Onsets of The Asian Summer Monsoon 1979–82

Abstract

Analyses of moisture and mean tropospheric enthalpy distributions based on FGGE data are described and interpreted for the months of May and June 1979. Their detailed spatial characteristics are inferred using 15-day means for the region 0?150°E, 40°S?40°N, and the detailed temporal evolutions from time series at daily intervals for two regions-the Arabian Sea and the larger area 0°?150°E, 22.5°5?41.25°N. Vertical motion fields are calculated using O'Brien's method and are used to determine the distributions of moisture convergence and heating. The onset is found to consist of two main phases: 1) a moisture buildup over the Arabian Sea during which synoptic- and mesoscale transient disturbances develop; the relationship of this buildup with planetary wave activity is discussed. This is followed by 2) a rapid intensification of the Arabian Sea winds and a substantial increase in latent heat release, essentially a large-scale feedback process. The fully established monsoonal flow is interpreted in terms of Gill's model based on linear equatorial beta-plane theory. Then the model flow is found to agree reasonably well with observations, with strong cross-equatorial flow in both lower and upper troposphere, maintained by latent heat release over a large region between 10 and 20°N. A major part of the moisture required for this is supplied by evaporation over the Indian Ocean, depending on the strength and configuration of the low-level winds. Similar analyses are carried out for 1980, 1981 and 1982, using ECMWF operational analyzed fields, although moisture fields are analyzed only for 1982. Although these show characteristics generally similar to those of 1979, there are some differences. Comparisons are made of the four years, and related to the commencement of rains over India.