Intraseasonal Variability of the Zonal-Mean Extratropical Tropopause Height

EOFs for the extratropical (32.5N-90N) zonal-mean tropopause height in the extended winter season (NDJFM) are presented. The first two EOFs are well separated, and correspond to wave-like patterns of the zonal-mean tropopause; no net variations of the extratropical tropopause are found. Both EOFs seem to follow a similar dynamical mechanism: a possible positive feedback between stratospheric polar jet intensity and planetary-wave upward propagation may lead to a north-south dipole of potential temperature in the lower stratosphere caused by the convergence of meridional eddy heat flux which would induce a meridional dipole in zonal-mean tropopause height; the higher altitude at which planetary waves break produces a positive anomaly of EP flux divergence at the tropopause level, causing an intensification of the zonal-mean zonal wind at this level. In the first EOF, the latitude of this positive anomaly coincides with the northern edge of the eddy-driven extratropical jet which, joint to a southernmost anomalous breaking of synoptic waves, seems to induce a latitudinal displacement of this jet, similar to that associated with the Zonal Index. Regression of the PC over the complete field shows that variations of tropopause height in the first EOF are higher over the Atlantic and Pacific stormtracks, while the second EOF presents a stronger signal over the pole and a more zonal structure. In the Southern Hemisphere, the first EOF shows a situation similar to that of the Northern Hemisphere, while the second EOF seems to be associated with a non-dynamical warming/cooling in the lower polar stratosphere.