Feedbacks between the isentropic slope and wave generation at tropospheric levels

The authors examine feedbacks between the isentropic slope and the generation of eddy activity in the Northern Hemisphere winter. The results reveal that periods of enhanced lower tropospheric eddy heat fluxes (and thus wave generation) are preceded by anomalously steep isentropic surfaces in the free troposphere, and followed by anomalously poleward eddy momentum fluxes near the tropopause. The lag between the isentropic slope and wave generation is consistent with our theoretical understanding of baroclinic instability. The lag between the eddy fluxes of heat and momentum suggests that variability in the momentum fluxes is driven in part by variability in wave generation near the Earth’s surface. The observed feedbacks between the isentropic slope and the generation of eddy activity in the lower troposphere have implications for the response of the extratropical circulation to a range of forcings. Here the authors examine the implications for the tropospheric response to stratospheric variability. It is demonstrated that the mass circulation anomalies associated with variations in the stratospheric flow account for a component of the observed changes in the tropospheric isentropic slope and lower tropospheric wave generation, but do not account for the observed changes in the wave fluxes of momentum at the tropopause level.