Changes in Overturning Circulation under Arctic Sea Ice Retreat in EC-Earth

Anais Bretones, Department of Earth Science, University of Bergen and the Bjerknes Centre for Climate, Bergen, Norway, Kerim Hestnes Nisancioglu, University of Bergen and the Bjerknes Centre for Climate Research, Bergen, Norway, Mari Fjalstad Jensen, University of Bergen, Bergen, Norway and Mehmet Ilicak, GFDL, Princeton, NJ, United States
While a rapid sea ice retreat in the Arctic has become ubiquitous, the potential weakening of the Atlantic Meridional Overturning Circulation (MOC), in response to rising greenhouse gases, is still under debate. Although climate models predict a weakening of the Atlantic MOC, observations are so far inconclusive.
It has been suggested that the strength and vertical extent of the Atlantic MOC responds to sea ice retreat, as deep mixing occurs in open ocean area close to the sea ice edge. Here, we investigate this hypothesis in a long simulation with the EC-Earth climate model under the emission scenario RCP8.5. Interactively coupled with a dynamic Greenland ice sheet, this model includes fresh water fluxes from the ice sheet to the ocean. Moreover the length of the run (years 1850-2300) documents the consequences of an ice-free Arctic in summer and in winter.
By calculating the Atlantic MOC beyond the north Pole to the Siberian Shelf, we investigate its spatial variations through time in response to changes in sea ice and the location of deep mixing in the central Arctic. We find that the regions with deep mixing move north, following the retreating winter sea ice edge. At the same time, the mixing in the Labrador and Greenland Seas is reduced. While the Atlantic MOC weakens, its Arctic extension is enhanced, suggesting that the location of deep water formation plays a decisive role in determining the structure and strength of the MOC.
Interestingly, the strengthening of the overturning circulation and deep water formation in the Arctic do not compensate for the weakening of the Atlantic MOC.