Simulated excitation of the Arctic Oscillation by orbital forcing

The observed solar forcing over the past 165,000 years is imposed on an atmosphere model coupled to a mixed layer ocean to study the response of the Arctic Oscillation to orbital forcing. The orbitally-forced changes in the surface pressure field project strongly onto the typical spatial patterns associated with both the Pacific and Atlantic centers of action of the Arctic Oscillation. Thus orbital forcing excites preferentially the characteristic mode of variability exhibited by the unforced climate system. Feedbacks between eddies and the mean flow are responsible for this, just as they are responsible for the existence of Arctic Oscillation in the unforced climate. When orbital forcing intensifies or weakens the northern hemisphere jet stream through an change in the equator-to-pole temperature gradient, the anomaly of the jet stream is reinforced where eddy activity is large. This results in a larger pressure signal in the storm track regions. Thus perturbations to the mean flow, whether internally-generated or orbitally-forced, are amplified the most in the storm track regions. These results have interesting implications for past variations of the Arctic Oscillation and NAO, particularly over the past 10,000 years.