Submesoscale Coherent Vortices in the South Atlantic Ocean: A Pathway for Energy Dissipation

Abstract

Mesoscale eddies propagate westward across the South Atlantic basin. As they reach the westernmost part of the basin, at approximately 20°S, they interact with a quasi-zonal seamount chain, the Vitória-Trindade Ridge (VTR). The interactions with the local topography lead to submesoscales instabilities, which ignite the formation of submesoscale coherent vortices (SCVs) such as those described in the present study for the first time in the VTR region. Here, using high-resolution hydrographic and microstructure measurements, we describe the dynamics of two adjacent SCVs wandering through the ridge. We find that the anticyclonic SCVs are characterized by a low potential vorticity and angular momentum signature, and are therefore prone to both centrifugal and symmetric instabilities. This dynamic regime suggests small-scale turbulence is actively cascading energy down to dissipation, diagnosed from turbulent kinetic energy dissipation estimates within one of the SCVs through microstructure measurements. The energy dissipation levels observed within the SCV are two orders of magnitude larger than in surrounding waters. The thermohaline signatures of each SCV reveal homogenized waters in their cores but with small thermohaline anomalies when compared to surrounding waters, suggesting a remote generation site. Here, we argue that such vortices are essential agents for energy dissipation in the ocean. We speculate that the observed SCVs were formed due to mesoscale eddy-topography interaction along the VTR and advected by the meandering South Equatorial Current to the location of field observations.

Key Points

• Submesoscale coherent vortices (SCV) are observed around the complex topography of the Vitória-Trindade Ridge

• Small-scale turbulence inside the SCVs can cascade energy down to dissipation via centrifugal and symmetric instabilities

• The amount of turbulent kinetic energy dissipation within the SCV is comparable to that of the mixed layer

Plain Language Summary

The interaction between topography and oceanic flows results in the development of small-scale turbulent phenomena. The occurrence of such phenomena is significant for the ocean energy balance due to energy dissipation, which occurs on spatial scales smaller than a centimeter. In this study, we describe, for the first time in the western South Atlantic Ocean, the physics of two adjacent submesoscale coherent vortices in the vicinity of the Vitória-Trindade Ridge. These vortices have radii of 12 and 16 km and a subsurface signature with intensified velocity and weak stratification. Since this type of vortex has no surface expression, it is challenging to detect it due to its small horizontal scale. Microstructure measurements collected by a ship inside and outside the vortex enable us to evaluate, for the first time, its influence on energy dissipation and ocean mixing.