Multiweek Tropical Cyclone Prediction in ACCESS-S1 and the role of the MJO

The Australian Bureau of Meteorology (BoM) seasonal forecasting system (ACCESS-S1), based on the UK Met Office seasonal system (GloSea5), is investigated for the prediction of the weekly occurrence of tropical cyclone (TC) activity in the Southern Hemisphere. On multi-week timescales the Madden-Julian Oscillation (MJO) is the primary driver of TC predictability. 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 enhanced TC activity with the enhanced convective phase of the MJO, as well as reduced TC activity both before and after the enhanced MJO phase. MJO modulated changes in the large-scale environment which are known to strongly influence 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 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 skillful forecasts of TC occurrence out to at least the end of week 3 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 everywhere and 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 skillful multi-week forecasts of TC occurrence for the Southern Hemisphere, greatly extending the current TC forecast availability beyond the 1-5 day timescale.