Impacts of Arctic precipitation changes on the downwelling limb of the Atlantic Meridional Overturning Circulation

According to the latest IPCC report, under the RCP 8.5 scenario precipitation in the Arctic region may increase by as much as 50%. The projected 21stcentury decline of the Atlantic Meridional Overturning Circulation (AMOC) is attributed in part to this increase in precipitation and the associated river run-off. Here we analyze a set of 44-year long coupled climate model simulations in which only the precipitation between 70N and 90N is varied, to assess the impacts of the projected Arctic precipitation increase on deep ocean convection and the downward transport of dense waters that constitutes the downwelling limb of the AMOC. With a 50% increase in Arctic precipitation, mixed layer depths in the Nordic Seas and Labrador Seas reduce substantially, albeit with a longer response time in the latter. A 400% precipitation increase completely inhibits deep mixing. At the same time, the simulations display a 10%-50% reduction of the overturning strength, showing that a shutdown of convection does not automatically result in a shutdown of the AMOC. Theoretical studies have pointed out that, while dense water formation in the marginal seas is important for the density structure of the deep ocean, the downward volume transport associated with the AMOC is governed by oceanic conditions along the continental boundary of the North Atlantic Ocean (Iceland Basin, Irminger and Labrador Seas). In addition, the overflows along the Greenland-Scotland Ridge contribute to the AMOC. In the simulation for the present-day climate, the contributions of these two processes are of similar magnitude. The overflow transports are unaffected by a 50% increase in Arctic precipitation. In contrast, the sinking along the boundary of the North Atlantic Ocean reduces by roughly 60%. When the Arctic precipitation is increased by 400%, a clear decline in the overflows is simulated as well. In this study, we analyze the physical processes responsible for these changes in the downwelling transport.