A one-dimensional model of the seasonal thermocline: I. A laboratory experiment and its interpretation

A simple model of the seasonal thermocline is examined theoretically and with the aid of a laboratory experiment. It is argued that all the heat and mechanical energy which affect the water column can be put in near the surface and propagated downwards, without being influenced significantly by horizontal velocities, advection or rotation. If all the kinetic energy of stirring is used to change the potential energy of the system, one can calculate the temperature and depth of the well-mixed surface layer as a function of time, given the heat input. This has been done explicitly for a saw-tooth seasonal heating function and constant stirring rate. A step-by-step heating process has been simulated by an intermittent input of buoyant fluid at the surface, both theoretically and experimentally. Many features observed in the ocean are reproduced by both the theory and the experiments. The depth and temperature dependence of the upper mixed layer as functions of time, and the relation to the heating and cooling cycle, are in good qualitative agreement. Most important, this model shows clearly that surface mixing affects only the properties down to the topmost density interface, and that deeper features laid down early in the heating season can persist until the well-mixed layer reaches them again late in the winter.