Diagnostic Case Studies of the Northern Annular Mode

Abstract

Recent observational studies of the northern annular mode (NAM) indicate that significant case-to-case variability exists in the structural evolution of individual events. In particular, certain NAM events remain confined to stratospheric altitudes whereas others readily penetrate downward into the troposphere. We perform observational diagnostic analyses that are targeted at identifying the physical mechanisms behind this distinction. Our results thereby provide a test of the different existing theories regarding stratospheric influences upon tropospheric climate. We contrast robust stratospheric NAM events with differing tropospheric signals in order to identify the underlying dynamical reasons for the observed differences. Piecewise potential vorticity (PV) inversions and Eliassen?Palm flux analyses are performed to study the roles of different forcing mechanisms during NAM onset. Our results indicate that variations in the tropospheric response are readily explained on the basis of piecewise PV inversions. Specifically, during individual cases, preexisting tropospheric PV anomaly features can mask the downward penetration of an initial stratospheric NAM signal into the troposphere. Analyses of PV inversions further suggest that a minimum requirement for a direct downward stratospheric influence is that the stratospheric NAM signal be robust in the lower stratosphere. Thus, whether or not a tropospheric NAM signal emerges from a stratospheric NAM event is largely dependent upon (i) whether stratospheric PV anomalies descend to sufficiently low altitudes within the stratosphere and (ii) the detailed nature of preexisting annular modes in the troposphere. Parallel Eliassen?Palm flux analyses further indicate that anomalous Rossby wave forcing is important for initiating NAM events in the midstratosphere and facilitating their downward advance into the lower stratosphere.