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Abstract: Seven-year (2005–2011) Synthetic Aperture Radar (SAR) images are applied to study oceanic eddies in the East China Sea. It is found that most of these eddies detected from the SAR images are less than 10 km, which are submesoscale eddies. Seasonal differences are evident in the distribution of eddies, with the highest and the lowest number of eddies noted in summer and winter, respectively. Since slick streaks in SAR images look dark, an eddy identified due to the slicks is referred to as “black eddy”. As a result of wave-current interactions in the zones of current shear, it can be seen that an eddy exhibits a bright curve, the eddy is called “white eddy”. During the seven years, 95 black eddies and 50 white eddies are identified in the study area. Black eddies are found in the whole study area while white eddies are mainly distributed in the vicinity of the Kuroshio Current. This study suggests that the distribution of the white eddy is denser around the Kuroshio because of the strong shear in the Kuroshio region. In terms of the eddy sizes, white eddies are generally smaller than black eddies.
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Key words:
- submesoscale /
- ocean eddy /
- Synthetic Aperture Radar /
- East China Sea
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Figure 1. An eddy identified by SAR image (a), black eddy (b, 2008/07/11 UTC 01:48), and white eddy (c, 2008/08/10 UTC 13:53) identified by the Envisat SAR. In a, the outer black ellipse is the eddy boundary; the aggregation point of black curves in the SAR image is the center of the eddy; the average length of eight yellow straight lines passing through the eddy center is the diameter.
Figure 2. The radius histogram of eddies identified in the SAR images in the East China Sea from 2005 to 2011. Most of the eddies identified in the SAR images are submesoscale eddies with radius less than 20 km. Most of the radius of the eddies is concentrated in the range of 0–10 km.
Figure 3. The radius histograms of black (a) and white (b) eddies in the East China Sea. The radii of most black and white eddies are mainly concentrated in the range of 0–10 km and the radii of white eddies are more concentrated in the smaller scale than the radii of black eddies.
Figure 4. Spatial distribution of eddies in the East China Sea. The red triangles represent the center locations of the eddies. The background color is bathymetry (m). The data is derived from gridded bathymetry data provided by the General Bathymetric Chart of the Oceans (GEBCO).
Figure 5. Comparison between the numbers of black eddies and white eddies. The blue, orange and yellow bars represent the number of anticyclonic eddies, cyclonic eddies and total number of eddies, respectively.
Figure 6. Spatial distribution of black eddies (a) and white eddies (b). The red triangles represent the center locations of the eddies. The background color is bathymetry (m). The black eddies are distributed over the whole area while the white eddies are mainly concentrated in the Kuroshio and its adjacent areas.
Figure 7. The spatial distributions of the shear deformation rate (ξ/f) of sea surface current and white eddies. The background color is the shear deformation rate and black points denote locations of eddies. Eddies are mostly distributed in areas of large shear deformation rates.
Figure 8. Annual distribution of the number of eddies in 2005–2011. The blue, orange and yellow bars represent the number of anticyclonic eddies, cyclonic eddies and total number of eddies, respectively. The largest number of eddies is identified in 2008 and the smallest number is recorded in 2006. The number of eddies identified in the other years ranges from 15 to 20.
Figure 9. Annual distribution of the number of eddies identified in each SAR image which can be used for eddy detection. The blue, orange and yellow bars represent the number of anticyclonic eddies, cyclonic eddies and total number of eddies, respectively. In 2006, the number of eddies identified in each SAR image is the largest (3.33) and the lowest (0.70) in 2005. In the other years, the number of eddies identified in each SAR image ranges from 1 to 2.
Figure 10. Seasonal distribution of the number of eddies in the East China Sea. The blue, orange and yellow bars represent the number of anticyclonic eddies, cyclonic eddies and total number of eddies, respectively.
Figure 11. Seasonal distribution of the number of eddies identified in each SAR image. The blue, orange and yellow bars represent the number of anticyclonic eddies, cyclonic eddies and total number of eddies, respectively.
Figure 12. Seasonal average of 10-m wind speeds from 2005 to 2011 for spring (a), summer (b), autumn (c), and winter (d).
Figure 13. A cyclonic eddy identified by the SAR image (a), geostrophic current field (b), sea level anomaly (c), and sea surface temperature (d) in the Bohai Sea on September 16, 2009.
Table 1. The temporal distribution of the number of SAR images (a total of 104) capturing eddies by season and year
Year Number Spring Summer Autumn Winter Total 2005 3 4 15 1 23 2006 0 3 0 0 3 2007 1 7 4 1 13 2008 5 12 1 8 26 2009 5 4 2 1 12 2010 2 5 2 0 9 2011 5 7 1 5 18 Total 21 42 25 16 104 
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