Seasonal, semiannual, and interannual variability seen in measurements of methane made by the UARS halogen occultation experiment

Variability in atmospheric methane on semiannual, seasonal, and interannual timescales is investigated using measurements of CH4 made by the Upper Atmosphere Research Satellite Halogen Occultation Experiment (HALOE). It is shown that there is a strong correlation between the appearance of the stratopause double peak in methane mixing ratio (which is thought to be associated with the semiannual oscillation (SAO)) and the phase of the quasi-biennial oscillation (QBO) in the equatorial lower stratosphere. In a descending westerly QBO phase the double peak is seen clearly in the HALOE methane data, but during the onset of the easterly phase the double peak is not observed. This is the opposite of that which might be expected if the cause was a straightforward modulation of the strength of the SAO due to wave filtering by the QBO. The intra-annual variability in HALOE methane is shown to be broadly consistent with previous observations. One interesting feature is the apparent occurrence of descent in the summer high latitudes near the stratopause. It is shown that this feature is, in fact, due to enhanced photochemical destruction in this region in the presence of weak transport.