Simulations of the quasi‐biennial oscillation and its effect on stratospheric H₂O, CH₄, and age of air with an interactive two‐dimensional model

A zonally averaged photochemical–dynamical model of the middle atmosphere is used to simulate the quasi‐biennial oscillation (QBO) and its effect on the distributions of stratospheric H 2 O, CH 4 , and age of air. Changes in planetary wave amplitudes of ±25% cause 2–3 month changes in QBO period. Comparable changes in prescribed tropical heating have a smaller effect on the QBO period. The response of tropical upwelling, and QBO period, to changes in extratropical forcing depends on the magnitude and location of the imposed changes. In the Southern Hemisphere, where the planetary wave forcing is smaller than in the Northern Hemisphere, increased forcing produces stronger equatorial upwelling and a longer QBO period. In the Northern Hemisphere, increased forcing produces weaker upwelling and a shorter QBO period due to the larger amplitude waves becoming saturated. Overall, the effect of the QBO is to produce a slightly younger mean age of air near the tropical stratopause due to the model not exactly reproducing the strength and duration of the observed westerly wind shear. The QBO in lower stratospheric H 2 O, but not CH 4 , results primarily from meridional circulation anomalies superimposed upon background gradients in H 2 O mixing ratio that are maintained by the annual cycle in lower stratospheric H 2 O. QBO modulation of horizontal eddy transport plays a much smaller role. A realistic annual cycle in equatorial H 2 O mixing ratio at the model tropopause produces a QBO variation in lower stratospheric H 2 O of 0.1–0.2 ppmv. The effect of the QBO on model tropical tropopause temperatures doubles the amplitude of the water vapor QBO at 20 km.