Intraseasonal Variability of the Zonal-Mean Extratropical Tropopause Height

The physical processes that drive the fluctuations of the extratropical tropopause height are examined with the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data. A composite zonal-mean heat budget analysis for the Northern Hemisphere winter shows that fluctuations in the extratropical tropopause height result not only from a warming of the troposphere but also from an even stronger cooling of the lower stratosphere. While the tropospheric warming is caused by a poleward eddy heat transport associated with baroclinic eddies, the stratospheric cooling is driven primarily by planetary-scale waves. The results from analyses of synopticand planetary-scale eddy kinetic energy and Eliassen–Palm fluxes are consistent with the planetary waves first gaining their energy within the troposphere, and then propagating vertically into the stratosphere. For the Southern Hemisphere, while lower-stratospheric temperature anomalies still play an important role for the fluctuations in the tropopause height, the temperature anomalies are accounted for primarily by a poleward eddy heat transport associated with synoptic-scale eddies, and by diabatic heating. These results indicate that, although the height of the extratropical tropopause is modulated by baroclinic eddies, which is consistent with existing theories, the amount of the modulation is highly influenced by stratospheric processes.