The Changing Tropical Cyclone Hazard in the South-West Indian Ocean and Madagascar Regions between 1981-2025

Abstract Tropical cyclones (TCs) are amongst the most destructive and costly natural hazards that occur around the world and severely affect vulnerable countries such as Madagascar and Mozambique. This study investigates changes in TC hazard in the South-West Indian Ocean (SWIO), the Main Tropical Cyclone Region (MTCR) and the Madagascar Region over 45 TC seasons (1981–2025) and examines if there is an association between TC hazard metrics (frequency, maximum sustained wind speed [MSW] and accumulated cyclone energy [ACE] and ocean temperatures or climate indices. Seasonal slopes in frequency and ACE showed no significant values, while MSW increased significantly in all regions (2.8 to 4.8 knots per decade), strongest in the Madagascar Region. The number of Category 4 TCs per decade was found to have increased significantly in the SWIO. Sea surface temperature (SST) and ocean temperatures at 35 m and 45 m depths warmed significantly in all regions (0.13 to 0.25 °C per decade), with the subsurface depths warming at a higher rate. The Southern Oscillation Index (SOI) saw the only significant long-term trend of all the climate indices, but there was little evidence to suggest that any long-term trends in the TC metrics could be explained by any long-term changes in the climate indices. Seasonal associations between TC metrics and climate indices were largely absent, apart from the El Niño–Southern Oscillation indices on TC metrics in the Madagascar Region, where La Niña-like conditions coincided with higher frequency, MSW and ACE. Exploratory analyses suggested higher TC formation and TC-days in Madden–Julian Oscillation phases 2–4 in the SWIO and MTCR. TC hazard in the SWIO and most clearly in the Madagascar Region has increased due to the significant rise in MSWs over the period 1981–2025.