The Bering Strait is grip on the northern hemisphere climate

The last 10,000 years have been characterized by distinctly stable climates. For the earlier glacial period (up to 125,000 years ago), climate records show long-lasting large-amplitude oscillations, generally known as Dansgaard-Oeschger (D/O) or Heinrich events. These fluctuations are believed to be a result of freshwater anomalies in the North Atlantic which dramatically reduce the transport of the meridional overturning cell (MOC). They are followed by a recovery of the MOC. Here, we propose that such long lasting instabilities in the meridional circulation are only possible during glacial periods when the Bering Strait (BS) is closed. An analytical ocean model which includes both wind and thermohaline processes) is used to show that, during interglacial periods (when the BS is open) perturbations in North Atlantic Deep Water (NADW) formation are quickly damped out. This new mechanism involves the strong winds in the Southern Ocean (SO) which, with an open BS, quickly [O(1-10 years)] flush any large low salinity anomalies out of the Atlantic and into the Pacific Ocean. During glacial periods, the stabilizing effect is prevented by the closure of the BS which traps the anomalies within the Atlantic, causing long lasting perturbations. We also show that no continuous fresh-water flux is needed in order to keep the collapsed MOC from a recovery in the closed basin case, but a relatively large continuous flux of 0.19 Sv is required in order to keep the collapsed state in the open BS case. With a smaller freshwater transport, the open MOC quickly recovers. This also indicates that an open BS is more stable than the closed BS case. Process-oriented numerical simulations using idealized geometry in a two-layer ocean support our analytical solutions and show that the flushing mechanism is active even when the shallow BS sill is included. (C) 2004 Elsevier Ltd. All rights reserved.