1998年夏季季風肇始期間南海地區之熱力與水氣分析

Thermodynamic and Moisture Analyeses over the South China Sea during the Summer Monsoon Onset in 1998

林松錦(Song-Chin Lin) ； 施佩君(Pei-Chun Shih) ； 郭勉之(Mien-Tze Kueh)

《大氣科學》 31卷4期 (2003/12) Pp. 395-411

摘要

本研究利用1998年南海季風實驗所提供的1°×1°經緯度網格資料，將1998年夏季季風的肇始過程分成三個階段，經由熱力與水氣變數場的分析，來了解南海地區夏季季風肇始的過程，以及季風肇始發展期間南海南北部的環流特徵與差異。
分析結果顯示，在1998年南海夏季季風肇始發展過程中，南海北部的大氣大多呈現對流不穩定的形勢，第一階段鋒前西南氣流造成低層強烈的水氣輻合，第二階段之後鋒面遠離且太平洋副熱帶高壓東退，來自孟加拉灣的西南氣流夾帶大量暖濕水氣使得南海北部在整個季風肇始期間都是有利於對流的狀況。而在南海南部，不同的是在第一與第二個階段，因水氣量的不足，造成夏季季風已在南海北部盛行時，南海南部地區卻沒有較明顯的變化。一直到第三個階段，太平洋副熱帶高壓完全退出南海，而華南低壓中心移入南海北部的低壓帶，與強盛的熱帶西南氣流結合，在南海南部形成強烈的西風與大量的水氣集結，低層對流不穩定度快速增加，以致於產生旺盛的對流。顯示南海南北部在肇始期間最主要的差異即在於水氣的多寡與到達的時間。

關鍵字

夏季季風肇始 ； 南海季風實驗 ； 熱力與水氣分析

並列摘要

By dividing the whole evolution of the 1998 summer monsoon onset process into stages and using 1°*1°grid observations provided by the SCSMEX (South China Sea Monsoon Experiment) Data Center, this study performs thermodynamic and moisture analyses to investigate the differences in the circulation features over the northern and the southern South China Sea (SCS).
Over the northern SCS, analysis results find the lower troposphere is convectively unstable during the entire onset period. The favorable atmospheric conditions for convection are attributed to low-level moisture convergence caused by the frontal southwesterly flow during the first stage, and the retreat of western Pacific subtropical high and warm-moist southwesterly flow from the Bay of Bengal during the later stage. While summer monsoon are prevailing over the northern SCS during the early two stages, the lower tropospheric conditions are still not favorable for the development of convection over the southern SCS due to lack of moisture. Until the third stage, when subtropical high retreats completely from the SCS and the low pressure system over southern China moves southeastward and embeds itself in the low pressure zone over the northern SCS, strong tropical southwesterly flow brings in ample amount of moisture and causes moisture convergence over the southern SCS under the influences of the low pressure zone. Deep convections bloom due to sudden increase of low-level convective instability. The major factors that influence the onset process over the northern and the southern SCS seem to be the amount of moisture and the time of arrival of moisture.

並列關鍵字

Summer Monsoon Onset ； South China Sea Monsoon Experiment ； Thermodynamic and Moisture Analyses