Seasonal stratification and complex local dynamics control the sub-surface structure of marine heatwaves in Eastern Australian coastal waters

Marine heatwaves are extreme seawater temperature events that can have severe impacts on marine life. The extent of the ecological damage depends not only on the easily observed surface signature but on the marine heatwave structure at depth. However, due to a paucity of in situ sub-surface observations the vertical structure of marine heatwaves is poorly understood. Here we analyse the sub-surface coherence and controls of marine heatwaves using one of the world's longest (28 years) records of daily sub-surface ocean temperature off Sydney, Australia. We show that seasonal stratification, large-scale circulation and local downwelling processes control the vertical coherence of coastal marine heatwaves. We define three classes of marine heatwaves which can extend through the water column, form in the shallow surface layer, or sub-surface independently, and are therefore not always evident in surface data. We conclude that sub-surface data need to be considered in monitoring marine heatwaves in coastal areas where maximum biological damage is reported. Coastal marine heatwaves off Eastern Australia that extend throughout the water column are linked to intrusion of the Eastern Australian Current, whereas those with no surface signature are related to downwelling favourable winds, according to a 28-year in-situ record of daily ocean temperatures.