Quasi‐biennial oscillation in vertical velocity inferred from trace gas data in the equatorial lower stratosphere

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Vertical velocity variations associated with the quasi‐biennial oscillation (QBO) in the equatorial lower stratosphere are investigated with the Halogen Occultation Experiment (HALOE) data from 1993 to 1998. The vertical velocity is inferred from ascent rates of an annual cycle in total water ([H2O] + 2[CH4]) profiles around the 20–60 hPa layer over the equator. The zonally averaged ascent rates of total water anomalies exhibit QBO‐related variations with anomalies of 0.10–0.15 mm s−1 between 20 and 40 hPa at the equator, whereas there scarcely exists an annual cycle in the ascent rates. The QBO variation of the ascent rates shows that its positive anomalies precede negative anomalies of temperature and ozone variations by about 2–3 months at the equator and 30–60 hPa. This phase relationship is in conflict with former results from several two‐dimensional model studies, which have reported that vertical velocity variations are almost out of phase with temperature anomalies preceding ozone anomalies by a few months. It is supposed that these differences could be caused by the observed large tendency of temperature anomalies, which is related to the observed rapid acceleration of the QBO westerly. An estimate of vertical advection of zonal momentum expected from the ascent rates suggests that the asymmetric acceleration of the zonal wind QBO can be explained by the QBO‐induced vertical velocity being asymmetric between the easterly and the westerly shear zones.