The Atmospheric Response to Meridional Shifts of the Gulf Stream SST Front and Its Dependence on Model Resolution

Abstract

The Gulf Stream (GS) plays a key role in shaping the North Atlantic climate. Moreover, the associated sea surface temperature (SST) front undergoes interannual-to-decadal variability that is thought to force significant atmospheric circulation anomalies. However, general circulation models do not accurately reproduce the atmospheric response to SST front variability as estimated from observations. In this work we analyze the atmospheric response to the GS SST front (GSF) shifts in a multimodel ensemble of atmosphere-only simulations forced with observed SSTs (1950–2014). The atmospheric response is found to be resolution dependent. Only the high-resolution simulations produce a wintertime response similar to observed anomalies. More specifically, (i) analysis of the atmospheric thermodynamic balance close to the GSF showed that the anomalous diabatic heating associated to the GSF displacement is mainly balanced by vertical motion and by meridional transient eddy heat transport (not the case for low-resolution models), while (ii) the large-scale response includes a meridional shift of the North Atlantic eddy-driven jet and storm track homodirectional to the GSF displacement. This atmospheric response is accompanied by changes in low-level baroclinicity close to and north of the GSF, resulting from the oceanic forcing and the zonal atmospheric circulation anomalies respectively. The low-level baroclinicity anomalies lead to changes in baroclinic eddy activity and, ultimately, in the jet via eddy–mean flow interaction. Considering the two-way nature of air–sea interactions, using historical atmosphere-only simulations is a powerful way to isolate the impact of realistic oceanic variability on the atmosphere. Our results suggest that interannual-to-decadal predictability may be higher than what low-resolution models currently indicate.