Keynote -- The Stratospheric Link Between the Sun and Climate
Mark P. Baldwin [mark@nwra.com], Northwest Research Associates, Inc., Bellevue, WA

Observations suggest that the atmosphere is approximately twice as sensitive to solar variability as would be expected from radiative calculations. This indicates that processes amplify an atmospheric solar signal to affect surface climate. On the 11-year time scale we observe solar/ozone heating in the tropical stratosphere. One way that this signal could be communicated downward is through an indirect mechanism involving planetary-scale waves during winter. The stratosphere appears to be sensitive to circulation anomalies in the tropical stratosphere, and during the winter season, zonal wind anomalies in the stratosphere tend to progress northward and downward through the stratosphere on a timescale of a few weeks. The process of wave, mean-flow interaction in effect amplifies the anomalies as the progress downward from ~1 hPa to the troposphere. This creates a plausible pathway for solar effects on climate during winter. Circulation anomalies just above the tropopause affect tropospheric weatheróbut the exact mechanisms are not clear.

The observed solar signal in the troposphere is not large, so we do not need to find mechanisms that are communicating a large signal. I will suggest a possible mechanism involving changes to the wave guide for tropospheric planetary-scale waves outside the winter season. If such a mechanism is at work, it would help explain why a year-round solar signal can be extracted from tropospheric observations.

These mechanisms are not specific to the 11-year solar cycle, and they could operate on much longer time scales. I will provide an overview of the winter mechanism for solar effects on climate, with examples from observations and simulations. I will also discuss the effects of lower stratospheric circulation anomalies on planetary-scale waves in the summer, and argue for the possibility that this mechanism can communicate stratospheric variability downward even in summer.