Turbulent diapycnal mixing in the subtropical northwestern Pacific: Spatial-seasonal variations and role of eddies

Abstract

Both spatial and seasonal variation of turbulent diapycnal mixing in the subtropical northwestern Pacific are evaluated by employing a fine-scale parameterization method based on profiles of potential density, which are obtained from CTD measurements during our recent hydrographic surveys implemented by the China National Key Basic Research Project from 2008 to 2010 and the World Ocean Circulation Experiment. Over smooth seafloor, the value of diffusivity away from the boundary is comparable with the values observed in the stratified midlatitude ocean interior, i.e., O (10(-5) m(2) s(-1)). On the other hand, enhanced diapycnal mixing, i.e., O (10(-4) m(2) s(-1)) or larger has been found over rough topography such as the Central Basin Trough, Okidaito Ridge, the origin of the Kuroshio Current, and especially Luzon Strait, which might result from dissipation of baroclinic energy generated when barotropic tides rub over rough topography. Over flat bathymetry, mixing is probably stirred by the wind work on near inertial motions in the upper 600 m and enhanced downward propagating energy has been found in the presence of anticyclonic eddies, which points to the important role of anticyclonic eddies in enhancing the diapycnal mixing at greater depth. The diffusivity also displays a distinct seasonal variation with strong (weak) mixing corresponding to strong (weak) wind-input energy in winter (summer), which, however, is only confined to upper 600 m. This is different from the midlatitude northwestern Pacific, where seasonality of diffusivity can be found at 1500-m depth.