The role of small-scale cells in the Mediterranean convection process

Data from the Gulf of Lions in the northwest Mediterranean in 1987 indicated that the deep convection known to occur in that region was organized into small-scale [O(1 km)] cells embedded in a larger-scale [O(50 km)] homogeneous "patch." Velocities from current meters showed that during the period of strong surface forcing a front of increased kinetic energy propagated downward, finally reaching the bottom. An analytic expression for the depth of penetration of this front as a function of time and surface buoyancy flux was derived, using a simple one-dimensional model of the density profile, which agreed well with the observations. 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 vertical velocity. Results indicate that the cells provided the turbulence necessary to efficiently mix the water column vertically, removing the existing weak but stable stratification. The cells were not directly responsible for any net volume transport downward but did contribute to the transport of fluid properties such as heat, energy, and chemical tracer concentration.