Response of the northern North Atlantic and Arctic oceans to a sudden change of the North Atlantic Oscillation

Abstract

The influence of a switch from a long‐time high North Atlantic Oscillation (NAO+) state to an enduring NAO− situation is investigated with a coupled sea ice–ocean model. We compare the response to a sustained shift to NAO− conditions with that to a short‐lived transition to NAO− and back to NAO+ forcing. Observed changes in sea ice and oceanic variables between high and low NAO states are well captured in the model response. The ocean circulation adjustment includes a fast barotropic anomaly, accompanied by an enhancement of the meridional overturning and the northward heat transport at 48°N. The slow response contains a substantial decrease of the northward heat transport, which is caused by a reduction of the transport of subpolar and subtropical gyres. After a delay of around 2 years, the oceanic heat transport reacts to counteract the meridional temperature gradient imposed by the surface forcing. The propagation time of baroclinic Rossby waves is irrelevant for the delay between the forcing and the oceanic response. Rather, the strength of the subpolar gyres is determined by the rapid spin‐up of a topographic Sverdrup circulation and the subsequent slower changes through the JEBAR (Joint Effect of Baroclinicity And Relief). In an experiment where only the wind stress is changed to NAO− conditions, the adjustment is dampened strongly by the NAO+ thermohaline forcing. The gyres decrease slower than in the NAO− case. Comparison with an experiment where only the wind stress is changed to NAO− conditions indicates that the thermohaline forcing is most important. The heat fluxes counteract the wind stress forcing such that the meridional overturning and the northward heat transport decrease only slightly compared to the control run.