Zonal-eddy coupling and a neutral mode theory for the Arctic Oscillation

Abstract

This study demonstrates that a dynamical mode that resembles the observed Arctic Oscillation (AO) pattern can be obtained as a least damped mode of a linear operator that expresses the dynamics of the zonally averaged flow interacting with stationary, zonally-asymmetric waves. The spatial pattern of the leading singular mode of the operator bears much resemblance to the observed Arctic Oscillation both in terms of zonal and associated wave components. The mode has a damping time scale of more than a month, and is well separated from subsequent modes. Therefore, it is plausible that it survives on monthly and longer time scales. The simulated zonally asymmetric waves produce a meridional tilt due to advection by the AO-like sheared zonal flow and positively feed back to the zonal flow through the momentum flux arising from the interplay with the climatological mean waves. The dominant contribution of this feedback is shown to come from the features in the North Atlantic sector, supporting the notion that the AO is a hemispheric extension of the more regional North Atlantic Oscillation.