The Semi-Diurnal Tide at the Equinoxes: MF Radar Observations for 1978–1982 at Saskatoon (52°N, 107°W)

Abstract

Observations of the semi-diurnal atmospheric tide have been made on a continuous daily basis since September 1978 by a medium frequency (MF) radar. The monthly mean zonal winds and tidal amplitudes and phases (70?110 km), are shown here for each month of the autumn and spring equinoxes (1981?82), and compared with previous years. A very regular and dramatic tidal pattern emerges, with March and November being winter-like, and the remaining four months being more summer-like. Wavelengths for the latter are 60?95 km, intermediate between winter (?45 km) and summer (?180 km) values. The less systematic and extended observations from other North American and European radar systems are consistent with these results when they are treated on a monthly rather than a seasonal basis. The equinoctial spectral model of Walterscheid and DeVore (1981), which uses improved heating rates and realistic mean winds and temperatures, is compared with the new observations. The model is quite successful in producing wavelengths of 70?90 km, comparable amplitudes to the observational and phases of eastward maxima within 1?3 hours of the observations. However, the model does not explain the differences between spring and autumn equinoxes, nor does it address the large and regular monthly changes within each equinox.