Frontogenesis in an Advective Mixed Layer Model.

Abstract Through an analysis of a multi-dimensional extension of the bulk mixed layer model of Kraus and Turner (1967) it is shown how, even under spatially uniform atmospheric conditions, an initially smooth horizontal temperature gradient in the surface mixed layer can still develop into a front. For this to occur, it is essential that the net downward heat flux at the air-water interface be related to the difference between the mixed layer temperature T and an apparent atmospheric temperature TA (Haney, 1971), so that the initial horizontal gradient in T corresponds to a horizontal variation in the surface buoyancy flux. As a result the mixed layer depth also differs from place to place. Depending on the direction of the wind-driven transport, this produces either a steepening or a flattening of the initial temperature profile. A lower bound condition for the initial horizontal advective heat flux is derived in terms of the initial surface heat flux, the stirring by the wind, and the time rate of chanc...