Marine heatwaves across the central South Pacific: characteristics, mechanisms, and modulation by El Niño Southern Oscillation

Abstract

Marine heatwaves (MHWs) are intensifying with climate change, endangering ecosystems such as coral reefs. Yet their regional characteristics and drivers remain poorly understood in many parts of the Pacific. Here we provide a comprehensive assessment of MHWs in the central South Pacific and across the five archipelagos of French Polynesia (FP; representing more than 5 million km2 of maritime area, a region as vast as Europe), using sea surface temperature observations and an ocean reanalysis to investigate underlying mechanisms. MHW characteristics vary widely across the region: its northern and southern parts (the Marquesas and Austral archipelagos, respectively) experience the highest number of MHW days and the strongest cumulative intensities, especially during the warm season (November–April). In contrast, its central part (the Society, Tuamotu, and Gambier Islands) exhibits more moderate MHW characteristics. Heat budget analyses highlight the seasonally and regionally diverse mechanisms shaping MHWs. In central FP during the warm season (austral summer), most MHWs are driven by air–sea heat fluxes, while in the northern part, those driven by oceanic horizontal advection dominate. During the cold season (austral winter), more MHWs driven by horizontal advection are observed in the whole region since the thicker seasonal mixed layer reduces the proportion of MHWs driven by air–sea fluxes. El Niño–Southern Oscillation (ENSO) strongly modulates MHW occurrences: El Niño favors MHW occurrences in northeastern FP, while La Niña increases MHW occurrence in the southwest with different spatial extent depending on ENSO flavors (Central or Eastern Pacific ENSO events). This modulation arises from reduced wind-evaporation cooling with reduced wind speed, shoaled mixed layers, and enhanced horizontal heat advection, occurring primarily to the northeast of French Polynesia during El Niño and to the southwest during La Niña. These results greatly improve our understanding of MHW characteristics, dynamics and variability in this ecologically-fragile region.