Can we forecast the retreat of northern Australia's rainy season?

Can we forecast the retreat of northern Australia's rainy season?

According to the Bureau of Meteorology, the northern Australian wet season extends from November through to April. As the tropical dry season transition begins, crop farmers need to prepare for annual crops or pasture/fodder harvest, whilst beef producers start making decisions regarding herd numbers and feed rationing. Knowing if the last wet season rains (i.e., northern rainfall retreat; NRR) are likely to be later or earlier than usual would be valuable for agricultural industries as well as the infrastructure and tourism sectors. While the Bureau provides forecasts of the northern rainfall onset prior to September, there are no forecasts of the NRR. Here, we draw on three different NRR definitions and investigate whether there is any skill in forecasting the chance of an early or late retreat.

Based on the Australian Gridded Climate Data from 1950-2019, we find that, typically, NRRs occur ~1 week later than normal across the far northern tropics following La Niña events, but there is little change for El Niño. Retreats are also more common when the Madden Julian Oscillation is in phases 6 and 7, when convection passes through the western Pacific. Utilising the Bureau's seasonal forecast system, ACCESS-S2, we find that the model is moderately skillful in forecasting the NRR timing across the far northern regions at a lead time of ~2½ months, however, the skill is relatively poor in the subtropics and arid locations. We will highlight the various challenges of predicting the timing of the NRR at such long lead times.

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