冬季西北太平洋热带气旋的生成与热带东风扰动的关系

Relationship between the formation of tropical cyclone and tropical easterly disturbances in the western North Pacific in boreal winter

摘要:

利用合成技术对1995—2006年冬季(11月—次年2月)生成在西北太平洋上的34个热带气旋(tropicalcyclone,TC)个例进行分析,研究冬季西北太平洋TC生成的大尺度环流特征及其生成机制,结果表明:冬季TC生成的大尺度环流特征型为东风波西传型;北半球冬季对流层低层出现的跨赤道气旋对是冬季北半球TC形成的重要特征;太平洋中部赤道混合Rossby重力波西北传,与强对流中心重合,性质转为"热带低压型扰动",为冬季热带气旋生成提供扰动源。对合成TC初始场的涡动扰动动能的收支分析表明,涡动有效位能和正压不稳定转换为TC形成提供了能量,这两种能量分别与积云对流加热和水平不均匀气流有关。正压不稳定能量转换为动能主要位于对流层中下层,而扰动有效位能的转换主要位于对流层中上层。低层热带东风波动从平均气流中获得正压不稳定能量,并与强积云对流耦合,热力和动力共同作用下形成TC。

关键词:热带气旋;东风波;赤道混合Rossby重力波;大尺度环流;冬季

Abstract:

Thirty four cases of topical cyclogenesis that occurred in boreal winter(November to February of next year) during 12-yr(1995—2006) have been analyzed by using the composite technique for further understanding of the larger-scale circulation features and the mechanism of the formation of tropical cyclone in the western north Pacific(WNP)during wintertime.It has been found that the easterly waves play a major role in the tropical cyclogenesis.The existence of low-level cross-equatorial tropical cyclone pairs is a remarkable feature in the formation of tropical cyclogenesis.The equatorial mixed Rossby-gravity(MRG) waves in lower troposphere propagated northwestward from the central Pacific.These waves developed into the TD-type disturbances in place where the strong convection overlaps,which provided disturbances for the tropical cyclogenesis.Further analysis in terms of eddy kinetic energy budgets for the TC(tropical cyclone) related with circulations indicates that the available potential energy of disturbances and the energy that converts barotropically from the mean kinetic energy facilitates the TC formation.These two kinds of energy are associated with cumulus convection heating and the asymmetric horizontal flow respectively.The barotropic energy conversion mainly occurs in the low and mid-troposphere while the available energy conversion locates in the mid and upper troposphere.The tropic easterly waves get barotropic instability energy from the mean flow and coupled with the deep convection,inducing the formation of TCs under the interaction of dynamics and thermodynamics.

Key words:tropical cyclone;easterly wave;equatorial mixed Rossby gravity wave;large-scale circulation;wintertime