Lagrangian Study of Several Long-Lived Agulhas Rings

By using a Lagrangian-averaged vorticity deviation (LAVD)-based vortex detection scheme, rotationally coherent Lagrangian vortices in the South Atlantic Ocean are detected. These vortices act as agents for water transport and can stay coherent in a limited time scale. Our study starts from the life cycle of several long-lived Agulhas rings detected with the LAVD-based vortex detection method. The life cycle of those long-lived Agulhas rings can be separated into two distinct stages: the growing stage and the decaying stage. It is fotuid that at the growing stage, the ambient water spins in and provides effective shielding for the coherent core. The rate of change of material belt width with respect to the detection time scale at the end of the growing stage can represent the decay rate of coherence. We further find a linear relationship between the mean strain rate and the mean square root of kinetic energy (KE1/2). Mean finite-time Lyapunov exponents (FTLE) increase monotonically with the mean strain rate or mean KE1/2. The long existence of the Agulhas rings can be partly attributed to the energetic boundaries around the rings. The ratio of the boundary kinetic energy to the spatial mean kinetic energy (KE/MKE) is also found to be a contributing factor that can influence the lifetime of Agulhas rings. In the retroflection area, the short-lived Agulhas rings might be attributed to the low KE/MKE in this area.