Climate Change Intensifies Surface-Subsurface Compound Marine Heatwaves

Marine heatwaves (MHWs) — transient events of anomalously high sea surface temperatures — pose severe threats to marine ecosystems. This is particularly critical for Large Marine Ecosystems (LMEs), which, despite covering only 22% of the global ocean, account for up to 95% of the world’s fisheries catch. However, the coastal complexity of LMEs and biases in coarse-resolution climate models have hindered a precise understanding of how climate change modulates MHWs in these vital regions.

This study employs high-resolution, eddy-resolving Earth system model projections to systematically investigate the future evolution of MHWs. Our high-resolution framework significantly improves the simulation of key MHW characteristics by better resolving mesoscale oceanic processes. For instance, it reduces the global bias in simulated mean annual surface MHW frequency by approximately 50% compared to conventional low-resolution models (from -0.60 to -0.31 events per year), thereby providing a more reliable basis for future projections.

Under a high-emission scenario (RCP8.5), projections using the conventional "fixed historical threshold" reveal a dramatic increase in the intensity and annual occurrence days of both surface and subsurface MHWs by the end of the century. For example, the global mean surface MHW intensity is projected to increase by about 1.2 °C. However, this conventional method conflates the effects of long-term mean warming and increased temperature variability, obscuring the critical role of the latter.

To isolate the contribution of enhanced temperature variability, we propose and apply a "future threshold" method, which defines MHWs relative to the shifting long-term mean climate. Strikingly, even after removing the background warming signal, surface and subsurface MHWs are projected to intensify globally, with the surface mean annual MHW days increasing by approximately 2.8 days. This underscores the pivotal role of amplified ocean temperature variability in driving future MHW increases.

This effect is especially pronounced in coastal LME regions. Our "future threshold" analysis indicates that in 83% of LME areas, the intensification of subsurface MHWs surpasses that of surface MHWs, primarily due to greater increases in subsurface temperature variability. This finding suggests that global warming is eroding the vertical thermal refuge for marine organisms, as their adaptive capacity to escape surface heat by moving deeper is increasingly constrained.

Furthermore, we document a profound increase in compound MHW events, where extreme heat co-occurs at the surface and subsurface simultaneously. The projected increase in the frequency of such compound events is about tenfold greater than that of single-layer events. This intensified coupling, also driven by enhanced variability, indicates a move towards more pervasive and vertically extensive marine heat stress. Coupled with concurrent stressors like ocean acidification and deoxygenation, these compound events represent a multi-dimensional threat to deep-sea ecosystem stability and biodiversity.

In summary, high-resolution modeling reveals that global warming threatens marine ecosystems via dual drivers: persistent mean warming and, more critically, amplified temperature variability. This drives an escalation of surface, subsurface, and compound marine heatwaves, especially in crucial Large Marine Ecosystems, underscoring an urgent need for mitigation strategies to protect ocean health and resources.