The Changing Tropical Cyclone Risk in the South-West Indian Ocean over the last 45 tropical cyclone seasons (1981-2025)

Tropical cyclones are among the most devastating natural hazards to occur in the South-West Indian Ocean basin (SWIO), posing considerable risk to vulnerable countries such as Madagascar and Mozambique. This study examines changes in tropical cyclone risk across the SWIO, the Main Tropical Cyclone Region, and the Madagascar Region over the last 45 cyclone seasons (1981–2025). Seasonal and monthly time-series of  key tropical cyclone metrics, namely frequency, maximum sustained wind (MSW), and accumulated cyclone energy (ACE) were computed and analysed for trends. The relationship these metrics have with major oceanic and atmospheric drivers, such as ocean temperature, the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Madden–Julian Oscillation (MJO), were examined.

Results indicate a significant decrease in tropical cyclone frequency in the SWIO and the Main Tropical Cyclone Region, while frequency remained relatively stable in the Madagascar Region. In contrast, MSW increased significantly across all regions (+3.91 Knots per decade), with the strongest intensification occurring within the Madagascar Region (+4.79 Knots per decade). This suggests risk has increased over time in the SWIO despite the occurrence of fewer storms.

Ocean temperatures exhibited significant warming at both surface and subsurface levels, with depths of 35 m and 45 m indicating the greatest warming trends and the strongest relationship with increased cyclone MSW. Cyclone frequency on the other hand was negatively correlated with ocean warming, suggesting warmer waters in the SWIO may create conditions less conducive to the formation of tropical cyclones.

ENSO was found to be a considerable driver of regional cyclone variability, with La Niña conditions associated with higher frequency and stronger cyclones. The MJO was also identified as a key modulator of cyclonic activity, particularly in the Madagascar Region, where active phases 3, 4, and 5 coincided with increased cyclone frequency and MSW. The IOD on the other hand showed little to no influence on cyclone metrics in the SWIO. The incorporation of this research into forecasting and intensity models has the potential to enhance early warning systems in the SWIO, thereby providing a valuable tool for the highly vulnerable region.

(Swan and Hallam, in prep, 2026)