LONG-PERIOD MOTIONS IN THE EQUATORIAL MESOPHERE

The seasonal variations in winds measured in the equatorial mesosphere and lower thermosphere are discussed, and oscillations in zonal winds in the 3-10 day period range are examined. The observations were made between January 1990 and June 1991 with a spaced-antenna MF radar located on Christmas Island (2-degrees-N, 157-degrees-W). The seasonal variations are analyzed in terms of the mean, annual, and semiannual (SAO) harmonic components. The SAO is the dominant component in the zonal winds, with the amplitude and phase characteristics being in good agreement with earlier rocketsonde measurements at Kwajalien (9-degrees-N) and Ascension Island (8-degrees-S). The annual and semiannual oscillations combine to produce a stronger change in zonal wind strength in the first half-year (January-June) than in the second half-year (July-December). An annual cycle dominates the meridional winds with maximum velocities (5-10 m s-1) attained at about 90 km. The meridional circulation at the solstices is consistent with a flow from the summer to the winter pole. Power spectral analyses indicate that motions in the 3-10 day period range occur mainly in the zonal winds, behavior which is interpreted as being due to eastward propagating Kelvin waves. Despite the intermittent nature there is an overall semiannual variation in Kelvin-wave activity. Maximum amplitudes are achieved at the mesopause in January/February and August/September which are times when the zonal winds are westward.