The influence of variability in meridional over turning on global ocean circulation
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 temperatures, 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.
We have studied the influence of a collapsed AMOC on global ocean circulation and surface climate and explored the hypothesis that AMOC and PMOC form a natural seesaw-like variability, such that a weakening (strengthening) of AMOC induces a strengthening (weakening) of PMOC, and vice versa. There are four possible states: 1) active AMOC with an inactive PMOC, which represents the oceanic mean state under modern conditions; 2) inactive AMOC and PMOC, which could be a state under future warmer climate condition; 3) active PMOC with an inactive AMOC, a condition that might be true during Heinrich events; and 4) active AMOC and PMOC, a condition that may be true during the Pliocene.
Our results further indicate that significant changes in AMOC affect ocean circulation not only in the Atlantic basin but also globally. For example, a collapsed AMOC weakens the Bering Strait and Indonesian throughflows and the Agulhas Current but strengthens the ACC (measured as the Drake Passage transport). The collapse of AMOC would not automatically cause a strengthening of the PMOC unless additional salt flux is added into the subpolar North Pacific under modern conditions. This is due to two reasons: 1) much higher precipitation (thus higher net freshwater input) in the subpolar North Pacific than that in the subpolar North Atlantic and 2) the weaker AMOC inducing a weakening (even reversing direction) of the Bering Strait throughflow, which leads to a reduced freshwater being transported into the Atlantic from the North Pacific via the Bering Strait (and thus more freshwater being kept in the subpolar North Pacific), leading to a much fresher surface water that prevents the deep convection to occur.
Our experiments also show that by hypothetically adding additional salt input in the subpolar North Pacific, PMOC can set up and this formation of PMOC does induce a weakening of AMOC if there is additional freshwater input into the subpolar North Atlantic, leading to a seesaw-like change between AMOC and PMOC (e.g., the AMOC and PMOC changes in PACSALT experiment). Further investigation using our experiments indicates that under modern-day climate conditions, the states of active PMOC or inactive AMOC are not stable states without the additional salt input in the subpolar North Pacific or additional freshwater flux into the subpolar North Atlantic because without these forcings, PMOC collapses and AMOC reactivates. This result implies that although AMOC and PMOC are capable of generating global-scale climate changes, these changes due to either AMOC or PMOC or both are not strong enough to fundamentally change the dynamics of atmospheric moisture transport, ocean freshwater redistribution, and local air-sea interactions. Once the perturbation is removed, the AMOC and PMOC will go back to their preferred state, and under modern-day conditions, these are the active AMOC and inactive PMOC states. Therefore AMOC and PMOC do not form a natural seesaw under modern climate and geographic conditions.