Divergent Circulations during the Onset of the 1978–79 Australian Monsoon

Abstract

Large scale numerical analyses of divergence and the divergent component of wind are examined at two levels in the lower and upper troposphere. The synoptic sequence studied includes the onset of the Southern Hemisphere summer monsoon. Comparison with satellite-observed cloudiness leads to the conclusion that the analyzed patterns of divergence contain synoptically realistic meteorological information. Them seems to be virtually no information, however, in the day-to-day changes in magnitude of analyzed divergence in the lower troposphere, and only a weak signal in the upper troposphere. The divergent wind analyses reveal the Intertropical Convergence Zone (ITCZ) to be a readily identifiable feature on individual days, and its location to he both vertically consistent and coincident with the satellite-observed cloud. Two days prior to monsoon onset the analyzed ITCZ moves poleward by 8° latitude. Monsoon convection exists at the intersection of Northern and Southern Hemisphere Hadley cells; it is well removed from the upward branch of any east-west Walker circulations in this situation. The concept of a divergent surge is introduced to denote vertically consistent divergent circulations extending over distances greater than 20° latitude. This concept is shown to be useful in the physical interpretation of the role of the Southern Hemisphere subtropics in the triggering of monsoon onset. Use of the concept is also helpful in relating the day-to-day changes in tropical convection to simultaneous changes in location and intensity of (mean sea level) subtropical high pressure cells in both hemispheres. In addition, solutions for the divergent component of wind calculated over a limited domain are compared with solutions calculated over a sphere.