孟加拉灣雨季肇始-亞洲夏季季風的開端

Abstract

摘 要 亞洲夏季季風最早的雨季出現於孟加拉灣,該區春末約五月初有突發性強降雨由赤道北傳,此為全年第一次季內震盪（Intra-Seasonal Oscillation,，簡寫ISO）北移,在動力上深具意義。經由此事件即孟灣肇始（Bay of Bengal Onset, BOB Onset）區分出春、夏季環流系統的重大特徵，並凸顯ISO是造成BOB Onset的重要因子。本文研究季內震盪尺度下BOB Onset前後變化，進而瞭解影響ISO北移的機制。 當三、四月日照北移，印度半島開始迅速增溫加上地形影響形成低層大氣的熱槽，進而使印度半島東、西側低層風場呈現相異的南風及北風。同時，熱帶印度洋上大氣低層水氣含量受制於SST東暖西涼影響而具有西部乾而東潮濕的情況。 結果顯示BOB Onset大致可分為三大階段： （1）貯能階段(Charge Period)：此階段溼相位ISO沿赤道東行,造成沿線北印度洋向赤道對流輻合的底層風場，恰巧與印度熱槽前之孟灣風場抵消,同時對流層中上層的沉降氣流使短波輻射增加，因之孟灣潛熱通量減少，短波幅射通量增加，約有三週之久，其海溫（Surface Sea Temperature, SST）持續升高、底層水氣含量增加的現象都較阿拉伯海明顯。 （2）激發階段(Trigger Period)：熱帶印度洋水氣東西不對稱背景場造成濕ISO自西印度洋向東移動至東印度洋時對流加劇，經過印度半島南緣濕ISO成長到最強進而激發出對稱於赤道南北的(N=1)羅士培波，此時孟灣北部仍為沈降區因此更強化孟灣SST南涼北熱之梯度。 （3）釋能階段(Discharge Period)：東印度洋SST及水氣的南北梯度驅使北支的羅士培波往東北方移動。南支的羅士培波受東南印度洋強風速，海溫偏低等不利條件而迅速衰退，北支羅士培波北移後開始消耗孟灣SST並且北側的水汽通量也逐漸減少，隨後對流的維持轉為其所帶來的西南氣流提供底層水氣輻合。當赤道ISO東移底層濕輻合超前深對流的重大特徵在ISO北移時消失。亞洲夏季季風環流系統中的南亞高壓在此階段迅速增強並且沿著中南半島向西北移動，孟灣北部則形成印緬槽讓此階段後期對流滯留於孟灣東北部機制除了地形影響外，尚有槽前有利對流發展的條件。 此ISO的北移機制，包含季節變化及季內尺度的交互作用，不僅造成BOB Onset也有助後續亞洲夏季季風環流系統之形成與發展。

ABSTRACT The earliest onset of rainy season for the Asian summer monsoon takes place in the Bay of Bengal (BOB) in early May. The BOB onset results from a rapid northeastward propagation of convection from the equatorial Indian Ocean to the northeastern rim of the Bay of Bengal, which also marks the first northward movement of ISO in a calendar year. The BOB onset is the event to change circulations of spring and summer in a global scale. The pre-condition of the BoB onset is set up as the following; in northern hemisphere sunlight increases rapidly after spring (March) equinox that warms the surface temperature of Indian peninsula in a fast pace due to the low albedo in peninsula, causing a hot peninsula effect. A hot trough near surface is formed, leading to meridional winds of opposite directions in the eastern Arabian Sea and western BOB. A composed study shows the evolution of BOB onset can be classified into three stages: (1) Charge stage: The wet phase ISO in western Indian Ocean moving eastward along the equator, thus draws winds from subtropical area; that is, makes the north wind stronger in the Arabian Sea and the south wind weaker in the BOB, also, the tropical ISO causes subsidence over the BOB, the short wave flux near surface increases.. Thus both effects increase SST in the interior of the BOB lasting about 3 weeks, the maximum SST at 13N indicates a reverse north-south temperature gradient. (2) Trigger stage: When eastward wet phase ISO reaches Sumatra, the very abundant supply of moisture and potential instability triggers an explosion of convection that excites an n=1 Rossby wave. Only the north component thrives under the favorable basic state, which starts the BOB Onset. (3)Discharge stage: The Rossby wave moves north by consuming warm SST on its northern flank. Concurrently within the BoB a deep trough, generally referred as the India-Burma trough, has been formed. The trough supplies further warm and moist tropical air to sustain convection around 15N. At the same time the 200-hPa South Asian High develops at great speed that sets up summer monsoon circulation for the next three months. A string of events occur including the Indo-China Peninsula onset, South China Sea onset, the formation of pre-monsoon front in Taiwan and Okinawa on the east of the BoB, the shaping of Somalia jet, the invasion of convection in the Arabian Sea, that eventually culminates to the grand event of the Indian onset around 6th of June.