[Response: It's related to planetary wave activity. (realclimate.org)
Magnusdottir, G., and P.H. Haynes, 1999: Reflection of planetary waves in three - dimensional tropospheric flows. (sites.uci.edu)
Perlwitz, J., and N. Harnik, 2003: Observational evidence of a stratospheric influence on the troposphere by planetary wave reflection. (pubs.giss.nasa.gov)