Drifter measurements during the Alborex experiment in the Western Mediterranean

Surface drifters deployed during the Alborex experiment in May 2014 allowed to investigate several aspects of the surface dynamics, such as the sub-mesoscale and mesoscale circulation and inertial currents, in the eastern Alboran Sea (in the vicinity of the Almeria-Oran Front), in the Algerian Current and the Western Mediterranean Sea. Complex demodulation was applied to the drifter velocity times series to extract the amplitude, phase and effective frequency of the near-inertial currents. These high-frequency currents were studied as a function of wind forcing and background circulation vorticity.

Near-inertial currents appear ubiquitously along the drifter trajectories, with amplitudes correlated with the wind forcing and reaching values in excess of 15 cm/s. Their effective frequency can be both larger or smaller than the theoretical inertial frequency, and the frequency shifts are correlated with the vorticity of the background circulation as derived from satellite altimetry products

The motions of drifters in closely-spaced clusters in the vicinity of the Almeria-Oran Front were decomposed linearly in terms of divergence/convergence, vorticity and stretching and shearing deformation rates. The front is essentially divergent at the surface, possibly corresponding to upwelling near its core, and has positive vorticity.