Marine heatwaves (MHWs) may have severe and long-lasting impacts on marine life which depend on species’ thermal thresholds and ability to adapt, but also depends on the characteristics of the events, including their depth structure. However, due to a paucity of measurements below the surface, the vertical structure of MHWs is poorly understood. Here we use one of the longest (27 years) records of daily sub-surface ocean temperature anywhere, spanning the water column in 65 m of water off Sydney, to explore the sub-surface coherence of MHWs and its controls.
We show that MHW events often extend over the whole water column in weakly stratified months and are the longest, driven by the intrusion onto the shelf of warm large-scale currents, while during periods of strong stratification, shallow and deep MHW events are typically uncoupled.
Shallow events are mostly driven by anomalous air-sea fluxes, while downwelling favourable winds trigger sub-surface MHWs. The sub-surface events tend to be the most intense, but also the shortest.
These results highlight that, when considering coastal areas where maximum biological damage is reported, sea surface temperature extremes such as MHWs, are not representative of the water column below the shallow mixed layer during stratified months.