從渦度收支探討南海夏季季風肇始期間之大氣環流演變

Evolution of the Atmospheric Circulation during the South China Sea Summer Monsoon Onset as Depicted by the Vorticity Budget Analysis

郭勉之(Mien-Tze Kueh) ； 蕭美菁(Mei-Ching Hsiao) ； 林松錦(Song-Chin Lin)

《大氣科學》 29卷1期 (2001/03) Pp. 1-20

摘要

本文選取1985～1993年的觀測資料來做南海地區渦度收支的合成分析，以探討南海夏季季風肇始期間的環流演變特徵。南海地區的低層大氣環流在季風肇始期間有結構性的演變。西太平洋副熱帶高壓脊於肇始前3天退至菲律賓以東，南海地區開始轉爲西南氣流所籠罩。肇始前2天至肇始當天，南海地區的季風槽逐漸建立。
由南海地區區域平均的相對渦度及渦度收支的垂直結構演變，可將季風肇始期間分成三個階段：（一）肇始前穩定期，在這段期間整個對流層都是負渦度，渦度收支方程中的各項相互抵銷，環流形勢變化不大。（二）肇始前轉換期，此期間低對流層大氣開始轉爲正渦度，幷向上發展。低對流層大氣正渦度趨勢增加，主要是因爲垂直平流項、渦度之輻散項及收支剩餘項的明顯增强。此外，整個對流層內的渦度之輻散項轉爲大於水平平流項的情況。（三）肇始後成熟期，包含肇始當天，各項強度較肇始前轉換期爲強，並處於相互抗衡的狀態，因此渦度趨勢微弱。此時，南海地區的氣旋式環流場已經發展成熟。最後，由850hPa面上渦度收支的水平分布可知，南海地區低層大氣相對渦度正趨勢，主要的貢獻來自熱帶系統。

關鍵字

南海夏季季風 ； 肇始 ； 渦度收支

並列摘要

In order to study the evolution characteristics of the atmospheric circulation during the South China Sea summer monsoon onset, this study conducted a composite vorticity budget analysis by the use of 1985-1993 observational data. The lower tropospheric circulation in the South China Sea area changes significantly during the monsoon onset period. Three days prior to the monsoon onset, the western Pacific high-pressure ridge retreats to the east of Philippine and the southwesterly prevails in the South China Sea area. The monsoon trough starts to deepen about two days prior to the onset.
Based on the vertical structure changes of relative vorticity and vorticity budgets averaged over the South China Sea area, the monsoon onset period can be divided into three stages:
1) The steady stage prior to the onset, anti-cyclonic vorticity in the entire troposphere and nearly zero vorticity change due to cancellation of forcing budget terms in the vorticity equation.
2) The transition stage prior to the onset, cyclonic vorticity increases in the lower troposphere and extends upward mainly attributed to the vertical advection and horizontal divergence effects of vorticity. The residual term also contributes positively to the cyclonic vorticity increase.
3) The mature stage after the onset, though all budget terms increase in magnitude, the vorticity tendency is very small due to cancellation among the budget terms. The atmospheric circulation pattern persists during this stage.
Finally, the 850hPa vorticity budget analyses reveal that the major contribution to the positive vorticity tendency comes from the tropics.

並列關鍵字

South China Sea summer monsoon ； Onset ； Vorticity budget