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.

Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

“…Submesoscale motions can trigger the formation of mesoscale meanders, but the opposite is also true. The interaction of mesoscale meanders with topography can ignite symmetric and inertial instabilities (Napolitano et al, 2021a), and generate Submesoscale Coherent Vortices (SCVs) (Lazaneo et al, 2022). Such processes can enhance the vertical exchange of tracers and lead to energy dissipation (Gula et al, 2016b;Lazaneo et al, 2022).…”

“…In the last sections, we have shown that during this process, the shear between the cyclones and the topography increases anticyclonic vorticity and generates downscale KE uxes. In this section, we aim to answer if this process is associated with the development of inertial/symmetric instabilities and, subsequently, with the generation of anticyclonic SCVs (e.g., Molemaker et al, 2015;Morvan et al, 2019;Napolitano et al, 2021b;Lazaneo et al, 2022).…”

“…Some can evolve into mesoscale eddies, while others can be destroyed when hitting the western boundary. The lifetime of these SCVs is smaller than those advected by the large-scale ow in the interior ocean (e.g., Dewar & Meng, 1995;Lazaneo et al, 2022). However, they play an important role in cascading energy to both larger and smaller scales.…”

“…However, they play an important role in cascading energy to both larger and smaller scales. In fact, Lazaneo et al (2022) observed that anticyclonic SCVs can lead to kinetic energy dissipation in the interior ocean. Therefore, the SCVs described here may constitute an important sink of energy for the Brazil Current.…”

The Circulation On The Rio Grande Rise: Mean Flow, Internal Tides, and Eddies

“…One of the processes responsible for enhancing the submesoscale dynamics are the interaction of local currents with the topography. Submesoscale studies about ow-topography interaction are recent (Ruan et al, 2017;Garabato et al, 2019;Napolitano et al, 2021;Lazaneo et al, 2022), as well as studies that investigate the dynamics of canyons through high resolution numerical modeling (Vic et al, 2018;Lahaye et al, 2019). The numerical models reveal that the large scale currents interacting with steep slopes, seamounts and ridges allow the formation of submesoscale currents (Gula et al, 2016b;Srinivasan et al, 2019;Vic et al, 2018) that create high rates of local kinetic energy dissipation (Gula et al, 2016b).…”