Speaker: Prof. Joseph LaCasce from University of Oslo

Title: "Weakening of the Atlantic Meridional Overturning Circulation in an extreme climate"

Abstract: The Atlantic Meridional Overturning Circulation (AMOC) regulates the global transport of heat, freshwater, trace gases and nutrients in the Atlantic sector. Published proxy records and modeling studies, reviewed by the IPCC, are consistent with a weakening AMOC in the warming climate. We examine AMOC changes in the quadrupled CO2 experiments conducted under the CMIP6 program. While the forcing is extreme, the abrupt transition facilitates diagnosing the sequence of significant events. AMOC weakens in response to freshwater input in the subpolar gyre, due primarily to sea ice melt. The resulting freshwater flows south along the eastern coast of North America, and then eastward, north of the Gulf Stream. This decreases the density gradient across the North Atlantic Current, reducing the associated vertical shear and consequently the transport. As such, the inflow to the northern downwelling regions is cut off. This is in contrast with the common perception that freshwater “caps the convection regions”, stifling deep water production. Changes in surface temperature have a weaker effect, and there are no consistent changes in local wind forcing among the models. The results indicate that increase in freshwater discharge, primarily from the Labrador Sea, is a precursor to AMOC weakening.