RECENT TRENDS IN AUSTRALIAN REGION TROPICAL CYCLONE ACTIVITY

The number of tropical cyclones observed in the Australian region (sou th of equator; 105-160 degrees E) has apparently declined since the st art of reliable (satellite) observations in the 1969/70 season. Howeve r, the number of more intense cyclones (with minimum pressures droppin g to 970 hPa or lower) has increased slightly. The numbers of weak (mi nimum pressures not dropping below 990 hPa) and moderate systems (mini mum pressures between 970 and 990 hPa) have declined. Possible reasons for these different trends are discussed. The decline in the number o f weaker cyclones may at least partly reflect improved understanding o f the nature of some weak systems. The decline in the number of cyclon es more intense than 990 hPa primarily reflects the downward trend in the Southern Oscillation Index (SOI). Previous work has demonstrated t hat the number of tropical cyclones observed in the Australian region each cyclone season is related to the value of the SOI prior to the st art of the cyclone season. This relationship is dearest with the numbe r of moderate cyclones. The SOI is only weakly related to the number o f intense or weak cyclones. The increase in the number of intense cycl ones is not attributable to the trend in the SOI. Nor is there clear r eason, at present, to suspect that it is artificial (i.e., due to chan ges in observing or analysis techniques).