Modeling quasi‐biennial variability in the semiannual double peak

Abstract

The interannual variability of the stratopause semiannual double peak as seen in methane measurements made by the Halogen Occultation Experiment (HALOE) is investigated using a two‐dimensional isentropic model. It is found that the observed variability is reproduced when the quasi‐biennial oscillation (QBO) of the tropical lower stratosphere is introduced into the model and that there are two distinct means by which the QBO affects stratopause tracer values. First, the QBO modulates the westerly phase of the stratopause semiannual oscillation (SAO) by the selective damping of the parameterized waves responsible for producing this phase of the SAO in the model. This results in a modulation of the strength and duration of the SAO circulation, with it being stronger 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 or eliminate the trough of the double peak. For a westerly QBO phase the downwelling, although weaker, is also more persistent, giving a deeper trough. Second, the tropical QBO influences the propagation of planetary waves in the winter hemisphere, thereby producing an extratropical QBO in the winter stratosphere and mesosphere. This alters the circulation in this region, giving more upwelling in the winter extratropics above 0.5 hPa when the QBO (defined at 15 hPa) is westerly. This causes the winter lobe of the double peak to form only when the QBO is westerly. The combined result of the two processes is to produce a double peak when the QBO winds are westerly but a single peak in the summer hemisphere when the QBO winds are easterly. This modeled result agrees well with the interannual variability displayed by the HALOE methane observations.