澳大利亚冷空气活动对西北太平洋热带气旋生成的影响

利用1980—2012年NCEP/NCAR再分析资料及中国气象局的最佳台风路径资料,研究澳大利亚冷空气活动对西北太平洋热带气旋生成的影响。研究发现,北半球夏季925 h Pa经向风超过6 m/s的频数在澳大利亚东北部海域最高,达40 d/a。为此,确定澳大利亚冷空气侵入南北半球低纬的关键区为澳大利亚东北部所罗门海地区,并用该区域经向风风速定义了一个澳大利亚冷空气活动强度指数。该指数与越赤道气流及赤道西风都有很好的相关关系,还与同期的SOI(Southern Oscillation Index,南方涛动指数)显著相关。当SOI偏低(高)时,关键区经向风风速偏强(弱)。合成分析和相关分析结果表明,澳大利亚冷空气活动强、弱年西北太平洋热带气旋生成的位置的变化与季风槽的变化一致,西北太平洋热带气旋生成总数则无显著差异。澳大利亚冷空气活动强年季风槽偏强偏东,热带气旋生成位置偏东偏南;而弱年季风槽偏弱偏西,热带气旋生成位置偏西偏北。低层涡度场、水汽输送、风垂直切变以及低纬地区对流活动的分布表明,澳大利亚冷空气活动强年有利于热带气旋生成位置偏东、偏南;弱年偏西、偏北。

Influence of the cold air activity over Australia on the tropical cyclone genesis over the western North Pacific

In the present study,based on the NCEP/NCAR reanalyses dataset and best track data of the typhoons provided by China Meteorological Administration(CMA),the influence of the Australian cold air activity on the genesis of tropical cyclones(TCs) over the western North Pacific was investigated.It was found that,off the northeastern coast of Australia,the occurrence frequency of the meridional wind larger than 6 m/s was the largest(50 days annually).Therefore,the Solomon Sea was determined as the key area of influence of the Australian cold air activity on the low latitudes in both hemispheres,and a cold air intensity index over Australia was defined by the low level meridional wind over the Solomon Sea(SSV).The SSV was shown to be closely related to the cross-equatorial flows and westerly over the Equator,as well as the SOI.As the SOI becomes weaker(stronger),the SSV becomes stronger(weaker).The location of the tropical cyclogenesis(TCG) over the western North Pacific coincides with the variation of the monsoon trough during the years of strong and weak cold air activities over Australia.However,the total TC number has no significant correlation with the SSV.The monsoon trough over the western North Pacific is stronger and located more eastward than normal,and the location of the TCG is more eastward and southward than normal during years of strong cold air activity.Meanwhile,the monsoon trough is weaker and located more westward than normal,and the location of the TCG is more westward and northward than normal during years of weak cold air activity.The distribution of vorticity in the lower troposphere,the moisture transport,the vertical shear of horizontal wind and the convections in low latitudes all indicate that the locations of the TCs over the western North Pacific are more eastward and southward(westward and northward) than normal during strong(weak) years of cold air activities over Australia.