Response of the stratosphere to interannual variability of tropospheric planetary waves

A primitive-equation global model of the middle atmosphere is used to investigate the response of the extratropical stratosphere to different levels of wave forcing from steady perturbations of the geopotential height near the tropopause. The response of the stratosphere is compared to that in quasi-geostrophic beta-plane models used in previous studies. The primitive-equation model exhibits three flow regimes under perpetual-January conditions: strong westerly, steady flow for small wave-amplitude forcing, strong westerly but unsteady flow for moderate wave-amplitude forcing and oscillations between easterly and westerly flow for large wave-amplitude forcing. The regimes for low and high forcing are analogous to solutions of the simpler Holton-Mass (HM) quasi-geostrophic model. The moderate-forcing regime does not occur in the HM model and it is attributed to instability of the strongly sheared flow generated by planetary waves in the upper stratosphere. We also show how the observed patterns of interannual variability in the winter stratosphere can be explained in terms of these three flow regimes: in the northern hemisphere the flow often enters the high-forcing regime, where variations in conditions in the early-winter how or quasi-steady upper-tropospheric planetary-wave amplitudes make similar contributions to the interannual variability in the stratospheric circulation. For the southern hemisphere, we suggest that the flow alternates between the low- and moderate-forcing regimes through year-to-year changes in the amplitude of quasisteady waves near the tropopause. This mechanism produces large enough changes to explain the interannual variability in the southern stratosphere.