Observing and forecasting the retreat of northern Australia's rainy season

According to the Australian Bureau of Meteorology, the northern Australian wet season extends through to April, which also formally marks the end of Australia's tropical cyclone season. Mid-autumn is when the tropical dry season transition period begins, when crop farmers prepare land for annual crops or pasture-fodder harvest, or when beef cattle producers make decisions regarding stock numbers and feed rationing. Potentially knowing if the last rains of the wet season will be later or earlier than normal would be valuable information for northern sectors such as agriculture, infrastructure and tourism. The Bureau of Meteorology provides seasonal forecasts of the Northern Rainfall Onset - the date when a location has accumulated 50 mm of rain from 1 September - yet there is currently no prediction of the rainy season retreat (the Northern Rainfall Retreat, NRR). In this study, we draw on three different NRR definitions and investigate how they vary with the El Nino-Southern Oscillation and the Madden-Julian Oscillation (MJO). In general, retreats occur similar to 1 week later than normal across the far northern tropics following La Nina events, but little change from normal occurs for El Nino. Although most retreats occur when the MJO is weak, if the MJO is active, retreats are mostly observed in phases 6 and 7, when convection is passing through the western Pacific. Utilising the Bureau of Meteorology's sub-seasonal to seasonal forecast system, ACCESS-S2, we show that the model has some skill in forecasting the NRR across the far northern regions at a lead time of similar to 2.5 months, but poor skill in the subtropics and arid locations. Verification of the 2023 NRR forecasts, highlights the challenges of predicting the timing and magnitude of daily rainfall at such a long lead time.