Unsteady, Turbulent Convection into a Rotating, Linearly Stratified Fluid: Modeling Deep Ocean Convection

Abstract

A laboratory experiment has been constructed to Model the deep convective processes in a stratified ocean driven by the energetic cooling at the ocean surface. For convenience in the laboratory the authors have examined the analogous but inverted problem of the growth of a convective mixed layer generated by a localized source of bottom heating in a rotating, thermally stratified fluid. The heating is provided by a heat exchanger mounted beneath a central circular plate of a diameter smaller than that of the main cylindrical tank. Following the initiation of the heating, a cylinder of well-mixed fluid forms above the heated plate and as it grows upward it steadily erodes the overlying ambient stratification. Measurements of the root-mean-square velocities in the mixed layer indicate that rotation affects but does not control the turbulence, and the nonrotating turbulent velocity and length scales are thus appropriate to describe the small-scale turbulence. Rotation initially confines the heated fluid to the region above the central plate, but as time progresses, the near-vertical front separating the well-mixed heated fluid and the surrounding ambient stratified fluid becomes unstable, eventually generating a field of baroclinic eddies. The breakdown is initiated by the development of a rim current centered on the periphery of the heated plate. Our experiments have confirmed that the current velocity scales as (Bt)1/2 and that it is also linearly dependent on the height within the mixed layer. The rim current eventually goes unstable, generating the eddies whose length scales measured in our experiments are in good agreement with the predictions of linear stability theory.