The 8-hour tide in the mesosphere and lower thermosphere over the UK, 1988–2004

Horizontal winds in the mesosphere and lower thermosphere at a height of ∼90–95 km have been measured by a VHF meteor radar in Castle Eaton, UK (52.6°N, 2.2°W). The measurements span the 16-year interval from January 1988 to January 2004. The complete dataset has been used to investigate the 8-h (terdiurnal) tide which at this latitude appears as a persistent feature of the atmosphere. Instantaneous amplitudes can be as large as ∼25 m s−1. However, the monthly mean values are smaller and range from ∼1 to ∼9 m s−1. A clear seasonal behaviour is evident, in which the largest amplitudes are observed in autumn and early winter (September–November) and the smallest amplitudes are observed in early summer (May). In general, the phase difference between the zonal and meridional components is close to 2 h, indicating a circularly polarised motion. Conspicuous inter-annual variability is seen in both the amplitude and phase of the tide. On shorter timescales the amplitude of the tide sometimes exhibits a periodic modulation at frequencies associated with planetary waves (particularly the 2-day wave), indicating a degree of non-linear coupling between the tide and planetary waves. Comparisons with observations made by another meteor radar at Esrange (68°N, 21°E) in the Arctic suggest that at higher latitudes the winter time amplitude maximum is absent below heights of ∼95 km.