MULTIPLE EQUILIBRIA AND TRANSITIONS IN A COUPLED OCEAN-ATMOSPHERE BOXMODEL

A six-box model is employed as a prototype of the coupled Atlantic oce an-atmosphere system. Ice dynamics are excluded. Numerical integration of this system shows that different thermohaline circulation patterns are possible under the same forcing conditions. They consist of a glo bal thermal mode with oceanic poleward surface flow, a global saline m ode with equatorward surface flow, and two intermediate modes that are combinations of the two global modes. The stability of the modern-day -like intermediate mode to finite amplitude freshwater flux perturbati ons in the high latitude North Atlantic (meant as a model of glacial m elting) is explored. It is found that freshwater fluxes of the proper inferred magnitude are close to critical and can induce a transition o f the coupled system to a saline mode. However, paleoclimatic data arg ues the last deglaciation was subcritical. Further, working in a reali stic subcritical parameter regime, the box model yields an unrealistic temperature record. This argues, in turn, that additional physics (e. g., sea-ice effects) must be included to properly describe the fundame ntal mechanics of the last glacial retreat.