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 intera nnual timescales is investigated using measurements of CH4 made by the Upper Atmosphere Research Satellite Halogen Occultation Experiment (H ALOE). It is shown that there is a strong correlation between the appe arance of the stratopause double peak in methane mixing ratio (which i s thought to be associated with the semiannual oscillation (SAO)) and the phase of the quasi-biennial oscillation (QBO) in the equatorial lo wer stratosphere. In a descending westerly QBO phase the double peak i s seen clearly in the HALOE methane data, but during the onset of the easterly phase the double peak is not observed. This is tile opposite of that which might be expected if the cause was a straightforward mod ulation of the strength of the SAO due to wave filtering by the QBO. T he intra-annual variability in HALOE methane is shown to be broadly co nsistent with previous observations. One interesting feature is the ap parent occurrence of descent in the summer high latitudes near the str atopause. It is shown that this feature is, in fact, due to enhanced p hotochemical destruction in this region in the presence of weak transp ort.