Long-term variability of the coastal ocean stratification in the Gulf of Naples: Two decades of monitoring the marine ecosystem at the LTER-MC site, between land and open Mediterranean Sea

We analyze 20 years (2001-2020) of temperature and salinity profiles at the LTER-MC coastal station in the Gulf of Naples, Mediterranean Sea. Surface and bottom layers show weak increases of temperature (+0.01 +/- 0.01$$ +0.01\pm 0.01 $$ and +0.03 +/- 0.02 degrees C/year$$ +0.03\pm {0.02}<^>{{}<^>{\circ}}\mathrm{C}/\mathrm{year} $$, 2005-2019); water-columns budgets (heat, freshwater) show pseudo-periodic oscillations every 3 to 5 years, and weak linear trends. Seasonal minimum of salinity occurs 2 months later than the runoff peak, pointing to the importance of horizontal circulation in regulating the inshore-offshore exchanges and the residence time of freshwater. Inter-annual variations of the mixed layer depth (MLD) indicate a shallowing (-1.27 +/- 0.38m/year$$ -1.27\pm 0.38\mathrm{m}/\mathrm{year} $$ during winter) and a shortened time span of the fully mixed water-column. A visible decadal shift in the external forcings suggests an influence of winterly wind stress in 2010-2019, that prevailed over dominant buoyancy fluxes in 2001-2009. Changes are visible in the large-scale indices of the North Atlantic and Western Mediterranean Oscillations and highlight the role of wind direction, offshore or inshore oriented, in disrupting the stratification driven by freshwater runoff. A Random Forest Regression confirms that role and quantifies the MLD's drivers importance. This allows for a reliable prediction of the stratification using external variables independent from the in-situ observations.