亚洲中高纬环流春夏季节转换过程的关键特征

Key Circulation Characteristics of Spring-to-summer Seasonal Transition Process over Mid-and High-Latitude Asia

亚洲中高纬环流春夏季节转换是亚洲大陆上发生的不同区域春夏季节转换中的一个重要组成部分,它为江淮流域梅雨形势的建立提供必要的中高纬环流条件。但是关于其独特性和关键特征,迄今为止尚没有系统性的总结。本文利用NCEP/NCAR再分析资料I的逐日数据,分析和总结了这一春夏季节转换过程的关键特征。亚洲中高纬环流春夏季节转换以500 hPa东北亚脊和"双阻型"环流形势的依次建立为重要标志。东北亚脊及其相关的海陆温度差异的形成主要归因于东北亚积雪融化和局地强烈增温过程。东北亚低压(850 hPa)的建立是亚洲中高纬环流春夏季节转换的另一个重要标志。当季节转换发生时,青藏高原上空的200 hPa急流轴从35°N向北跳到37°N,与此同时亚洲温带急流彻底消失。伴随着季节转换,亚洲中高纬地区近地面温度经向梯度减弱,高频瞬变斜压扰动随之减弱;与之形成鲜明对比,低频天气系统,包括亚洲阻塞高压和东北冷涡系统,则成为该地区主导天气系统。本文也从春夏季节转换早晚的角度,探讨了亚洲中高纬环流和天气系统的演变特征,由此进一步补充了春夏季节转换的关键信息。

The spring-to-summer seasonal transition over mid-and high-latitude Asia(MHASST)is an important part of several spring-to-summer seasonal transitions in different regions of the Asian continent.It provides the necessary circulation conditions in the mid-and high-latitudes for the establishment of the Meiyu rainfall in the Yangtze and Huaihe River basins.However,so far,there is no systematic summary of its uniqueness and key characteristics.This paper analyzes and summarizes the key characteristics of the MHASST process based on the daily data of NCEP/NCAR reanalysis data I.The MHASST is symbolized by the establishment of the Northeast Asian ridge at 500 hPa and the“double blocking”circulation pattern.The formation of the Northeast Asian ridge and its related land-sea temperature difference is mainly attributed to the snow melting process and consequently the local strong warming process in Northeast Asia.The establishment of the northern East Asian low(850 hPa)is another important sign of the MHASST.When the MHASST occurs,the 200-hPa Asian jet axis over the Tibetan Plateau jumps northward from 35°N to 37°N,while the Asian temperate jet disappears completely.With the seasonal change,the meridional gradient of the nearsurface temperature in the mid-and high-latitude Asia weakens,thus causing the attenuation of high-frequency transient baroclinic disturbances.In contrast,low-frequency weather systems,including the Asian blocking high and the Northeast China cold vortex system,become the dominant weather systems in the same region.From the perspective of the early and late timing of the MHASST,this paper also discusses the evolution features of the circulation and weather system over the mid-and high-latitude Asia.The obtained results further supplement the key information of the climatic MHASST.