Eddies along the South-West Coast of Norway

The image was acquired off the southwest coast of Norway on 6 May 1993. The eddy structures are presumably responsible for collecting the surfactants into the curved and spiral patterns observed. The abundance of surfactant material would be coincidental with the spring blooms of algae and other planktonic organism which release hydrophobic oils.

The low wind band traversing the right side of the image exposes a train of bright internal wave crests propagating westward. The synoptic wind recorded close to the time of acquisition was 10 knots from the northwest (see Weather Map )

The SAR image expresses a cyclonic eddy feature through the spiral shaped lines associated with small scale turbulence aligned in the direction of the larger scale eddy orbital motion. The turbulence, in turn, leads to convective motion in the water that can bring organic material present in the upper layer to the surface where it can remain as a microlayer of natural surface film. As the concentration of this surface film (surfactant molecules) increases, it can reach sufficient surface tension to inhibit growth of capillary and short gravity waves. In addition, the film edge may reflect the short waves that propagate at oblique angles to the edge, thus limiting the advance of short wave roughness through the slick-covered region.

In turn the lack of small scale surface roughness prevents the radar echo from the surface having sufficient strength, leading to manifestation of low backscatter, dark features from surface slicks. The SAR image displays no variations in intensity of the dark spiraling lines due to radar look angle dependence.
The expressions of dark spiral lines are expected to disappear at higher wind speeds (7 m/s) since wind-induced mixing in the upper layer will redistribute the surface slicks and prevent such damping.
Hence, the fully developed 0.07-0.08 m waves necessary to provide resonant Bragg backscatter are formed in the eddy region, and the presence of surface film at sufficient concentration can in turn form areas of slicks which leads to the backscatter contrast of about 6-10 dB. In addition, the backscatter front and region of low radar return in the eastern sector is caused by the wind dropping below the threshold.
The spiraling lines suggest convergence towards the eddy center. Indications of the convergence suggest that this cyclonic eddy may be important for the distribution and concentration of chlorophyll a, algae and pollutants such as oil spills.

The synoptic manifestations of the eddy rotational direction, horizontal dimension and eventual surface convergence offer a valuable opportunity for comparison and validation of model simulations of surface current pattern. Moreover, since there is no sequence of SAR images of this case, the temporal characteristics of such eddies, fully developed or decaying, cannot be determined, unless one combines this kind of observation with modelling tools.

NERSC, Bergen, Norway

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry