Scale‐Dependent Drivers of Marine Heatwaves Globally

Marine heatwaves (MHWs) are prolonged extreme warm water events, threatening marine ecosystems. Understanding drivers of MHWs over the global ocean is essential for their forecast. Here, we use an eddy‐resolving coupled global climate model with improved realism of MHWs to evaluate the drivers of MHWs at different spatial scales, that is, MHWs defined based on temperature anomalies at different spatial scales. The properties of MHWs are scale‐dependent, being generally weaker, less frequent, and longer with increasing spatial scales. The primary driver of MHWs shifts from local oceanic intrinsic advection to atmospheric forcing as their spatial scale becomes larger. The transition spatial scale between the ocean and atmosphere‐driven regimes varies geographically, being larger in eddy‐rich regions but smaller in gyre interior. This study suggests the complicated dynamics of MHWs at different spatial scales and provides guidance on improving their forecast capacity. Plain Language Summary Increasing greenhouse gas emission causes ocean warming, triggering frequent extreme warm water events known as marine heatwaves (MHWs). An in‐depth knowledge of the drivers of MHWs globally is essential for improving their forecast capacity. In this study, we demonstrate the dominant drivers of MHWs vary with their spatial scales based on a state‐of‐the‐art high‐resolution global climate simulation. Smaller‐scale MHWs are primarily driven by oceanic processes, whereas atmospheric processes play a dominant role in driving larger‐scale MHWs. The transition spatial scale between the ocean and atmosphere‐driven regimes is region‐dependent. It is generally larger in regions with energetic ocean currents such as the western boundary currents as well as their extension, but smaller in the gyre interior where the ocean is more quiescent. Key Points Dominant drivers of Marine heatwaves (MHWs) shift from oceanic to atmospheric processes as their spatial scale increases The transition spatial scale from ocean to atmosphere‐driven MHWs varies geographically The transition spatial scale is larger in eddy‐rich regions while smaller in gyre interior