A series of numerical experiments have been carried out to explore the variability of the thermohaline circulation and the energetics of halocline catastrophe. It is found that when the amplitude of surface freshwater flux is smaller than a critical value, the thermohaline circulation is in a thermal mode, with deep water formed in the north. When the freshwater flux amplitude is supercritical, the thermohaline circulation 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 stress, the saline mode is characterized by sinking along the equator. For cases with wind stress, the saline mode is characterized by intermediate water formation at midlatitude. During the saline mode phase, the deep water gradually becomes warm and salty. Thus at the end of the saline mode phase, within the northern basin there is cold and relatively fresh water lying on top of warm and salty water. Such a vertical structure is potentially very unstable because small perturbations can grow, supported by the release of potential energy during strong cooling. A quantity called the diabatic available potential energy index is introduced as an indicator for such convective instability. Thus a final equilibrium in the saline mode cannot be reached; instead, the model ocean flips to a very energetic thermal mode in which violent overturning destroys the vertical stratification. After the energy is released, the saline cell supported by precipitation in the subpolar basin advances southward and the model ocean returns to the saline mode. The whole cycle will be repeated. ¿ American Geophysical Union 1994