Diagnostic Case Studies of the Northern Annular Mode

Abstract

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 ...