Subsurface thermal stratification as a driver of Mediterranean climate dynamics based on historical time series

This study explores the role of subsurface thermal stratification in shaping Mediterranean climate dynamics over the past century. A nonlinear regression model is applied to historical temperature-depth time series (1900–2019) from the SeaDataNet archive, focusing on four key subregions: the Aegean Sea, the Gulf of Lion, the Strait of Gibraltar, and the Strait of Sicily. The analysis employs the Plateau Followed by One Phase Decay (PFOPD) model to extract depth-resolved thermal metrics, including thermocline depth, surface and deep-water temperatures, and decay rates. The results indicate a long-term pattern of summer shoaling and winter deepening of the thermocline, coupled with regional variability and basin-wide subsurface warming. The PFOPD model demonstrates high explanatory power (R 2 > 0.95 in most summer time series), enabling the detection of robust climatic signals from fragmented historical data. Stratification metrics show strong covariation with atmospheric circulation regimes, highlighting their diagnostic value for climate variability assessments and early-warning systems. These findings are further contextualized by the 2024 flood event in Valencia, presented as an illustrative example of how persistent marine stratification may modulate atmospheric instability, although a formal attribution of the event would require dedicated coupled modeling. Beyond methodological innovation, the study proposes a reconceptualization of the Mediterranean Sea as a depth-integrated climatic system, where subsurface processes shape regional climate feedbacks. By bridging historical ocean data with present-day environmental challenges, this research supports improved monitoring strategies and informs climate-sensitive marine and coastal planning under ongoing warming and intensifying stratification.