Periodic variability in the large-scale middle latitude atmosphere

Periodic behavior in the climate system has important implications not only for weather prediction, but also for understanding and interpreting the physical processes that drive climate variability. Here we demonstrate that the large-scale middle latitude atmospheric circulation exhibits marked periodicity on timescales of ~20- 30 days. The periodicity is most clear in the Southern Hemisphere but is also apparent in the Northern Hemisphere. It emerges in hemispheric-scale averages of the eddy fluxes of heat, the eddy kinetic energy, and precipitation. Observational and theoretical analyses suggest that the ~20-30 oscillation in the middle latitude circulation derives from feedbacks between the extratropical baroclinicity, the wave fluxes of heat, and radiative damping. The oscillation plays a potentially important role in driving large- scale climate variability throughout much of the midlatitudes.