Oceanic Preconditioning and Wind-Driven Amplification of the 2024/2025 Marine Heatwave off Western Australia

The western coast of Australia is a global hotspot for intense marine heatwaves (MHWs). During the austral summer of 2024/2025, an extreme and persistent MHW affected the North West Shelf of Western Australia, lasting more than seven months and spanning over a large shelf and slope region, influenced by a weak La Niña. The event was associated with severe ecological impacts, including widespread fish kills and coral reef degradation, and was characterised by pronounced warming throughout the upper ocean, with subsurface temperature anomalies extending to depths of approximately 200 m.

We investigate the physical processes underpinning this event, with a particular focus on the interplay between atmospheric forcing and oceanic preconditioning. We examine the hypothesis that a period of elevated ocean heat content along the continental shelf, in the absence of strong atmospheric forcing, first triggered subsurface MHW that remained weak or undetectable at the surface. We also examine whether the emergence of a surface-intensified MHW depends on the timing and magnitude of atmospheric anomalies acting on this preconditioned ocean state.

Enhanced subsurface heat storage, suppressed vertical mixing in the water column, anomalous air–sea heat fluxes, and variability in boundary current transport may interact to promote the surface expression, vertical extent, and persistence of MHWs. Our results suggest that elevated ocean heat content alone does not consistently lead to surface extremes, while its coincidence with favourable atmospheric conditions may contribute to particularly intense and vertically extensive events.

Finally, we place the event in a broader climate context by considering how projected changes in both background ocean heat content and air–sea heat fluxes in CMIP6 simulations may favour more frequent, persistent, and vertically extensive subsurface and surface MHWs along the North West Shelf of Western Australia. Increasing ocean heat content, together with shifts in the magnitude, timing, and persistence of atmospheric forcing, may enhance the likelihood that subsurface warming is expressed at the surface and sustained over longer periods. These insights aim to advance process-based understanding of extreme MHWs and support the development of early warning and ecosystem risk assessment frameworks for vulnerable shelf regions.