Tropical Cyclone Trends

Trends in tropical cyclone activity in the Australian region (south of equator; 105 - 160°E) show that the total number of cyclones has decreased in recent decades. However, the number of stronger cyclones (minimum central pressure less than 970 hPa) has not declined.

The overall decrease may partly be due to an improved discrimination between tropical cyclones and sub-cyclone intensity tropical lows. Tropical cyclone numbers in the Australian region are influenced by the El Niño-Southern Oscillation phenomenon and the decrease in total cyclone numbers may be associated with an increased frequency of El Niño events. A number of long-term trends and oscillations have been observed in other parts of the world, extending over many decades. It is difficult to sort these natural trends from those that may result from global warming.

Potential changes in tropical cyclone occurrence and intensity are discussed in detail in the 2007 report, Climate Change in Australia, Technical Report - Chapter 5: Regional climate change projections (8.9MB) See: Chapter 5.9.1 Severe weather: Tropical cyclones. There is substantial evidence from theory and model experiments that the large-scale environment in which tropical cyclones form and evolve is changing as a result of greenhouse warming. Projected changes in tropical cyclones are subject to the sources of uncertainty inherent in climate change projections. These include errors in the modelled tropical cyclone climatology and regional patterns and magnitude of change for various fields and climate patterns such as ENSO. Consequently there is large uncertainty in the future change in tropical cyclone frequency projected by climate models.

IPCC (2001) concluded that 'there is some evidence that regional frequencies of tropical cyclones may change but none that their locations will change. There is also evidence that the peak intensity may increase by 5% to 10% and precipitation rates may increase by 20% to 30%. There is a need for much more work in this area to provide more robust results.'

Since that time there has been a growing number of studies that indicate a consistent signal of fewer tropical cyclones globally in a warmer climate. However, there are significant regional variations in the direction of the changes and these vary between models. Substantial disagreement remains between climate models concerning future changes in tropical cyclone intensity, although the highest resolution models show evidence of an increase in tropical cyclone intensity in a warmer world.

There have been three recent studies producing projections for tropical cyclone changes in the Australian region. Two suggest that there will be no significant change in tropical cyclone numbers off the east coast of Australia to the middle of the 21st century. The third study, based on the CSIRO simulations, shows a significant decrease in tropical cyclone numbers for the Australian region especially off the coastline of Western Australia. The simulations also show more long-lived eastern Australian tropical cyclones although one study showed a decrease in long-lived cyclones off the Western Australian coast.

Each of the above studies finds a marked increase in the severe Category 3 - 5 storms. Some also reported a poleward extension of tropical cyclone tracks.

Projected changes in tropical cyclone characteristics are inherently tied to changes in large-scale teleconnection patterns such as ENSO, changes in sea surface temperature and changes in deep convection. As global climate models improve, their simulation of tropical cyclones is expected to improve, thus providing greater certainty in projections of tropical cyclone changes in a warmer world.