RELATIONSHIP OF GENERAL CIRCULATION TO NORMAL WEATHER OVER SOUTHERN ASIA AND THE WESTERN PACIFIC DURING THE COOL SEASON

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Abstract
As a first step in a study of cool-season (November through April) weather over southern Asia and the western Pacific, an analysis of the relationship of the general circulation to normal weather has been attempted. In October, a jet stream suddenly appears along the southern edge of the Himalayas. Thereafter it varies little in position until April, when it begins to waver prior to disappearance in early summer. This sequence, and also the fact that no comparable jet is found north of the Himalayan-Tibetan massif, are explained in terms of mechanical lifting and the effect of snow cover on insolation. Nearly all extra-tropical depressions form in and move along the jet stream, while even rapidly moving cold-fronts are temporarily, though strongly, intensified as they pass beneath it. The increasing velocity of the jet stream after it leaves the mountains seems due to confluence, over northeast India, between the westerlies and upper southwesterlies of the Bay of Bengal. Vigorous convergence in the southwesterlies over Burma, and between the southwesterlies and the westerlies of the jet stream over northeast India, probably results in the intense subsidence and aridity observed downstream. The subtropical ridge is displaced northward and becomes nearly vertical, and south of it deep baroclinic easterlies prevail. East of the Philippines this distribution, and over the southwest Bay of Bengal a combination of lowlevel trough and high-level divergence, are thought to favor the formation of tropical storms.

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