On the Structure and Kinematics of an Algerian Eddy in the Southwestern Mediterranean Sea

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.

Non‐Local Eddy‐Mean Kinetic Energy Transfers in Submesoscale‐Permitting Ensemble Simulations

“…scales. Its time integrated contribution, however, is of the same order of magnitude than EDDYFLX but slightly weaker, supporting mixed barotropic-baroclinic instability processes for driving the growth of Algerian Eddies as proposed earlier (Obaton et al, 2000;Poulain et al, 2021). It is interesting to compare these estimates to the total IKE and…”

“…Around 5 • E, the Algerian Current detaches from the coast, forming a 'loop current', a region of intense meso-scale eddies formation through mixed baroclinic-barotropic instabilities (e.g. Obaton et al, 2000;Poulain et al, 2021). We will focus on the eddy dynamics of this region in the following.…”

Climatic, Decadal, and Interannual Variability in the Upper Layer of the Mediterranean Sea Using Remotely Sensed and In-Situ Data

“…Moreover, these variations show prominent long-term trends, which however differ between various sub-basins [11,14,15,17]. These differences are probably due to the local circulation [18][19][20][21], thermohaline changes [22][23][24], and relatively short time-series where it has been calculated from [25,26]. A quick summary of the trend analysis recently conducted in the Mediterranean describes a scenario of increasing freshwater deficit [27], sea surface temperature (SST) and sea level [11,[14][15][16], and a basin-scale multi-decadal salinification [11,23,[28][29][30].…”

Comparing the Currents Measured by CARTHE, CODE and SVP Drifters as a Function of Wind and Wave Conditions in the Southwestern Mediterranean Sea

“…This might be expected, since the CARTHE drifter is closer to the sea surface and has a smaller drag area ratio compared to the CODE. We therefore conclude that the two types of surface drifters, CARTHE and CODE, measure the currents in approximately the same way and that their data can be combined to investigate the surface kinematics and dynamics [ 24 , 25 , 36 ].…”