A dissection of the physics of seasonal cycle of oceanic upper layer stratification is necessary to improve climate predictions of biogeochemical cycles. We present a time series of vertical profiles of ε, the dissipation rate of turbulent kinetic energy, obtained from a microstructure profiler during the destratification period (summer-to-winter) at a mid-latitude 75m-deep coastal site. Significant correlation is obtained in the mixed layer depth (MLD) with a model combining effects of wind, wave, and buoyancy forcings, estimated from bulk parameters ~10 hours before observations, and used to identify the dominant forcings leading to MLD deepening. Intermittency at surface is correlated with seasonal storminess, and we observe a quadratic relation between kurtosis and skewness for ε statistics. By splitting the time series into layers, we observe the co-location of patches of higher ε with the shear maxima of the two first baroclinic modes, and significant correlations with surface wind stress in the transitional layer the past 24 hours, and at longer scale (4.25 days) in the baroclinic layer, suggesting that internal waves activity influences the setup of mixing intensity despite the lack of tidal forcing. The low-passed microstructure shear distribution seems to support this hypothesis despite possible signal contamination. In the highly stratified layers associated to salt-fingering (MLD’s basis and below), the buoyancy Reynolds number indicates a buoyancy regime control with low mixing value (0.2 x 10e-5 m²/s). More turbulent flows are identified in both surface and bottom layers (0.6 - 0.8 x 10e-5 m²/s), suggesting a seasonal erosion of the stratification by the boundary processes.

This is a short communication about the inter-annual recurring presence at the coastal site in the Gulf of Naples of density staircases visible below the mixed surface layer of the water-column, from the end of summer to the beginning of winter, each year during nearly two decades of survey (2001 to 2020). We repetitively observe sequences from 1 to 4 small vertical staircases structures (~ 3 m thick) in the density profiles (~ Δ0.2 kg/m³), located between 10 m to 50 m deep below the seasonal mixed layer depth. We interpret these vertical structures as the result of double diffusive processes that could host salt-fingering regime (SF) due to warm salty water parcels overlying on relatively fresher and colder layers. This common feature of the Mediterranean basin (i.e., the thermohaline staircases of the Tyrrhenian sea) may sign here for the lateral intrusions of nearshore water masses. These stably stratified layers are characterized by density ratio Rρ 5.0 to 10.0, slightly higher than the critical range (1.0 - 3.0) generally expected for fully developed salt-fingers. SF mixing, such as parameterized (Zhang et al., 1998), appears to inhibit weakly the effective eddy diffusivity with negative averaged value (~ - 1e-8 m²/s). A quasi 5-year cycle is visible in the inter-annual variability of the eddy diffusivity associated to SF, suggesting a decadal modulation of the parameters regulating the SF regime. Even contributing weakly to the turbulent mixing of the area, we hypothesis that SF could influence the seasonal stratification by intensifying the density of deep layers. Downward transfer of salt could have an impact on the nutrient supply for the biological communities, that remains to be determined.

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 layer show increases of temperature (+0.01 and +0.03°C/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 two 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 contribution. Inter-annual variations of the mixed layer depth (MLD) exhibit a shallowing (-1.27m/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 drivers importances. This allows for a reliable prediction of the stratification using external variables independent from the in situ observations.