北印度洋季風與季風低壓－(1)自然季節

North Indian Ocean Monsoon and Monsoon Depressions Part Ⅰ: Natural Seasons

吳宜昭(Yi-Chao Wu) ； 林和(Ho Lin)

《大氣科學》 34卷1期 (2006/03) Pp. 47-65

摘要

雖然季節轉變的外力來自平滑、週期性及緯向對稱的季節性日照，季風區的季節轉換卻呈現突兀、如階梯般的跳躍，這樣的演變同時展現在大尺度環流及大氣系統上。相較於以熱帶氣旋生成前數日的合成來代表大尺度的環境，本研究以季節平均環流為背景，詮釋大尺度環流型態及條件對北印度洋季風低壓（monsoon depression，為北印度洋氣旋擾動的通稱）生成的影響。使用1959年至1998年的ERA-40資料氣候平均場及北印度洋（包括孟加拉灣及阿拉伯海）季風低壓觀測，根據環流結構的轉變及季風低壓的生成頻率分佈，北印度洋季風區的年旋環可以劃分成暑（第24至32候）、夏（第33至50候）、秋（第51至60候）及初冬（第61至71候）四個自然季節。本研究考量季風環境中影響季風低壓生成最重要的四個條件，包括海面溫度、200hPa及925hPa間之垂直風切、500hPa的相對溼度及925hPa相對渦度，探討隨著季節的不同，以上諸條件如何調節季風低壓生成在時間及空間上的分佈結果顯示海面溫度、垂直風切及相對渦度為強勢制約條件，對暑、夏兩季的季風低壓生成尤其有決定性的影響。

關鍵字

北印度洋 ； 季風 ； 季風低壓 ； 自然季節

並列摘要

Though forced by smooth, periodic, and meridionally symmetric seasonal insolation, the season transitions in monsoon regions exhibit abrupt, stepwise jumps. Such abrupt changes display in both large-scale and synoptic scale. In contrast to the studies that adopt the composite of circulation fields several days before tropical cyclones form, this study examines the influences that large-scale patterns and conditions have on the formation of monsoon depression (a generic term for all cyclonic disturbances in North Indian Ocean) based on the seasonal mean flow. The climatology derived from 1959-1998 ERA-40 data and monsoon depressions of the North Indian Ocean (including Arabian Sea and Bay of Bengal) in the same 40-year-long period are analyzed. According to the changes in large-scale circulation structure and monsoon depression formation frequency, the annual cycle of North Indian Ocean monsoon region can be divided into four natural seasons, pre-summer (pentad 24-32), summer (pentad 33-50), autumn (pentad 51-60), and early winter (pentad 61-71). Sea surface temperature, vertical wind shear between 200hPa and 925hPa, 500hPa relative humidity, and 925hPa relative vorticity are considered four dominant factors that influence monsoon depression formation in monsoon regions. The current study investigates how these four factors, varying with seasons, regulate the temporal and spatial distribution of monsoon depression formation. Sea surface temperature, vertical wind shear; and relative vorticity are regarded stronger constraints. They exert essential impacts on regulating the monsoon depression formation in pre-summer and summer seasons.

並列關鍵字

North Indian Ocean ； Monsoon ； Monsoon depression ； Natural seasons