The life cycle of a coherent Lagrangian Agulhas ring

TL;DR

This paper investigates the long-term evolution and coherence of an Agulhas ring using satellite data and nonlinear dynamical systems, revealing a multi-year lifespan with intermittent coherence and a complex genesis process.

Contribution

It introduces an objective eddy detection method that captures the ring's coherence dynamics and details its lifecycle, including formation, partial coherence, and eventual splitting.

Findings

The Agulhas ring remains a coherent entity for about 2 years.

Short-term coherent boundaries shield the ring from mixing.

Ring coherence can be regained temporarily after initial loss.

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.