北太平洋冬季海-气耦合的主模态及其与瞬变扰动异常的联系

利用45年的ECMWF再分析资料,使用SVD方法研究了冬季北太平洋地区表层海温(SST)异常与大气环流异常间的主要耦合模态,探讨了大尺度海-气耦合型与天气尺度瞬变扰动的相互关系。分析结果表明,中纬度北太平洋地区冬季存在两种主要的海-气耦合型,第1种耦合型反映了与ENSO紧密相关的中纬度北太平洋冬季海温异常分布型以及大气的PNA型,第2种耦合型SST异常集中在东亚沿海以及中纬度北太平洋海流区,相应的大气场则为暖(冷)SSTA上空东西向带状区域内位势高度偏高(低),明显独立于ENSO型。进一步的合成分析表明,在第1种耦合型SST正(负)异常年里,冬季阿留申低压主体位置偏西南(东北),从东北亚到北美西海岸的西北—东南向带状区域内是低层大气温度正(负)异常区和高层西风负(正)异常区,西风负(正)异常中心位于西风急流出口处的北太平洋中东部,而西风急流主体区的风速变化很小。在第2种耦合型东亚沿海至中纬度北太平洋海流区SST偏暖(冷)时,阿留申低压整体偏弱(强),SST暖(冷)异常上空的大气温度偏暖(冷),高层西风急流区西风偏弱(强)。两种耦合型均显示出在北太平洋中纬度地区大气和海洋的异常相关中心有很好的空间对应性。在两种耦合型下,中纬度北太平洋冬季的大气斜压性也发生截然不同的改变,引起中纬度天气尺度瞬变扰动活动异常。瞬变扰动异常的动力强迫作用对北太平洋西风异常的形成存在正反馈作用,而其热力作用则试图破坏与两种海-气耦合模态相关的大气温度异常型。

LEADING COUPLED MODES BETWEEN THE ATMOSPHERE AND OCEAN IN THE NORTH PACIFIC IN WINTER AND THEIR ASSOCIATION WITH THE TRANSIENT EDDY ANOMALIES

45 year's ECMWF reanalysis data ERA40 are used in singular value decomposition(SVD) analyses to investigate the leading coupled modes between the sea surface temperature(SST) and the time-mean atmospheric circulation in the North Pacific region in winter season,and their interactions with synoptic scale transient eddy activities.Results show that there are two leading coupled modes between the atmosphere and ocean in the North Pacific region in winter.The first mode reflects the winter SST anomalies in the middle latitude North Pacific and the atmospheric Pacific-North American(PNA) pattern,which is closely associated with the El NiDo-Southern Oscillation(ENSO) events.The second mode demonstrates the SST anomalies confined mainly to the East Asian coastal region and the middle latitude Pacific area,and corresponding atmospheric anomalies pattern shows above-(below-) normal geopotential height zonally overlapped on the warm(cold) SST anomalies region,which is obviously independent upon the ENSO mode.The further composite analysis suggests that in the positive(negative) first mode SST anomalies years,the main body of the Aleutian low in winter season shifts southwestwards(northeastwards),then,the positive(negative) atmospheric temperature anomalies at the lower level and negative(positive) westerly wind anomalies at the upper level occur in northeast Asia to the west coast of North American.The negative(positive) center of the westerly wind anomalies appears in the central-east North Pacific of the jet exit,with small wind changes in the main body region of the westerly jet at the upper level.During the positive(negative) second mode SST anomalous periods,the warmer(colder) SST appears in the east coasts of Asia to the mid-latitude North Pacific region,then the entire Aleutian low is weakened(intensified),the warmer(colder) atmosphere and weakened(strengthened) westerly jet overlaps on the warm(cold) SST anomalous region.Both of the two leading modes display that the anomalous region of the atmosphere has good local correlation with that of the ocean over the middle latitude North Pacific.The two leading modes cause different changes in the low-level atmospheric baroclinicity,thus inducing the synoptic scale transient eddy activity anomalies in the middle latitude regions.The dynamic forcing of the transient eddies has a positive feedback to the zonal wind anomalies,meanwhile,its thermodynamic forcing attempt to destroy the atmosphere temperature anomalies pattern associated with the two leading modes.