Satellite observations of mean winds and tides in the lower thermosphere: 2. Wind Imaging Interferometer monthly winds for 1992 and 1993

Abstract

Satellite observations of monthly (i.e., 60‐d averages centered on the 15th of each month) zonal mean and migrating tide winds in the region of 90–120 km and 40°S–40°N from the Wind Imaging Interferometer are presented. The zonal mean zonal winds are characterized by annually varying eastward winds at midlatitudes with a maximum in summer and semiannually varying westward winds in the tropics with maxima at equinox below 105 km. The zonal mean meridional winds are characterized by a summer‐to‐winter flow below 100 km at solstice and by a cell‐like structure at equinox; the latter is the first such global observation of this tidally driven structure. Two distinct types of diurnal tide are observed: an upward propagating one and an evanescent one of comparable magnitude. The propagating tide, which is dominant in the subtropics below about 105 km, exhibits a semiannual variation with a primary maximum in March/April and a secondary maximum in September/October. The evanescent tide, which is dominant poleward of 20°N/S and above 100 km, has an annual variation with a maximum in the summer months. The coupling of the two produces a double‐peaked structure in the vertical profile of the diurnal amplitude. The semidiurnal tide is generally weaker than the diurnal tide. The maximum semidiurnal winds are at 30–40°N/S and at 105–110 km. Both zonal and meridional components are stronger in April–September than in October–March in both hemispheres, but in the equatorial region in April–September the meridional component has a maximum whereas the zonal component has a minimum.