.The correlation coefficient between sea ice coverage index of the Chukchi Sea and that of the East Siberian Sea is high,suggesting good consistency of ice variation.In the Atlantic sector,sea ice changes are coincided with each other between the Kara Sea and the Barents Sea as a result of warm inflow into the Kara Sea from the Barents Sea.Sea ice changes in the central Arctic are affected by surrounding seas.     

Severe winter weather as a response to the lowest Arctic sea-ice anomalies

Possible impact of reduced Arctic sea-ice on winter severe weather in China is investigated regarding the snowstorm over southern China in January 2008. The sea-ice conditions in the summer （July-September） and fall （September-November） of 2007 show that the sea-ice is the lowest that year. During the summer and fall of 2007, sea ice displayed a significant decrease in the East Siberian, the northern Chukchi Sea, the western Beaufort Sea, the Barents Sea, and the Kara Sea. A ECHAM5.4 atmospheric general circula- tion model is forced with realistic sea-ice conditions and strong thermal responses with warmer surface air temperature and higher-than-normal heat flux associated with the sea-ice anomalies are found. The model shows remote atmospheric responses over East Asia in January 2008, which result in severe snowstorm over southern China. Strong water-vapor transported from the Bay of Bengal and from the Pacific Ocean related to Arctic sea-ice anomalies in the fall （instead of summer） of 2007 is considered as one of the main causes of the snowstorm formation.     

Linkages among halocline variability,shelf-basin interaction,and wind regimes in the Beaufort Sea demonstrated in pan-Arctic Ocean modeling framework

To address the mechanisms controlling halocline variability in the Beaufort Sea, the relationship between halocline shoaling/deepening and surface wind fields on seasonal to decadal timescales was investigated in a numerical experiment. Results from a pan-Arctic coupled sea ice-ocean model demonstrate reasonable performances for interannual and decadal variations in summer sea ice extent in the entire Arctic and in freshwater content in the Canada Basin. Shelf-basin interaction associated with Pacific summer and winter transport depends on basin-scale wind patterns and can have a significant influence on halocline variability in the southern Beaufort Sea. The eastward transport of fresh Pacific summer water along the northern Alaskan coast and Ekman downwelling north of the shelf break are commonly enhanced by cyclonic wind in the Canada Basin. On the other hand, basin-wide anti-cyclonic wind induces Ekman upwelling and blocks the eastward current in the Beaufort shelf-break region. Halocline shoaling/deepening due to shelf-water transport and surface Ekman forcing consequently occur in the same direction. North of the Barrow Canyon mouth, the springtime down-canyon transport of Pacific winter water, which forms by sea ice production in the Alaskan coastal polynya, thickens the halocline layer. The model result indicates that the penetration of Pacific winter water prevents the local upwelling of underlying basin water to the surface layer, especially in basin-scale anti-cyclonic wind periods.     

Arctic salinity anomalies and their link to the North Atlantic during a positive phase of the Arctic Oscillation

Many of the changes observed during the last two decades in the Arctic Ocean and adjacent seas have been linked to the concomitant abrupt decrease of the sea level pressure in the central Arctic at the end of the 1980s. The decrease was associated with a shift of the Arctic Oscillation (AO) to a positive phase, which persisted throughout the mid 1990s. The Arctic salinity distribution is expected to respond to these dramatic changes via modifications in the ocean circulation and in the fresh water storage and transport by sea ice. The present study investigates these different contributions in the context of idealized ice-ocean experiments forced by atmospheric surface wind-stress or temperature anomalies representative of a positive AO index.Wind stress anomalies representative of a positive AO index generate a decrease of the fresh water content of the upper Arctic Ocean, which is mainly concentrated in the eastern Arctic with almost no compensation from the western Arctic. Sea ice contributes to about two-third of this salinification, another third being provided by an increased supply of salt by the Atlantic inflow and increased fresh water export through the Canadian Archipelago and Fram Strait. The signature of a saltier Atlantic Current in the Norwegian Sea is not found further north in both the Barents Sea and the Fram Strait branches of the Atlantic inflow where instead a widespread freshening is observed. The latter is the result of import of fresh anomalies from the subpolar North Atlantic through the Iceland-Scotland Passage and enhanced advection of low salinity waters via the East Icelandic Current. The volume of ice exported through Fram Strait increases by 20% primarily due to thicker ice advected into the strait from the northern Greenland sector, the increase of ice drift velocities having comparatively less influence. The export anomaly is comparable to those observed during events of Great Salinity Anomalies and induces substantial freshening in the Greenland Sea, which in turn contributes to increasing the fresh water export to the North Atlantic via Denmark Strait. With a fresh water export anomaly of 7 mSv, the latter is the main fresh water supplier to the subpolar North Atlantic, the Canadian Archipelago contributing to 4.4 mSv.The removal of fresh water by sea ice under a positive winter AO index mainly occurs through enhanced thin ice growth in the eastern Arctic. Winter SAT anomalies have little impact on the thermodynamic sea ice response, which is rather dictated by wind driven ice deformation changes. The global sea ice mass balance of the western Arctic indicates almost no net sea ice melt due to competing seasonal thermodynamic processes. The surface freshening and likely enhanced sea ice melt observed in the western Arctic during the 1990s should therefore be attributed to extra-winter atmospheric effects, such as the noticeable recent spring-summer warming in the Canada-Alaska sector, or to other modes of atmospheric circulations than the AO, especially in relation to the North Pacific variability.     

