CONVECTIVE BUILDING OF A PYCNOCLINE - LABORATORY EXPERIMENTS

The convective building of a pycnocline is examined using a laboratory model forced by surface fluxes of saline water at one end and fresh w ater at the other. A deep recirculation evolves in the tank, which hom ogenizes the interior fluid by repeated passes through the dense, desc ending plume. A thin, fresh surface layer develops and modifies the ef fective buoyancy flux into the dense plume, causing the interior veloc ities to fall to an intermediate-time minimum. Adding bottom topograph y under the dense source greatly reduces the amount of entrainment tha t the descending plume undergoes. In this case, the tank fills with a deep, heavy layer, which causes the plume to ''lift off'' the bottom o f the tank and detrain at successively higher depths in the water colu mn. A simple numerical ''plume'' model shows that this cannot be a ste ady state, as it is not in diffusive balance; the plume must eventuall y return to the bottom of the tank and ventilate the interior waters. Adding rotation increases the surface mixing, thickens the halocline, and increases the observed variability in the salinity field.