南亞夏季季風肇始過程及其與聖嬰現象的關聯
Other Title
Summer Monsoon Onset Process over South Asia and Its Relationship with ENSO
Journal
2024臺大學士論文獎
Date Issued
2024
Author(s)
Tsung-Hsun Lin
Advisor
Abstract
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.
Subjects
Indian summer monsoon onset
intraseasonal oscillation
northward propagation
moist static energy budget
background convective instability
cross-equatorial westerlies
ENSO
Publisher
國立臺灣大學大氣科學系
Description
指導教授：盧孟明
Type
thesis
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