Australian heavy-rain days and associated east coast cyclones: 1958-92

The authors present a comprehensive climatology of heavy rain and east coast cyclones from January 1958 to September 1992. A total of 80 cyclones, including nondeveloping systems, were objectively identified from daily rainfall and surface wind observation at 28 stations along the east coast of Australia. The method used first identifies heavy-rain days, then uses the wind observations to differentiate east coast cyclones from other rain-producing systems. This method is found to be reliable and with modifications may be to identify other mesoscale systems. In general, onshore southeasterly to southerly flow is most commonly associated with heavy rain along the east coast of Austraila. Local convective heavy-rain events are most common in the Tropics, and the maximum occurrence of heavy-rain days propagates poleward from summer to winter. The latitudinal position and movement of the subtropical anticyclone, and variations in the Southern Oscillation index, have been found to be major factors in the variability of costal heavy-rain occurrences. Consistent with previous studuesm it is found that east coast cyclones occur primarily in winter and form in regions of warm sea surface temperature anomalies. Intensification of east coast cyclones is associated with strong zonal sea surface temperature gradients, greater that 4°C within 50 km of the coastline. Significant correlations exist between the occurrence of east coast cyclones, the Southern Oscillations index, and the latitudinal position of the subtropical anticyclone. The strongest correlations, however, suggest a preference for east coast cyclones to form between extreme episodes (negative to positive) of the Southern Oscillation index. A long-term annual trend toward increased numbers of east coast cyclones has been identified, along with an apparent decrease of local convective heavy-rain events, particularly events, particularly for coastal stations at higher latitudes. No corresponding trend is found for heavy-rain occurrences, the Southern Oscillation index, or the latitudinal position of the subtropical anticyclone.