EVOLUTION OF THE INSTABILITY OF A MIXED-LAYER FRONT

Observations from the Frontal Air-Sea Interaction Experiment (FASINEX) , indicating the presence of small-scale cold-core features in the Nor th Atlantic Subtropical Convergence Zone, motivated a recent linear an alysis of the instability of a geostrophically balanced mixed-layer fr ont. The results of that analysis suggested that the instability would preferentially form small-scale cold-core eddies at finite amplitude. In the present study, the finite-amplitude evolution of the fastest g rowing mode of this nongeostrophic baroclinic instability is investiga ted numerically. The linear prediction of cold-core eddy formation is confirmed by the nonlinear calculation. There are large horizontal and vertical heat and potential vorticity fluxes associated with the deve loping disturbance. The heat flux is confined above the thermocline, i n the region of sloping frontal isotherms that provide the energy sour ce for the instability, but the potential vorticity fluxes are maximum 50-75 m deep er and reach into the thermo dine. A tongue of low-poten tial-vorticity fluid is advected 50-75 m downward along isopycnal surf aces from the cold side of the front into the thermocline at the mixed -layer base. The small-scale potential vorticity structure has similar ities to estimates of the upper ocean potential vorticity field obtain ed previously from FASINEX observations. The calculations illustrate t he role that frontal instabilities may play in the flux of heat and po tential vorticity from the mixed layer into the thermocline. The evolu tion of the disturbance resembles baroclinic wave life cycles obtained in atmospheric models.