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 cyc lones, including nondeveloping systems, were objectively identified fr om daily rainfall and surface wind observations at 28 stations along t he east coast of Australia. The method used first identifies heavy-rai n days, then uses the wind observations to differentiate east coast cy clones from other rain-producing systems. This method is found to be r eliable and with modifications may be used to identify other mesoscale systems. In general, onshore southeasterly to southerly flow is most commonly associated with heavy rain along the east coast of Australia. Local convective heavy-rain events are most common in the Tropics, an d the maximum occurrence of heavy-rain days propagates poleward from s ummer to winter.;The latitudinal position and movement of the subtropi cal anticyclone, and variations in the Southern Oscillation index, hav e been found to be major factors in the variability of coastal heavy-r ain occurrences. Consistent with previous studies, it is found that ea st coast cyclones occur primarily in winter and form in regions of war m sea surface temperature anomalies. Intensification of east coast cyc lones is associated with strong zonal sea surface temperature gradient s, greater than 4 degrees C within 50 km of the coastline. Significant correlations exist between the occurrence of east coast cyclones, the Southern Oscillation index, and the latitudinal position of the subtr opical anticyclone. The strongest correlations, however, suggest a pre ference for east coast cyclones to form between extreme episodes (nega tive to positive) of the Southern Oscillation index. A long-term annua l trend toward increased numbers of east coast cyclones has been ident ified, along with an apparent decrease of local convective heavy-rain events, particularly for coastal stations at higher latitudes. No corr esponding trend is found for heavy-rain occurrences, the Southern Osci llation index, or the latitudinal position of the subtropical anticycl one.