Vertical mixing in mesoscale loops of the western tropical Atlantic region (North Brazil Current - Equatorial Undercurrent retroflexion)

Planella-Morató, J.1, Roget, E.1, Pelegrí, J.L.2, Puigdefàbregas, J.3
1Physics Department, University of Girona, Catalonia, Spain
2Physical and Technological Oceanography Dept., Institut de Ciències del Mar, CSIC, Catalonia, Spain
3Maritime Engineering Laboratory, Polytechnic University of Catalonia, Catalonia,Spain

The western tropical Atlantic region plays an important role in both interhemispheric and west-to-east exchange of water-mass properties. In the upper layers, warm and salty waters from the South Atlantic subtropical gyre enter teh basin carried by the North Brazil Current (NBC) along the eastern Brazil continental slope. As the NBC reaches the equator, some waters retroflect eastward as the Equatorial Undercurrent (EUC) while generating mesoscales structures, such as rings and loops that vary in space and time. As a result, the NBC-EUC retroflexion is a complex structure, transportin different types of waters masses across the region.

Here we use data collected during a field campaign in late spring 2018 to analyze the ocean dynamics and mixing processes in the upper layer of the equatorial retroflection zone. 24 hydrographic stations were distributed along two meridional transects across the retroflected loops near the equator. Microstructure measurements were performed at 7 stations (4-6 profiles per station) using a MSS profiler. The velocity field was continuously recorded using a ship mounted ADCP ($\textit z$ >35 m). Stability of the water column was evaluated using the Richardson number $\textit Ri$ and the density ratio $\textit R_ρ$, obtained from the Turner angle $\textit Tu$.

Data analysis reveals the presence of warm and salty water embedded in the stratified upper thermocline (UT), which corresponds to Saline Maximum Water (SMW) advected from the subtropics via the NBC and EUC (core velocities of ~ 1.5 m·s$^{-1}$). Above the SMW, we find tropical waters, slightly warmer and fresher. Underneath the saline current, the weakly stratified lower thermocline (LT) contains colder and fresher central waters, which retroflect reaching maximum speeds of ~ 1 m·s$^{-1}$ in the EUC core. Different water masses in the column favor the conditions for salt-fingering in most of the segments analyzed (about 70% of the segments). However, the velocity field causes strong vertical shear gradients (maximum values of $\textit Sh^2$ ~ 10$^{-3}$ s$^{-2}$), resulting in low $\textit Ri$ scattered through the water column ($\textit Ri$ ~ 1 in 50% of the segments). This leads to conclude that vertical mixing in the upper layer of the retroflected area can be induced by both salt-fingering and/or shear.

Microstructure data shows that about 60% of the segments are active ($\textit Re_b$ > 200), with geometric mean dissipation and Richardson values of $\tilde{\varepsilon}_{act}$ ~ 4 × 10$^{-8}$ W·kg$^{-1}$ and $\widetilde{Ri}_{act}$ = 0.70. In this range, mixing is dominated by shear. Within the thermocline, it is found that $\tilde{\varepsilon}_{act,UT}$ ~15$\tilde{\varepsilon}_{act,LT}$, indicating very energetic events in the SMW layer of the EUC and NBC. For weak turbulence ($\textit Re_b$ < 30; 10% of the data), about 25% of the segments exhibit the most favourable conditions for salt-fingering (1 < $\textit R_ρ$ < 2, $\textit Ri$ >1). The mean salt-finger dissipation $\tilde{\varepsilon}_{SF}$ is ~ 3.5 × 10$^{-9}$ W·kg$^{-1}$, in agreement with results reported in literature for other main streams [e.g. Nagai et al., 2015]. In the transitional stage between weak to energetic turbulence (30 < $\textit Re_b$ < 200; 30% of the segments), the dissipation is similar to values reported for low turbulent activity ($\tilde{\varepsilon}_{wk}$ ~ 5 × 10$^{-9}$ W·kg$^{-1}$, $\widetilde{Ri}_{wk}$ = 1.70, $\tilde{R}_{\rho,SF}$ =2.15) and mixing is possibly affected by salt fingering at a transitional stage between weak and energetic turbulence.