Analyses of Turbulent Submeso- and Mesoscale Processes using SAR Data
Contents
Submesoscale Eddies in the Mediteranean and Red Sea:
Mediterranean Sea
We used about 1700 Envisat ASAR WS and ERS-2 SAR medium resolution images acquired in 2009-2011 over the Black and eastern Mediterranean seas to reveal the spatial and temporal (seasonal) distribution of submesoscale eddies and thus investigate submesoscale eddy activity in these basins and eddy visibility in SAR imagery.
In total, about 13.000 submesoscale eddies were found. Eddies were detected in about 46% of the images analyzed. Most of the eddies were cyclonic (about 90%) though in the Aegean Sea the percentage of anti-cyclonic eddies was higher due to local coastal topography. About 63% of eddies manifested in the SAR imagery due to surfactant films, i.e. they were “black” eddies, while the rest 37%, through wave-current interactions (“white” eddies). Most of the “black” eddies (about 64%) were detected in the Black Sea presumably due to higher bioproductivity and resulting higher concentration of surfactants there. “White” eddies were distributed more or less evenly between the two basins thus letting us conclude about close wind conditions above them, which cause similar effects on surface currents. Most of the images analyzed covered the western part of the Black Sea, eastern edge of the Mediterranean Sea as well as the Aegean Sea and therefore most of the eddies detected were also located there. However, the normalized densities (number of eddies per image within a resolution cell) show evidence that the observed heterogeneity is not simply due to the inhomogeneous coverage by SAR imagery, especially for “white” eddies.
“Black” eddies were found mostly in coastal areas where there are more favourable conditions for eddy generation (e.g., via later friction) and visualization (due to higher concentration of surfactants). “White” eddies were found both in the coastal area and further offshore where both wind and surface currents can reach higher speeds. Area with especially frequent observation of “white” eddies was one along the western coast of the Black Sea. In the Mediterranean Sea such an area was the southern part of the Levantine Basin.
Due to seasonal variability of the near-surface wind speed and availability of the surfactant films “black” eddies were observed mostly during the warm period (spring and summer), while “white” ones, during the cold period (autumn and winter). In the area along the western coast of the Black Sea, where winds and resulting currents are strong enough throughout the year, “white” eddies were found during all the seasons. In the Mediterranean Sea, eddies of both types of visualization were mostly detected during autumn when there are apparently favourable conditions for eddy generation (due to thin upper mixed layer) and visualization (due to available surfactants and required near-surface wind speeds).
Red Sea
We used more than 500 Envisat ASAR WS images acquired in 2006-2011 over the Red Sea to study the spatial and temporal distribution of eddies at sub-meso-, meso-, and basin-scale. Most of the images covered the northern part of the basin and therefore, most of the eddies detected were also located there. However, the normalized densities (i.e. the numbers of detected eddies per image resolution cell) show evidence that the observed heterogeneity is not simply due to the inho-mogeneous coverage by SAR imagery, but that a general trend exists of a greater number of sub-mesoscale eddies in the northern part of the basin (north of 20°N).
In total more than 1000 sub-mesoscale eddies were found, which is about two eddies per SAR image. This is generally less than previous studies revealed for the Baltic, Black, and Caspian seas. We hypothesize that this finding is linked to the greater depth of the upper mixed layer in the Red Sea (about 100 m), but also to the overall shape of the (narrow) basin and to the different circulation within the basin.
“Black” eddies, i.e. eddies manifesting in SAR images through the accumula-tion of surfactants along the shear lines, were found mostly in coastal areas, while “white” eddies, i.e. manifesting through wave-current interactions, were found further offshore. In general, “black” eddies visualize in SAR images at lower wind speeds, since the surfactants start to disrupt when the wind speed increases.
We also found about 50 meso- and basin-scale eddies with diameters up to ap-prox. 200 km. Their rotation was both cyclonic and anti-cyclonic; however, most of the basin-scale eddies (with diameters exceeding 100 km) were found to be an-ti-cyclonic, which is in contrast to the smaller, sub-mesoscale eddies. Those basin-scale eddies were mainly found in SAR images acquired in spring, and they were located between 21°N and 24°N, which supports the hypothesis of Quadfasel and Baudner (1993) that wind forcing and interactions with the local topography in that area are the main generation mechanisms.
SAR imagery of six consecutive years is certainly not sufficient for the deriva-tion of complete climatologies, particularly if the entire basin was not covered homogeneously. However, our results indicate very well that the systematic anal-yses of SAR imagery with respect to the detection of sub-mesoscale, mesoscale, and basin-scale eddies has great potential to further the knowledge about the hy-drodynamics in certain sea areas or in enclosed or semi-enclosed seas.
Baltic Sea
We discuss the spatio-temporal distribution of submesoscale eddies seen in Envisat Advanced Synthetic Aperture Radar (ASAR) imagery of the Baltic Sea. A total of 1250 ASAR images acquired between 2009 and 2011 form the basis of our studies and show imprints of almost 7000 submesoscale eddies. Since the visibility of vortical structures in synthetic aperture radar (SAR)SAR imagery significantly depends on the near-surface wind speed, wind data from a numerical model of the Baltic Sea were additionally used to get improved eddy statistics. Seasonally averaged fields of near-surface wind speed, surface currents, sea surface temperature (SST), and SST gradient were also analyzed in order to reveal the role of these hydrophysical parameters in the observed spatial and temporal variation of submesoscale eddies.
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