Long-term study of the MHWs in the Mediterranean Sea and the Red Sea: an assessment of their trends, drivers, and their early indicators

MHWs have become one of the most significant manifestations of ocean warming, with profound implications for marine ecosystems, regional climate, and extreme weather events. This thesis provides a comprehensive analysis of MHW dynamics in two thermally sensitive semi-enclosed basins, the Mediterranean Sea and the Red Sea, by integrating surface and subsurface observations, atmospheric forcing, ecological responses, and interactions with compound and extreme events. The objective is to move beyond a surface-based description of MHWs and develop an integrated understanding of their physical drivers, vertical structure, and impacts within the coupled ocean–atmosphere system.
Using daily datasets spanning 1982–2024, MHWs were identified following the Hobday et al. (2016) framework, while additional atmospheric and oceanic variables were analyzed to characterize the mechanisms controlling their evolution. The results show a significant intensification of MHWs in both basins, with increasing frequency, duration, and cumulative intensity over recent decades, consistent with global trends. However, strong regional variability is observed. In the Mediterranean Sea, a west–east contrast emerges, with more intense and variable events in the western basin and more persistent events in the eastern basin. In the Red Sea, a pronounced north–south gradient reflects the influence of stratification and basin-scale circulation.
The contribution of long-term warming is found to be a key factor in MHW occurrence, with a substantial fraction of events directly attributable to the upward shift in background sea surface temperature. Nevertheless, detrended analyses reveal that internal variability remains essential, indicating that MHWs arise from the interaction between gradual warming and short-term atmospheric and oceanic processes. Atmospheric forcing is identified as the primary trigger of MHW development, with events consistently associated with positive air temperature anomalies, reduced wind speed, and enhanced surface heat fluxes. Strong air–sea coupling is confirmed by the high correlation between sea surface temperature and near-surface air temperature, particularly in the Mediterranean basin.
Beyond surface processes, this thesis demonstrates that MHWs exhibit a pronounced vertical structure. Subsurface temperature analyses reveal that MHWs extend well below the mixed layer, with anomalies penetrating to depths of several hundred meters. This vertical extent is modulated by stratification and mixed layer dynamics, which control the redistribution and retention of heat within the upper ocean. The presence of subsurface heat reservoirs provides a mechanism for ocean memory, allowing thermal anomalies to persist beyond the duration of atmospheric forcing and influencing the development of subsequent events.
The ecological impacts of MHWs are assessed through the analysis of mixed layer depth, chlorophyll-a, and nitrate concentrations. Results show that MHWs lead to enhanced stratification, reduced nutrient availability, and decreased primary productivity in both basins, with stronger effects in oligotrophic regions such as the eastern Mediterranean and the southern Red Sea. A compound stress framework further reveals that physical and biogeochemical anomalies frequently co-occur, amplifying ecosystem stress. Complementary laboratory experiments on zooplankton (Acartia tonsa) demonstrate that elevated temperatures significantly increase mortality rates, indicating direct biological impacts in addition to large-scale environmental changes.
The analysis of compound marine and atmospheric heatwaves shows that the most extreme events occur when MHWs and atmospheric heatwaves co-occur. These compound events are characterized by stronger intensity, longer duration, and enhanced spatial coherence, reflecting nonlinear amplification driven by coupled ocean–atmosphere feedbacks. Their frequency has increased over time, particularly during summer, when stratification and radiative forcing maximize air–sea interaction.
A novel aspect of this thesis is the investigation of interactions between MHWs and Mediterranean tropical-like cyclones (Medicanes). The results show that Medicanes preferentially develop over regions with positive sea surface temperature anomalies and elevated ocean heat content, suggesting that MHWs provide favorable conditions for storm intensification. In turn, Medicanes induce strong ocean feedbacks through vertical mixing and surface cooling, leading to the redistribution of heat within the water column. This bidirectional interaction highlights the role of MHWs within a broader system of coupled extreme events.
Overall, this thesis demonstrates that MHWs in the Mediterranean Sea and the Red Sea are not isolated surface phenomena but rather emerge from the interaction of atmospheric forcing, ocean stratification, and subsurface heat storage, with important implications for ecosystems and extreme weather processes. The results provide a unified framework for understanding MHW dynamics in semi-enclosed basins and highlight the growing importance of compound and vertically structured extremes under ongoing climate change.