Characteristics of semidiurnal tides observed by the Kyoto meteor radar and Saskatoon medium‐frequency radar

Abstract

We have compared seasonal variations of semidiurnal tides in the upper middle atmosphere observed by the Kyoto meteor radar and the Saskatoon medium‐frequency (partial reflection) radar in 1983–1985. Amplitudes of semidiurnal tides are generally larger at Saskatoon than at Kyoto. Seasonal variations of phases at 90 km agree fairly well for both zonal and meridional components between Kyoto and Saskatoon. The seasonal variation is mainly composed of two fundamental states appearing in summer‐centered (April/May–September/October) and winter‐centered months (November–February/March), which are characterized by long and relatively short vertical wavelengths, respectively. Clear transitions between the two states occur in equinoxes (February–April and October–November). Using determinations at Kyoto and Saskatoon with 4‐ to 5‐day observation intervals, we have studied the behavior of semidiurnal tides during the transitions in October–November in 1983–1985 and February–April in 1985. Precise comparisons have shown that the transitions are slightly delayed at Saskatoon in both vernal and autumnal equinoxes. Abrupt changes in phases of semidiurnal tides coincide with reduction and pulsating fluctuations of their amplitudes, suggesting the existence of interference between fundamental and higher modes which are dominant in summerlike and winterlike months, respectively.