Solar cycle influence in the equatorial lower stratosphere amplified by wave-mean flow interactions and extending to polar latitudes

In several papers, the solar cycle (SC) effect in the lower atmosphere has been linked observationally to the Quasi-biennial Oscillation (QBO) of the zonal circulation at low latitudes. Salby and Callaghan (J. Clim., 2000) in particular analyzed 40 years of wind measurements to show that the QBO contains a large SC signature at 20 km altitude. These observations are qualitatively reproduced with the 3D version of the Numerical Spectral model (NSM) that generates the QBO with parameterized small-scale gravity waves (GW) [Mayr et al., GRL, 2006]. The same model run generates a SC-modulated 12-month Equatorial Annual Oscillation [EAO, Mayr et al., GRL, 2005], which is apparently the pacemaker and pathway for the QBO. While the modeled oscillations of the zonal winds are confined to low latitudes, they extend in the temperature to the Polar Regions to produce measurable variations near the tropopause. We report here the results from a more recent analysis, which demonstrates that the SC modulations of the QBO and EAO are amplified by the GW interaction with the flow. The GW momentum source clearly shows a SC modulation that is in phase with the corresponding modulations of the QBO and EAO. By tapping the momentum from the upward propagating GWs, the QBO and EAO apparently serve as conduits that amplify and transfer to lower altitudes the larger SC variations in the UV absorbed in the stratosphere and mesosphere. In search for observational support, we initiated a study of the zonal mean temperature variations in the global NCEP (National Center for Environmental Prediction) data set issued by the Climate Prediction Center (CPC). The data extend up to 55 km altitude (0.4 hPa) and cover about 27 years. Our spectral analysis reveals a pronounced hemispherically symmetric 12-month annual oscillation (HSAO) that dominates at low latitudes and has properties similar to those reported for the EAO temperature variations generated in the NSM. The downward propagating HSAO also extends with enhanced amplitudes into the Polar Regions, and thus supports our suggestion that this wave driven symmetric annual oscillation, originating at low latitudes, may be involved in generating the arctic oscillation [Thompson and Wallace, GRL, 1998]. The data-based results indicate that the HSAO contains a significant 5-year modulation, and such a periodicity is generated in our model by the interaction between the 30-month QBO and the annual cycle.