Marine Heatwaves across the central South Pacific: characteristics, mechanisms, and modulation by the 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 (representing ~77 atolls, more than half of world’s atolls, and 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 exposure varies 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 (Nov–Apr). 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, most of MHWs are driven by air–sea heat fluxes, while in the northern part, those driven by oceanic horizontal advection dominate. During the cold season, more MHWs driven by horizontal advection are observed in the whole region since the thicker seasonal mixed layer reduce the proportion of MHWs driven by air-sea fluxes. El Niño– Southern Oscillation (ENSO) strongly modulates MHWs occurrence: El Niño favors MHWs occurrence in northeastern FP, while La Niña increases MHW occurrence in the southwest with different spatial extent across ENSO flavors (Central and Eastern Pacific ENSO events). This modulation arises from reduced wind-evaporation cooling with reduced wind speed, shoaled mixed layers, and enhanced horizontal heat advection. These results greatly improve our understanding of MHW characteristics, dynamics and variability in this ecologically-fragile region.