Structure of an eddy measured with drifters

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Abstract
Drifters are used to measure the structure of an eddy on Western Bank, Scotian Shelf over a 5 day period. A regression method that uses both the temporal and spatial structure of flow field observations is presented and used to analyze this eddy. The eddy has vorticity −1.9×10−5 s−1 with divergence, stretching deformation and shearing deformation smaller by more than an order of magnitude. The eddy is centered on the crest of Western Bank. The motion residual to the eddy had a slab like part that moved all points on a plane in a circle without rotation. This latter type of motion can cause substantial error when vorticity and other eddy properties are estimated using methods that operate on a single drifter track. The Eulerian statistics of residual motion show correlations greater than 0.5 are restricted to spatial separations less than 2 km and temporal lags less than 3 h, but motion directed along the vector separating two velocity measurements has a significant Eulerian correlation for timescales of more than 40 hours and length scales of order 5 km. Transverse motion is positively correlated over lesser scales. Much cluster distortion results from subtidal frequency motion that has spatial scales that are seldom coherent over drifter separation scales greater than 2–3 km. The long Lagrangian timescale is inconsistent with a description of horizontal mixing in terms of an eddy diffusion coefficient. Rather, we might picture drifters being dispersed by slowly evolving, narrow streams of current that have larger spatial scales along their axis than across their axis.

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