Abstract

In 1976–1977, 16 surface drifters were seeded regularly in an area about 100 km in diameter, providing a good opportunity to study diffusion induced by the mesoscale eddies present in this part of the eastern North Atlantic. It is found herein that Taylor's formula relating the diffusivity (rate of dispersion) of marked fluid parcels to the integral of the Lagrangian correlation function is accurate up to 30 days. The diffusivity, initially zero, reaches a plateau after about 10 days. The diffusivities associated with the time scale of the energy containing eddies (30 days) are 2.3 107 cm2/s and 1.7 107 cm2/s in the zonal and meridional direction respectively. This is small in view of the surface eddy kinetic energy of the order of 100 cm2/s2. Accordingly the Lagrangian integral time scales are as short as 2 days. After 30 days, the diffusivity decreases and the dispersion saturates. The Lagrangian eddy energy content is therefore weak at low frequencies in contrast with subsurface Eulerian energy content obtained in the same area. The center of gravity of the Lagrangian and Eulerian energy spectrum indicates time scales respectively of 12 and 26 days. At frequencies higher than 0.1 cpd the Lagrangian velocity spectrum falls off with a −2 slope. This may be an indication of a balance between the Lagrangian acceleration of water parcels and the essentially white noise wind stress computed from a nearby weathership. Eulerian spectra at greater depths which had a similar −2 slope in this band but a much reduced energy level suggest an oceanic response strongly trapped at the surface.

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