3D Aspects of Lagrangian Coherent Structures in Oceanic Flows

In three-dimensional flows, Lagrangian Coherent Structures (LCS) are identified by 2D surfaces that attract or repel nearby particles for the longest time. In ocean flows very little is known about these structures since many ocean models do not provide prognostic vertical velocities. In fact it is not even clear that 2D velocities can accurately delineate these structures. Here we present an efficient approach of constructing attracting and repelling 2D LCS using Finite-Time Lyapunov Exponents (FTLEs) computed from 2D velocity data from ocean models. We study the pinch-off of a large anticyclonic ring from the Loop Current in the Gulf of Mexico. The separation is characterized by the formation of a strong hyperbolic region between the Loop Current and the ring. The intersection of the attracting and repelling surfaces was traced nearly vertically down to 700 meters in the water column.