The influence of variability in meridional overturning on global ocean circulation

The Atlantic meridional overturning circulation (AMOC) is an important global-scale circulation and changes in AMOC can induce significant regional and global climate impacts. Here we study the stability of AMOC and its influence on global ocean circulation and the surface climate though analyzing a set of sensitivity experiments using the Community Earth System Model version 1 (CESM1). Results show that a collapsed AMOC can induce changes in global ocean circulation, such as reduced (or reversed) Bering Strait transport and weakened Indonesian Throughflow and Agulhas Current, but strengthened Drake Passage transport. It also changes the global wind pattern and surface temperature, such as a seesaw-like surface temperature change between Northern and Southern Hemispheres, a weakening of the Indian–Australian summer monsoon, and a southward shift of the Southern Ocean westerlies. We also found that AMOC and the Pacific deep meridional overturning circulation (PMOC) do not form a natural seesaw under modern-day climate and geography. A collapsed AMOC (active PMOC) is not stable under modern conditions if there is no additional freshwater (salt) input in the subpolar North Atlantic (Pacific), suggesting that the modern mean state of AMOC (PMOC) does not depend on local haline forcing although its variability and changes do.