Mean winds and the semiannual oscillation in the mesosphere and lower thermosphere at Christmas Island

In this paper we analyze mesospheric wind data collected from August 1988 to November 1989 and January 1991 to December 1991 at Christmas Island (1.95°N, 157.30°W) using the Meteor Echo Detection and Collection system. A brief overview of the data analysis techniques used on these data sets and meteor arrival statistics are presented. The mean winds are computed for 80 to 100 km and compared with the CIRA(86) model of mesospheric equatorial mean winds. Examination of the zonal mean winds and a Fourier decomposition of the zonal wind field shows a strong semiannual oscillation (SAO). To determine the vertical characteristics of the zonal SAO, a least squares fit was computed at each height. The results show a semiannual oscillation that peaks in amplitude at 84 km and decreases with increasing height. The SAO at Christmas Island is compared with data collected at Ascension Island and Kwajalein Island and with the CIRA(86) model. This comparison shows good phase agreement between the sites and some variability in the amplitudes. To fully understand the SAO in the context of the theory presented by Delisi and Dunkerton (1988), the tropospheric mean winds and wind variances are computed to determine the extent of tropospheric gravity wave activity present. Finally, the amplitude and phase of the SAO in temperature are computed indirectly from the Christmas Island wind data and compared to temperature results from SME data. Our results show reasonable agreement below 87 km and a divergence in amplitude near 90 km. One probable reason for this difference is contamination of the SME data from the diurnal tide as noted by Garcia and Clancy (1990).

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