北印度洋熱帶氣旋特性與大尺度環流之關係;Tropical Cyclone Characteristics in the Northern Indian Ocean and Their Relationship with Large-Scale Circulation

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题名:	北印度洋熱帶氣旋特性與大尺度環流之關係;Tropical Cyclone Characteristics in the Northern Indian Ocean and Their Relationship with Large-Scale Circulation
作者:	陳柏佑;Chen, Po-Yu
贡献者:	大氣科學學系
关键词:	北印度洋;熱帶氣旋;亞洲季風;年循環;Northern Indian Ocean;Tropical Cyclone;Asian Monsoon;Annual Variation
日期:	2025-07-17
上传时间:	2025-10-17 11:04:15 (UTC+8)
出版者:	國立中央大學
摘要:	北印度洋熱帶氣旋的活躍程度在一年中呈現雙峰特性。這兩個高峰分別發生在亞洲季風盛行前與盛行後，因此被稱為 pre-monsoon season 與 post-monsoon season。由此可知，北印度洋熱帶氣旋活動的年循環與亞洲季風的發展密切相關。春季時，太陽由南向北加熱北半球，開啟了北印度洋的 pre-monsoon season，直到7、8月，亞洲季風的強勁西南風在北印度洋面產生巨大的垂直風切，導致北印度洋熱帶氣旋進入間歇期，pre-monsoon season 結束。由於「中南半島-馬來半島」是往東南方深入海洋大陸的陸橋，在 pre-monsoon season 中，太陽輻射加熱源沿著陸橋由東南向西北傳遞，讓北印度洋熱帶氣旋的生成熱區由東南向西北移動。由於孟加拉灣在阿拉伯海東側，這讓孟加拉灣比起阿拉伯海較早進入、也較早結束 pre-monsoon season，阿拉伯海的 pre-monsoon season 則是比孟加拉灣較晚開始、較晚結束。9月過後，亞洲季風帶來的強烈垂直風切逐漸減弱，北印度洋的熱帶氣旋又開始活躍，post-monsoon season 開始。由於太陽輻射加熱源同樣沿著中南半島-馬來半島陸橋由西北往東南撤退，這導致位在西側的阿拉伯海 post-monsoon season 較早開始也較早結束，位在東側的孟加拉灣則較晚開始也較晚結束 post-monsoon season。透過輻散環流場的分析，發現在 pre-monsoon season 中，低層的氣候平均輻合中心一路由海洋大陸往北移至青藏高原東南側，帶動「南北向的季風槽」隨著亞洲季風往北推進，同時讓熱帶氣旋生成位置逐漸往北移，直到南北向季風槽與垂直風切小於 10 m/s 的區域進入南亞陸地後，pre-monsoon season 結束；在 post-monsoon season 中，則是「東西向的ITCZ-季風槽」為有利熱帶氣旋生成的環境。在低層的氣候平均輻合中心從青藏高原東南側往南移回海洋大陸的過程中，帶動亞洲季風與垂直風切小於 10 m/s 的區域往南撤退進入北印度洋洋面上，讓北印度洋9月至11月的熱帶氣旋逐漸增加，進入 post-monsoon season 的高峰。12月以後，太陽直射南半球使得北半球熱量逐漸減少，post-monsoon season 宣告結束。此外，藉由渦度收支方程式得以發現，在熱帶氣旋生成時，相對渦度的水平平流項對熱帶氣旋為負貢獻，絕對渦度的渦旋伸展項對熱帶氣旋則為正貢獻，且渦旋伸展項的量值略大於水平平流項，讓熱帶氣旋得以順利生成。;Tropical cyclone activities in the northern Indian Ocean exhibit double-peak characteristics. These two peaks occur before and after the Asian monsoon season and are referred to as the "pre-monsoon season" and "post-monsoon season", respectively. The annual variability in tropical cyclone activity in the northern Indian Ocean is closely related to the development of the Asian monsoon. In spring, the overhead solar heating gradually shifts from the Southern Hemisphere to the Northern Hemisphere, initiating the pre-monsoon season in the northern Indian Ocean. The "Indochina–Malay Peninsula" acts as a land bridge extending southeastward into the Maritime Continent so that the heat source from solar radiation propagates northwestward along this land bridge, driving the hot zone of tropical cyclone genesis to migrate northwestward accordingly. By July and August, the intensified southwesterly associated with the Asian monsoon system induced strong vertical wind shear over the northern Indian Ocean, causing a break in tropical cyclone activity and marking the end of the pre-monsoon season. Since the Bay of Bengal lies east of the Arabian Sea, it enters and exits the pre-monsoon season earlier than the Arabian Sea, starting and ending later. Beginning in September, the vertical wind shear caused by the Asian monsoon system gradually weakens, allowing tropical cyclones to become active again and signaling the onset of the post-monsoon season. The solar heating source retreats southeastward along the Indochina–Malay Peninsula land bridge, causing the post-monsoon season in the Arabian Sea (located to the west) to start and end earlier, whereas the Bay of Bengal (to the east) begins and ceases later. Analyzing the divergent circulation reveals that during the pre-monsoon season, the low-level climatological convergent center moves northward from the Maritime Continent to the southeastern slopes of the Tibetan Plateau. This migration drove the "north–south-oriented monsoon trough" to progress northward following Asian monsoon development, gradually guiding the genesis location of tropical cyclones further northward. The pre-monsoon season ends when the north–south-oriented monsoon trough and regions of vertical wind shear below 10 m/s move into the South Asian landmass. During the post-monsoon season, the favorable environment for tropical cyclone formation is characterized by an east–west-oriented ITCZ–monsoon trough couplet. As the low-level climatological convergent center retreats southward from the southeastern slopes of the Tibetan Plateau back to the Maritime Continent, the Asian monsoon system and regions with vertical wind shear rates below 10 m/s also withdraw southward from the South Asian landmass to the ocean. This southward retreat led to an increase in tropical cyclone activity from September to November, peaking during the post-monsoon season. By December, as the Sun shifts to the Southern Hemisphere and heating in the Northern Hemisphere diminishes, the post-monsoon season ends. Furthermore, analysis via the vorticity budget equation reveals that during tropical cyclone genesis, the horizontal advection term of relative vorticity makes a negative contribution, whereas the divergent term of absolute vorticity (vortex stretching) makes a positive contribution. The magnitude of the vortex stretching term slightly exceeds that of the horizontal advection term, enabling the successful genesis of tropical cyclones.
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