The long-held view among atmospheric scientists is that the troposphere, with its abundance of mass and momentum, dominates lower atmospheric dynamics while the stratosphere is more passive, contributing little influence on tropospheric circulation. In light of recent observational and modeling studies, this view is changing as the stratosphere has been shown to contribute directly and indirectly to the tropospheric circulation. In particular, the mid-to high-latitude troposphere is sensitive to variations in the stratospheric polar vortex related to the Northern Annular Mode (NAM). We study this interaction on intraseasonal timescales to deduce the potential mechanisms responsible for current mid- to high latitude climate variability in the Northern Hemisphere.

Composite and regression analyses are presented showing the structure and time evolution of a typical NAM event. During intraseasonal events there is a strong signal that propagates from the stratosphere downward to the surface. Zonal-mean and three-dimensional eddy-flux diagnostics are used to study the role of synoptic and low frequency waves in forcing the anomalous zonal winds characteristic of the NAM. Anomalous zonal wind tendencies are analyzed at various heights and latitudes in conjunction with the wave driving and Coriolis torque in an attempt to attribute the accelerations to specific dynamical processes. These are complemented with parallel potential vorticity inversion diagnoses to assess the direct dynamic interaction among separate flow anomaly features.