The thermal structure of the upper ocean

The salient feature of the oceanic thermal structure is a remarkably shallow thermocline, especially in the tropics and subtropics. What factors determine its depth?; The first part of this dissertation explores the hypothesis that the constraint of a balanced heat budget for the ocean exerts a strong control on the depth of the thermocline. This constraint implies that an increase in heat loss in the subtropics can result in a shoaling of the tropical thermocline, whereas a decrease in heat loss can cause a deepening of the thermocline. Arguments based on a balanced heat budget yield an expression for the depth of the thermocline and for the static stability of the ocean, in terms of certain parameters such as oceanic diffusivity, the imposed surface winds, and atmospheric surface temperature gradients; previous scaling arguments are recovered as special cases of a general result. Numerical simulations with a general circulation model agree well with the predictions of the theory.; Processes responsible for satisfying the constraint of a balanced heat budget, such as the thermohaline circulation and the ventilated thermocline, are often described as having characteristic timescales. The second part of this dissertation investigates the implications such timescales have for the variability of the mean state of the ocean. This is first illustrated through a series of numerical simulations of the formation of a thermocline from an initially isothermal ocean. It is found that, in the presence of a basin-wide heat budget constraint on the steady state solution, the relevant timescale for the formation of a thermocline is a short one, associated with the exchanges of heat at the surface and their interaction with the wind-driven dynamics of the ocean. The response of the low latitude thermocline to mid-latitude heat flux variability is also investigated by studying the response of a closed basin to periodic forcing. The diabatic control of thermocline structure results in a low-pass filter behavior for a periodically forced oceanic basin.