Quantifying eddy feedbacks and forcings in the tropospheric response to stratospheric sudden warmings
The equatorward shift of the zonal mean midlatitude tropospheric jet following a stratospheric sudden warming in a comprehensive, stratosphere-resolving model is found to be well quantified by a simple model, due to Lorenz and Hartmann, of the tropospheric eddy feedbacks. This permits a decomposition of the shift into a component driven by the stratospheric anomalies, and a component driven by tropospheric feedbacks. This is done by extending the simple model to include three effective forcing mechanisms by which the stratosphere may influence the tropospheric jet. These include (1) the zonally-symmetric adjustments associated with the mean meridional circulation, and direct influence of the stratospheric anomalies on (2) the tropospheric synoptic-scale or (3) the tropospheric planetary-scale eddies. Although the anomalous tropospheric winds are primarily maintained against surface friction by the synoptic-scale eddies, this response can be entirely attributed to the eddy-feedback term. The response of the planetary scale eddies, in contrast, is found to be directly influenced by the stratosphere. The zonally-symmetric tropospheric circulation associated with downward control is found to play little role in driving the tropospheric response. The prospects of applying this methodology to reanalysis data is also considered, but statistical limitations and the relatively weak projection of the vertically integrated composite wind anomalies onto the leading EOF preclude any conclusions from being drawn.