THE ROLE OF SMALL-SCALE CELLS IN THE MEDITERRANEAN CONVECTION PROCESS

Data from the Gulf of Lions in the northwest Mediterranean in 1987 ind icated that the deep convection known to occur in that region was orga nized into small-scale [O(1 km)] cells embedded in a larger-scale [O(5 0 km)] homogeneous ''patch.'' Velocities from current meters showed th at during the period of strong surface forcing a front of increased ki netic energy propagated downward, finally reaching the bottom. An anal ytic expression for the depth of penetration of this front as a functi on of time and surface buoyancy flux was derived, using a simple one-d imensional model of the density profile, which agreed well with the ob servations. An analytic expression for the kinetic energy density was derived as a function of time, dissipation rate, and rate of change of potential energy, assuming conservation of total energy. Estimates of net volume transport were made from frequency distributions of vertic al velocity. Results indicate that the cells provided the turbulence n ecessary to efficiently mix the water column vertically, removing the existing weak but stable stratification. The cells were not directly r esponsible for any net volume transport downward but did contribute to the transport of fluid properties such as heat, energy, and chemical tracer concentration.