Abstract

The time evolution of the general circulation over Asia during an 80-day period from mid-April to early July 1979 is studied using objectively analyzed FGGE Level II-b data. Through the analyses of the wind, temperature, precipitation, outgoing longwave radiation (OLR) flux, and heat and moisture budgets in the area 0°–50°N, 40°–130°E, the major changes of the circulation leading to the onset of the summer monsoon and the role of the Tibetan Plateau in these changes are examined.

During the analyzed period the general circulation underwent two distinct stages of abrupt transitions resulting in the successive onsets of early summer rains over Southeast Asia and the Indian summer monsoon. The first transition occurred in May in which low-level southwesterlies began over the longitudes east of 80°E (from the Bay of Bengal to the South China Sea), resulting in the spreading of early summer rains over Assam, the Bay of Bengal coasts of Burma and the Malay Peninsula, Thailand, Indochina and the South China Sea. The center of the South Asian anticyclone at 200 mb abruptly moved northward from 10° to 20°N with an increase of the upper tropospheric temperature over the eastern Tibetan Plateau and the South China Plain and the reversal of the meridional gradient of temperature on the south side of the Plateau. During the second transition, which took place in June, the southwesterlies developed over the Arabian Sea and the monsoon rains commenced along the west coast of India. A new center of the 200 mb anticyclone formed over the Saudi Arabia–Iran region with a large increase of the upper tropospheric temperature over the Iran–Afganistan–western Plateau region.

The analyses of the time sequences and 80-day mean distribution of the vertically integrated heat sources and moisture sinks, precipitation and the OLR flux reveal that the release of latent heat of condensation is the primary heat source driving the Asian summer circulation. Over the Tibetan Plateau, however, there is continuous sensible heating from the ground surface throughout the analyzed period.

A prominent feature seen in the general circulation during this period is the presence of the vertical circulation induced by the Tibetan Plateau which coexists with the principal monsoon system migrating northward. The sensible heating from the elevated surface and the radiative cooling in the environment maintain the temperature contrast that drives intense ascent over the Plateau and compensating descent in the surrounding areas. The examination of the warming process of the 200–500 mb layer during the two transition periods shows that the adiabatic warming due to large-scale subsidence is the dominant mechanism for the temperature increase. The subsidence on the northeast and west sides of the Plateau appears to provide a key mechanism for the observed warming associated with the distinct changes in the general circulation.

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