Skilful multiweek tropical cyclone prediction in ACCESS-S1 and the role of the MJO

The skill of predicting the weekly occurrence of tropical cyclone (TC) activity in the Southern Hemisphere is investigated in the Australian Bureau of Meteorology seasonal forecasting system (ACCESS‐S1). On multiweek time‐scales, the Madden–Julian Oscillation (MJO) has previously been shown to be a major driver of TC variability. ACCESS‐S1 shows high skill for predictions of the MJO out to a lead time of ∼30 days and is able to reproduce the observed modulation of TC activity by the MJO in the Southern Hemisphere. In particular, ACCESS‐S1 shows a clear eastward propagation of increased TC activity with the enhanced convective phase of the MJO. MJO modulated changes in the large‐scale environment associated with TC genesis, such as 850‐hPa absolute vorticity, 600‐hPa relative humidity and 850–200 hPa vertical wind shear, are well captured by ACCESS‐S1 except for off the northwest coast of Australia. There, the change in the large‐scale environment and associated TC activity is too weak in the model. Probabilistic forecast verification shows that ACCESS‐S1 is able to provide skilful forecasts of TC occurrence into week 5 if forecasts are calibrated to take into account model biases in TC frequency. Two different calibration strategies are tested: the first is a simple constant scaling factor applied across the Southern Hemisphere for all forecast leads; the second applies a different scaling factor for different regions and lead times. Results of this study suggest that ACCESS‐S1 can provide skilful multiweek forecasts of TC occurrence for the Southern Hemisphere, greatly extending the current TC forecast availability beyond the 1–5 day time‐scale.