Downstream Development in the Southern Hemisphere Monsoon during FGGE/WMONEX

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Abstract
Evidence is presented of a downstream development mechanism operating across the entire longitudinal span of the 1978/79 Southern Hemisphere monsoon. Observationally it is seen as progressive cyclonic and anticyclonic vorticity increases that develop eastward in the monsoon trough at a speed of approximately 5 m s−1. The process results in many tropical cyclone and tropical depression formations over northern Australia and the South Pacific. It is shown that the downstream development process is generally consistent with linearized barotropic dynamics, and that the Southern Hemisphere monsoon, because of an intrinsic westerly basic state, is a particularly suitable region for downstream events. It is also shown that some apparent contradictions in previous observational studies can be rationalized by the theory. The interactions between the regional components of the monsoon (Indonesian, Australian and South Pacific sectors) can also he better understood. We further suggest that the process has implications for other features of the monsoon circulation, namely onset and 40–50 day events.

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