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7. Observed trends in tropical cyclone activity in the Australian Region

7.1 Introduction

This Chapter reviews trends in tropical cyclonic activity in the Australian Region

during the last thirty years, and possible explanations.

7.2 Observations by Nicholls et al (1998)

A review of cyclonic activity in Australia from 1969/70 to 1995/6 was given by

Nicholls et al (1998). A downward trend in the total number of tropical cyclones

observed in the Australian Region was observed. However, there was a slight

upward trend in the number of intense cyclones, defined as those with a central

pressure of 970hPa or less, as shown in Figure 7.1.

Nicholls et al noted that at least part of the downward trend in overall cyclone

numbers can be explained by changes in the way storms are classified as cyclones.

For example Cyclone ‘Wanda’ in 1974 had a central pressure of 1000 hPa, and would

not have been classified as a cyclone in later years. This has an effect on the trend in

the number of cyclones with central pressures higher than 990hPa, but negligible

effect on stronger ones.

The downward trend in overall numbers was primarily explained by the negative

trend in the Southern Oscillation Index (SOI) – otherwise known as the El Nino

phenomenon – over the time period in question. The SOI is the difference in mean

sea level atmospheric pressure between Tahiti and Darwin, standardized to a mean of

zero and standard deviation of 10, and low values are associated with droughts in

Australia, as well as fewer tropical cyclones. No explanation was given for the

increase in more intense cyclones, although this trend is also predicted by climate

models for the east coast of Australia as a consequence of increasing greenhouse gas

concentrations (see Section 8.4).

Figure 7.1 Trend in number of cyclones with central pressures of 970 hPa or less in

the Australian Region during 1969-96 (from Nicholls et al, 1998).

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