Why does seasonal wind shift occur earlier in the northern Thailand?
Jun Matsumoto (1)
(1) University of Tokyo
It is found that prevailing wind at the surface shifts from winter-time northerly or easterly to summer-time southerly or westerly in the midst of winter at some stations located in the northern Thailand, whereas it occurs in May or June in the southern Thailand, accompanied with the onset of summer monsoon. Why does seasonal wind shift occur so early in Northern Thailand? This question cannot be answered by the seasonal north-south migration of the major wind systems.
By inspecting large-scale wind, height and temperature fields at 850 hPa, after the beginning of retreat of summer monsoon season in September, westerly wind is replaced by easterly wind from the north. However, southwesterly flow is predominant throughout the year in the northwestern part of the Indochina Peninsula. After January, the regional extent of this southwesterly wind system extends both southward and eastward directions. This westerly wind system is different from the summer-time monsoon westerly embedded by upper easterly and is regarded as a part of mid-latitude westerly. The boundary between this westerly and easterly shifts southward from December to April to be located around 10N over the Indochina Peninsula in April. Due probably to the dynamical reason, the center of the subtropical high is located in central (not northern) India in December. Then warming over northern India and Indochina even from February induces the heat trough to be located in northern India and it gradually extends southward. As a result, mid-latitude westerly in the north of the subtropical high expands southward from December to April. In short, warming of the lower troposphere in mid-winter over South and Southeast Asia is the main reason why southerly or westerly wind establishes in the midst of winter in northern Thailand then proceeds in central Thailand during the winter-spring seasonal transition, and finally to southern Thailand in so-called monsoon onset phase. Such surface wind conditions are due partly to the dynamical situation of Hadley circulation over South and Southeast Asia and to the surface warming there even in the midst of winter.
Submittal Information
Name :
Date :
Jun Matsumoto
29-May-01-16:20:38
Organization :
Theme :
Department of Earth and Planetary Science, University of Tokyo
