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

摘要

We analyze 20 years (2001–2020)of temperature and salinity profiles at the LTER–MCcoastal station in the Gulf of Naples, Mediterranean Sea. Surface and bottom layersshow weak increases of temperature (+ 0.01 ± 0.01 and + 0.03 ± 0.02℃∕year,2005–2019);water-columnsbudgets (heat, freshwater) show pseudo-periodicoscillationsevery 3 to 5 years, and weak linear trends. Seasonal minimum of salinity occurs2 months later than the runoff peak, pointing to the importance of horizontalcirculation in regulating the inshore–offshoreexchanges and the residence time offreshwater. Inter-annualvariations of the mixed layer depth (MLD) indicate a shallowing(? 1.27 ± 0.38m∕year during winter) and a shortened time span of the fully mixedwater-column.A visible decadal shift in the external forcings suggests an influence ofwinterly wind stress in 2010–2019,that prevailed over dominant buoyancy fluxes in2001–2009.Changes are visible in the large-scaleindices of the North Atlantic andWestern Mediterranean Oscillations and highlight the role of wind direction, offshoreor inshore oriented, in disrupting the stratification driven by freshwater runoff. ARandom Forest Regression confirms that role and quantifies the MLD's drivers importance.This allows for a reliable prediction of the stratification using external variablesindependent from the in-situobservations.