The life cycle of a coherent Lagrangian Agulhas ring

Abstract

Abstract. We document the long-term evolution of an Agulhas ring detected from satellite altimetry using a technique from nonlinear dynamical systems that enables objective (i.e., observer-independent) eddy framing. Such objectively detected eddies have Lagrangian (material) boundaries that remain coherent (unfilamented) over the detection period. The ring preserves a quite compact material entity for a period of about 2 years even after most initial coherence is lost within 5 months after detection. We attribute this to the successive development of short-term coherent material boundaries around the ring. These boundaries provide effective short-term shielding for the ring, which prevents a large fraction of the ring's interior from being mixed with the ambient turbulent flow. We show that such coherence regain events cannot be inferred from Eulerian analysis. This process is terminated by a ring-splitting event which marks the ring demise, near the South American coast. The genesis of the ring is characterized by a ring-merging event away from the Agulhas retroflection, followed by a 4-month-long partial coherence stage, scenario that is quite different than a simple current occlusion and subsequent eddy pinch off.

Agulhas Ring Transport Efficiency From Combined Satellite Altimetry and Argo Profiles

“…Interactions with such topographic features are not surprising given that the vertical extent of the eddies inferred from the profiles of density anomalies was deeper than 1,500 m. The loss of volume induced by such interactions is consistent with results by Frenger et al () who showed that bottom interactions are one of the most important processes to dissipate mesoscale energy in the Southern Ocean. Between the two ridges the eddies showed reduced volume losses (<0.5 × 10 13 m 3 ), as observed in Froyland et al () and Wang et al (). Other studies (Doglioli et al, ; van Sebille et al, ) have reported larger volume losses.…”

“…The ∼0.5 × 10 13 m 3 of water advected by eddy B12 west of 30°W corresponds to ∼33% of the initial volume of the eddy after the Mid‐Atlantic Ridge, and to ∼12.5% of the initial combined volume of eddies AN1 and AS2 east of the Walvis Ridge. These percentages are lower than those found by Froyland et al () and Wang et al (), but the difference can be mostly explained by the depth variation in eddy volume, which was not considered in those studies.…”

“…In case they were not, our results indicate that coherent Lagrangian vortices (Haller & Beron‐Vera, ) are not the only mechanism for efficient westward transport in the South Atlantic, and thus, the contribution of incoherent eddies should also be estimated and included in studies focusing on cross basin eddy transport. Indeed, the contribution of incoherent transport due to eddy far‐field effects has been already recognized and discussed in studies based on coherent‐vortex detection(e.g., Abernathey & Haller, ; Wang et al, ). On the other hand, in case they were coherent, our results indicate that even such eddies are not completely impermeable to their surroundings due to mixing‐induced exchanges.…”

“…In recent years, these estimates have been further refined with the emergence of Lagrangian techniques capable of detecting coherent mesoscale eddies (Haller & Beron‐Vera, , ). Compared with traditional Eulerian eddy detection (e.g., Chaigneau et al, ; Chelton et al, ; Nencioli et al, ), studies based on these novel techniques can identify eddies capable of transporting water within their core with no noticeable leakage (Froyland et al, ; Wang et al, , ). Thus, these Lagrangian studies can provide more accurate estimates of the associated eddy transport.…”

Anticyclonic Eddies Connecting the Western Boundaries of Indian and Atlantic Oceans

“…Using different eddy detection methods, several authors have attempted to reconstruct and analyze Agulhas Rings trajectories in and across the South Atlantic (e.g., Byrne et al, ; Gordon & Haxby, ; Souza, de Boyer Montégut, Cabanes, & Klein, ; Wang et al, ). In the published studies, most reconstructions of the trajectories of Agulhas Rings leaving the Cape Basin are identified initially well within the Cape Basin and not at the Agulhas Current Retroflection where they are believed to originate (e.g., Byrne et al, ; Guerra et al, ; Souza, de Boyer Montégut, Cabanes, & Klein, ; Wang et al, , ). Taking into account the separation of an eddy into smaller structures, to which, in what follows, we will refer to as an eddy splitting event, Dencausse et al () tracked the Agulhas Rings formed in the Agulhas Retroflection area and entering the Cape Basin.…”