Intraseasonal Dynamical Evolution of the Northern Annular Mode

Abstract

The Northern Hemisphere annular mode (NAM) accounts for a significant fraction of the extratropical wintertime atmospheric variability. The dynamics of NAM events have been studied on monthly time scales, but little is known about the physical mechanisms that give rise to NAM variability on shorter time scales. Composite diagnostic analyses based on daily NAM indices are performed with a goal of identifying the dominant processes responsible for the growth and decay of large-amplitude positive and negative NAM events on short intraseasonal time scales. Transformed Eulerian mean, piecewise potential vorticity inversions, and regional Plumb flux diagnoses are employed to deduce the proximate forcings of the zonal-mean wind tendency during maturing and declining NAM stages. A remarkable degree of reverse symmetry is observed between the zonal-mean dynamical evolution of positive and negative NAM events. Anomalous equatorward and downward (poleward and upward) Eliassen?Palm fluxes are observed during the maturation of positive (negative) NAM events, consistent with index of refraction considerations and an indirect downward stratospheric influence. The associated patterns of anomalous wave driving provide the primary forcing of the zonal wind tendency field. Spectral analyses reveal that both the stratospheric and tropospheric patterns of wave driving are primarily due to low-frequency planetary-scale eddies. Regional wave activity flux diagnoses further illustrate that this wave-driving pattern represents the zonal-mean manifestation of planetary-scale anomalies over the North Atlantic that are linked to local anomalies in stationary wave forcing. The decay of NAM events coincides with the collapse in the pattern of anomalous stationary wave forcing over the North Atlantic region. Our diagnostic results indicate that both (i) synoptic eddies and (ii) direct downward stratospheric forcing provide second-order reinforcing contributions to the intraseasonal dynamical evolution of NAM events.