LONG-TERM MF RADAR OBSERVATIONS OF SOLAR TIDES IN THE LOW-LATITUDE MESOSPHERE - INTERANNUAL VARIABILITY AND COMPARISONS WITH THE GSWM

Long-term MF radar wind measurements in the 80-100 km height range mad e at three equatorial and subtropical sites (Adelaide, Christmas Islan d, and Kauai) are used to produce climatologies and to study interannu al variability of solar tides. Twelve years of data were available for Adelaide and up to 6 years at the other sites and are analyzed in 30- day intervals. The climatological values are compared with the Global Scale Wave Model (GSWM). Good agreement between the measured and model amplitudes and phases is found for the diurnal tide, but the semidiur nal model values agree less well with the observations. The diurnal ti dal amplitudes and phases show strong seasonal variability. Maximum am plitudes are attained in March, and subsidiary maxima are observed in July/August and October, while the phase shows an annual cycle at Adel aide and Kauai, with the phase advancing by similar to 4-6 hours from summer to winter. Amplitudes of the semidiurnal tide rarely exceed 10 m s(-1). The phases undergo rapid shifts around the equinoxes at Adela ide and Kauai, but there is a more complicated phase variation at Chri stmas Island. The diurnal tide shows strong interannual variability in amplitude, especially near the March equinox. There appears to be an association with the quasi-biennial oscillation (QBO) in zonal winds i n the equatorial stratosphere, with the amplitudes being larger than t he climatological average in years when the stratospheric winds are ea stward and smaller than average when the QBO is in its westward phase. In contrast, the phase of the diurnal tide, as well as the semidiurna l tide, shows little systematic interannual variability.