Seasonal variability and multi-decadal trends of the mixed layer depth in the central Mediterranean

Abstract

This study examined seasonal variability and multi-decadal trends of the Mixed Layer Depth (MLD) in the central Mediterranean. The primary objectives included addressing regional research gaps and evaluating three estimation methodologies to determine the most physically consistent approach for the region. The methods examined were the vertical temperature gradient, fixed temperature threshold, and density threshold (TEOS-10). MEDSEA_MULTIYEAR_PHY_006_004 physical reanalysis product from Copernicus Marine Service was used to provide daily temperature and salinity fields at a 1/24° (~4.5 km) horizontal resolution across 141 vertical levels. These reanalysis data were used to identify long-term trends and shifts in the MLD through daily estimates analyzed over 39 years, from 1 January 1987 to 1 January 2026. The findings indicate that the density threshold method (TEOS-10) was the most suitable framework. The most noteworthy result was a statistically significant spring shoaling trend of -0.87 m per decade that is indicative of seasonal stratification occurring earlier every decade. A possible association between MLD anomalies and to the presence of a gyre is identified. The results contribute to current regional knowledge of seasonal variability and multi-decadal trends, identify a statistically significant shoaling trend, and identify a possible association of MLD anomalies to the Malta-Sicily Gyre (MSG).