Anticyclonic and cyclonic eddies of subtropical origin in the subantarctic zone south of Africa

Michel Arhan, Sabrina Speich, Christophe Messager, Guillaume Dencausse, Rana A Fine, Marie Boye

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Abstract

Two eddies, one anticyclonic and the other cyclonic, intersected in the Subantarctic Zone south of South Africa during a hydrographic transect, are described using a large set of measurements including full depth hydrography, Acoustic Doppler Current Profiler velocities, biogeochemical tracers, air-sea fluxes and altimetric sea surface height. Both eddies have a subtropical origin. The anticyclone is an Agulhas ring with convected core water of ∼12°C, and swirl velocities of 1 m s-1. It was 9.5 months old when sampled and had crossed the Agulhas Ridge. Though sampled in summer, it was releasing ∼200 W m-2 (sensible plus latent heat flux) to the atmosphere. It was observed adjacent to the Subantarctic Front, illustrating the usual encounters of such structures with this front. The cyclone, marked by pronounced low oxygen and CFC anomalies revealing an origin at the continental slope, was 4.5 months old. It had swirl speeds of 0.3 m s-1, and was coupled with the anticyclone when observed. From their kinematics and water mass properties both structures were found to transport subtropical water down to ∼900 m, the water trapped below this depth being either from the northern Subantarctic Zone, or local water. The two structures illustrate the capacity of eddies in the region to transfer subtropical and alongslope water properties into the Subantarctic Zone.

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ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Oceanography

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