Westward Propagating Predecessors of Monsoon Depressions

Abstract

By examining maps of 24 h change of sea level pressure during July and August over the 10-year period 1969?78, we find that 45 (87%) of the 52 lows and depressions that formed in the Bay of Bengal were associated with predecessor disturbances coming from the cast. In 12(23%)instances, the predecessor was associated with a typhoon or named tropical storm in the South China Sea, while the remaining 33 (64%) were weaker systems originating over a broad region of land and sea. From examination of time sections over the same, period from eastern Thailand to the Burmese coast, we identified 50 westward-moving disturbances, with consider able vertical extent, of which 64% developed into lows or depressions on reaching the Bay. In 60% of the 50 instances, the disturbance could be traced to the South China Sea, 32% being typhoons and named tropical storms and 28% weaker circulations. The remaining 40% of the 50 disturbances appeared to originate over land. For the three stations along the section line, cross-correlation and power spectrum analyses of departure of the 24 h pressure change at each station from the average of the three station values confirms the existence of westward-moving waves. The isallobaric maps and time sections and the statistical analysis indicate a period of slightly less than five days, a westward phase speed slightly less than 6 m s?1 and a wavelength of ?2300 km. The amplitude of the oscillation of the 700 mb meridional wind component in northwest Thailand was ?5 m s?1. The maps indicate an isallobaric amplitude of ?3 mb day?1 while data along the section line indicate a value only about half as large, owing mainly to the failure of the isallobaric centers to lie on the line. These waves appear to be morphologically similar to waves found in the lower and middle troposphere in other tropical longitudes of the Northern Hemisphere. Although we have not addressed the important physical problems of the origin of the disturbances, or of their development at the surface on arrival in the Bay of Bengal, it appears that our empirical results can be directly applied to forecasting practice and should show considerable skill in predicting the appearance of monsoon depressions.