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Marine heatwaves (MHWs) are episodes of anomalous warming in the ocean, responsible for widespread impacts on marine ecosystems. For the first time summer MHW variability at surface and three ecosystem-relevant depths of the upper Mediterranean Sea are assessed here for 1982-2017. We apply a MHW detection algorithm on a hindcast simulation, performed with a high-resolution, fully coupled regional climate system model. Identified surface events last, on average, 15 days with a mean intensity of 0.6 degrees C above threshold and a maximum sea surface coverage of around 39%. Subsurface events are seasonally shifted and appear, on average, longer and more intense but less frequent and less extended in space than surface MHWs. We also find significant trends of increase in most MHW properties throughout the period, with severe surface MHWs detected for the first time in 2012, 2015, and 2017. However, MHW spatial << hot spots >> are inhomogeneously distributed in surface and deeper layers.

Plain Language Summary Marine heatwaves (MHWs) are episodes of prolonged and extended anomalous warming in the ocean, responsible for widespread impacts on marine ecosystems and related fisheries. Here, we identify for the first time all the MHWs that have occurred at surface and three ecosystem-relevant depths of the Mediterranean Sea between 1982 and 2017. To do this, we use the output from the latest version of a regional, high-resolution climate system model and a consistent detection framework. We find that past surface events, on average, last around 2 weeks and cover almost half the basin with above-normal temperatures by more than half a degree. Subsurface events appear seasonally shifted and on average, longer and more intense but less frequent and less extended in space than surface MHWs. Following an increase of extremes temperatures with time, surface MHWs appear to have become longer, more severe and spatially extended, while record-breaking MHWs are identified for the first time in 2012, 2015, and 2017 in addition to the well-documented 2003 event. However, spatial distribution of MHW << hot spots >> differs at surface and depth.