近20年北大西洋中纬度海温与江南5月降水关系增强的可能原因

Possible Reasons for the Stronger Relationship Between the North Atlantic Mid-Latitude Sea Surface Temperature and Precipitation in Jiangnan in May in the Past 20 Years

本文以1980—2021年江南5月降水为研究对象,利用美国国家气候预测中心(Climate Prediction Center, CPC)降水资料、美国环境预测中心(National Centers for Environment Prediction, NCEP)大气环流和Hadley中心的海表面温度资料,分析了江南5月降水极端性的变化及其与大气环流、北大西洋海温的联系。结果表明,自2004年江南5月降水年际差异显著增大以来,200 hPa上的Rossby波能量从北大西洋向下游传播,一部分能量沿副热带西风急流传播,形成南支波列;另一部分沿不列颠群岛向西伯利亚地区传播,上述两支波列在旱涝年的不同配置一方面可以通过影响江南地区高空200 hPa附近的辐合辐散影响江南地区的垂直运动;另一方面波列下游的中国东南沿海200 hPa的异常气旋或反气旋具有准正压结构,对应于对流层中下层西北太平洋异常气旋或反气旋在孟加拉湾、中南半岛和南海上空位置和强弱变化,影响水汽向江南地区的输送。在偏涝年形成正位相的欧亚遥相关波列,利于冷空气在贝加尔湖附近积累并向江南地区输送,它和低纬暖湿气流在江南地区附近相遇,形成降水。2004年以来,欧亚大陆上空与5月不列颠群岛附近海温异常有关的波列分布发生了改变,影响了江南地区高空200 hPa的辐合辐散。上述遥相关波列增强了和西北太平洋异常反气旋的关系,进一步与江南5月降水建立起紧密联系。2004年以来自江南5月偏涝年前的冬季开始,北大西洋中纬度通常出现冷海温异常并随时间东移,于5月降水同期东移到不列颠群岛附近,偏旱年反之,这对江南5月降水具有一定的预报意义。

Due to the frequent occurrence of dramatic droughts and floods in Jiangnan area in recent years,this paper explores Jiangnan precipitation in May from 1980 to 2021 based on CPC precipitation,NCEP atmospheric circulation and Hadley sea surface temperature(SST)data and analyze the changes in precipitation anomalies in Jiangnan in May,and its relationship with atmospheric circulation and North Atlantic SST.The results show that since the interannual difference of precipitation in May in Jiangnan has increased significantly since 2004,when the Rossby wave energy over the North Atlantic passes through the Eurasian continent.Rossby wave can propagate along the subtropical westerly jet,forming a southern branch wave train.Some Rossby wave energy spreads from the British Isles to Siberia,forming a positive phase Eurasian teleconnection(EU)in wet years,which is conducive to the accumulation of cold air near Baikal Lake and its transport to Jiangnan,where it meets the warm and humid air at low latitudes to form a front.The wave action fluxes of the two wave trains in wet years and dry years converge in the area from the southeastern coast of my country to Japan and over the Mongolian Plateau,respectively.On the one hand,it can affect the vertical motion of Jiangnan by affecting the convergence and divergence near 200 hPa in the upper air of Jiangnan.The anomalous cyclone or anticyclone has a quasi-barotropic structure,which corresponds to the changes in the position and intensity of the Northwest Pacific anomalous anticyclone(WNPAC)in the middle and lower troposphere over the Bay of Bengal,Indo-China Peninsula and South China Sea,which affects the transport of water vapor to Jiangnan.The combined effect of low-latitude warm and humid air and northerly cold air affect the amount of precipitation in Jiangnan.The pattern of wave trains in Eurasia related to the SSTA near the British Isles in May has changed since 2004,affecting the convergence and divergence at 200 hPa in the upper air of Jiangnan,and strengthening the relationship with WNPAC and the precipitation in Jiangnan.Usually,the anomaly of cold SST in the mid-latitudes of the North Atlantic maintains and moves eastwards in the early stage of the wet years in Jiangnan in May since 2004,and vice versa in the dry years,which can be used as an indicator to predict the extreme precipitation in Jiangnan in May.