ZONAL ASYMMETRIES IN SAMS STRATOSPHERIC METHANE AND NITROUS-OXIDE

Stratospheric and Mesospheric Sounder (SAMS) methane (CH4) and nitrous oxide (N2O) constituent measurements were taken a decade before the U pper Atmosphere Research Satellite (UARS) observations and are general ly of lesser quality. However, SAMS data are important because of the limited lifetime of the UARS and because they provide a unique, histor ical data series for these gases involved in greenhouse and ozone-depl etion 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 perturb ations can be successfully investigated, provided sufficient care is t aken and aliasing considered. At the lowest SAMS level, 20 hPa (simila r to 28 km altitude) zonal-wave-1 CH4 and N2O data reveal similar feat ures for latitudes 45 degrees N-65 degrees N during stratospheric warm ing 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 19 81 (during stratospheric sudden warming) southward winds over North Am erica transported air with low mixing ratios from polar latitudes, whi le northward winds over Siberia transported air with high mixing ratio s from subtropical latitudes. The effect produced strong wave-1 amplit udes in both CH4 and N2O mixing ratios. Cross-correlations between wav e-1 CH4 and N2O are large and positive in middle and high latitudes (c onsistent with ideal tracer behaviour for both constituent gases) but weak over the tropics. The cause of the latter remains an open issue.