Subsurface intensification of marine heatwaves off southeastern Australia: The role of stratification and local winds

Marine heatwaves (MHWs) are becoming more common with record events occurring around the world and unprecedented biological impacts including mass mortality and habitat shifts. However, little is known about the statistical characteristics of MHWs due to the lack of long-term in situ observations. Using two historical data sets spanning from 1953 (and 1992) to 2016, we use a seasonally varying climatology and temperature anomalies to identify and characterize MHW events down to 100 m depth in coastal waters off southeastern Australia. We show that MHWs regularly extend the full depth of the water column, with a maximum intensity below the surface. Extreme temperatures at depth are driven by local downwelling favorable winds that mix the water column and reduce the stratification. These results show the importance of considering subsurface hydrography and that sea surface temperature is insufficient to fully understand MHWs which are having disastrous ecological consequences in coastal regions globally.