We use an empirical statistical model to demonstrate significant skill in forecasts of the monthly-mean Arctic Oscillation (AO). Forecast skill derives from long-lived circulation anomalies in the lowermost stratosphere and is greatest during the extended winter season when the stratosphere and troposphere are coupled by vertically-propagating planetary-scale waves. A comparison to the Southern Hemisphere, together with modeling evidence, indicates that predictability of the AO depends on 1) the long timescale of circulation anomalies in the lowermost stratosphere and 2) the vertical coupling of the troposphere and stratosphere by planetary-scale waves. Circulation anomalies in the lowermost stratosphere affect the troposphere through a mechanism involving changes to waves and momentum fluxes in the upper troposphere, which induce surface pressure changes corresponding to the AO.

I will also summarize the results of the workshop "The role of the stratosphere in tropospheric climate" held in Whistler, BC, Canada, April 29 to May 2 2003.