Spatial structure of turbulent mixing of an anticyclonic mesoscale eddy in the northern South China Sea
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Abstract: Upper turbulent mixing in the interior and surrounding areas of an anticyclonic eddy in the northern South China Sea (SCS) was estimated from underwater glider data (May 2015) in the present study, using the Gregg-Henyey-Polzin parameterization and the Thorpe-scale method. The observations revealed a clear asymmetrical spatial pattern of turbulent mixing in the anticyclonic eddy area. Enhanced diffusivity (in the order of 10–3 m2/s) was found at the posterior edge of the anticyclonic mesoscale eddy; on the anterior side, diffusivity was one order of magnitude lower on average. This asymmetrical pattern was highly correlated with the eddy kinetic energy. Higher shear variance on the posterior side, which is conducive to the triggering of shear instability, may be the main mechanism for the elevated diffusivity. In addition, the generation and growth of sub-mesoscale motions that are fed by mesoscale eddies on their posterior side may also promote the occurrence of strong mixing in the studied region. The results of this study help improve our knowledge regarding turbulent mixing in the northern SCS.
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Key words:
- mesoscale eddy /
- turbulent mixing /
- South China Sea /
- GHP parameterization /
- Thorpe-scale method
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Figure 1. Trajectories of the anticyclonic eddy and underwater glider. a. Sea level anomaly (SLA) data for May 18, 2015. The red line indicates the track of the studied anticyclonic eddy. The black line shows the track of the underwater glider. The studied anticyclonic eddy is marked with a blue dashed line. The locations of the eddy center and glider on May 3, May 8, May 13, May 18, May 23, and May 28 are indicated. b. Absolute dynamic topography (ADT) on March 1, 2015. The red line indicates the track of the studied anticyclonic eddy. The yellow dash line in a and white dash line in b show an ADT of 118 cm. Bathymetry of the South China Sea with the 200 m, 1 000 m, 2 000 m and 3 000 m isobaths overlaid in a and b.
Figure 2. Mean T-S diagrams of water masses within and outside of the studied eddy (with the potential density of
$ {\sigma }_{\theta } $ in kg/m3 contours overlaid). The blue, red, and yellow lines show data for the water forming the within eddy (115.5°–117°E), outside eddy left (114°–115.5°E), and outside eddy right (117°–118.5°E) sections, respectively. The black dashed line shows data for the Kuroshio Current water based on historical Argo T-S profiles within the black boxes of the inset figure.
Figure 3. Variations in the physical field of the studied anticyclonic eddy, including the eddy radius (a), rotational speed (b), and amplitude (c).
Figure 4. Vertical profiles of physical fields of the eddy. Potential temperature (a), salinity (b), potential density along the glider track (c), and baroclinic geostrophic velocity across the track (d). The white dashed lines indicate the eddy center derived from SLA data. The positive and negative values in d represent northward and southward velocities, respectively.
Figure 5. Estimates of GHP parameterization. a. 3D view of diffusivity, Kρ, along the glider track. The colors indicate SLA values on May 18, 2015; the black line shows the edge of the anticyclonic eddy, identified following Chelton et al. (2011); and the red dashed line indicates the eddy center. b. Averaged diffusivities by depth related to the eddy anterior side (blue) and posterior side (red). The dashed line indicates the averaged values. c. Variation in diffusivity by longitude. The vertical red and black dashed lines indicate the eddy center and edge, respectively.
Figure 6. An example of overturn identified at 19.86°N, 117.031°E in the northern SCS. a. Vertical profiles of potential temperature. b. An overturn detected from the original and sorted potential temperature profiles in the depth range indicated by the orange box in a.
Figure 8. Variation in diffusivity derived with Thorpe-scale method by longitude. The vertical red and black dashed lines indicate the eddy center and edge, respectively.
Figure 9. Comparison of eddy kinetic energy, diffusivity, and surface velocity. a. Eddy kinetic energy (EKE) corresponding to the moving path of the studied anticyclonic mesoscale eddy and the bin-averaged diffusivities by longitude. R2 is the linear fitting coefficient of determination. b. Bin-averaged surface velocity by longitude. The velocity was derived from HYCOM.
Figure 10. Fine structure shear variances derived from geostrophic velocities. a. Section display. b. Averaged shear variance in the posterior edge region (red), anterior edge region (blue), and the peripheral region (yellow).
Figure 11. Contour map of density anomalies δρ. The white line shows the core of the eddy. The anterior and posterior edges are marked with pink boxes.
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