Seasonal cycles and QBO variations in stratospheric CH4 and H2O observed in UARS HALOE data

Abstract

Measurements of stratospheric methane (CH4) and water vapor (H2O) are used to investigate seasonal and interannual variability in stratospheric transport. Data are from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) spanning 1991-97. Profile measurements are binned according to analyzed potential vorticity fields (equivalent latitude mapping), and seasonal cycles are fit using harmonic regression analysis. Methane data from the UARS Cryogenic Limb Array Etalon Spectrometer and water vapor from the Microwave Limb Sounder are also used to fill in winter polar latitudes (where HALOE measurements are unavailable), yielding complete global seasonal cycles. These data reveal well-known seasonal variations with novel detail, including 1) the presence of enhanced latitudinal gradients (mixing barriers) in the subtropics and across the polar vortices, 2) strong descent inside the polar vortices during winter and spring, and 3) vigorous seasonality in the tropical upper stratosphere, related to seasonal upwelling and the semiannual oscillation. The observed variations are in agreement with aspects of the mean meridional circulation derived from stratospheric meteorological analyses. Interannual variations are also investigated, and a majority of the variance is found to be coherent with the equatorial quasibiennial oscillation (QBO). Strong QBO influence is found in the tropical upper stratosphere: the double-peaked "rabbit ears" structure occurs primarily during QBO westerlies. The QBO also modulates the latitudinal position of the tropical "reservoir" in the middle stratosphere.