DENSITY COMPENSATED THERMOHALINE GRADIENTS AND DIAPYCNAL FLUXES IN THE MIXED-LAYER

Density-compensated temperature and salinity gradients are often obser ved in mixed layer fronts. A possible explanation of this observation is that there is a systematic relation between the ''strength'' of a f ront, defined as the buoyancy jump across the front, and the thickness of a front. If stronger fronts tend to be thicker, then in an ensembl e of random fronts, in which the temperature and salinity jumps are in dependent random variables, the temperature and salinity gradients wil l be correlated. This correlation between the thermohaline gradients i s such that heat and salt make antagonistic contributions to the buoya ncy gradient-that is, there is buoyancy compensation. The statistics o f heat and salt fluxes across nearly compensated fronts are counterint uitive: strong heat fluxes can occur across a front with weak thermal gradients and strong salinity gradients, and vice versa. As a specific model that relates the width of a front to the strength of a front, a pair of coupled nonlinear diffusion equations for heat and salt are u sed. The nonlinear diffusion coefficient, proportional to the square o f the buoyancy gradient, arises from quasi-steady shear dispersion dri ven by thermohaline gradients. This nonlinear mixing prevents stirring by mesoscale advection from indefinitely filamenting mixed layer trac er distributions. The model predicts that the thickness of a front var ies as the square root of the strength and inversely as the one-quarte r power of the mesoscale strain.