Meanders in the Mediterranean

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Instabilities in ocean currents can be identified through synergistic use of data from the Copernicus Sentinel-3 satellites.

Date & Time
4 November–2 December 2019
Satellites
Sentinel-3A and B
Instruments
OLCI, SLSTR
Channels/Products
Sea Surface Temperature, Chlorophyll-a concentration

By Aida Alvera Azcarate (University of Liege) and Hayley Evers-King (EUMETSAT)

In the southern part of the Mediterranean Sea, the Algerian current runs along the coast of Northern Africa. The current is part of the general circulation pattern in the western Mediterranean, which can be complex, including features such as the highly variable Alboran Gyres.

Figure 1 shows the current along the Algerian coast and a clear connection through both temperature and the overlaid currents, to the waters in the west of the image.

Figure 1
 
Figure 1: Sentinel-3A Sea Surface Temperature (in degrees celsius) 4 November. Currents from CMEMS (Mediterranean sea physics analysis and forecast) overlaid. The connection between the Algerian current and the Alboran gyre can be seen in the temperature signature (and the currents that have been overlaid).
 

The Algerian current frequently becomes unstable, forming meanders and eddies. The animation in Figure 2 shows a series of images, as in Figure 1, of sea surface temperature from the Sentinel-3 SLSTR instruments.

Figure 2
 
Figure 2: Sentinel-3 Sea Surface Temperature, 28 November and 2 December. The series of images shows growing instability and the formation of a meander in the current.
 

As the meanders travel off-shore they generate a current that results in upwelling of colder waters. These physical dynamics create a biological response in the phytoplankton (microscopic, plant-like organisms), that can be seen through ocean colour imagery.

In Figure 3, an image from the Sentinel-3 OLCI sensor, shows increased chlorophyll-a concentrations, associated with colder, more nutrient rich waters of the meander. The meanders can eventually break off entirely to form large eddies, which can be more than 100 km in diameter, and may persist at time frames between several months and years. Comparison with the SST images in Figure 2 shows how cold waters, induced by the circulation, are likely a source of nutrients for the increased phytoplankton growth evidenced by the distribution of chlorophyll-a concentration. Estimates of high chlorophyll-a concentration, very near the coast, may also be affected by the presence of re-suspended sediments.

Figure 3
 
Figure 3: Sentinel-3A OLCI Chlorophyll-a concentration (log10 mg m-3), 30 November.
 
 
 

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