To identify a mechanism by which solar variability influences climate we investigate how the dynamics of the interaction between planetary waves and zonal flow responds to changes in solar UV flux. The dynamics is characterized by the state of the North Annular Mode (NAM). The response depends on seasonal variations of stratospheric circulation and the phase of the tropical Quasi Biennial Oscillation (QBO). Using the assimilated data from the National Centers for Environmental Prediction (NCEP) Reanalysis we show that the extratropical signature of the QBO is seen mostly in the NAM. To understand this we discuss two earlier suggested mechanisms for extratropical manifestation of the QBO: (1) the QBO modulation of the planetary wave flux that drives the NAM, (2) the effect of the QBO-induced meridional circulation on the NAM. We find that both mechanisms are involved but affect different extratropical areas, and the first one is seen in the beginning of the winter for wave 1 and in the end of winter for wave 2. We investigate the time dependence of the QBO-NAM coupling using an exact differential equation, which relates a proxy for the NAM index (the polar-cap averaged temperature) with the stream function of the meridional circulation.
We use the NCEP Reanalysis data and a simple nonlinear model to identify the major attractors of the stratospheric dynamics, which are displayed with the help of the joint probability functions of dynamical variables. The response to the solar variability is seen through change in occupation frequencies of the attractors.