Short-Term Fluctuations in the Australian Monsoon during Winter Monex

Abstract

Satellite cloud imagery and single station rainfall data are used to define large-scale cloudy and clear phases of the Australian monsoon for the Winter (W) Monex period 20 December 1978 to 31 January 1979. To examine the flow characteristics and possible forcing mechanisms of these phases, large-wale, objective wind and mean sea level pressure analyses have been produced using W Monex IIB data. Composited Row configurations for each phase show marked differences in areal mean divergence and vertical motion over the monsoon region. The other main distinguishing features are the strength of the northeas trades, the strength of the southeast trades over the Indian Ocean, the amplitude of Southern Hemisphere midlatitude upper troughs and the location of the Northern Hemisphere jet maximum. No obvious differences in the wind field are evident over the deep tropics. For each mean phase, diagnosed divergent wind analyses indicate that the ITCZ is farther south and the Southern Hemisphere Hadley cell more organized for the cloudy phase. The effect on the individual cloudy and clear events of Northern Hemisphere cold surge events, cross-equatorial flow, Southern Hemisphere trade-wind changes and changes in the monsoonal westerlies is examined. For this season it is suggested that large-scale convective changes were associated with local Hadley cell variations in the Southern Hemisphere. An important part of the Hadley cell intensification for convectively active periods seemed to be the strengthening of the southeast trade-wind maximum off the West Australian cost. The Northern Hemisphere circulation generally played a relatively passive but co- operative role in the short-term variations of the monsoon.