Dichotomy between freshwater and heat flux effects on oceanic conveyor belt stability and global climate

The Atlantic meridional overturning circulation is an important global-scale oceanic circulation, and its changes may be responsible for past abrupt climate change events. By using two versions of a coupled climate model, here we show that the stability of this circulation depends not only on the background climate, but also on the type of primary external forcing: freshwater vs. greenhouse gases. When freshwater forcing is dominant, hysteresis of this circulation (an abrupt collapse/reactivation) becomes possible only under simulated glacial conditions with closed Bering Strait. Under present day and future conditions, both freshwater and greenhouse gas forcings could collapse this circulation, but only greenhouse gas forcing produced a bi-stable equilibrium state comparable to abrupt climate change. Our results demonstrate that the Bering Strait status (open vs. closed) may facilitate or prohibit the existence of this circulation's hysteresis, irrespective of the background climate conditions, but is directly related to the primary forcing. The collapse of the Atlantic meridional overturning circulation may be triggered by either freshwater or greenhouse gas forcing under present or future conditions, but only by freshwater forcing under glacial conditions, according to climate model simulations

Automated summary

The stability of the Atlantic meridional overturning circulation depends on both the background climate and the type of primary external forcing. The collapse of this circulation can be triggered by freshwater or greenhouse gas forcing under present or future conditions, but only by freshwater forcing under glacial conditions. The status of the Bering Strait is directly related to the presence of hysteresis in this circulation, regardless of background climate conditions.