Study On Formation Causes Of Strong And Weak Cross-equatorial Flow To The North Of Australia

Using the NCEP/NCAR reanalysis data during 1980-2004,this paper firstly samples the obvious reinforcement and decline processes of cross-equatorial flow to the north of Australia,then the southern and northern circulation patterns are summarized from the samples so as to discuss the weather causes of the cross-equatorial flow's reinforcement and decline.Secondly,the strong and weak extremums of cross-equatorial flow during May to August are chose as typical samples,then their relevant configure features of circulation system,in southern and northern hemisphere and equatorial regions,are analyzed so as to discuss the climate causes of strong and weak cross-equatorial flow.Thirdly,some forcing factors are defined and its impacts on circulation factors and cross-equatorial flow are studied with diagnosis analysis and numerical simulation.Main conclusions are shown as follows:1.Corresponding to the obvious reinforcement process of cross-equatorial flow, there is always circulation adjustment in the tropical and subtropical zone to the south or north of the channel,and the adjustment mainly denotes the Australia cold air activity in Southern Hemisphere or the variation of convergence zone in Northern Hemisphere.The circulation feature in southern hemisphere can be divided into four types according to the configuration of cold air trough and Australia High,and the circulation feature in northern hemisphere can be divided into three types according to the relative position of convergence zone and Western Pacific Subtropical High(WPSH).2.Corresponding to the reinforcement process of cross-equatorial flow,there is always equatorial ridge or equatorial anticyclone,and its circulation background is the counterwork process between WPSH and convergence zone.3.The strong cross-equatorial flow mainly shows high-standard-maintenance feature in the month,which is always caused by the strong convergence zone in northern hemisphere,or strong Australia High in southern hemisphere,or frequent Australian cold air activities,and it always corresponds to the circulation situation that the location of the WPSH is to the east or north as compared with the normal.4.The weak cross-equatorial flow mainly shows low-standard-maintenance or large-amplitude-oscillation feature.The former always corresponds to the circulation situation that the location of the WPSH is to the west or south as compared with the normal,and there is always weak convergence zone to the north of channel.The latter is always caused by the frequent counterwork processes between WPSH and convergence zone,and there is always strong convergence zone to the north of channel.5.The cold Australia land always corresponds to strong Australian High or frequent cold air activities,which is favorable to the formation of strong cross-equatorial flow;The cold underlaying surface to the south of channel always corresponds to weak Walker circulation,which is also favorable to the formation of strong cross-equatorial flow;The warm Indo-China peninsula and cold South China Sea,as well as the strong heat source near the Philippines,all can correspond to strong convergence zone in north hemisphere,which is favorable to the formation of strong cross-equatorial flow.The features of forcing factors and circulation factors corresponding to weak cross-equatorial flow are opposite to the above features.This paper mainly includes the following innovations:(1)This paper samples the obvious reinforcement and decline processes of 105°E and 125°E cross-equatorial flow,and summarizes their relevant circulation patterns in southern and northern hemisphere.Corresponding to the reinforcement process,the circulation feature can be divided into four types in southern hemisphere and three types in northern hemisphere.The obvious decline process always corresponds to the counterwork process between WPSH and convergence zone.(2)This paper analyzes the formation causes of strong and weak cross-equatorial flow from the view of circulation configuration.Strong cross-equatorial flow is always caused by the strong convergence zone in northern hemisphere,or strong Australia High in southern hemisphere,or frequent Australian cold air activities,and it always corresponds to the circulation situation that the location of the WPSH is to the east or north as compared with the normal.Weak cross-equatorial flow is always caused by the circulation situation that the location of the WPSH is to the west or south as compared with the normal,or by the frequent counterwork processes between WPSH and convergence zone.(3)Using diagnostic analysis and numerical experimentation,this paper studies the impacts of some forcing factors on local circulation and cross-equatorial flow. The cold Australia land is favorable to the formation of strong Australian High or frequent cold air activities,and the weak heating of underlaying surface to the south of channel is favorable to the formation of weak Walker circulation,and the strong heat difference between South China Sea and Indo-China peninsula or the strong convection heating near the Philippines is favorable to the formation of strong convergence zone to the north of chancel,which all can correspond to the strong cross-equatorial flow to the north of Australia.