Oceanic mesoscale eddies as revealed by Lagrangian coherent structures

We demonstrate the feasibility of using dynamical systems tools to unambiguously identify mesoscale oceanic eddies from surface ocean currents derived using climatological hydrography and altimetry. Specifically, our analysis is based on extracting Lagrangian coherent structures (LCSs) from finite-time Lyapunov exponent (FTLE) fields. The FTLE fields reveal with unprecedented detail an intricate tangle of LCSs, which are hidden in ocean surface topography maps but sometimes are apparent in ocean color images. These LCSs delineate fluid domains with very different advective properties, and thus their detection provides an objective (i.e., frame-independent) means of identifying eddy boundaries. The importance of considering LCSs in quantifying transport by eddies is highlighted. Such a quantification does not rely on the common assumption-which is shown to be generally not valid-that transport is largely effected by the trapping and subsequent translation of water slugs inside eddies defined as the regions enclosed by sea height (streamfunction) contours within which rotation dominates over strain. LCSs are calculated for the whole globe and compared with satellite-tracked drogue drifter trajectories within a selected region of the South Atlantic.