Anticyclonic eddy in the Mediterranean Sea

By Aida Alvera-Azcárate (University of Liège), Hayley Evers-King (EUMETSAT)

The ocean is a highly dynamic place in which water masses of different characteristics can be brought together by ocean currents. On 29 May, an anticyclonic eddy formed between the islands of Mallorca and Sardinia, as it can be seen in this image of the sea surface temperature taken by the SLSTR sensor onboard Sentinel-3 (Figure 1).

This eddy was formed by relatively warm water (18°C) located in the southern part of the western Mediterranean Sea, combined with the colder waters (less than 16°C) coming from the Gulf of Lion in the north. The relatively large eddy was accompanied by a very rich field of fine structures that show the complexity of the dynamics that occur in the ocean. For example, three smaller eddies rode alongside the central eddy, and swirls and filaments can be seen all over the image.

The currents associated to this eddy field act as a 'freeway' that favours the transport of colder waters from the north to the south of the basin, as it can be seen in the surface current field (Figure 2).

If there isn't an eddy, the waters of the Gulf of Lion travel southwestward, following the path of the North Current along the French and Spanish coasts.

Colder waters are richer in nutrients, and, therefore, the presence of these features can have an impact in the availability of these nutrients in the southern Mediterranean Sea, influencing the ecosystem of this basin, by favouring primary production.

The SST field of Figure 1 is rich with structures at various spatial scales, and there is a large correspondence between these structures and the underlying currents distributing water masses in the domain, as it can be observed in Figure 3, in which these two fields have been superimposed. For example, northerly winds in the Gulf of Lion produce a strong surface current towards the south and cool the surface temperature in this region. The southward cold current between the Catalan coast and Mallorca is part of the northern current system, but a warm branch flowing northeastwards along Mallorca can be also observed.