Climate models overestimate the spatial coherence of global marine heat waves

Marine heat waves (MHWs) are among the most impactful oceanic extremes, yet their spatial structure remains poorly constrained in climate models. Here we show that low-resolution (LR) climate models systematically overestimate the spatial coherence of marine heat waves, producing events that are unrealistically connected across large ocean regions. Using the same LR–high-resolution (HR) model hierarchy previously employed in Gou et al. (2024) and Gou et al.(2025), we quantify MHW spatial structure using decorrelation length scales and snapshot-based connectivity diagnostics, and compare simulations to satellite-based observations. Observed MHWs exhibit rapid spatial decorrelation and fragmented event patterns. In contrast, LR simulations show decorrelation lengths that are too long and MHW snapshots dominated by basin-scale connected components. Increasing horizontal resolution substantially reduces spatial coherence, increases the number of effective spatial degrees of freedom, and brings both decorrelation scales and event connectivity closer to observations. The strongest resolution sensitivity is found in the Southern Ocean, where LR models produce unrealistically synchronized extremes. Our results demonstrate that model resolution fundamentally controls the spatial organization of marine heat waves, with important implications for impact assessments and future projections.