Identification and Tracking of Coherent Agulhas Current Rings

Recent advancements in nonlinear dynamical systems theory have led to the development of a methodology, called geodesic eddy detection, for the objective (i.e., frame-independent) framing and tracking of coherent Lagrangian eddies in two-dimensional unsteady flows. Such eddies have material boundaries that do not stretch or fold over long periods of time, thereby providing an explicit mechanism of transport of fluid and distinguished properties, such as temperature and salinity. Eddies identified from Eulerian criteria (e.g., as regions enclosed by streamlines where rotation dominates over strain instantaneously) do not possess this property as such criteria either depend on the choice of reference frame or fail to reveal long-term material response of the flow field. Applying geodesic eddy detection on surface currents derived from the over two-decade-long record of satellite altimetry measurements, we isolated coherent Lagrangian Agulhas Current rings capable of traversing the South Atlantic basin with no noticeable signs of filamentation. The rings are found to acquire material coherence from incoherent fluid away from the Agulhas retroflection region in the South Atlantic, revealing that their genesis mechanism differs substantially from the commonly accepted picture in which rings are shed from the Agulhas retroflection as a result of occasional occlusions. While ability of Agulhas rings in transporting fluid over long distances is confirmed, this is found to be more restricted than suggested by earlier assessments. First, coherent transport estimates are found to be small compared to estimates from Eulerian analysis. The reason for this is that Eulerian analysis significantly overestimates the volume of the coherently transported fluid. Second, the coherent transport estimates are smaller than the total Agulhas leakage reported in numerical studies. These conclusions remain unaltered by the discovery that Agulhas rings can be shielded by Atlantic Ocean water as a result of successive short-term material coherence regain events. In fact, while the volume of fluid trapped by these short-term shields is larger than that from a long-term material coherence assessment, the fraction of such volume traceable into the Indian Ocean is in general rather small.