Variability on the century time scale and regime changes in a stochastically forced zonally averaged ocean-atmosphere model

The response of a zonally averaged climate model of the atmosphere-ocean system to a stochastic freshwater forcing depends on its strength. Three regimes are found: For small perturbations of the system, the ocean responds with irregular oscillations around a state which is characterized by a strong Atlantic overturning circulation. For intermediate to large amplitude forcing the thermohaline circulation in the Atlantic ocean exhibits pronounced regular overturning (flushing) events. The third regime is a complete collapse of the Atlantic overturning. Transitions can be induced between the three regimes. While for moderate values the transitions are reversible, for large forcing amplitudes the Atlantic overturning circulation collapses irreversibly to the third regime.