我国东部夏季降水季节内振荡与低频大气环流演变特征分析

Characteristics of intra-seasonal oscillation of summer precipitation in eastern China and its related low frequency atmospheric circulation

利用1979-2017年夏季逐日气象数据,使用经验正交函数(Empirical Orthogonal Function,EOF)分解和超前滞后回归方法分析了我国东部地区夏季降水季节内振荡(Intra-seasonal Oscillation,ISO)的时空特征及其与低纬和北太平洋中高纬度低频大气环流的联系。结果表明:EOF分解的第一模态表现为我国东南部地区ISO降水显著偏多,而我国北方地区偏少的空间型,相应的低频向外长波辐射(Outgoing Long wave Radiation,OLR)场表现为我国东南部地区低频对流异常活跃,而北方地区异常不活跃的南北偶极子型分布。位于南海至副热带西太平洋上空的低频对流异常活跃区与低频上升气流和低频水汽辐合带协同北传和西北传,在我国东南部地区ISO降水异常偏多位相峰值之前一周抵达华南上空,我国东部地区ISO降水开始呈现出南北偶极子空间型;与此同时,200 hPa低频经向风在北太平洋中高纬度表现为一个纬向低频波列形式,并向西传播,当低频波列中的低频南风异常西传到东亚-日本岛上空时,与低纬北传的高层异常北风形成经向对比,造成我国东南部地区上空出现强的低频散度异常。以上低纬度低频信号西北传与北太平洋中高纬度低频波列西传的协同作用,使得异常活跃的对流活动、水汽异常辐合区、异常上升运动和高层异常辐散区均锚定在我国东南部上空,形成该地区ISO降水异常偏多位相。

Based on the meteorological daily data from 1979 to 2017,the spatial and temporal characteristics of the Intra-seasonal Oscillation(ISO)of summer precipitation in eastern China and its relationship with low-frequency atmospheric circulation in the low and middle-high latitudes North Pacific were analyzed by using Empirical Orthogonal Function(EOF)and lead-lag regression method.The first EOF mode shows that the ISO precipitation in southeastern China is above-normal and that in northern China is below-normal.The low frequency Outgoing Longwave Radiation(OLR)field indicates more active convection activity in southeastern China and less one in northern China.The anomalous low-frequency convective zone,located between the South China Sea and the subtropical western Pacific Ocean,propagates northward and northwestward,together with the low-frequency updraft and the low-frequency water vapor convergence zone.They arrive at south China one week before the positive phase peak of ISO precipitation in southeast China.The ISO precipitation in eastern China begins to show the north-south dipole pattern.At the same time,the low-frequency meridional wind at 200 hPa shows a zonal wave-train in the middle-high latitudes North Pacific.This low-frequency wave-train propagates westward.When a south wind anomaly among the wave-trains move to the region over East Asia-Japan Island,a meridional contrast is formed between it and the north wind anomaly which originates from the low latitude.This pattern results in a strong low-frequency divergence anomaly over southeast China.The northwestward propagation of the above low-frequency signals in the low latitudes cooperates with the westward propagation of low-frequency wave-train in the mid-high latitudes North Pacific.Thus,the above-normal convection activity,water vapor convergence,upward motion and high-level divergence anomaly are anchored over the southeastern China,contributing to the positive phase of ISO precipitation locally.