Three Decades of Mediterranean Mixed Layer Depth Change: Basin Trends and Regional Contrasts

Javad Babagolimatikolaei^*

• The University of Manchester, Manchester, United Kingdom

The mixed layer depth (MLD) regulates air–sea heat fluxes, nutrient entrainment, and ecosystem dynamics in the Mediterranean Sea, making its long-term evolution a sensitive indicator of climate-driven changes in ocean stratification and surface thermal conditions. Using a high-resolution Mediterranean physics reanalysis and daily mixed-layer estimates (1990–2019), we quantify basin-wide changes in MLD, their seasonal and regional variability, and the physical drivers behind them. Across the Mediterranean, MLD exhibits strong seasonal and interannual variability, but a persistent long-term shoaling of ≈0.5–0.6 m per decade is evident. This trend is most robust in spring and late autumn (notably April, November and December) and is linked primarily to enhanced near-surface stratification driven by surface warming and subsurface salinification; surface Brunt–Väisälä frequency (N²) increased by ≈4.26×10⁻⁷ s⁻² per decade in the upper 0–50 m. Changes in wind stress are spatially heterogeneous and too weak to explain the basin-scale shoaling, although localized winter intensification (e.g., Gulf of Lion) produced episodic deepening and strong convection in the 1990s–2000s. Sub-basin contrasts are pronounced: the Levantine Basin shows consistent shoaling, the Southern Adriatic displays modest variability, and the NW Mediterranean experienced transient deepening episodes linked to dense-water formation. Continued shoaling has important implications for winter ventilation, nutrient supply to the euphotic zone, and the vulnerability of Mediterranean ecosystems to prolonged marine heatwaves.