Longer term study of past cyclones reveals averages and trends over time.
The Australian cyclone season officially runs from November to April, although very few have occurred in November.
Since the 1970s, when reliable tropical cyclone data became available, the earliest cyclone of a season to directly impact Australia was severe tropical cyclone Ines, which crossed the northern Kimberley coast on 21 November 1973. One of the latest cyclones in a season was tropical cyclone Herbie that formed near Cocos Islands and passed over Shark Bay on 21 May 1988.
Further west in the Indian Ocean, cyclones can occur all year around although the risk of one in the winter months is very low.
On average about eleven cyclones form in the Australian region (90-160° E) each cyclone season. This represents about 13% of the global total. About half of these occur in the western region and about half of the total number of tropical cyclones become severe. Tropical cyclones in the western and eastern basins have around 25% chance of making landfall, while those in the northern basin have an 80% chance of making landfall.
Tropical cyclones in the Australian region are influenced by several factors, and in particular variations in the El Niño – Southern Oscillation. In general, more tropical cyclones cross the coast during La Niña years, and fewer during El Niño years.
Tropical cyclone climatology maps show the average number of tropical cyclones through the Australian region and surrounding waters in El Niño years, La Niña years, neutral years and using all years of data. These maps show that an area north of Port Hedland has the maximum occurrence of cyclones.
Analysis of historical tropical cyclone data has limitations due to changes in observing practices and technology that have occurred over time. With new and improved meteorological satellites our ability to detect tropical cyclones has improved, as has our ability to differentiate tropical cyclones from other tropical weather systems such as monsoon depressions, which in the past may have been incorrectly named as tropical cyclones. A particularly important change occurred in the late 1970s when regular satellite images became first available from geostationary satellites above the Earth's equator.
The time series of analysed tropical cyclone activity in the Australian region (south of the equator; 90-160°E) show that the total number of cyclones appears to have decreased. However, there was a change to the definition for tropical cyclones in 1978 which led to some systems which would previously have been classified as tropical cyclones instead being considered sub-tropical systems. This contributes somewhat to the apparent decline in total numbers.
The number of severe tropical cyclones is dominated by variability with periods of lower and higher frequencies of occurrence. There is less confidence in the earlier intensity data with continuous satellite coverage commencing in 1979.
Potential changes in tropical cyclone occurrence and intensity (a measure of wind speed alone rather than the amount of precipitation or coastal flooding) are discussed in CSIRO and BoM (2015: see Sections 4.2.7 and 7.3.2). 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 global warming. Projected changes in the number and intensity of tropical cyclones are subject to the sources of uncertainty inherent in climate change projections. There remains uncertainty in the future change in tropical cyclone frequency (the number of tropical cyclones in a given period) projected by climate models, with a general tendency for models to project fewer tropical cyclones in the Australia region in the future climate and a greater proportion of the high intensity storms (stronger wind speeds and heavier rainfall).
Wind speed is only one aspect of tropical cyclones and their impacts. The amount of heavy precipitation from all weather systems, including tropical cyclones, is likely to increase. Increased rainfall intensity from tropical cyclones is pertinent to Australia, since these storms have historically been associated with major flooding.
Additionally, increases in storm surges and extreme sea-levels are very likely to occur in association with tropical cyclones under future climate change. This change is independent of changes in tropical cyclone intensity and is directly related to increases in global mean sea-level due to global warming.
Projected changes in tropical cyclone characteristics are inherently tied to changes in large-scale patterns such as the El Niño - Southern Oscillation, 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
CSIRO and Bureau of Meteorology (BoM), 2015: Climate Change in Australia Information for Australia's Natural Resource Management Regions: Technical Report, CSIRO and Bureau of Meteorology, Australia.