Abstract
An Algerian Eddy, anticyclonic vortex generated by the instability of the Algerian Current in the southwestern Mediterranean Sea, is studied using data provided by drifters (surface currents), Argo floats (temperature and salinity profiles), environmental satellites (absolute dynamic topography maps and ocean color images) and operational oceanography products. The eddy was generated in May 2018 and lasted as an isolated vortex until November 2018. Its morphology and kinematics are described in June–July 2018 when drifters were trapped in its core. During that period, the eddy was slowly moving to the NE (~2 km/day), with an overall diameter of about 200 km (slowly growing with time) and maximal surface swirl velocity of ~50 cm/s at a radius of ~50 km. Geostrophic currents derived from satellite altimetry data compare well with low-pass filtered drifter velocities, with only a slight overestimation, which is expected as its maximum vorticity corresponds to a small Rossby number of ~0.6. Satellite ocean color images and some drifters show that the eddy has an elliptical spiral structure. The looping tracks of the drifters trapped in the eddy were analyzed using two statistical methods: least-squares ellipse fitting and wavelet ridge analysis, revealing a typical eccentricity of about 0.5, a wide range of inclination and a rotation period between 3 and 10 days. Clusters of drifters on the northeastern limb of the eddy were also considered to estimate divergence and vorticity. The results indicate convergence (divergence) and downwelling (upwelling) at scales of 20–50 km near the northeastern (northwestern) edge of the eddy, in agreement with the quasi-geostrophic theory. Vertically, the eddy extends mostly down to 250 m depth, with a warm, low-salinity and low-density signature and with geostrophic currents near 50 cm/s in the top layer (down to ~80 m) reducing to less than 10 cm/s near 250 m. Near the surface, colder water is advected into it.
Highlights
Vortices are ubiquitous circulation features of the world’s oceans
Drifter track segments were overlaid on maps of absolute dynamic topography (ADT), absolute geostrophic velocities (AGV) and chlorophyll concentration to describe the structure of the AE
We focus on the southwest Mediterranean and consider weekly ADT maps (Figures 2 and 3)
Summary
Vortices are ubiquitous circulation features of the world’s oceans. Their size varies from ~100 km (mesoscale) to ~10 km (submesoscale). Mesoscale eddies and gyres are detected in sea surface height data provided by satellite altimeters. 2021, 13, 3039 drifters and floats at the sea surface and in the ocean interior, respectively. If drifters or floats are trapped in vortices, they can provide unique measurements about their structure and kinematics [1,2,3,4], trapping and the persistence in the eddies can generally be due to several complex factors [5], including the fact that the instruments do not follow exactly the currents
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