Estimating Surface Divergence of Ocean Eddies Using Observed Trajectories from a Surface Drifting Buoy

Abstract

A method is described that estimates the time evolution of surface divergence and other secondary circulation properties of an ocean eddy. The method is novel because it is applied to the observations of a single surface drifting buoy. Surface drifting buoys located on ocean eddies provide Lagrangian trajectories that orbit the local extremum in geopotential. At each instance, the position of the buoy lies on the boundary of a closed material surface over the eddy. Assuming, on an ocean eddy, that the material boundary points vary smoothly in time, a method is developed to estimate all points along this boundary. An ellipse is fitted to the approximated material boundary to provide a continuum of properties, including centroid, aspect ratio, orientation, and area. The time rate of change of these properties provides approximations to the eddy velocity, moment of inertia, secondary rotation, and surface divergence. Two surface drifting buoys deployed on an anticyclonic eddy in the East Australian Current are used to demonstrate the analysis method. The estimated surface divergence is compared and interpreted using the observed separation of the two drifting buoys and other independent observations.