Nonlinear behavior of the thermohaline ocean circulation
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.