Nonlinear behavior of the thermohaline ocean circulation

Ulrike Feudel

Carl von Ossietzky University Oldenburg, Germany

Two conceptual box models describing the interhemispheric thermohaline circulation and a part of it, the open ocean deep convection, are studied with respect to bifurcations. Both models exhibit coexisting stable states. Freshwater fluxes and atmosperic temperatures are the main control parameters of the system: they determine the stable states and transitions between the different stable states. For the large scale thermohaline circulation it is shown that two different mechanisms can lead to the loss of stability of the present thermohaline ocean circulation: the saddle-node bifurcation can only occur for a change in total freshwater input into the North Atlantic catchment, but not for a redistribution of freshwater between the low and high latitudes of the Northern Atlantic. The Hopf bifurcation, in contrast, depends on both freshwater fluxes. The conceptional model of the deep ocean convection possesses also two stable states, where convection is "on" and convection is "off". Using data from the Labrador Sea we find that the model is in a bistable regime. Stochastic forcing can trigger a hopping process between convecting and non-convecting states.