Stratospheric impact on surface climate
Is it real, and how does it work?
There are several lines of evidence linking stratospheric conditions to surface climate. In most cases, the tropospheric signal manifests itself in the “annular mode” patterns of the northern and summer hemispheres, which are characterized principally by shifts in the latitude of the eddy-driven jet. While no single piece of observational evidence demonstrates unambiguously that the tropospheric flow is influenced by stratospheric conditions, the sum total of the evidence is highly suggestive. The clearest evidence, however, comes from models: there is now a substantial literature demonstrating the tropospheric impact of imposed perturbations to the stratospheric state.
The annular modes themselves are inherently dependent on the interaction between the zonal flow and baroclinic eddy activity within the troposphere. For one thing, this interaction tends to amplify and to organize any perturbation to the climate system, such that the response is “annular mode”-like even when the perturbation is not. For another, this fact makes diagnosis of the role of such perturbations less straightforward than one might expect: the immediate causes of changes in the zonal flow are changes in baroclinic eddy fluxes, irrespective of the perturbation. A third consequence is that most models, which tend to exaggerate the time scale of the annular modes, are too sensitive to external perturbations – this prediction of the “fluctuation-dissipation theorem” has been validated in model experiments.
The dynamics of the stratosphere-troposphere coupling that is manifested in much model experiments remains somewhat unclear. The most likely candidates are the meridional circulation induced by stratospheric perturbations, and modulation of the planetary-scale Rossby waves that dominate the winter/spring stratosphere.