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Wind-induced subduction at the South Atlantic subtropical front

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Abstract

The South Atlantic Subtropical Front, associated with the eastward-flowing South Atlantic Current, separates the colder, nutrient-rich waters of the subpolar gyre from the warmer, nutrient-poor waters of the subtropical gyre. Perturbations to the quasi-geostrophic, eastward flow generate meanders and filaments which induce cross-frontal exchange of water properties. Down-front winds transport denser waters from the South over warm waters from the North, inducing convective instability and subduction. Such processes occur over spatial scales of the order of 1 km and thus require high horizontal spatial resolution. In this modeling study, a high-resolution (4 km) regional grid is embedded in a basin-wide configuration (12 km) of the South Atlantic Ocean in order to test the importance of submesoscale processes in water mass subduction along the subtropical front. Stronger and more numerous eddies obtained in the high-resolution run yield more intense zonal jets along the frontal zone. Such stronger jets are more susceptible to instabilities, frontogenesis, and the generation of submesoscale meanders and filaments with \(\mathcal {O}(1)\) Rossby number. As a consequence, vertical velocities larger than 100 md 1 are obtained in the high-resolution run, one order of magnitude larger than in the low-resolution run. Wind-driven subduction occurs along the frontal region, associated with negative Ertel potential vorticity in the surface layer. Such processes are not observed in the low-resolution run. A passive tracer experiment shows that waters with density characteristics similar to subtropical mode waters are preferentially subducted along the frontal region. The wind-driven buoyancy flux is shown to be much larger than thermal or haline fluxes during the wintertime, which highlights the importance of the frictional component in extracting PV from the surface ocean and inducing subduction, a process that has been overlooked in subtropical mode water formation in the region.

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Acknowledgements

This research was funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Bolsa de Produtividade em Pesquisa (Process: 306971/2016-0) and Project 457118/2012-1. Funding from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) - Projeto REMARSUL (Processo CAPES 23038.004299/2014-53) is also acknowledged. The author would like to thank one anonymous reviewer for comments that significantly improved the manuscript.

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  1. Laboratório de Dinâmica e Modelagem Oceânica (DinaMO), Instituto de Oceanografia - Universidade Federal do Rio Grande, Av. Itália, km 08, Rio Grande, RS, 96201-900, Brazil

    Paulo H. R. Calil

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  1. Paulo H. R. Calil
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Correspondence to Paulo H. R. Calil.

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Responsible Editor: Amala Mahadevan

This article is part of the Topical Collection on the 48th International Liège Colloquium on Ocean Dynamics, Liège, Belgium, 23-27 May 2016

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Calil, P. Wind-induced subduction at the South Atlantic subtropical front. Ocean Dynamics 67, 1351–1365 (2017). https://doi.org/10.1007/s10236-017-1090-z

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  • DOI: https://doi.org/10.1007/s10236-017-1090-z

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