18.104.22.168 Tropical Cyclones
A number of recent regional model-based studies of changes in tropical cyclone behaviour in the Australian region have examined aspects of number, tracks and intensities under enhanced greenhouse conditions (e.g., Walsh and Katzfey, 2000; Walsh and Ryan, 2000; Walsh et al., 2004). There is no clear picture with respect to regional changes in frequency and movement, but increases in intensity are indicated. For example, Walsh et al. (2004) obtained, under tripled CO2 conditions, a 56% increase in storms with a maximum wind speed greater than 30 m s–1. It should also be noted that ENSO fluctuations have a strong impact on patterns of tropical cyclone occurrence in the region, and therefore uncertainty with respect to future ENSO behaviour (see Section 10.3) contributes to uncertainty with respect to tropical cyclone behaviour (Walsh, 2004). See Section 10.3.6.3 for a global assessment of changes in tropical cyclone characteristics.
The MMD ensemble mean projected change in winter sea level pressure is shown in Figure 10.9. Much of Australia lies to the north of the centre of the high-pressure anomaly. With the mean latitude of maximum pressure near 30°S at this season, this corresponds to a modest strengthening of the mean wind over inland and northern areas and a slight weakening of the mean westerlies on the southern coast, consistent with Hennessy et al. (2004b). Studies of daily extreme winds in the region using high-resolution model output (McInnes et al., 2003) indicate increases of up to 10% across much of the northern half of Australia and the adjacent oceans during summer by 2030. In winter, the pressure gradient is projected to increase over the South Island of New Zealand (see Figure 10.9), implying increased windiness. This increase is present in all of the MMD-A1B projections.