基于CryoSat-2数据的2010-2017年北极海冰厚度和体积的季节与年际变化特征

北极海冰变化影响着全球物质平衡、能量交换和气候变化。本文基于CryoSat-2测高数据和OSI SAF海冰密集度及海冰类型产品，分析了2010-2017年北极海冰面积、厚度和体积的季节和年际变化特征，结合NCEP再分析资料探讨了融冰期北极气温异常和夏季风异常对海冰变化的影响。结果表明，结冰期海冰面积的增加量波动较大，海冰厚度的增加量呈明显下降趋势。融冰期海冰厚度的减小量波动较大，2013年以后融冰期海冰面积的减小量逐年增加。海冰体积的变化趋势和面积变化更相似，融冰期的减小速率大于结冰期的增加速率。融冰期北极海表面大气温度异常与海冰融化量正相关。夏季风影响海冰的辐合和辐散，在弗拉姆海峡海冰的输运过程中起关键作用，促进了北冰洋表层水向大洋深层的传输。     

Observations and modeling of the ice-ocean conditions in the coastal Chukchi and Beaufort Seas

The Chukchi and Beaufort Seas include several important hydrological features： inflow of the Pacific water, Alaska coast current （ ACC ）, the seasonal to perennial sea ice cover, and landfast ice ＇along the Alaskan coast. The dynamics of this coupled ice-ocean system is important for both regional scale oceanography and large-scale global climate change research. A mumber of moorings were deployed in the area by JAMSTEC since 1992, and the data revealed highly variable characteristics of the hydrological environment. A regional high-resolution coupled ice-ocean model of the Chukchi and Beaufort Seas was established to simulate the ice-ocean environment and unique seasonal landfast ice in the coastal Beaufort Sea. The model results reproduced the Beaufort gyre and the ACC. The depthaveraged annual mean ocean currents along the Beaufort Sea coast and shelf hreak compared well with data from four moored ADCPs, but the simulated velocity had smaller standard deviations, which indicate small-scale eddies were frequent in the region. The model resuits captured the sea,real variations of sea ice area as compared with remote sensing data, and the simulated sea ice velocity showed an ahnost stationary area along the Beaufort Sea coast that was similar to the observed landfast ice extent. It is the combined effects of the weak oceanic current near the coast, a prevailing wind with an onshore component, the opposite direction of the ocean current, and the blocking hy the coastline that make the Beaufort Sea coastal areas prone to the formation of landfast ice.     

冬季白令海海冰范围变化及其对大气响应机制研究

白令海是冬季北极海冰变化最明显的区域之一,该区域海冰的季节和长期变化与局地的气候、水文环境和生态系统密切相关,并会影响我国的天气气候过程。为了识别该区冬季海冰的长期变化,基于Hadley中心数据,采用滑动t检验和线性回归分析方法对白令海1960–2020年海冰范围的变化趋势及其空间差异进行分析,并分析了海冰变化对大气环流等大气强迫的影响。结果表明:白令海冬季海冰范围在1960–2020年显著减小,20世纪70年代和2000年前后白令海海冰范围存在显著的均值突变。其过程中伴随着阿留申低压中心低压加强、核心位置向白令海西部偏移以及对应风场分布的变化,这个过程存在一个近20 a周期的振荡。同时,太平洋年代际震荡的相位变化可以通过改变海平面气压来调节经向风,改变进入白令海的热平流,进而影响白令海冬季海冰范围。因此,阿留申低压系统和北太平洋年代际振荡对冬季白令海海冰的变化起到重要的调节作用。