| description abstract | An analysis is performed on the dynamical coupling between the variability of the extratropical stratospheric and tropospheric circulations during the Northern Hemisphere winter. Obtained results provide evidence that in addition to the well-known Charney and Drazin mechanism by which vertical propagation of baroclinic Rossby waves is nonlinearly influenced by the zonal mean zonal wind, topographic forcing constitutes another important mechanism by which nonlinearity is introduced in the troposphere?stratosphere wave-driven coupled variability. On the one hand, vortex variability is forced by baroclinic Rossby wave bursts, with positive (negative) peaks of baroclinic Rossby wave energy occurring during rapid vortex decelerations (accelerations). On the other hand, barotropic Rossby waves of zonal wavenumbers s = 1 and 3 respond to the vortex state, and strong evidence is presented that such a response is mediated by changes of the topographic forcing due to zonal mean zonal wind anomalies progressing downward from the stratosphere. It is shown that wavenumbers s = 1 and 3 are the dominant Fourier components of the topography in the high-latitude belt where the zonal mean zonal wind anomalies are stronger; moreover, obtained results are in qualitative agreement with the analytical solution provided by the simple topographic wave model of Charney and Eliassen. Finally, evidence is provided that changes of barotropic long (s ≤ 3) Rossby waves associated with vortex variability reproduce a NAO-like dipole over the Atlantic Ocean but no dipole is formed over the Pacific Ocean. Moreover, results suggest that the nonlinear wave response to topographic forcing may explain the spatial changes of the NAO correlation patterns that have been found in previous studies. | |
