Record-breaking 2023 marine heatwaves

The year 2023 witnessed an extraordinary surge in marine heatwaves (MHWs) across Earth's oceans, setting new records in duration, extent, and intensity, with MHW activity totaling 53.6 billion °C days square kilometer-more than three standard deviations above the historical norm since 1982. Notable events include the North Atlantic MHW (276-year return period) and the Southwest Pacific (141 years). Using ECCO2 (Estimating the Circulation and Climate of the Ocean-Phase II) high-resolution daily data, we conducted a mixed-layer heat budget analysis and identified region-specific drivers: enhanced shortwave flux and a shallower mixed layer in the North Atlantic and North Pacific, reduced cloud cover and increased advection in the Southwest Pacific, and oceanic advections in the Tropical Eastern Pacific. The 2023 MHWs highlight the intensifying impacts of a warm climate and the challenges in understanding extreme events.