Projections of the North Atlantic warming hole can be constrained using ocean surface density as an emergent constraint

Abstract

There are large uncertainties in the projections of the future of the warming hole, which is defined as the cooling trend in the subpolar North Atlantic Ocean despite of global warming. Here, we found that the uncertainty in the future changes of the warming hole is mainly due to model bias rather than either natural variability or climate change scenario. Observations and model results constrain the future warming hole intensity in terms of its relationship with the present-day surface density in the subpolar North Atlantic. Models with a low present-day surface density tend to project a weaker warming hole intensity (i.e., a stronger temperature increase) due to a suppressed oceanic deep convection in the future than models with a high surface density. This relationship was robust across all scenarios and was used to reduce the uncertainty of the future warming hole intensity by 39% in the high emissions scenario. Use of present-day surface ocean water density as an emergent constraint substantially reduces the uncertainty in Earth system model projections of the North Atlantic warming hole.