Multi-stability of the Global Overturning Circulation: A Conceptual Approach

The Global Overturning Circulation (GOC) is characterized by deep water formation in the subpolar North Atlantic, which feeds the southward-flowing branch of the Atlantic Meridional Overturning Circulation (AMOC). In contrast, the North Pacific lacks deep water formation and therefore does not host an analogous Pacific Meridional Overturning Circulation (PMOC). Proxy records, however, indicate that this asymmetric pattern of deep water formation has varied in the past, suggesting that a PMOC likely existed during earlier climate states. Recent studies further show that the development of a PMOC influences the future weakening of the AMOC: climate models that develop a PMOC in response to warming exhibit a stronger decline in AMOC strength. It therefore becomes important to understand under what circumstances a PMOC is likely to develop.

Here, we extend the pycnocline model of Gnanadesikan (1999) to a two-basin configuration, consisting of a narrow basin representing the Atlantic and a wide basin representing the Pacific. By including salinity as a prognostic variable, we find that this two-basin box model may exhibit three distinct overturning states under identical, longitudinally symmetric forcing: (1) an active narrow-basin sinking state, (2) an active wide-basin sinking state, and (3) a state with active sinking in both basins. Overturning states confined to a single basin are stabilized by the salt-advection feedback, whereas the state with sinking in both basins is maintained by a meridional temperature contrast. We find that this latter state becomes the preferred equilibrium when the interhemispheric temperature contrast increases, the northern gyre transport strengthens, and the hydrological cycle weakens. Moreover, we show that this state is more sensitive to high-latitude freshwater fluxes, indicating that a transition to such a state would enhance the projected future weakening of the AMOC. We verify these findings in an uncoupled global circulation model (MITgcm) with a simplified model geometry.