Stratospheric-Troposphere coupling during intraseasonal variations in the Arctic Oscillation

The long-held view among atmospheric dynamicists is that the troposphere, with its abundance of mass and momentum, dominates global dynamics while the stratosphere is a more passive component, contributing little downward influence to the troposphere. In light of recent observational and modeling studies, this view is changing as the stratosphere has been shown to contribute both 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 Arctic Oscillation (AO). We study this dynamic interaction on intraseasonal timescales to deduce the role of potential coupling mechanisms in the mid- to high-latitude troposphere and stratosphere.

We first apply lag regression analysis to daily observational data to isolate the vertically coupled three-dimensional linear and nonlinear dynamical structure of the AO on intraseasonal timescales. We then use the regression results along with a transformed Eulerian Mean analysis to trace various components of the zonal-mean dynamic evolution of AO events. This analysis includes a two-dimensional diagnostic modeling component based upon observed E-P fluxes. Three-dimensional Rossby wave activity flux diagnoses are used to characterize the basic dynamic characteristics of the zonally asymmetric component of the AO evolution. Piecewise potential vorticity (PV) inversions are also performed to study the dynamic interaction of PV anomalies at different vertical levels. The results indicate that, on intraseasonal timescales, zonal stratospheric anomalies may act to initiate tropospheric AO events.