Multiple equilibria and transitions in a coupled ocean-atmosphere box model

A six-box model is employed as a prototype of the coupled Atlantic ocean-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 global thermal mode with oceanic poleward surface flow, a global saline mode 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 perturbations in the high latitude North Atlantic (meant as a model of glacial melting) is explored. It is found that freshwater fluxes of the proper inferred magnitude are close to critical and can induce a transition of the coupled system to a saline mode. However, paleoclimatic data argues the last deglaciation was subcritical. Further, working in a realistic 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 fundamental mechanics of the last glacial retreat.