THERMOHALINE CIRCULATION - ENERGETICS AND VARIABILITY IN A SINGLE-HEMISPHERE BASIN MODEL

A series of numerical experiments have been carried out to explore the variability of the thermohaline circulation and the energetics of hal ocline catastrophe. It is found that when the amplitude of surface fre shwater flux is smaller than a critical value, the thermohaline circul ation is in a thermal mode, with deep water formed in the north. When the freshwater flux amplitude is supercritical, the thermohaline circu lation does not reach a single steady state. Instead, the model ocean is in a continuous transition between a slow, quasi-steady saline mode and an energetic, unsteady thermal mode. For cases with no wind stres s, the saline mode is characterized by sinking along the equator. For cases with wind stress, the saline mode is characterized by intermedia te water formation at midlatitude. During the saline mode phase, the d eep water gradually becomes warm and salty. Thus at the end of the sal ine mode phase, within the northern basin there is cold and relatively fresh water lying on top of warm and salty water. Such a vertical str ucture is potentially very unstable because small perturbations can gr ow, supported by the release of potential energy during strong cooling . A quantity called the diabatic available potential energy index is i ntroduced as an indicator for such convective instability. Thus a fina l equilibrium in the saline mode cannot be reached; instead, the model ocean flips to a very energetic thermal mode in which violent overtur ning destroys the vertical stratification. After the energy is release d, the saline cell supported by precipitation in the subpolar basin ad vances southward and the model ocean returns to the saline mode. The w hole cycle will be repeated.