Variability and Multiple Equilibria of the Thermohaline Circulation Associated with Deep-Water Formation

In this study the variability of the thermohaline circulation on decadal and centennial time scales that is related to the process of deep-water formation is investigated. This is done within the context of a simple geostrophic three-layer ocean model with a rectangular closed basin. When slowly varying the atmospheric forcing the model response shows sudden transitions, characterized by local changes in convective activity. Many different equilibria were found within the thermally driven regime (in Stommel's sense, i.e., downwelling occurs near the poles). Next, the deep-water formation process was analyzed with a one-dimensional box model. In this box model four different regimes can be identified: convective, nonconvective, periodic, and a regime where both convection and no convection are possible with the same mixed boundary conditions. These regimes were identified in the circulation of the ocean model. A region was traced where convection is possible according to the authors’ analysis but not occurring. In this potentially convective region convection can be triggered easily. Similarly. there exists a region where the deep-water formation is easily suppressed. These sensitive areas generate multiple equilibria in the ocean model and contain a simple mechanism for variability on decadal and centennial time scales in the climate system.