Constraining CMIP6 simulations for the Atlantic Water in the Arctic using an AMOC-SST index

Marion Devilliers ^1* Steffen Malskær Olsen ¹ Helene Langehaug ² Chuncheng Guo ¹ Rashed Mahmood ¹ Tian Tian ¹ Shuting Yang ¹

• ¹ Danish Meteorological Institute (DMI), Copenhagen, Denmark
• ² Nansen Environmental and Remote Sensing Center, Bergen, Hordaland, Norway

The Atlantic Water plays a key role regarding the future changes in the Arctic Ocean. It contributes to the Atlantification by transporting salt and heat within the Arctic Ocean basins. Many studies also attribute the amplified Arctic Ocean warming to an increase in poleward ocean heat transport by warming currents or increasing strength of ocean currents. Global models are needed to predict reliable and consistent trends of heat transport as large scale processes are at play. They are however too coarse to resolve key ocean processes and deal with the complex interplay between ocean dynamics and bathymetry of the Arctic region. Here we propose to construct a sub-ensemble of simulations based on 235 historical simulations from 12 CMIP6 models that best represents the downstream drivers of Atlantic warming. We select the model ensemble members showing the closest agreement with observed surface temperature variability over 1960-1990 in the subpolar gyre (SPG). More specifically we use a recent index that links the surface temperature in the SPG to the Atlantic Overturning Meridional Circulation (AMOC): the AMOC-SST index. The subsampled ensemble shows a better correlation with the observed AMOC-SST index over the last 35 years of the historical period . It also displays over the latter period a reduced error and better correlation for the Atlantic Water core temperature and depth in the Eurasian Arctic Ocean, when compared to reanalysis and observations. Overall, the AMOC-SST index based selection leads to a systematic improvement of the representation of the Atlantic Water layer in the Eurasian Arctic region, suggesting a clear connection between the Arctic Ocean and the surface temperature in the subpolar region, and by extension, possibly the AMOC.