A Study of a Monsoon Depression Bringing Record Rainfall over Australia. Part II: Synoptic–Diagnostic Description

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Between 12 and 15 March 1989, a monsoon depression moved southeastwards across Australia, producing record rainfalls in the interior of the continent and caused major flooding over a wide area of inland Australia. Mills and Zhao present a series of numerical predictability experiments for this situation. Using the “late data” assimilated analyses prepared by Mills and Zhao, the synoptic evolution of this major precipitation event is described. It is shown that prior to intensification of the low there was a sustained period of horizontal differential temperature advection over Australia, leading to the development of an intense baroclinic zone stretching almost across the entire continent. With the establishment of this slow-moving upper-tropospheric pattern, two things happened. First, the low in the monsoon trough intensified as a surge of southeasterly wind moved around an anticyclone, which developed south of Western Australia. Second, the jet stream associated with the baroclinic zone strengthened over Western Australia, and the direct transverse vertical circulation at its entrance established an isallobaric fall center over the interior of Western Australia, well to the southeast of the monsoon low. Increased advection of warm, moist air southward by the monsoon low circulation entered the ascending branch of the jet entrance circulation, leading to the heavy precipitation in this area. Latent heat release from this precipitation probably contributed to the development of a baroclinic cyclone, as suggested by Mills and Zhao, white the original monsoon low moved inland and weakened. The baroclinic low then moved southeastward, extending the rainfall to a wide area of southern Australia. It is shown that the development of the baroclinic low was not well resolved by the standard observing network, as it occurred over a sparsely observed area of Australia, but could be inferred by careful subjective analysis. This event is compared with the extratropical interactions of Hurricane Hazel and Tropical Storm Agnes over the United States, and several similarities are found.