Characteristics and evolution of an Agulhas ring

Abstract

A South Atlantic ring is studied through remote sensing altimetry, hydrographic stations, and drifters' trajectories. The ring's core was characterized by warmer and saltier Indian Ocean waters. At the time of the cruise, the ring's signature extended radially out to 124 km and vertically down to 2000 m, and its core absolute dynamic topography (ADT) exceeded the surrounding Atlantic Ocean waters in 0.4 m. The geostrophic velocities were anticyclonic with maximum speeds about 35 cm s−1 at 100 m and reaching negligible values near 4500 m. The rotational transport inside the ring was 33 Sv in the thermocline and intermediate layers. The drifters' data distinguish a 30‐km core revolving as a solid body with periodicity near 5 days and a transitional band that revolves with constant tangential velocity, resembling a Rankine vortex. The ADT data identify the ring's track, showing that it was shed by the Agulhas Current retroflection in November 2009 and propagated northwest rapidly during the first 2 months (mean speed of about 10 cm s−1) but slowed down substantially (3–4 cm s−1) between March and July 2010, when it was last detected. The altimetry data also outlines the evolution of the ring's core ADT, radius, vorticity, and, through a simple calibration with the cruise data, rotational transport. In particular, the ring surface and vertical‐mean vorticity decay with time scales of 373 and 230 days, respectively, indicating that most of the property anomalies contained by the ring are diffused out to the subtropical gyre before it reaches the western boundary current system.

Agulhas Ring Transport Efficiency From Combined Satellite Altimetry and Argo Profiles

“…The Gaussian profile was also chosen based on the radial distribution of in situ observations, and the sections that did not show an accurate fit were removed from the analysis. Finally, the resulting eddy radii were within the range reported in previous studies based on in situ measurements (e.g., Casanova‐Masjoan et al, ) as well as satellite‐based coherent‐vortex detection methods (e.g., Wang et al, ). Future studies in which the different approaches are applied to dedicated numerical model experiments will undoubtedly contribute to better assess the accuracy of the various boundary definitions and the resulting volume estimates for Agulhas rings.…”

Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation

“…However, observational estimates of Agulhas leakage remain scarce. Even though Agulhas rings have a diameter of up to 300 km, their deep velocities down to more than 1,000 m prevent the quantification of total leakage from satellite altimetry alone (Casanova‐Masjoan et al, ; van Aken et al, ). Richardson () estimated a 15‐Sv leakage in the upper 1,000 m from subsurface floats and surface drifters.…”

Anticyclonic Eddies Connecting the Western Boundaries of Indian and Atlantic Oceans

“…However, this procedure proved to be very sensitive, as experts often disagreed. Finally, Mkhinini et al () and Casanova‐Masjoan et al () undertook a more objective, albeit still qualitative, assessment of the skill of their method by using respectively, 10 and 2 surface drifters trapped in specific anticyclonic eddies.…”