Abstract

AbstractThe transfer of Indian Ocean thermocline and intermediate waters into the South Atlantic via the Agulhas leakage is generally believed to be primarily accomplished through mesoscale eddy processes, essentially anticyclones known as Agulhas Rings. Here we take advantage of a recent eddy tracking algorithm and Argo float profiles to study the evolution and the thermohaline structure of one of these eddies over the course of 1.5 years (May 2013–November 2014). We found that during this period the ring evolved according to two different phases: During the first one, taking place in winter, the mixing layer in the eddy deepened significantly. During the second phase, the eddy subsided below the upper warmer layer of the South Atlantic subtropical gyre while propagating west. The separation of this eddy from the sea surface could explain the decrease in its surface signature in satellite altimetry maps, suggesting that such changes are not due to eddy dissipation processes. It is a very large eddy (7.1×1013 m3 in volume), extending, after subduction, from a depth of 200–1,200 m and characterized by two mode water cores. The two mode water cores represent the largest eddy heat and salt anomalies when compared with the surrounding. In terms of its impact over 1 year, the north‐westward propagation of this long‐lived anticyclone induces a transport of 2.2 Sv of water, 0.008 PW of heat, and 2.2×105 kg s−1 of salt. These results confirm that Agulhas Rings play a very important role in the Indo‐Atlantic interocean exchange of heat and salt.

Highlights

  • Mesoscale eddies are ubiquitous in the World Ocean (Chelton et al, 2007, 2011).Their energy generally exceeds that of the mean flow by an order of magnitude or more (Stammer, 1997; Wunsch, 1999) and they are thought to have a major impact on the spreading of hydrological properties by advecting them for considerable distances and over long periods

  • Rings are defined as the anticyclonic eddies which are initially detected in the Southeast Indian Ocean and enter and propagate into the South Atlantic

  • We will make use of the concept of the Agulhas Ring Eddy Network (AREN) of trajectories introduced by Laxenaire et al (2018)

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Summary

Plain Language Summary

We take the advantage of the complementary information provided by eddies detected from satellite maps and free-drifting profiling floats measuring the upper 2000 m of the ocean, to reconstruct the internal structure of one of them. This reconstruction shows that this Agulhas Ring quits the surface layer and slowly sinks into the subsurface as it drifts westward into the Atlantic Ocean. The ring is a very large eddy extending down to 1200 m of depth and characterized by two cores showing temperature anomalies up to 4◦C when compared with the surrounding South Atlantic waters These results confirm that Agulhas Rings play a very important role in the Indo-Atlantic interocean exchange of heat and salt

Introduction
The South Atlantic Eddy Atlas
Sea Surface Temperature
Argo profiles data and eddy colocation
Results
Evolution of the Agulhas Ring surface dynamics
Evolution of the internal structure
Reconstruction of the subsurface structure of an Agulhas Ring
Integral properties of the subsurface Agulhas Ring
Comparison with Agulhas Rings identified in the literature
Agulhas Rings general vertical structure and classification
Agulhas Rings Mode Waters
Summary and conclusions
Full Text
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