Spatial structure of turbulent mixing of an anticyclonic mesoscale eddy in the northern South China Sea

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-HenyeyPolzin 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.

Variations of mesoscale eddy SST fronts based on an automatic detection method in the northern South China Sea

SST fronts at the mesoscale eddy edge(ME fronts)were investigated from 2007–2017 in the northern South China Sea(NSCS)based on an automatic method using satellite sea level anomaly(SLA)and SST data.The relative probabilities between the number of anticyclonic/cyclonic ME fronts(AEF/CEF)and the number of anticyclones/cyclones reached 20%.The northeastern and southwestern parts of these anticyclones had more fronts than the northwestern and southeastern parts,although CEFs were nearly equally distributed in all directions.The number of ME fronts had remarkable seasonal variations,while the eddy kinetic energy(EKE)showed no seasonal variations.The total EKE at the ME fronts was three times of that within the MEs,and it was much stronger in AEFs than in CEFs.The interannual variability in the number of ME fronts and EKE had no significant correlation with the El Ni?o-Southern Oscillation(ENSO)index.Possible mechanisms of ME fronts were discussed,but the contributions of mesoscale eddies to SST fronts need to be quantified in future studies.

Eddy covariance measurements of turbulent fluxes in the surf zone

Turbulent eddies play a critical role in oceanic flows. Direct measurements of turbulent eddy fluxes beneath the sea surface were taken to study the direction of flux-carrying eddies as a means of supplementing our understanding of vertical fluxes exchange processes and their relationship to tides. The observations were made at 32 Hz at a water depth of ~1.5 m near the coast of Sanya, China, using an eddy covariance system, which mainly consists of an acoustic doppler velocimeter(ADV) and a fast temperature sensor. The cospectra-fit method-an established semi-empirical model of boundary layer turbulence to the measured turbulent cospectra at frequencies below those of surface gravity waves-was used in the presence of surface gravity waves to quantify the turbulent eddy fluxes(including turbulent heat flux and Reynolds stress). As much as 87% of the total turbulent stress and 88% of the total turbulent heat flux were determined as being at band frequencies below those of surface gravity waves. Both the turbulent heat flux and Reynolds stress showed a daily successive variation;the former peaked during the low tide period and the later peaked during the ebb tide period.Estimation of roll-off wavenumbers, k0, and roll-off wavelengths, λ0(where λ0=2π/k0), which were estimated as the horizontal length scales of the dominant flux-carrying turbulent eddies, indicated that the λ0 of the turbulent heat flux was approximately double that of the Reynolds stress. Wavelet analysis showed that both the turbulent heat flux and the Reynolds stress have a close relationship to the semi-diurnal and diurnal tides, and therefore indicate the energy that is transported from tides to turbulence.