Direct and Indirect Impacts of Positive Subtropical Indian Ocean Dipole (SIOD) events on Cyclonic Activity in the South West Indian Ocean (SWIO) Basin

The positive (resp. negative) phase of the Subtropical Indian Ocean Dipole (SIOD) is caracterized by a positive (resp. negative) anomaly of sea surface temperature South East of Madagascar and a negative (resp. positive) anomaly West of Australia. This variability mode of the Indian Ocean is phase locked to Austral summer. It usually begins in October and peaks in January. Positive SIOD events are known to bring above average precipitation over South East Africa and lower rainfall over Western Australia (Behara and Yamagata, 2001). This dipole also has an impact on tropical cyclone trajectories in the SWIO basin (0S-40S, 30E-90E) (Ash and Matyas, 2010). However, the potential impact of SIOD on both maritime ITCZ activity and cyclonic activity is much less documented. The two most recents positive SIOD events (2010/2011) and (2016/2017) occurred one year after moderate to strong positive ENSO events. Over these two periods the cyclonic activity in the SWIO basin was very low both in terms of frequency and accumulated cinetic energy (ACE). The activity was, in particular, almost totally inhibited between November and January (1 system in 2010-2011, none in 2016-2017). The current study investigates the possible relationships between cyclonic activity and space-time distribution of water vapor in the basin. Using GPS total zenithal delay from several island of the south Indian Ocean and NCEP reanalysis we show that the observed weak cyclonic activity in 2011 and 2017 was well correlated with a dry anomaly in total water content within a large band extending from 0S 35E to 30S 100E. The causes of this dry anomaly are an above than average large scale subsidence over the tropical south central Indian Ocean and colder than average sea surface temperature west of Australia.