Intra-seasonal variability of Australian rainfall

Spectral and cross spectral analysis is applied to daily rainfall on a one degree by one degree grid covering most of the Australian continent. While there is considerable variance on the intra-seasonal time scales from 30 to over 100 days over most of northern Australia, no particular narrow range of periodicities (eg 40-60 days) or frequencies dominates the spectrum. Significant spectral peaks at these frequencies are found in the subtropics over southwest Western Australia and in eastern Australia, outside the area normally considered to be influenced by the equatorial Madden-Julian oscillation.

Cross spectral analysis between Darwin and the rest of Australia does not reveal significant propagation of rainfall anomalies in the Australian monsoon region, unlike the Indian monsoon which shows pronounced south to north propagation of rainfall on 40-50 day timescales. A similar result is found for the coherence between zonal winds, measured either at Darwin or close to the equator, and Australian monsoon rainfall. A composite analysis of rainfall anomalies relative to strong westerly winds through the equatorial regions of Indonesia (100oE to 150oE) suggests that the strongest westerly bursts are associated with drier conditions over most of northern Australia,while wetter conditions are found 5 to 10 days after the maximum wind is observed at the equator for more moderate westerly wind bursts. This suggests that the strongest events propagate zonally across Indonesia and into the Western Pacific with minimal impact on north Australia.