Deep marine heatwaves: relating drivers and characteristics.

Ocean temperature extremes occur at different scales, both in time, considering the frequency and duration of the events, and space. Here we focus on the vertical extent of marine heatwaves (MHWs) and show that extreme anomalous temperatures extend deeper than the surface mixed layer, with significant anomalies reaching hundreds of meters in the open ocean. Interestingly, the depth extent of MHWs, sub-surface anomaly and seasonality are not a simple function of the surface signature of the events, and vary between the coastal and the open ocean due to different drivers.

Using in situ temperature observations from shelf moorings and Argo floats off southeastern Australia, we contrast the predominant effect of wind stress and mesoscale ocean dynamics in driving deep MHWs. We show that downwelling-favorable winds and anticyclonic eddies are responsible for the deep anomalous warm water on the shelf and offshore, respectively, while anomalous air-sea fluxes tend to generate shallow ocean temperature anomalies. As a result, satellite observations of sea surface temperature associated with sea level anomaly from altimetry and wind stress forecasts could be used as a first step to predict the sub-surface intensity and environmental impacts of MHWs.