Influence of the Madden-Julian Oscillation on multiweek prediction of Australian rainfall extremes using the ACCESS-S1 prediction system

We assessed the ability of the Bureau of Meteorology’s ACCESS-S1 dynamical forecast system to simulate and predict high rainfall extremes for each season over Australia, especially focusing on the role of the Madden-Julian Oscillation (MJO). Using retrospective forecasts for the period 1990–2012, we show that ACCESS-S1 simulated the observed modulation of extreme weekly mean rainfall by each phase of the MJO reasonably well; however the simulated changes in probabilities tended to be weaker than those observed, especially across the far north during the austral summer season. The ability of the model to (i) simulate the observed modulation of extreme rainfall and (ii) predict the MJO to a lead time of four weeks, translated to enhanced forecast skill for predicting the occurrence of extreme weekly mean rainfall across much of Australia at times when the MJO was strong, compared to when the MJO was weak, during the austral spring and summer seasons in weeks 2 and 3. However, skill reduced across the central far north during the summer when the MJO was strong, suggesting the model is not good at depicting the MJO’s convective phases as it protrudes southward over northern Australia. During autumn and winter, there was little indication of changes in forecast skill, depending on the strength of the MJO. The results of this study will be useful for regional applications when the MJO is forecast to be strong during spring and summer, particularly where the swing in probability of extreme rainfall is large for specific phases of the MJO.