There is an increasing recognition of the important influence that annular modes have upon extratropical climate variability. Recent observational studies further posit that the stratosphere may modulate tropospheric climate in association with annular modes. In particular, during boreal winter tropospheric Northern Annular Mode (NAM) events are often preceded by variations in the strength of the stratospheric polar vortex. However, not all NAM cases follow the statistical prototype of mid-stratospheric initiation followed by downward signal movement into the troposphere as some robust stratospheric NAM events are not linked to subsequent tropospheric NAM events. We perform observational diagnostic studies of NAM events to identify the physical mechanisms responsible for intraseasonal NAM evolution and associated Northern Hemisphere climate variability.

The problem is approached in two general ways: First we analyze the composite evolution of intraseasonal NAM events to diagnose the physical mechanisms responsible for typical NAM variability. Second, we study selected case evolutions to identify the mechanisms accounting for case-to-case variability. In the latter, we contrast stratospheric NAM events with and without strong succeeding tropospheric signals to identify the reasons for observed differences. The basic input data are the daily averaged NCEP-NCAR reanalyses.

Our analysis procedure begins with a detailed assessment of eddy heat and momentum flux anomalies to deduce the respective roles of synoptic and low frequency eddies in forcing the zonal wind anomalies characteristic of NAM events. Zonal wind tendencies are then quantified at various heights and latitudes in conjunction with eddy forcing and Coriolis torques in order to attribute local accelerations to specific physical processes. The eddy flux analyses are complemented with parallel potential vorticity (PV) inversion diagnoses to assess the direct interaction between stratospheric and tropospheric dynamical structures. 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. The composite analyses indicate that (a) upward propagating Rossby waves force initial variations in the stratospheric polar vortex and (b) lower stratospheric PV anomalies provide a substantial feedback to the subsequent tropospheric circulation. The case studies further illustrate that, in individual cases, pre-existing tropospheric PV anomaly features can mask the downward penetration of an initial stratospheric NAM signal to tropospheric levels.