Response of the stratosphere to interannual variability of tropospheric planetary waves

A primitive-equation global model of the middle atmosphere is used to inves tigate the response of the extratropical stratosphere to different levels o f wave forcing from steady perturbations of the geopotential height near th e 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-Ja nuary conditions: strong westerly, steady flow for small wave-amplitude for cing, strong westerly but unsteady flow for moderate wave-amplitude forcing and oscillations between easterly and westerly flow for large wave-amplitu de forcing. The regimes for low and high forcing are analogous to solutions of the simpler Holton-Mass (HM) quasi-geostrophic model. The moderate-forc ing regime does not occur in the HM model and it is attributed to instabili ty of the strongly sheared flow generated by planetary waves in the upper s tratosphere. We also show how the observed patterns of interannual variabil ity in the winter stratosphere can be explained in terms of these three flo w regimes: in the northern hemisphere the flow often enters the high-forcin g regime, where variations in conditions in the early-winter how or quasi-s teady upper-tropospheric planetary-wave amplitudes make similar contributio ns 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 eno ugh changes to explain the interannual variability in the southern stratosp here.