Barrier Layers in the Tropical South Atlantic: Mean Dynamics and Submesoscale Effects*

Abstract

Barrier layers are generated when the surface mixed layer is shallower than the layer where temperature is well mixed, in geographical regions where salinity plays a key role in setting up upper-ocean density stratification. In the tropical oceans, thick barrier layers are also found in a latitude range where spiraling trajectories from surface in situ drifters suggest the presence of predominantly cyclonic submesoscale-like vortices. The authors explore these dynamical processes and their interplay in the present paper, focusing on the tropical South Atlantic Ocean and using a high-resolution modeling approach. The objective is threefold: to investigate the mean dynamics contributing to barrier-layer formation in this region, to study the distribution and seasonality of submesoscale features, and to verify whether and how the submesoscale impacts barrier-layer thickness. The model used is the Regional Ocean Modeling System (ROMS) in its Adaptive Grid Refinement in Fortran (AGRIF) online-nested configuration with a horizontal resolution ranging between 9 and 1 km. The simulated circulation is first described in terms of mean and submesoscale dynamics, and the associated seasonal cycle. Mechanisms for barrier-layer formation are then investigated. The results confirm previous hypotheses by Mignot et al. on the relevance of enhanced winter mixing deepening the isothermal layer, whereas the salinity stratification is sustained by advection of surface fresh waters and subsurface salinity maxima. Finally, submesoscale effects on barrier-layer thickness are studied, quantifying their contribution to vertical fluxes of temperature and salinity. Submesoscale vortices associated with salinity fronts are found to have a significant effect, producing thicker barrier layers (by ~20%–35%) and a shallower mixed layer because of their restratifying effect on salinity.

Subduction by Submesoscales

“…The simplest suggestion h = MLD yields zero subduction rate, see equation (27). Such a suggestion is also not in agreement with several studies (Badin et al, 2011;Luneva et al, 2015;McWilliams, 1985;Mensa et al, 2013;Ramachandran et al, 2014;Thomas, 2008;Treguier et al, 1997;Veneziani et al, 2014) that showed that h > MLD implying that below the ML, the flow is still diabatic. The inadequacy of the mixed layer depth to represent the near-isothermal upper layers was also discussed by Gregory (2000, section 2).…”

“…Since the results we obtain depend on the underlying instabilities represented by BIs and wind stress, use of an ocean general circulation model (OGCM) to compute the 2‐D velocity that appears in does not allow us to identify the relative contributions of the two processes to the SM subduction rates. To do so, one needs an analytic treatment in which we take to be the sum of geostrophic and a‐geostrophic components, , where where τ ( z ) is the wind stress and where in the first relation in we have used a constant in the mixed layer as suggested by the results of numerical simulations (Mensa et al, ; Veneziani et al, ). We begin with the geostrophic component .…”

“…where τ(z) is the wind stress and where in the first relation in (18) we have used a constant ∇ H b in the mixed layer as suggested by the results of numerical simulations (Mensa et al, 2013;Veneziani et al, 2014). We begin with the geostrophic component.…”

Submesoscale contribution to subduction: Tracer and momentum fluxes

“…However, the SM parameterizations (6)–(9) do not provide a model for h, specifically, its relation with the mixed‐layer depth H. Analytic studies presented below indicate that the sign of tracer fluxes depend on the difference and whether in a given location there is subduction or obduction depends on whether (11) is positive or negative. Previous studies [ McWilliams , ; Mensa et al ., ; Veneziani et al ., ] concluded that but since they were not specific to the ACC, it may not be legitimate to extrapolate (12) to that region. The model we present in section 5 reproduces relation (12) away from the ACC and shows that in the latter there are three distinct regions characterized by h > H, h = H and h < H (see Figure ).…”

“…To better understand these features, we derive the analytic form of the tracer fluxes. To do so, we take the 2‐D mean velocity as the sum of geostrophic and a‐geostrophic (wind‐driven) components where is the wind stress and in the second relation we used the fact that the horizontal buoyancy gradient is constant in the mixed layer [ Mensa et al ., ; Veneziani et al ., ]. Using the second of (13) in (7) and then in (6), we obtain that at z = −H,…”