Rainfall regimes over northwestern Australia

Annual precipitation in northwestern Australia has been increasing since the 1950s. The work presented here investigates the causes of rainfall using K-means clustering to classify daily rainfall patterns into regimes, demonstrating their link to known synoptic structures in northern Australia and explaining the overall change in rainfall in terms of these regimes. After first removing tropical cyclones and assigning them to two regimes based on their location, six additional rainfall regimes are identified. The eight regimes fall into four groups: two associated with localized thunderstorms, two associated with monsoonal lows, two associated with tropical cyclones and two involving interactions with the midlatitudes. The increase in rainfall is dominated by an increase in the number of tropical cyclones in the Pilbara region of northwestern Australia. There is also an increase in the frequency of monsoonal lows and midlatitude interactions at the expense of days with isolated storms. The change in the rainfall intensity of each regime is far less important than the change in their frequency. The trend in precipitation is therefore largely due to an increase in organized rain-bearing synoptic structures, as opposed to a change in the precipitation associated with these structures.