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The Halogen Occultation Experiment (HALOE) is in orbit on NASA's Upper Atmosphere Research Satellite (UARS), and has been used to make measurements of a number of stratospheric and mesospheric constituents since October 1991. These include, amongst others, water vapour, H2O, and methane, CH4, two principal components of the total hydrogen budget of the middle atmosphere. The third main component is molecular hydrogen, H2, which is not measurable by HALOE or any other UARS sensor. By making the assumption that the total hydrogen content of the middle atmosphere is a conserved quantity, and that these three constituents dominate the budget, it is possible to infer the H2 fields in the mesosphere from the HALOE H2O and CH4 measurements.

Abstract.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.

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.

The improved stratospheric and mesospheric sounder (ISAMS) used the pressure modulation technique to observe infrared emissions from the atmosphere at 7.4/•m and 7.8/•m. The target gases, methane and nitrous oxide, both emit at these wavelengths and so their concentrations have been determined by joint retrieval from the available signals. This paper describes the measurement technique, the current retrieval scheme (version 10), and the data set produced. The version 10 data set comprises typically 2600 profiles per day on 180 days between September 26, 1991, and July 29, 1992. Retrieved profiles extend in altitude from 7 mbar to 0.08 mbar for methane and from 7 mbar to 0.8 mbar for nitrous oxide. The precision of the data is better than 20% over much of the range and estimated systematic uncertainties are less than 30%. Comparisons with coincident measurements show that the systematic uncertainties are a reasonable estimate for the methane data but reveal a much larger positive bias for the nitrous oxide data relative to other measurements. Accounting for aerosol contamination effects and a priori biases, ISAMS methane data are recommended for use in scientific studies at altitudes between 7 mbar (5 mbar in the tropics) and 0.1 mbar. ISAMS nitrous oxide data are recommended for use between 7 mbar and 1.0 mbar where relative rather than absolute values are required. Introduction Observations of methane and nitrous oxide in the middle atmosphere have proved useful for a number of reasons. Both gases are produced almost entirely at the Earth's surface and are well-mixed in the troposphere. In the stratosphere, they have relatively long photochemical lifetimes which decrease rapidly with height from over 100 years at 20 km to a few months or less at 40 km [Solomon et al., 1986; Stanford and Ziemke, 1991]. Particularly in the lower and middle stratosphere, these lifetimes are much greater than the timescales of dynamical activity, so that these gases act as long-lived tracers of the atmospheric motions. The end prod-1Atmospheric, ucts of their destruction mechanisms are also important, the reaction of N20 with O( • D) providing the primary source of odd nitrogen in the stratosphere [WMO, 1986], and CH4 oxidation dominating the formation of stratospheric water vapor [Brasseur and Solomon, 1986]. The only global satellite observations of CH4 and N20 available before the launch of the Upper Atmosphere Research Satellite (UARS) [Reber et al., 1993] were those from the stratospheric and mesospheric sounder (SAMS) [Drummond et al., 1980]. These data, as described by Jones and Pyle [1984], revealed a number of interesting features of the atmospheric circulation, including the "double peak" observed at the equinoxes in the middle stratosphere and, subsequently, shown to be related to the semi-annual oscillation [Gray and Pyle, 1986, 1987]. The SAMS data were also used to explore the chemistry of the middle atmosphere, e.g., the relationship between CH4 oxidation and water vapor [Jones, 1984], but nonetheless important questi...