Mesoscale eddies in the Algerian Basin: do they differ as a function of their formation site?

Abstract

The circulation of the western Mediterranean Sea (WMED) is dominated by highly variable and heterogeneous mesoscale circulation that is strongly driven by the formation and propagation of eddies (cyclonic and anticyclonic) mainly acting in the Algerian Basin.In order to investigate the spatial and temporal distribution of eddy generation and their respective paths in the Algerian Basin, the most energetic WMED portion, we use an automated detection and tracking hybrid method applied to 24 years of sea level anomaly (SLA) data. The algorithm is based on the computation of the Okubo–Weiss parameter in SLA closed loops and has been modified in order to fill the gaps in single eddy tracks. In this work we analysed both cyclonic and anticyclonic structures, but the conclusions will be focused mainly on anticyclones with a lifespan longer than 3 months, as they are characterized by higher kinetic energy, thereby potentially contributing to a large extent to the mesoscale characterization of the basin.In particular, we find that anticyclonic short-life eddies mostly occur in the northern portion of the domain, north of 39° N along the North Balearic Front (NBF). Such short-life eddies, labelled frontal eddies (FEs), are characterized by low translational velocity and a highly variable direction of propagation. We found a weak seasonality in their formation, with maxima in fall and winter. By contrast, anticyclonic longer-life eddies tend to arise in the southern part of the basin along the Algerian Current, with a clear maximum in spring. All the structures (both cyclonic and anticyclonic) originating along the Algerian Current are known as Algerian eddies (AEs). According to previous studies, we observe that these anticyclonic eddies mainly form east of 6.5° E and move eastward along the African coast to the Sardinia Channel, where they detach from the coast, continuing offshore and following the cyclonic intermediate circulation. We detect a region between 4.5 and 6.5° E where such eddies tend to converge and terminate their life.Finally, the analysis suggests that eddies formed in the northern and in the southern part of the Algerian Basin present some physical differences such as lifetime, kinetic energy and vorticity. Furthermore, the connection between the two parts in terms of eddy tracks is limited to a very small number of southbound (FEs) or northbound (AEs) structures crossing 39° N.