Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 5 2007 10.5194/acpd-7-14747-2007 http://www.atmos-chem-phys-discuss.net/7/14747/2007/ http://www.atmos-chem-phys-discuss.net/7/14747/2007/acpd-7-14747-2007.html http://www.atmos-chem-phys-discuss.net/7/14747/2007/acpd-7-14747-2007.pdf 14747 14765 2007-10-16 The wintertime two-day wave in the Polar Stratosphere, Mesosphere and lower Thermosphere D. J. Sandford d.j.sandford@bath.ac.uk M. J. Schwartz N. J. Mitchell Centre for Space, Atmospheric {&} Oceanic Science, Department of Electronic and Electrical Engineering, University of Bath, Bath, BA2 7AY, UK Microwave Atmospheric Science Group, Jet Propulsion Laboratory, 4800 Oak Grove Dr, Pasadena, CA 91109-8099, USA Recent observations of the polar mesosphere have revealed that waves with periods near two days reach significant amplitudes in both summer and winter. This is in striking contrast to mid-latitude observations where two-day waves maximise in summer only. Here, we use data from a meteor radar at Esrange (68° N, 21° E) in the Arctic and data from the MLS instrument aboard the EOS Aura satellite to investigate the wintertime polar two-day wave in the stratosphere, mesosphere and lower thermosphere. The radar data reveal that mesospheric two-day wave activity measured by horizontal-wind variance has a semi-annual cycle with maxima in winter and summer and equinoctial minima. The MLS data reveal that the summertime wave in the mesosphere is dominated by a westward-travelling zonal wavenumber three wave with significant westward wavenumber four present. It reaches largest amplitudes at mid-latitudes in the southern hemisphere. In the winter polar mesosphere, however, the wave appears to be an eastward-travelling zonal wavenumber two, which is not seen during the summer. 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