Abstract

Off the coast of central Chile, subsurface anticyclonic eddies are a salient feature of the oceanic circulation, transporting a significant fraction of coastal water that is rich in nutrients and poor in dissolved oxygen offshore. In this study, the formation mechanism of these eddies is analyzed through a high-resolution (similar to 0.3 km) and low-resolution (similar to 3 km) oceanic model that realistically simulate the regional mean circulation, including the Peru-Chile Undercurrent (PCUC). An analysis of the vorticity and eddy kinetic energy in both simulations indicated that the subsurface eddies can be triggered through a combination of processes that are associated with instabilities of the PCUC. In the high-resolution simulation, we observed that the interaction between the PCUC and topographic slope generates anticyclonic vorticity and potential vorticity close to zero in the bottom boundary layer. The separation of the undercurrent from the slope favors the intensification of anticyclonic vorticity. It reaches magnitudes that are larger than the planetary vorticity while kinetic energy is converted from the PCUC to the eddy flow. These processes set the necessary conditions for the development of centrifugal instabilities, which can form submesoscale structures. The coalescence of submesoscale structures generates a subsurface anticyclonic mesoscale eddy. In the low-resolution simulations (>3 km) centrifugal instabilities are not simulated, and the barotropic conversion of the mean kinetic energy into eddy kinetic energy appears as the main process of eddy formation. We showed that the vertical structure of these eddies is sensitive to the spatial resolution of the model. Plain Language Summary Subsurface mesoscale eddies are swirling masses of water observed below the surface layer of the ocean (around 100- to 400-m depth). Off central Chile, these eddies have typical diameters of few tens of kilometers. They are formed near the coast, where an intense subsurface poleward flow, namely, the Peru-Chile Undercurrent (PCUC), interacts with the continental slope and the seaward border of the continental shelf. These eddies can travel long distances, toward the open ocean, transporting coastal waters with low dissolved oxygen and high nutrient concentrations and impacting the regional marine ecosystems. We use a high-resolution numerical oceanic model (similar to 0.3 km) to analyze the formation of an eddy near 33.5 degrees S off Chile. We showed that the eddy formation process requires the undercurrent to destabilize and detach from the coast, promoting the generation of submesoscale eddies (diameters -10 km). This means that in regions of eddy formation, initially, the PCUC drifts offshore transferring momentum to submesoscale eddies. Later on, these eddies begin to coalesce to form an eddy with larger dimensions. We also showed that the spatial resolution of the numerical model can impact the mechanism of transfer of momentum and the vertical structure of eddies.