The weakening AMOC under extreme climate change

Abstract

We examine changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO2 experiments conducted under the sixth Coupled Model Intercomparison Project (CMIP6). The increase in CO2 triggers extensive Arctic warming, causing widespread melting of sea ice. The resulting freshwater spreads southward, first from the Labrador Sea and then the Nordic Seas, and proceeds southward along the eastern coast of North America. The freshwater enters the subpolar gyre north of the separated Gulf Stream, decreasing the density contrast across the current. The current weakens in response, reducing the inflow to the deepwater production regions. The AMOC cell weakens in tandem, first near the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perception that freshwater caps the convection regions stifling deep water production; rather, it is the inflow to the subpolar gyre which is suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local or remote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is the precursor to AMOC weakening in these simulations.

The interaction of Solar Radiation Modification and Earth System Tipping Elements

“…Gregory et al (2016) found that for forcings derived from doubling CO2 gradually over 70 years (1pctCO2), only heat flux changes lead to significant AMOC weakening, whereas freshwater flux other than ice sheet runoff has no significant impact. However, a recent preprint (Madan et al , 2023) suggests that for instantaneous CO2 quadrupling in CMIP6, freshwater forcing from sea ice melt weakens AMOC. Liu, Fedorov and Sévellec (2019) also suggested that changes in sea ice cover may impact AMOC through changes in freshwater input (freezing, advection and melting of ice floes) and heat flux (e.g., shielding ocean water from atmospheric influences).…”