The Capture of Tropical Cyclones by Cold Fronts off the West Coast of Australia

Abstract

A climatology of all tropical cyclones that have affected the subtropical west coast of Australia during the 82-year period 1909?1990 is presented. A total of 39 events were recorded, an average of 0.5 per year, although a more realistic estimate of the frequency of occurrence, based on a dataset that included satellite surveillance (1964?1990), suggested a value of 0.7 cyclones per year. Northern sections of the west coast experienced nearly twice the number of cyclones as central and southern sections. Cyclone occurrence was confined to the later part of the tropical cyclone season (nominally November to April) and was most common in March. An examination of mean sea level pressure (MSLP) analyses associated with tropical cyclones in the period 1964?1990 revealed two distinct synoptic patterns. One group of cyclones was characterized by a steady southward movement into a region of environmental easterly flow, and the other by rapid acceleration due to the approach of a frontal system from the central Indian Ocean. The MSLP patterns for the latter group were composited nearest to the time of acceleration. The key feature of the composite analysis was a highly meridional front/ridge pair over the eastern Indian Ocean to the southwest of the cyclone. The acceleration toward the southeast began, on average, when the surface cold front was located approximately 1700 km to the southwest. On the basis of the composite analysis, interpretation of satellite imagery and a case study of a recent tropical cyclone event, a conceptual model for acceleration and transformation of tropical cyclones into the midlatitudes is proposed. Weather changes accompanying the transformation process are described.