Analyses of Turbulent Submeso- and Mesoscale Processes using SAR Data

Contents

  1. Analyses of Turbulent Submeso- and Mesoscale Processes using SAR Data
    1. Submesoscale Eddies in the Mediteranean and Red Sea:
      1. Mediterranean Sea
      2. Red Sea
    2. Description of DTeddie Data Project:

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.

Description of DTeddie Data Project:

In this project we will develop algorithms to detect, track and measure small scale eddies using high resolution SAR-Data from both RADARSAT-2 and TerraSAR-X satellites. The principle areas to be investigated are the Black Sea and the Baltic Sea. In previous work we have already shown, that mesoscale surface currents in the Baltic Sea may be detected and measured by tracking natural surface lms using SAR-images or optical images. The algorithm used was a combination of maximum-cross-correlation and optical flow techniques. In the frame of this e ort, our promising results will be extend to the region of the Black Sea and to the study of small-scale eddies. Special attention will be paid to dynamics of small-scale fronts in semi-enclosed seas. The input of frontal instabilities in the structure formation process will be highlighted. High resolution radar data provides valuable information on the position of fronts, their origin, peaking, transition and destruction. They will be used to receive statistical and other information about their variability, to observe the formation of meanders along the fronts and their departure with the formation of vortices. Type, form and dynamic characteristics of the phenomena stipulated by a front and observed in its immediate proximity: jets, spiral eddies; vortical dipoles, internal waves etc. will be analyzed.

With the high resolution data provided by RADARSAT-2 and TerraSAR-X it will be possible even to study small scale eddies with sizes of less than 1 km. Using data from both satellites we will bene t from the high spatial resolution, the multiple modes of polarisation (more robust feature detection and classi cation) and the higher temporal resolution.

Principal investigator

Co-investigators

FEW3O/SAREddies/SAREddiesintern To get to the internal pages click here.

FEW3O ... back to the FEW3O Page (in German) ...