Enhancement of Interdecadal Climate Variability in the Sahel by Vegetation Interaction

border regions of Russia, Mongolia, China, and Kazakhstan (Fig. 4E). These mountain waves are present at all heights, reaching the uppermost levels of the stratosphere (Fig. 4C). There is also evidence of more isolated mountain wave activity over the Alps and the AlaskaYukon region. Mountain waves are largely absent in both the observations and model results over other mountainous regions, such as western North America, the Himalayas, Greenland, and Scandinavia. This absence occurs in the MWFM results because regional winds inhibit mountain wave propagation into the stratosphere. Despite the basic similarities, specific differences between the CRISTA data and MWFM results are also evident in Figs. 3 and 4. Shortcomings in the model may be implicated here, highlighting the potential value of observations such as these in improving global mountain wave parameterizations. Figure 4C also shows large T̂ amplitudes at z 5 45 km at the northernmost latitudes, which cannot be explained in terms of polar mountain waves (Fig. 4F). This observation is consistent with large gravity-wave amplitudes observed previously in ground-based soundings of the high-latitude winter upper stratosphere (28). The feature has been attributed to enhanced exp(z/2Hr) amplitude growth of weaker “background” gravity-wave activity, due to the temperature structure of the winter polar stratosphere (13, 29), although longitudinal asymmetries in stratospheric winds can modulate the effect substantially (30). This indicates that CRISTA measured gravity waves from sources other than mountains as well.