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 Nordic Seas Overturning Circulation (NOC) is projected to increase. So far, no causal link has been proposed between these two opposing trends. Here we propose that a density reduction in the subpolar North Atlantic will 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. Using high resolution climate model data and a box model, we find that in response to combined global warming and freshwater input the NOC initially increases moderately as the AMOC weakens, 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.