Study On The Diapycnal And Isopycanl Mixing In The South Chian Sea And The Western North Pacific

Observations by CTD, ADCP, LADCP, TurboMAP and VMP in western North Pacific(WNP) during October to November, 2005 and off Vietnam during August, 2007 are used to study the diapycnal and isopycnal mixing in the South China Sea(SCS) and WNP.TurboMAP observations suggest that, the turbulent dissipation ratesεin Vietnam cold eddy(VCE) are 10^-10¹0-8W·kg^-1, and diapycnal diffusivitiesκare 10-7¹0-6m2s^-1. There are no distinct difference ofεandκbetween the eddy core and edge. The dissipation rates in VCE are higher than the magnitudes in the north of SCS, but with diffusivities alike. Inferred upwelling due to diapycnal mixing based on vertical advective-diffusive balance equation is about 10-8m/s, maximum of which up to 7×10-8m/s (equivalent to 0.60cm/day) appears in the core. While the maximum of vertical velocity via the numerical solution of 2-D column coordinate equation is up to 7.31m/day, which is about 700 times the ocean interior values and far lager than that due to diapycnal mixing, suggesting that the vertical motion in eddy is dominant by the geostropic current field. The distributions of vertical velocity are opposite to the driving term in the right side of equation, suggesting that the distributions of vertical motion are dominant by the nonlinear interaction between the components of the horizon geostropic velocity. Using"mixing length"theory, the lateral eddy scale is estimated to be 50～210km, and isopycnal diffusivity decrease from 4500 m2 s^-1 at surface to 500 m2 s^-1 at 500m. The timescale of Vietnam cold eddy estimated using 1D heat diffusive equation is about 12.4 days.Compared with the observations of VMP, finesale parameterizations based on shear and strain are better than overturn method based on Thorpe-scale. The turbulent dissipation ratesεin WNP interior are 10^-11¹0^-10W·kg^-1, and diapycnal diffusivitiesκare 10-6¹0-5m2s^-1, with the higher value of the water volumn adjacent to than that off the Philippine Islands. The enhanced mixing at 21°N may due to the energy supplied by the internal tide generated in Luzon Strait. The elevated mixing level appeared at 2⁶°N can be resulted from low water depth and the interaction between flows and the rough topography. The diffusivities in SCS are one to two orders higher than the values at the same depth in WNP, the reason of which is that, on one hand, the internal tide generated in Luzon Strait propagates to the supercritical continental slope and is reflected to the deep basin of SCS; on the other hand, the nonlinear interaction between the internal tide and the background internal wave field results to wave breaking and dissipation, both supply great deal of energy to turbulent mixing there.