Direct microstructure observation in the northern South China Sea

Microstructure observation was conducted in upper layer of the northern South China Sea, along with horizontal velocity, temperature and salinity measurements. Based on these data, a spatial distribution of mixing diffusivity is given, which shows a decreasing variability from the north to the south. During the observation period, a warm eddy existed over the field experiment area. The measurements spanning the mesoscale eddy are analyzed. Result shows the mixing level in the eddy is elevated significantly in the mixed layer when compared to that outside the eddy. In contrast, the mixing levels in and outside the eddy differ slightly below the mixed layer. Further analysis suggests that in the mixed layer the diffusivity value in the margin of the eddy is clearly larger than that in the center of the eddy. The associated mechanism is explored, which indicates that a potential candidate for this high diffusivity values in the margin of the eddy is sub-mesoscale process. This study provides a way to assess the influence of the mesoscale eddy on the small scale mixing.