Timothy J. Dunkerton and Mark P. Baldwin

Northwest Research Associates

Abstract:

The Arctic and Antarctic Oscillations (AO, AAO) are the surface manifestation of the Northern and Southern Annular Modes (NAM, SAM) which represent the primary mode of axisymmetric variability in the high-latitude troposphere and stratosphere. Although the time scale of annular-mode variation is quite different between the troposphere and lower stratosphere, being considerably shorter in the troposphere, the state of the winter tropospheric annular-mode anomaly is biased in the direction of the stratospheric anomaly after the onset of a large positive or negative stratospheric anomaly. This bias lasts for 60 days or more, much longer than the time scale of tropospheric annular-mode variation (~10 days). The stratosphere annular mode provides an apparent predictive skill for the troposphere in the subseasonal range, beyond the limit of numerical weather prediction. In fact, the stratospheric annular-mode signature provides a better explanation of future tropospheric AO or AAO variability than the tropospheric anomaly itself.

Enhanced coupling and predictive skill are observed only at those times of year when planetary-scale Rossby waves propagate from the troposphere to middle atmosphere. This time span corresponds to the extended winter season in the northern hemisphere, but is restricted to the beginning and end of the southern-hemisphere extended winter season.