Property and energy transfer by an Agulhas Ring

Abstract

We use hydrographic observations in March 2022, combined with drifter and altimetry time series, to investigate the structure and time evolution of an Agulhas Ring (AR). This multi-platform approach enables a comprehensive assessment of the ring’s water mass properties, heat and salt anomalies, and energy content, offering insights beyond those available from satellite alone. The anticyclonic AR transported relatively (to the background state) warm (>2°C), salty (>0.3) and low-oxygen (≈35 μmol/kg) Indian Ocean waters into the South Atlantic in its upper core. Its velocity field exhibits a coherent solid-body rotating core (~38 km radius) and a radially decaying baroclinic structure in the periphery. The ratio between rotational and translational speeds shows that the eddy maintains a dynamically trapped core down to at least 595 m, representing a lower bound for its vertical extent. Heat and salt anomalies range from 3.4 × 1019 to 4.9 × 1019 J and from 1.1 × 1012 to 1.5 × 1012 kg, respectively, with annualized transports of 1 × 10-3 – 2× 10–3 PW and 3.3 × 10-5–4.7 × 10–5 Sv. Available potential energy (1.9–7.4 × 1015 J) exceeds kinetic energy by two orders of magnitude, indicating a strong dominance of potential energy and a tendency toward instability. The observed weakening and fragmentation of the ring near the Walvis Ridge is consistent with the release of this energy during its evolution. These results show that even after several months of propagation, Agulhas Rings remain dynamically active structures that contribute significantly to the transport of heat, salt, and water masses across the South Atlantic.