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Stratospheric Processes And their Role in Climate  (SPARC)

 

Authors:  Peter Haynes

 

Published By:  "SPARC" - A Project of the World Climate Research Programme

 

Published online:   July, 2005

 

Introduction:
It is widely accepted that the troposphere has a strong dynamical effect on the stratosphere, primarily through the upward propagation of waves, both
low-frequency large-scale Rossby waves (‘planetary waves’) and high-frequency inertia-gravity waves. Understanding of this effect is based on simple theories of wave propagation (including the wellknown Charney-Drazin criterion for vertical Rossby wave propagation), experiments in many different types of numerical models, plus observational indicators such as differences in stratospheric circulation between summer and winter, and between the hemispheres. An important aspect of this effect is that in the stratosphere there is a two-way interaction between waves and mean flow. Breaking or
dissipating waves exert a systematic mean force G that changes the mean flow. The mean flow, on the other hand, affects the propagation, breaking and dissipation of waves and hence itself affects G . The twoway interaction can lead, for example, to sensitivity to initial conditions, or to internal
dynamical variability. Yoden et al. (2002) and Haynes (2005) review some these issues.

Nonetheless, it is still not yet the case that our understanding of the dynamical effect of the troposphere on the stratosphere can be said to be complete. For example, for events such as stratospheric sudden warmings (including the unexpected Southern Hemisphere sudden warming of September 2002), in which the stratospheric circulation is highly perturbed, it does not seem possible to identify unambiguously anomalously large tropospheric wave
forcing as the cause. (See for example papers in the recent special issue of Journal of Atmospheric Sciences, volume 62, number 3.) One of the reasons may be that the view of the troposphere having a one-way dynamical effect on the stratosphere is seriously limited. There are plenty of reasons why the coupling should be two-way. The large-scale extratropical dynamics of the atmosphere is inherently non-local in both horizontal and vertical. Changes in the stratosphere must inevitably affect the troposphere and vice versa – the key question, of course, is how much?

 

Link to full article: http://www.atmosp.physics.utoronto.ca/SPARC/News25/coupling.html

 

 

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