Study examines the resilience of the Atlantic Meridional Oscillating Circulation (AMOC) to climate extremes and greenhouse gases. The study uses 34 climate models models to demonstrate that AMOC is resilient to extreme conditions.

Key points of the study:

• The importance of AMOC: The AMOC is important for the northward transport of heat in the Atlantic Ocean and is therefore important for global climate. The AMOC is projected to weaken during the 21st century due to increasing greenhouse gas concentrations and freshwater input into the North Atlantic.
• AMOC resilience: Despite predictions of a weakening or collapse of the AMOC, the study shows that the AMOC is resilient to extreme climate conditions. Upwelling in the Southern Ocean, driven by winds, maintains the weakened AMOC and prevents it from completely collapsing.
• Pacific Meridian Overturning Circulation (PMOC): The study found that the PMOC appears in almost all models, but is too weak to balance out the upwelling in the Southern Ocean, suggesting that AMOC collapse is unlikely this century.
• Upwelling paths: The study quantifies the AMOC upwelling pathways that return deep North Atlantic waters to the surface through wind-driven upwelling in the Southern Ocean or diffusion in the Atlantic and Indo-Pacific Oceans. It is shown that wind-driven upwelling in the Southern Ocean maintains a weakened AMOC.
• Experiments: Two extreme forcing scenarios in CMIP6 climate models were used to examine the AMOC: a sudden fourfold increase in atmospheric carbon dioxide and a freshwater input to the North Atlantic.
• Findings: All models show a weakening of the AMOC that stabilizes over 90 years. The weakening of the AMOC ranges from 20 % to 81 % 90 years after the fourfold increase in CO2.
• Southern Ocean: The Southern Ocean upper cell is strengthened by the fourfold increase in CO2, primarily due to increased westerly winds. Conversely, the Southern Ocean upper cell is weakened by the influx of freshwater into the North Atlantic.
• Dependence of future AMOC strength: The future strength of the AMOC depends on the future strength of the Southern Ocean upper cell and the future strength of the PMOC.
• Implications for the future: The findings suggest that a better understanding of the Southern Ocean and Indo-Pacific circulation is needed to make more accurate predictions of AMOC changes. The study also highlights that changes in the hydrological cycle or changes in the transport of freshwater into the Indo-Pacific Ocean may be critical to AMOC tipping.
• The importance of observation: The study highlights the need for improved observations of the Southern Ocean and Indo-Pacific circulation, as well as their heat and freshwater transport. This would allow climate models to be identified with realistic Southern Ocean upper cell forcings and to detect changes in AMOC upwelling paths due to global warming.

Overall, the study suggests that a complete collapse of the AMOC is unlikely in the 21st century, but a better understanding of the Southern Ocean and Indo-Pacific circulation is needed for more accurate predictions of AMOC changes. Spring

Glossary of key terms

• AMOC (Atlantic Meridional Overturning Circulation): Atlantic Meridian Overturning Circulation; a system of currents in the Atlantic Ocean that transports heat northward.
• PMOC (Pacific Meridional Overturning Circulation): Pacific Meridian Overturning Circulation; a potential circulation in the Indo-Pacific Ocean that could arise from a weakening of the AMOC.
• CMIP6 (Coupled Model Intercomparison Project Phase 6): A climate model intercomparison project that coordinates simulations from different global climate models.
• 4xCO2: An extreme forcing scenario in which atmospheric CO2 concentration suddenly quadruples from pre-industrial levels.
• u03_hos: An extreme forcing scenario in which a constant inflow of 0.3 Sverdrups of freshwater is added to the North Atlantic.
• Sverdrup (Sv): Unit of volumetric flow rate; 1 Sv = 10^6 m^3/s.
• Southern Ocean (SO): Southern Ocean; the ocean surrounding Antarctica.
• Upwelling: The rise of deep waters to the ocean surface.
• Downwelling: The sinking of surface waters into the depths of the ocean.
• SO upper cell: Wind-driven circulation in the Southern Ocean, which has an important influence on the AMOC.
• Atlantic-Pacific seesaw: A phenomenon in which changes in the AMOC and PMOC interact.
• Eddy compensation: A process in the Southern Ocean where eddies compensate for the effect of wind on circulation.
• Buoyancy flux: The buoyancy flow on the ocean surface, influenced by temperature, salinity, and precipitation.
• IndoPac_ResidualUp: Upwelling of AMOC water in the Indo-Pacific Ocean, which subsequently does not rise in the Southern Ocean.
• SouthernOcean_Up: Upwelling of AMOC waters in the Southern Ocean.
• Atlantic_Up: Upwelling of AMOC waters in the Atlantic Ocean.