Dynamical Prototype of the Arctic Oscillation as Revealed by aNeutral Singular Vector

Abstract

Origin of the Arctic Oscillation (AO) was explored from a dynamical perspective, using a primitive equation model linearized about the observed winter climatology. In order to obtain a leading low-frequency mode of the model atmosphere, singular vector analysis is performed on the linear dynamical operator. The singular mode (v vector) with the smallest singular value, referred to as the neutral mode, has a considerable similarity to the observed AO in many aspects, suggesting that, at least in a linear dynamical framework, the AO is a dynamically consistent mode of variability that arises from the zonal asymmetry of the time-mean state. Diagnosis of the neutral mode shows that a zonal flow?stationary wave interaction and an interaction between anomalous and climatological stationary waves are both of importance for maintenance of the spatial structure. Besides, vortex stretching?shrinking is significant in the upper levels, which indicates that a baroclinic process has a certain role as well. It is found that the formation of a hemispheric scale of the neutral mode is accomplished within several days via propagation of Rossby wave packets along jet streams that act as a waveguide. The neutral mode structure is robust for a wide range of model damping parameters, and especially preferred when the damping is weakened in the free troposphere. The corresponding optimal forcing (u vector) indicates that, though its robustness is relatively ambiguous, the neutral mode is most effectively excited by a certain pattern of the extratropical thermal forcing, in addition to a modest sensitivity to the deep heating anomaly over the tropical Indian Ocean.