Abstract
The onset and interannual variability of the Asian summer monsoon in relation to land–sea thermal contrast and its contributing factors are studied using a 14-yr (1979–1992) dataset. The onset of the Asian summer monsoon is concurrent with the reversal of meridional temperature gradient in the upper troposphere south of the Tibetan Plateau. The reversal is the result of large temperature increases in May to June over Eurasia centered on the Plateau with no appreciable temperature change over the Indian Ocean. In spring the Tibetan Plateau is a heat source that is distinctly separate from the heat source associated with the rain belt in the equatorial Indian ocean. The Tibetan heat source is mainly contributed by sensible heat flux from the ground surface, while the oceanic heat source is due to the release of latent heat of condensation. It is the sensible heating over the Plateau region in spring that leads to the reversal of meridional temperature gradient. Despite its intensity the condensational heating over the Indian Ocean does not result in tropospheric warming because it is offset by the adiabatic cooling of ascending air.
A monsoon intensity index, based on the magnitude of the summer mean vertical shear of zonal wind over the North Indian Ocean, is used to compare the years of strong and weak Asian summer monsoon circulation. The strong (weak) Asian summer monsoon years are associated with (a) positive (negative) tropospheric temperature anomalies over Eurasia, but negative (positive) temperature anomalies over the Indian Ocean and the eastern Pacific; (b) negative (positive) SST anomalies in the equatorial eastern Pacific, Arabian Sea, Bay of Bengal. and South China Sea, but positive (negative) SST anomalies in the equatorial western Pacific; and (c) strong (weak) heating and cumulus convection over the Asian monsoon region and the western Pacific, but weaker (stronger) heating and convection in the equatorial Pacific.
