夏季印度洋海盆模与MC区域降水异常联系的进一步分析

摘要

利用英国哈德莱中心的逐月海表温度资料及NCEP/NCAR月平均再分析资料等,通过在印度洋海盆模IOBM指数(IIOB)中扣除长期趋势和两类ENSO的同期信号后,得到了修正的IOBM指数(ImIOB),并由此分析了IOBM的变化及与海洋性大陆区域降水异常的联系.结果表明:印度洋IOBM为暖位相时,不同季节的印度洋地区均呈现异常偏暖,但大气是上升还是下沉运动则在印度洋不同季节和不同区域存在很大变化.就夏季而言,印度洋大部分地区存在上升运动,这与海温异常偏暖有关.在北半球夏季,指数ImIOB存在3～5a的周期变化.当IOBM处于正位相时,印度洋至我国东海地区大范围海温偏暖.MC(Maritime Continent,海洋性大陆)区域西部降水正异常,而MC区域东北部降水为负异常.造成这种降水分布的原因是:当指数为正时,在MC区域的西部对流层低层辐合、高层辐散,上升运动增强,且水汽辐合,而MC区域的东北部对流层低层辐散、高层辐合,上升运动不明显,水汽辐散,不易形成降水.而在对流层低层与西太平洋辐散中心对应,南北半球出现关于赤道对称的反气旋对,赤道印度洋上的异常加热激发东传的Kelvin波,加强东风异常,同时加强了KMC(海洋性大陆的核心区域)之外南北半球热带地区的这对Rossby波型.以上这些结果有利于深刻理解MC降水异常成因及热带海陆气相互作用过程.%By using the monthly mean Sea Surface temperature(SST) data from Hadley Centre and the NCEP/NCAR monthly reanalysis,and removing out the long term trend and simultaneous influences of two kinds of ENSO from the Indian Ocean Basin-wide Mode(IOBM) index IIOB,the modified IOBM index as ImIOB was defined and the variations of IOBM and its associated precipitation anomalies over the Maritime Continent(MC) were analyzed.Results demonstrate that,when IOBM in its warm phase,Indian O cean region undergoes an anomalous warm state in different seasons.The vertical motion of atmosphere over Indian Ocean changes large from a season to another.During boreal summer,the atmospheric updraft in most areas of Indian Ocean is possibly induced by the warmer sea temperature anomalies.It is found that the IOBM index ImIOB varies with periods of 3-5 years.When it is in its positive phase,SST in regions from the Indian Ocean to East China Sea is anomalously warmer.Precipitation in the western part of MC region is significantly more than the normal while it is less than the normal in the northeastern part of MC region.The anomalous convergence in the lower troposphere and anomalous divergence in the upper troposphere lead to both the strengthening upward motion and water vapor convergence there,inducing more than normal rainfall in the western part of MC.In the northeastern part of MC region,the anomalous divergence in the lower troposphere and anomalous convergence in the upper troposphere is observed,which do not facilitate occurrences of more precipitation there.In the lower troposphere,the divergent flows emanating from eastern part of MC and West Pacific induce a pair of anomalous anticyclones to merge in northern and southern tropics,which look almost symmetrical about the equator.The abnormal diabatic heating over equatorial Indian Ocean excites the eastward propagating Kelvin wave,strengthening easterly anomaly along equator.This strengthened easterly facilitates the intensification of Rossby waves in the north and south tropical regions outside the MC region.These results are helpful for our better understanding the formation of MC precipitation anomaly and tropical sea-land-atmosphere interactions.