MODELING QUASI-BIENNIAL VARIABILITY IN THE SEMIANNUAL DOUBLE PEAK

The interannual variability of the stratopause semiannual double peak as seen in methane measurements made by the Halogen Occultation Experi ment (HALOE) is investigated using a two-dimensional isentropic model. It is found that the observed variability is reproduced when the quas i-biennial oscillation (QBO) of the tropical lower stratosphere is int roduced into the model a,nd that there are two distinct means by which the QBO affects stratopause tracer values. First, the QBO modulates t he westerly phase of the stratopause semiannual oscillation (SAO) by t he selective damping of the parameterized waves responsible for produc ing this phase of the SAO in the model. This results in a modulation o f the strength and duration of the SAO circulation, with it being stro nger when the QBO is easterly than when it is westerly. However, when the QBO is easterly, the effect of stronger downwelling in the tropics is negated by subsequent upwelling; the overall effect is to reduce o r eliminate the trough of the double peak. For a westerly QBO phase th e downwelling, although weaker, is also more persistent, giving a deep er trough. Second, the tropical QBO influences the propagation of plan etary waves in the winter hemisphere, thereby producing an extratropic al QBO in the winter stratosphere and mesosphere. This alters the circ ulation in this region, giving more upwelling in the winter extratropi cs above 0.5 hPa when the QBO (defined at 15 hPa) is westerly. This ca uses the winter Ic,be of the double peak to form only when the QBO is westerly. The combined result of the two processes is to produce a dou ble peak when the QBO winds are westerly but a single peak in the summ er hemisphere when the QBO winds are easterly. This modeled result agr ees well with the interannual variability displayed by the HALOE metha ne observations.