Large-scale drivers of extreme humid heat on a tall tropical island: small-scale response and future projections

Extreme heat stress is a growing concern, particularly in the tropics. Indeed, high humidity limits the efficiency of thermoregulation through reduced evaporation of sweating, which may become critical with global warming as reported by numerous studies. Besides continents, our previous study based on statistical downscaling of global climate model ensembles showed that small tropical islands are also increasingly exposed to extreme humid heat. However, the drivers of such extreme events have not been investigated for these islands.

We use the ERA5 reanalysis for the 2000-2024 period and daily high-resolution (3 km) gridded 2-m air temperature (T2m) and relative humidity observations over Réunion island in the tropical South-West Indian Ocean (SWIO) to assess the large-scale conditions favourable for the occurrence of extreme humid heat in this tall volcanic island during the warm and humid season. Hierarchical cluster analysis applied to values of the National Oceanic and Atmospheric Administration Heat Index (HI) exceeding the 95^th percentile is used to divide the island into 7 areas (4 coastal and 3 mountainous) with coherent timings of extreme heat occurrence. Such information is combined with a weather regime analysis based on empirical orthogonal functions and k-means clustering of 850 hPa geopotential height from ERA5, allowing to identify weather regimes over the SWIO most favourable to island humid heat extremes. Composite analysis of sea level pressure, 10-m winds, T2m, sea surface temperature (SST) and total column water vapour from ERA5, as well as island T2m, humidity and HI is further used to assess the large-scale drivers of these events and their local responses.

The results show that humid heat extremes over Réunion island are up to ~50% more (respectively less) likely when a mid-latitude low (resp. high) pressure anomaly is located to the south, driving northwesterly (resp. southeasterly) wind anomalies and the anomalous poleward (resp. equatorward) advection of warm and humid (resp. cool and dry) air and warm (resp. cool) SST towards the island. Under the regime most favourable to extreme heat stress, the island experiences widespread warm and humid anomalies during the occurrence of HI extremes over coastal areas. Interestingly though, this is not the case when HI extremes occur over mountain areas: then, such anomalies are typically restricted to the northwest, windward side, while the southeast, leeward side experiences warm, yet dry anomalies as a result of orographic blocking.

Finally, a regional climate model of the SWIO at 12 km resolution, statistically-downscaled over Réunion island using the aforementioned gridded data is used to discuss future projections of extreme island heat stress and of its large-scale drivers over the 21^st century.