MJO活动对云南5月降水的影响

Impact of MJO Activities on Precipitation in May over Yunnan

本文分析了1979～2008年5月MJO(Maddenand Julian Oscillation)不同位相上大尺度环流对流和水汽输送的异常情况及其对云南5月降水的影响。按MJO活动中心位置从西向东分为8个位相,在不同位相上,云南5月降水呈现出明显的差异:第4～6位相(MJO对流中心位于赤道印度洋中部至西太平洋)降水偏多,而第7～8位相(赤道太平洋中部以东)和第1～3位相(赤道印度洋中西部)降水偏少,其中以第6位相的降水正异常和第2位相的负异常最为显著。在MJO1～8位相中,对流主体从热带印度洋东移。在第1～3位相,孟加拉湾还未形成西南向水汽输送,而云南又处于水汽辐散区,降水较少;第4位相时对流主体到达90°N附近,部分对流云系向孟加拉湾北传,并在孟加拉湾生成气旋性环流,向云南输送水汽,云南降水增多;第5位相时对流主体传到南海,部分对流云系在南海北传,同时在南海形成北传的气旋性环流;第6位相时赤道MJO对流主体虽然东移出孟加拉湾,但孟加拉湾和南海的两个气旋性环流依然继续北传,孟加拉湾气旋东部的西南风和南海气旋西部的东北风在云南交汇,云南被强烈的水汽辐合区控制,降水最充沛。第7～8位相时,对流主体减弱,东移到南海和西太平洋一带,孟加拉湾转向为偏北风,停止向云南输送水汽,且云南处于水汽辐散区控制,降水偏少。因此,MJO主体在东传过程中,激发了热带对流在孟加拉湾和南海两条通道上的北传,强盛的水汽输送和两个海区气旋环流的有利配置是造成云南5月降水的重要原因。

This paper analyzes the abnormal conditions of large-scale circulations,convection,and water vapor transportation and their effects on Yunnan precipitation at various MJO（Madden and Julian Oscillation） phases in May during 1979-2008.During this period,Yunnan precipitation showed obverse differences at eight MJO phases that were divided by the central location of MJO from west to east.Positive precipitation anomalies occurred at phases 4-6,where the main MJO convective body was situated along the equator from the central Indian Ocean to the western Pacific Ocean.Negative precipitation anomalies appeared at phases 7-8,when the main MJO convective body appeared east of the equator at the central Pacific Ocean,and at phases 1-3,when it was situated over the central and western Indian Ocean.The positive anomalies at phase 6 and the negative anomalies at phase 2 are the most significant.The main convective body moved northward and eastward from the tropical Indian Ocean.At phases 1-3,the southwestward water vapor transport had not yet formed in the Bay of Bengal（BOB）;therefore,the precipitation in Yunnan at various divergent regions decreased.At phase 4,the main convective body moved near 90°E,and a partial convective cloud system began a northward transfer to the BOB.As a result,cyclonic circulation formed and transported water vapor to Yunnan;therefore,its precipitation showed positive anomalies.At phase 5,the main convective body moved to the South China Sea（SCS）,from which a partial convective cloud system began a northward transfer with northward cyclonic circulation.At phase 6,the main convective body of the equatorial MJO had moved eastward from the BOB.However,the two cyclonic circulations in the BOB and SCS continued northward,and the southwestern wind along the eastern cyclone of the BOB and the northeastern wind along the western cyclone of the SCS met and joined in Yunnan.Because they were controlled by a strong water vapor convergent region,and precipitation in this region shows the highest increase.At phases 7-8,the main convective subject was weakened and moved eastward to the SCS and the western Pacific Ocean.The wind in the BOB shifted to the north and no longer transported water vapor to Yunnan,where it was controlled by divergent water vapor to decrease precipitation.Therefore,northward convection propagation along paths in the BOB and SCS during the eastward migration process of MJO created strong water vapor transportation.Their advantageous coordination in the two sea regions is the main reason for Yunnan precipitation in May.