There is an increasing recognition of the potentially important influence that the stratosphere has upon tropospheric variability. Recent observational and numerical modeling studies posit that the stratosphere may provide direct and/or indirect influences on tropospheric climate. In particular, the tropospheric circulation is strongly coupled to variations in the strength of the stratospheric polar vortex, especially during extreme phases of the Northern Annular Mode (NAM). The hypothesized indirect influence is linked to stratospheric modulation of tropospheric planetary wave propagation whereas in the direct forcing mechanism stratospheric potential vorticity anomalies induce annular tropospheric circulation anomalies. We perform diagnostic case studies of the NAM to test the extent to which the proposed mechanisms can account for idiosyncracies among the different cases.

A key result of Baldwin and Dunkerton's 1999 observational study is that not all NAM cases follow the composite model of mid-stratospheric initiation followed by downward signal movement into the troposphere. In particular, certain stratospheric NAM events are not linked to subsequent tropospheric events and vice-versa. We test the role of the direct and indirect forcings in explaining such case to case variability. Focusing on strong stratospheric NAM events, we contrast those with and without strong succeeding tropospheric signals with a goal of identifying dynamical reasons for observed differences. We first apply potential vorticity (PV) methods to daily observational analyses to diagnose the direct dynamic interaction between the troposphere and stratosphere during NAM cases of interest. In this approach, circulation anomalies are decomposed into separate parts related to distinct PV anomaly features, permitting a diagnosis of far-field circulations associated with local PV structures. PV analyses are complemented with parallel eddy-flux diagnostics to study the role of wave driving in locally forcing the anomalous zonal winds characteristic of the NAM. Our results indicate that, for individual cases, pre-existing tropospheric PV anomaly features can preclude the downward penetration of an initial stratospheric NAM signal to tropospheric levels.