Conference Overview

To first order, the coupling between the stratosphere and troposphere is mediated by wave dynamics. Planetary-scale Rossby waves, gravity waves, and equatorially trapped Kelvin and mixed Rossby-gravity waves typically originate in the troposphere, propagate upward into the stratosphere, and then dissipate causing variability of the stratospheric flow. The conventional view through the 1990s was that the resulting interactions are principally one way, i.e., that tropospheric waves influence the stratospheric circulation, but that stratospheric circulation anomalies did not have significant effects on tropospheric weather and climate. However, in the past ~5-10 years, the prevailing view has changed, and variability in the extratropical atmospheric flow is now recognized to reflect "two-way" interactions between the stratospheric and tropospheric circulations. The change in view is in response to the increasing amount of observational evidence (e.g., Kodera et al., 1990) that suggests stratospheric processes play an important role in tropospheric climate variability across a wide range of timescales. For example:

• On intraseasonal timescales, observations show that large amplitude anomalies in the strength of the Northern Hemisphere wintertime stratospheric polar vortex frequently precede long-lived (up to ~two months) changes to the tropospheric circulation (Baldwin and Dunkerton 1999; 2001). These changes modulate not only average weather, but also the likelihood of extreme events on timescales longer than the limit of deterministic weather prediction (Thompson et al., 2002).
• On interannual timescales, the stratospheric quasi-biennial oscillation has been found to exhibit a signature in surface climate (Coughlin and Tung, 2001; Thompson et al., 2002).
• On timescales of several years, volcanic eruptions that inject sulphate aerosols into the stratosphere also noticeably influence tropospheric climate both radiatively and dynamically (Robock and Mao, 1992; Graf et al., 1994; Kodera and Yamazaki, 1994; Stenchikov et al., 1998; 2004).
• On decadal timescales, Antarctic ozone depletion appears to have had a demonstrable impact not only on stratospheric temperatures and circulation, but on surface climate as well (Thompson and Solomon, 2002).
• On timescales up to centuries, stratospheric mechanisms have been proposed to explain a larger than expected impact of solar variability at the surface.

Below is a list of the key questions and topics we propose to address at the conference.

1. What determines the flux of planetary wave activity into the stratosphere?
2. Dynamical mechanisms
3. Effects of climate change on stratospheric circulation
4. How will climate change affect ozone recovery?
5. How will the tropopause layer change?
6. How will the oceans and the stratosphere interact?
7. Forecasting, modelling priorities and strategies

The Whistler meeting in 2003 led to publication of a Perspective in Science Magazine on stratosphere-troposphere coupling (Baldwin et al., 2003). The scientific outcomes from the current meeting will be published in a lengthier paper for AGU Reviews of Geophysics. The topics to be covered in the synthesis will be discussed in a panel discussion on the last day of the meeting. Key questions that will be addressed will include: What is the state of our understanding of stratsophere/troposphere coupling? What constitutes "downward influence"? What observational and data assimilation products are needed to foster continued scientific progress? What new stratospheric modeling capabilities are needed to improve climate forecasts? Is the current observational network sufficient to monitor stratospheric climate change and its effects on tropospheric weather? What new directions are key for understanding the dynamics of stratosphere/troposphere coupling?

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