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 made at three equatorial and subtropical sites (Adelaide, Christmas Island, and Kauai) are used to produce climatologies and to study interannual 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 semidiurnal model values agree less well with the observations. The diurnal tidal amplitudes and phases show strong seasonal variability. Maximum amplitudes are attained in March, and subsidiary maxima are observed in July/August and October, while the phase shows an annual cycle at Adelaide 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 Adelaide and Kauai, but there is a more complicated phase variation at Christmas 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 in the equatorial stratosphere, with the amplitudes being larger than the climatological average in years when the stratospheric winds are eastward and smaller than average when the QBO is in its westward phase. In contrast, the phase of the diurnal tide, as well as the semidiurnal tide, shows little systematic interannual variability.