Probabilistic Forecasts of the Onset of the North Australian Wet Season

Fiona Lo, Matthew C. Wheeler, Holger Meinke, and Alexis Donald

2007: Mon. Wea. Rev., 135, 3506-3520.

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The amount and timing of early wet-season rainfall is important for the management of many agricultural industries in north Australia. With this in mind, a wet-season onset date is defined based on the accumulation of rainfall to a pre-defined threshold, starting from 1 September, for each square of a 1-degree gridded analysis of daily rainfall across the region. Consistent with earlier studies, the interannual variability of the onset dates is shown to be well related to the immediately preceding July-August Southern Oscillation Index (SOI). Based on this relationship, a forecast method using logistic regression is developed to predict the probability that onset will occur later than the climatological mean date. This method is expanded to also predict the probabilities that onset will be later than any of a range of threshold dates around the climatological mean. When assessed using cross-validated hindcasts, the skill of the predictions exceeds that of climatological forecasts in the majority of locations in north Australia, especially in the Top End region, Cape York, and central Queensland. At times of strong anomalies in the July-August SOI, the forecasts are reliably emphatic. Further, predictions using tropical Pacific sea-surface temperatures (SSTs) as the predictor are also tested. While short-lead (July-August predictor) forecasts are more skilful using the SOI, long-lead (May-June predictor) forecasts are more skilful using Pacific SSTs, indicative of the longer-term memory present in the ocean.