南海夏季风建立期间副高带断裂和东撤及其可能机制

使用1998年南海夏季风试验（SCSMEX）资料和日本气象研究所（MRI）所提供的TBB资料，分析了南海夏季风建立期间副高带断裂和东撤过程的主要特征及其可能机制。发现北半球副热带高压带的断裂（低层早于高层）和印缅槽（或孟加拉湾槽）的形成是南海夏季风建立的重要前期征兆之一，也为副高东撤和南海夏季风的建立提供了重要条件。斯里兰卡附近低涡的持续北移是副高断裂和印缅槽建立过程的显特征。分析表明，南海夏季风建立之前，印度半岛的感热加热和中南半岛的潜热加热所激发的气旋性流场在孟加拉湾地区是相互迭加的，这有利于孟加拉湾低涡活动和低槽的形成，这可能是副高断裂和印缅槽活跃的机制。伴随着印缅前西南气流和赤道副高东撤、季风加强和对流加热之间存在一种正反馈作用，这导致了副高的连续东撤和南海夏季风的“爆发性”以及各种要素的突变特征。当南海夏季风对流减弱后，西太平洋副高则会西伸。

Splitting and Eastward Withdrawal of the Subtropical High Belt during the Onset of the South China Sea Summer Monsoon and Their Possible Mechanism

In terms of the 1998 South China Sea Monsoon Experiment (SCSMEX) data and the T BB data provided by the Meteorological Research Institute (MRI)of Japan, main features and the possible mechanism of the splitting and withdrawal processes of the subtropical high belt during the onset of the 1998 South China Sea summer monsoon are analyzed in this paper. Major results are as follows: (1) It is found that the splitting (earlier at lower levels than at upper levels) of the subtropical high belt in the Northern Hemisphere and the establishment of India_Burma trough are one of the important precursors for the establishment of the South China sea summer monsoon, and they also provide favorable conditions for the eastward withdrawal of the West Pacific subtropical high and the onset of the South China Sea summer monsoon. The continuous northward movement of the vortex near Sri Lanka is a distinctive feature of the processes of the splitting of the subtropical high and the establishment of the India_Burma trough. The analysis suggests that the cyclonic circulations, which are induced by the sensible heating over Indian peninsula and the latent heating over the Indo_China peninsula before the establishment of the South China Sea summer monsoon, overlap each other in the Bengal bay area. This is responsible for the splitting of subtropical high and the activeness of the India_Burma trough. (2) The splitting and eastward withdrawal of the subtropical high belt and the establishment of the India_Burma trough create a necessary condition for the encounter of warm and wet flow and cold air on the western or northwest edge of the West Pacific subtropical high, and the cyclonic vorticity, which is resulted from the convective precipitation heating brought about by the interaction of the lower and middle latitude systems, conversely promotes the eastward withdrawal of the subtropical high. When the Australian south wind disturbances which propagate northwards arrive at the South China Sea, the South China Sea summer monsoon bursts over the whole SCS area. (3) During the onset period of the South China Sea summer monsoon, there is a positive feedback among the eastward withdrawal of the subtropical high, the strengthening of the summer monsoon and the heating of convective precipitation, which leads to the explosive feature of the onset of the South China Sea summer monsoon and the sudden changes of various meteorological elements.