A New Model of Current Retroflection Applied to the Westward Protrusion of the Agulhas Current

Wilton Arruda Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, and Instituto de Matemática, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil

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Volodymyr Zharkov Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, Florida

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Bruno Deremble Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida

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Doron Nof Department of Earth, Ocean and Atmospheric Science, and Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, Florida

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Eric Chassignet Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, Florida

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Abstract

The dynamics of current retroflection and rings shedding are not yet fully understood. In this paper, the authors develop an analytical model of the Agulhas Current retroflection dynamics using three simple laws: conservation of volume, momentum balance, and Bernoulli’s principle. This study shows that, for a retroflecting current with a small Rossby number, this theoretical model is in good agreement with numerical simulations of a reduced-gravity isopycnal model. Otherwise, the retroflection position becomes unstable and quickly propagates upstream, leaving a chain of eddies in its path. On the basis of these findings, the authors hypothesize that the westward protrusion of the Agulhas retroflection and the local “zonalization” of the Agulhas Current after it passes the Agulhas Bank are stable only for small Rossby numbers. Otherwise, the retroflection shifts toward the eastern slope of the Agulhas Bank, where its position stabilizes due to the slanted configuration of the slope. This study shows that this scenario is in good agreement with several high-resolution numerical models.

Corresponding author address: Doron Nof, Department of Earth, Ocean and Atmospheric Science, 419, 117 N. Woodward Ave., Florida State University, Tallahassee, FL 32306. E-mail: nof@fsu.edu

Abstract

The dynamics of current retroflection and rings shedding are not yet fully understood. In this paper, the authors develop an analytical model of the Agulhas Current retroflection dynamics using three simple laws: conservation of volume, momentum balance, and Bernoulli’s principle. This study shows that, for a retroflecting current with a small Rossby number, this theoretical model is in good agreement with numerical simulations of a reduced-gravity isopycnal model. Otherwise, the retroflection position becomes unstable and quickly propagates upstream, leaving a chain of eddies in its path. On the basis of these findings, the authors hypothesize that the westward protrusion of the Agulhas retroflection and the local “zonalization” of the Agulhas Current after it passes the Agulhas Bank are stable only for small Rossby numbers. Otherwise, the retroflection shifts toward the eastern slope of the Agulhas Bank, where its position stabilizes due to the slanted configuration of the slope. This study shows that this scenario is in good agreement with several high-resolution numerical models.

Corresponding author address: Doron Nof, Department of Earth, Ocean and Atmospheric Science, 419, 117 N. Woodward Ave., Florida State University, Tallahassee, FL 32306. E-mail: nof@fsu.edu
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