Nordic overturning increases as AMOC weakens in response to global warming

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is weakening in response to global warming, while the Nordic Seas Overturning Circulation (NOC) is projected to strengthen. So far, the causal link between these two opposing trends has been little explored. Using an idealized box model of the Atlantic Ocean and the Nordic Seas and a high resolution climate model, we propose that a density reduction in the subpolar North Atlantic might weaken the AMOC by reducing the density difference with lighter waters further south, while at the same time strengthening the NOC by increasing the density difference with the heavier waters further north. The box model shows that the NOC initially increases moderately as the AMOC weakens in response to combined global warming and freshwater input, while a tipping point may be reached later if deep convection in the Nordic Seas shuts down and the NOC collapses together with the AMOC. These results are supported by GCM simulations.