Observations of Middle Atmosphere Dynamics over Antarctica

Abstract

This thesis is concerned with the dynamics of the middle atmosphere over the southern-most continent on our planet, Antarctica. Building on previous observational and theoretical efforts to understand the dynamics of this region of the atmosphere, the work encompasses instrument improvements as well as data analysis studies of gravity waves in the lower and middle stratosphere, tides in the upper mesosphere and lower thermosphere, and short-period planetary waves in the stratosphere and mesosphere. An upgrade of the Medium-Frequency Spaced Antenna (MFSA) radar at Scott Base, Antarctica, was carried out in 2004/5 in order to ensure continued operation and enhance its capabilities to measure gravity waves. As a result, the quality of the wind measurements was greatly enhanced and the amount of data collected is now greater by a factor of approximately 15 compared with before the upgrade. Analysis of over two decades of wind velocity data from Scott Base yields a reliable climatology of the dynamics of the upper mesosphere and lower thermosphere in this area, namely the tidal oscillations and the background winds. In addition, interannual variability is assessed and periodicities of approximately 11 years and strong positive trends in tidal amplitudes are reported. Mechanisms that could explain the observed behaviour are proposed. The data are then combined with wind measurements from Halley, the longitudinal conjugate site, in order to study the zonal character of the semi-diurnal tide. Zonal wavenumber 1 and 2 waves are both found and mechanisms that could explain the generation of a wavenumber 1 component are suggested. Two further sets of MFSA radar wind measurements are used to investigate the behaviour of planetary waves with periods of between two and four days in the Antarctic middle atmosphere. Satellite temperature measurements further help to create a more complete picture of these waves. Baroclinic and barotropic instabilities, which result from shears of the zonal wind, appear to be responsible for much of the observed wave activity. In addition, a quasi-to day wave event in mid-May 2005 with unusually large amplitudes is examined and suggested to be linked to a solar proton event. Gravity wave activity over Antarctica is studied using temperature profiles obtained through the satellite radio occultation technique. Although the measurements are restricted to below 35 km altitude, high-resolution temperature profiles allow conclusions to be drawn about the seasonal, geographical, and height distribution of gravity wave activity. Mountain waves are found to be important over the Antarctic Peninsula and the Transantarctic mountains where they contribute more than 20% of the observed wave activity in the lower stratosphere. In addition, the analysis indicates the importance of critical-level filtering and Doppler-shifting.