Large-scale aggregation of humid heatwaves exacerbated by coastal oceanic warming

Large-scale concurrences of humid heatwaves substantially escalate the risks of heat-related mortality. However, quantifying the origins of these concurrent extremes remains challenging. Here we use a complex-network approach applied to ERA5 reanalysis and OISST data. Our study reveals that the observed intensification of humid heatwaves is closely associated with coastal oceanic warming over the period 1982–2023. This linkage is more pronounced for the large-scale aggregation of extreme humid heatwaves than for the locally confined events. In particular, approximately 50% and 64% of the upward trends in humid heatwave frequency and spatial-aggregation strength over hotspot regions are linked to their adjacent oceans, respectively.

Subsequently, we elucidate the mechanisms using a composite analysis and conducting CESM model experiments. These land–ocean linkages largely arise from tropical ocean-driven moisture transport towards land regions and from coupled terrestrial–oceanic warming in the mid-to-high latitudes associated with atmospheric Rossby waves. One example is that North Indian Ocean warming triggers the occurrence of large-scale humid heatwaves over South Asia and Western Asia, by exciting a low-level cyclonic circulation anomaly over the Arabian Sea. Enhanced moisture is transported into the land area, which aggravates the terrestrial humid-heat risks. Overall, compared with mid-to-high latitudes, the tropics encompass most high-risk areas and exhibit stronger land–ocean linkages, highlighting the critical role of tropical oceans. Climate model (CESM and CMIP6 multi-model) experiments further demonstrate the influence of tropical oceans on adjacent terrestrial humid heatwaves. Our study provides insights that coastal sea surface temperature can potentially be a crucial precursor of the large-scale aggregation of humid heatwaves.

Reference: https://doi.org/10.1038/s41561-026-01952-z.