A simple model for large-scale thermohaline convection

Abstract

Abstract Large-scale thermohaline circulation cells in the world oceans exist due to horizontal density gradients. Owing to differential heating between equator and pole a north-south density gradient is present at the ocean surface. An additional mechanism for producing north-south density gradients is the heat and salinity transport by boundary currents in the upper parts of the world oceans. In the North Atlantic, high density sinking water is produced by a convergence of salt water from the south and cold water from the north. We have investigated large-scale thermohaline convection cells by time integrating a two-dimensional Boussinesq model. The flow is driven by an imposed density gradient at the upper boundary (mixed layer) and an externally prescribed, meridionally varying density flux in the interior of the model domain. We perform numerical simulations and through time integrations we determine equilibrium states of the model. By changing the forcing parameters slowly during the course of a time integration, we have identified regions in parameter space where a hysteretic behaviour may be found. For a given set of the forcing parameters the model has two stable equilibrium states which differ in the direction of the circulation. We have thus found that due to the nonlinear nature of the balance between advection, internal mixing and forcing more than one stable equilibrium circulation pattern can be found under a given set of external forcing parameters. This result may offer a potential explanation for the existing asymmetries between the thermohaline circulation patterns in the Atlantic and Pacific Oceans as well as possible implications for the climatic state of the oceans under changing external conditions.