Zonal asymmetries in SAMS stratospheric methane and nitrous oxide

Abstract

Stratospheric and Mesospheric Sounder (SAMS) methane (CH4) and nitrous oxide (N2O) constituent measurements were taken a decade before the Upper Atmosphere Research Satellite (UARS) observations and are generally of lesser quality. However, SAMS data are important because of the limited lifetime of the UARS and because they provide a unique, historical data series for these gases involved in greenhouse and ozone-depletion effects. While most prior SAMS analyses focused on zonal means, this paper assesses the strengths and weaknesses of analysing zonally asymmetric perturbations in SAMS data. It is shown that wave-1 perturbations can be successfully investigated, provided sufficient care is taken and aliasing considered. At the lowest SAMS level, 20 hPa (∼ 28 km altitude) zonal-wave-1 CH4 and N2O data reveal similar features for latitudes 45°N–65°N during stratospheric warming events and break-up of the polar vortex. Large wave-1 anomalies in the upper stratosphere (2 and 0.6 hPa) were found to be out of phase with the corresponding anomalies at 20 hPa. In one episode in early 1981 (during stratospheric sudden warming) southward winds over North America transported air with low mixing ratios from polar latitudes, while northward winds over Siberia transported air with high mixing ratios from subtropical latitudes. the effect produced strong wave-1 amplitudes in both CH4 and N2O mixing ratios. Cross-correlations between wave-1 CH4 and N2O are large and positive in middle and high latitudes (consistent with ideal tracer behaviour for both constituent gases) but weak over the tropics. the cause of the latter remains an open issue.