The Modulating Influence of the Indian Ocean Subtropical Dipole on Australian Region Seasonal Tropical Cyclone Counts

The seasonal count of Australian region tropical cyclones (TCs) has exhibited a reliable and robust statistical relationship with the phase of ENSO, with above (below) average counts during La Niña (El Niño); but this relationship has weakened since 1998.  We show, for the first time, that the seasonal Australian TC count also is strongly modulated by the Indian Ocean Subtropical Dipole (IOSD).  Active (inactive) seasons are linked to a negative (positive) phase of the IOSD, when sea surface temperatures northwest of Australia are anomalously warm (cool), the low-level relative vorticity is anomalously cyclonic (anticyclonic) over a similar region, and mid-tropospheric relative humidity is higher (lower) than average. This recent weakened ENSO-TC count relationship is associated with a sudden drop in correlation between ENSO and the IOSD.  The IOSD outperforms the commonly used Niño 4 sea surface temperature as a predictor of TC counts (39% skill increase), and when used in conjunction with Niño 4 gives a large skill increase (47%) over climatology.  Based on these results, more accurate seasonal forecasts of TC counts in the Australian region require that both the IOSD and ENSO be included in future statistical forecast models.