Establishing a monitoring approach for marine heatwaves in the Black and Mediterranean Seas

Increased frequency and intensity of marine heatwaves (MHWs) are contributing to cumulative thermal stress on marine systems, as extreme temperature events interact with long-term ocean warming. These impacts are particularly relevant in semi-enclosed basins such as the Mediterranean and Black Seas, where surface warming and rising ocean heat content (OHC) have been especially pronounced over the past decade. While MHW-related effects have been documented in the Mediterranean Sea, assessments remain limited for the Black Sea. Most studies to date have focused on sea surface manifestations of MHWs, although it is well established that such extremes can also extend into near-surface layers below the mixed layer. However, the vertical structure and persistence of MHWs beneath the surface remain less understood, particularly in the Black Sea. In this context, this study aims to identify and characterize MHWs in both surface and subsurface waters in the Mediterranean and Black Seas, and to provide a framework for monitoring MHWs within the upper water column. To this end, MHWs were detected using both sea surface temperature (SST) and OHC anomalies down to 40 m depth, allowing for the assessment of their spatial structure and vertical consistency. SST- and OHC-derived metrics generally show good agreement, particularly in the event's duration and frequency. Seasonal analysis further reveals that summer MHWs are typically more intense and show weaker surface–subsurface correlations, whereas winter events are more vertically coherent. Discrepancies between surface and subsurface MHW metrics are particularly evident in the Black Sea, where circulation patterns associated with the Rim Current appear to influence the vertical structure of heat extremes, leading to subsurface events that are not captured by surface-based metrics. In the Adriatic Sea, enhanced MHW occurrence promotes persistent stratification and reduced winter mixing, affecting both surface and subsurface layers. In 2024, both the Black Sea and Mediterranean Sea experienced record-high MHW intensities and durations, with events lasting over 30 d and temperature anomalies exceeding 4.6 °C in key regions such as the Anatolian coast, where summer upwelling commonly influences local ocean conditions. To improve MHW monitoring, we recommend integrating both SST- and OHC-based methods to capture surface and subsurface events in these regions. Nevertheless, due to regional variability, multiple MHW definitions such as threshold-based approaches, including the Hobday framework which also defines MHW categories, are necessary to accommodate diverse monitoring objectives and management strategies.