Elevated Mixing in the Periphery of Mesoscale Eddies in the South China Sea

Direct microstructure observations across three warm mesoscale eddies were conducted in the northern South China Sea during the field experiments in July 2007, December 2013, and January 2014, respectively, along with finestructure measurements. An important finding was that turbulent mixing in the mixed layer was considerably elevated in the periphery of each of these eddies, with a mixing level 5-7 times higher than that in the eddy center. To explore the mechanism behind the high mixing level, this study carried out analyses of the horizontal wavenumber spectrum of velocities and spectral fluxes of kinetic energy. Spectral slopes showed a power law of k(-2) in the eddy periphery and of k(-3) in the eddy center, consistent with the result that the kinetic energy of submesoscale motion in the eddy periphery was more greatly energized than that in the center. Spectral fluxes of kinetic energy also revealed a forward energy cascade toward smaller scales at the wavelength of kilometers in the eddy periphery. This study illustrated a possible route for energy cascading from balanced mesoscale dynamics to unbalanced submesoscale behavior, which eventually furnished turbulent mixing in the upper ocean.