Issues in stratosphere-troposphere coupling

While the role of the troposphere in forcing the stratosphere is undisputed, it has generally been assumed in the past that the stratosphere acts only passively. Recent evidence from a variety of directions suggests that the stratosphere may play a more active role, on both short and long timescales. This has important implications for tropospheric climate change and climate variability. In investigating this possibility, it is important to frame the questions in a physically consistent manner. What is the appropriate null hypothesis for the stratosphere providing a passive upper boundary condition for the troposphere? What do we mean by downward influence? This talk will review the evidence for two-way (non-passive) stratosphere-troposphere coupling; discuss possible mechanisms, as well as constraints on those mechanisms; and attempt to frame some meaningful open questions.