Tropical cyclones in the Australian region (90°E–180°E) are split into two sub-regions: the Southwest Pacific Ocean basin (SWPAC) (130°E–180°E) and the Southeastern Indian Ocean basin (SEIND) (90°E–130°E). This is done in order to understand the similarities and differences between the mechanisms influencing the two basins. The TCs that crossed over the 130°E dividing line between the two basins are counted in both basins. The trend in the number of TCs in the SWPAC is a general decrease for all TCs and weak TCs (>970 hPa). For intense TCs in the SWPAC there is smaller decrease in the number of intense TCs (<970 hPa). In the SEIND, there is again a general decrease in TC numbers for all TCs and weak TCs; however, the number of intense TCs has no trend.

In this study, through the use of climate indices, various atmospheric patterns that explain the variations in TC numbers in the Australian regions are identified. At least ten indices are linearly correlated with the number of TCs in both regions. Some indices are found to affect both basins, while others only affect one of the basins. From these non-independent predictors, a principal component analysis and logistic regression is performed to determine a smaller set of independent predictors from the climate indices to provide a predictive equation for TC variability within a given year.