On the instantaneous distribution of vertical velocity in the monsoon field and structure of the monsoon circulation

Abstract

An approximate representation of the instantaneous three-dimensional circulation of the Asian summer monsoon has been obtained using data and analysis at a given map time in July 1963 during the International Indian Ocean Expedition (1963–65). For this purpose, vertical velocity was computed using the continuity equation and by stepwise integration of the divergence field, at the standard isobaric levels at every five-degree grid-point along seven meridional sections extending from 50° E to 140° E between latitudes 35° N and 25° S. In conjunction with the meridional and zonal components of the winds, the vertical velocities yielded the instantaneous vertical circulation patterns and these are presented along seven meridional and four zonal sections. The isobaric distribution of vertical velocity is presented along four standard pressure surfaces, viz. 850, 700, 500 and 100 mb. The computed vertical velocities show three well-defined updraft areas over the landmass of Asia, one over Arabia, one over India, and the third over China. A few localized updraft cells of feeble intensity appear over the sea areas, e.g. one in the equatorial region between Gan Island (00°41? S, 73°09? E) and Indonesia and another in western central Pacific, east of the Philippines. Extensive downdraft areas are found over the land area between India and Arabia, over western Himalayas and Tibet, and over most of the sea areas lying to the south of the continental landmass. Practically the whole of the west Arabian Sea appears under intense downdraft. Over many areas, however, the computed vertical velocities increase continually with height showing little or no vertical compansation. This is, of course, due to analysis Notwithstanding this basic problem, the computed vertical velocity patterns are found to be compatible with the observed distribution of temperature, humidity, vorticity, rainfall, and jet streams. The presence of a belt of equatorial westerlies (south of the Equator) in the Indian Ocean area would also seem to support computed updraft in this region. The evidences furnished in the paper strongly suggest that it is only a three-dimensional thermal convective model that can describe the giant monsoon circulation over the north Indian Ocean and adjoining Asian continent adequately. DOI: 10.1111/j.2153-3490.1968.tb00403.x