A note about density staircases in the Gulf of Naples : 20 years of
persistent weak salt-fingering layers in a coastal area
Abstract
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.