Role of the Drake Passage in Controlling the Stability of the Ocean’s Thermohaline Circulation

The role of a Southern Ocean gateway in permitting multiple equilibria of the global ocean thermohaline circulation is examined. In particular, necessary conditions for the existence of multiple equilibria are studied with a coupled climate model, wherein stable solutions are obtained for a range of bathymetries with varying Drake Passage (DP) depths. No transitions to a Northern Hemisphere (NH) overturning state are found when the Drake Passage sill is shallower than a critical depth (1100 m in the model described herein). This preference for Southern Hemisphere sinking is a result of the particularly cold conditions of the Antarctic Bottom Water (AABW) formation regions compared to the NH deep-water formation zones. In a shallow or closed DP configuration, this forces an exclusive production of deep/bottom water in the Southern Hemisphere. Increasing the depth of the Drake Passage sill causes a gradual vertical decoupling in Atlantic circulation, removing the influence of AABW from the upper 2000 m of the Atlantic Ocean. When the DP is sufficiently deep, this shifts the interaction between a North Atlantic Deep Water (NADW) cell and an AABW cell to an interaction between an (shallower) Antarctic Intermediate Water cell and an NADW cell. This latter situation allows transitions to a Northern Hemisphere overturning state.