Three-dimensional characterization and tracking of an Agulhas Ring

A novel probabilistic methodology is applied to identify optimally coherent structures associated with Agulhas Rings, within a time varying velocity field in the South Atlantic Ocean, as simulated by an eddy-permitting ocean general model. It is shown that this technique provides a way of identifying the three-dimensional shape of a particular Ring in the upper ocean and tracking its evolution over space and time. Based on this three-dimensional representation we can accurately measure the amount of water mass remaining in an Agulhas Ring over time and consequently how much heat or salt is released from the structure as it decays. Identification techniques based on relative vorticity or the Okubo-Weiss parameter have previously been developed for a surface snapshot. Extending these methods in the vertical direction in the upper ocean and comparing the decay of all three-dimensional structures obtained by different methods, we demonstrate that our technique is able to define structures that are more coherent over time than classical methods. While our investigation concentrates on a single Agulhas Ring located in the Cape-Basin from May 2000 over 6 months, the technique may be extended to examine multiple Rings and other coherent structures that are involved in the Agulhas leakage.