Patch diffusion computed from Lagrangian data, with application to the atlantic equatorial undercurrent

Abstract

Listen

Translate article

Abstract Radar‐tracked drifter trajectories reported by Fahrbach et al. (1986) are reanalysed. The data are obtained from 4 drifter clusters that were deployed in the Atlantic Equatorial Undercurrent for approximately 48 h each. The ratio of Lagrangian integral length scale to the standard deviation of drifter positions ranged from C = 0.02 to 0.36. However, on average it was 0.12 in the meridional direction, thus confirming Fahrbach et al.’s (1986) calculations of the meridional diffusion of salt from the Atlantic Equatorial Undercurrent. Making a quasi‐stationary assumption we find that relative velocities had Lagrangian integral time‐scales of about 1 h when relative velocities were large O(0.1 m s−1) and about 3 h when they were smaller O(0.05 ms−1 ). The Eulerian integral time‐scale for relative velocities is only slightly greater than the Lagrangian integral time‐scale. Joint space‐time correlations indicated that the eddy velocity changes rapidly compared with time‐scales required to advect a particle over the eddy length scale. In the case of D1, D2 and D3 experiments (which had large relative velocities) the relative velocity became spatially decorrelated at scales small compared with cluster dimensions. The spatial scales of the relative velocity were somewhat larger for the CIPREA experiment, which had weaker relative velocities than D1, D2 and D3.