Storm-track weakening in response to Arctic sea ice loss arises with ocean coupling

The storm tracks play a major role in shaping the midlatitude climate through important contributions to the atmospheric heat transport between tropical and polar regions and its associated effects on energy balances. In response to changes in the climate from anthropogenic forcing, the location and intensity of the storm tracks is projected to change, with the North Atlantic storm track projected to shift poleward, affecting the weather and climate of western Europe. The poleward shift of the North Atlantic storm track arises from competing effects on the region's meridional temperature gradient. Arctic Amplification weakens the climatological gradients, hence weakening the storm track, while on the other hand, low latitude warming has the opposite effect by strengthening the climatological gradient. To determine the mechanisms driving these competing changes, we use the atmosphere-land general circulation model (AGCM) and extended ocean-atmosphere coupled simulations (CGCM) from the Polar Amplification Model Intercomparison Project (PAMIP). These serve to separate the effects of sea ice loss and low latitude warming. We also complement the standard PAMIP CGCM experiments with CO2 doubling experiments in which the sea ice state is fixed, using a full ocean model and a slab-ocean model coupled to WACCM4. In the coupled simulations, the sea ice is constrained using a novel hybrid nudging method that combines ghost-flux forcing and direct sea ice area nudging. We find that coupled simulations with either a full ocean model or a slab ocean model capture the weakening of the storm track, but atmosphere-only simulations fail to capture this response, potentially due to the weaker and more meridionally confined Arctic warming in the uncoupled simulations. We carry additional atmosphere-only simulations with meridionally varying prescribed sea surface temperature warming that is attributable to sea ice loss, to investigate the role of the meridional extent of the warming on the storm tracks.