Control by the Circulation Adjustment Outside the Arctic on Transient Response of AMOC to Global Warming

The Atlantic Meridional Overturning Circulation (AMOC), a key component of the climate system, is projected to weaken in the 21st century. Using the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR), we conduct a set of numerical experiments to investigate the transient response of AMOC to anthropogenic warming with quadrupled carbon dioxide (4×CO2). The results suggest that circulation adjustment outside the Arctic dominates the AMOC weakening. In warming conditions, northward advection transport from the southern flank of the Atlantic subpolar region will increase, which is expected to enhance the upper ocean stratification over deep convection zones and inhibit deep-water formation, thus weakening the AMOC largely. Stratification enhancement is more pronounced in Nordic Seas than that in the Labrador Sea, implying a more direct role of Labrador Sea in evolution of still-active AMOC. In Nordic Seas, decreased ocean convection is dominated by temperature contributions due to a substantial increase of northward advective heat transport. While in Labrador Sea, both surface heat flux and advective heat transport matter, with comparable thermohaline contributions.