Drivers of Global Marine Heatwaves in a Warming World

Global warming has exacerbated occurrence of extreme events, threatening the environment of human living. Marine heatwaves (MHWs) are prolonged extreme warm water events in the ocean, exerting devastating impacts on marine ecosystems. Comprehensive knowledge of physical processes controlling MHW life cycles is pivotal to improving MHW forecast capacity, yet it is still lacking. Here, we use a historical simulation from a global eddy-resolving climate model with an improved representation of MHWs, and innovatively show that heat flux convergence by oceanic mesoscale eddies acts as a dominant driver of MHW life cycles over most parts of the global ocean. In particular, the mesoscale eddies make an important contribution to growth and decay of MHWs, whose characteristic spatial scale is comparable or even larger than that of mesoscale eddies. Moreover, our results proved that features of global MHWs are scale-dependent. The primary drivers of MHWs shift from oceanic advection to atmospheric forcing as their spatial scale becomes larger. There is evident geographic heterogeneity in the transition scale between these oceanic and atmospheric-process dominated regimes. Our study reveals the crucial role of mesoscale eddies in controlling the global MHW life cycles and highlights that using eddy-resolving ocean models is essential for accurate MHW forecasts. Another contribution is we clarified the transition scale of global MHWs, which is essential for parameterization of MHWs forecasting in a warmer future.