盧孟明Tsung-Hsun Lin林宗勳2024-09-262024-09-262024https://ntu.primo.exlibrisgroup.com/permalink/886NTU_INST/14poklj/alma991039261579104786https://scholars.lib.ntu.edu.tw/handle/123456789/721565指導教授：盧孟明Monsoons over South Asia influence the climate at both regional and global scales, with monsoon onset marking a seasonal transition to summer-type rainfall and a moist atmosphere. However, the physical processes of the monsoon onset over India and the Arabian Sea (AS) are not fully understood, especially processes induced by intraseasonal timescale perturbations during the onset stage and their interaction with the seasonal background environment. In this study, a northward propagating convective phase of intraseasonal oscillations (ISO), associated with a cyclonic circulation, is identified as a crucial factor in initiating the monsoon onset. The northward propagation is sustained by a positive moist static energy (MSE) tendency ahead of the convective center. Results from the MSE budget diagnosis indicate that the MSE tendency dipole can be attributed to moisture advection. With a wetter (dryer) background environment over southeastern (northwestern) AS, the intraseasonal cyclonic circulation can enhance (reduce) the MSE to its north (south). In addition, the northward propagation of the ISO that triggers the monsoon onset is controlled by a meridional asymmetry of background convective instability (BCI). During the pre-onset stage, an accumulation of background low-level moisture over northern AS, which is transported by the cross-equatorial southwesterlies, markedly enhances local BCI. A relatively unstable background environment over AS, compared to the equatorial western Indian Ocean (WIO), may destabilize Rossby waves and facilitate the northward propagation of ISOs. Different ENSO events can influence the monsoon onset through their distinct effect on the BCI as well.The influence is exerted through both oceanic and atmospheric pathways. During post-La Niña Springs, the meridional sea surface temperature (SST) gradient over WIO is enhanced. Furthermore, anomalous westerlies over BOB, induced by the strengthened Walker Cell, stimulate convection locally and support an earlier westward extension of the South Asian High (SAH) to the AS. Both the enhanced SST gradient and coverage of SAH lead to stronger cross-equatorial low-level southwesterlies over the AS, which transport more moisture to the north. This increases the meridional asymmetry of BCI and advances the arrival of monsoon onset ISO. Conversely, during post-El Niño Springs, these anomalies are roughly opposite, resulting in delayed monsoon onsets.南亞夏季季風對於區域及全球氣候皆有不可忽略的影響，且南亞夏季季風的肇始也象徵該區整體環境由春季轉為夏季多雨潮濕的氣候。然而，目前我們對於印度、阿拉伯海區域季風肇始的重要物理過程了解仍然有限，尤其是季內尺度的變化以及其與背景環境的互動。本研究使用時間濾波方法將再分析資料根據時間尺度拆解，藉以探討1980 至2021 年間季內尺度過程在季風肇始中扮演的重要角色。首先，本研究發現此區域的季風肇始是由一個在西印度洋生成後向北移動、並有一氣旋式環流環繞的季內尺度對流所觸發。對其進行濕靜能收支分析後可以知道在其移動過程中主要透過水氣平流來增加（減少）其北側（南側）大氣中的能量，因此對流得以持續北移。由於阿拉伯海背景環境有東南方濕、西北方乾的水氣分佈特徵，因此該季內對流的氣旋式環流可以在其北側（南側）透過東風（西風）帶來正（負）的水氣平流效果，進而增加（減少）該處大氣中的能量。此外，本研究也發現決定此季內對流是否北移的關鍵因素在於背景對流不穩定性在南北方向上的不對稱。在季風肇始前，隨著背景的跨赤道西南風逐漸北移進入阿拉伯海，低層會有越來越多的水氣被送往北方並累積在阿拉伯海北部。低層大量水氣的存在會大幅增加該處的對流不穩定性，而當位於北方的阿拉伯海環境愈趨不穩定，熱帶的季內震盪便容易從南方相對穩定的赤道西印度洋海域北移進入此區。此對流不穩定性的建立在年際間的差異也有助於了解聖嬰／反聖嬰事件帶來的環境變化如何影響南亞夏季季風的肇始時間。以反聖嬰事件為例，其對於印度洋海溫的影響會增加西印度洋的經向海溫梯度並加強該處的跨赤道流。而增強的沃克環流也會使孟加拉灣地區渦度增加、旺盛的對流更早開始發展並支持南亞高壓擴張、西伸至伊朗高原、阿拉伯海一帶，在高層有更多輻散下使得低層流入的西南風也增強。此二者都會使得背景的西南風更早將大量水氣帶入阿拉伯海、增加背景對流不穩定性，最後使能夠引發季風肇始的季內對流提早抵達。而聖嬰事件帶來的變化則大致相反，因而會延後季風肇始。本研究共有三個主要結論：一、從能量觀點來看，觸發季風肇始的季內尺度對流是由水氣平流來維持其北移；二、阿拉伯海區域的對流不穩定度需足夠高，造成一個北側不穩定、南側穩定的環境條件，生成於西印度洋的季內對流才會進入此區；三、聖嬰／反聖嬰事件對南亞所帶來的環境變化可以影響第二點中不穩定度的建立時間點，進而影響到季風肇始時間。未來若要對此主題進一步研究，需多加考慮聖嬰／反聖嬰事件對赤道上季內震盪生成的影響，另外也可以對本研究中提到的過程進行更多定量分析以確認其在季風肇始過程中實際的重要程度。Indian summer monsoon onsetintraseasonal oscillation印度季風肇始northward propagation季內震盪北傳moist static energy budget濕靜能收支background convective instability背景對流不穩定性cross-equatorial westerlies跨赤道西風ENSO聖嬰現象thesis