台灣乾濕梅雨期之平均環流特徵

Characteristics of the Mean Circulation Patterns for the Wet and Dry Mei-Yu Seasons in Taiwan

陳泰然(George Tai-Jen Chen)

《大氣科學》 15卷1期 (1987/03) Pp. 17-30

摘要

本文利用台灣地區降水量選取乾濕梅月，並利用NCAR網格資料，以分析5～6月平均環流之特徵及乾濕梅月之環流條件。結果顯示6月時，相當正壓之暖心阻塞高壓脊於鄂霍次克海區850mb以上各層建立，且印度北方暖心結構之季風環流系統（低對流層季風低壓之上為高對流層季風高壓）發展完成。台灣地區梅雨季之5月與6月雨量多寡（乾濕梅月）之決定因素，為低對流層氣流之源地及強度。當源自孟加拉灣且較強之西南氣流控制時，該月多雨屬濕梅月；當源自西太平洋高壓脊環流之東南、南或西南氣流或東亞主槽後之大陸性西北氣流控制時，該月少雨屬乾梅月。台灣地區低對流層平均氣流之源地及強度，則由季風低壓、西太平洋高壓脊、東亞主槽及鄂霍次克海阻塞高壓等時間平均行星尺度環流系統之位置適當配置及強度來決定。此外，發現中緯度斜壓過程為濕梅月之重要因子，6月時之對流性降水對月雨量之貢獻較5月更為重要，且非鋒面性之強迫機制（例如：局部環流）亦可主宰月雨量之多寡。

並列摘要

A study of the planetary-scale circulation systems during the Taiwan Mei-Yu period was carried out. Rainfall data in Taiwan area were used to define and to choose the dry-and wet-Mei-Yu months. Objectively analyzed grid-point data of NCAR were used to reveal the long-term monthly mean circulations of May and June and to study the accompanying circulation systems in dry-Mei-Yu and wet-Mei-Yu months.Long-term mean circulations revealed that an equivalent barotropic warm core blocking system developed in June a t and above 850 mb level over the Sea of Okhotsk. At the same time, a warm -core structure monsoon circulation system with monsoon low in the lower troposphere and monsoon high in the upper troposphere established over northern India. The controlling factor for the wet-Mei-Yu and dry-Mei-Yu months to occur in Taiwan area was found to be the origin and the intensity of lower-tropspheric flows. The wet-Mei-Yu month occurred when Taiwan area was under the influence of stronger low-level southwesterlies originated from the Gulf of Bengal. When the low-level flows over Taiwan area originated from the Western Pacific ridge circulation or from continent to the west of the East Asia major trough, a dry-Mei-Yu month was expected. The origin and intensity of lower-tropospheric flows over Taiwan area were determined collectively by the intensities and proper locations of the time-mean planetary-scale circulation systems. These included the monsoon low, the Western Pacific high pressure ridge, the East Asia major trough and the Okhotsk blocking. It was also found that the mid-latitude baroclinic processes were of primary importance in causing the rainfall in the wet-Mei-Yu months. The contribution of convective precipitation to the monthly rainfall seemed to be more important in June than in May. Meanwhile, the non-frontal forcings such as local circulations in June appeared to be important in determining the monthly rainfall.