Stationary Planetary Waves Inferred from WINDII Wind Data Taken within Altitudes 90–120 km during 1991–96

Abstract

A climatology of stationary planetary waves (SPWs) in horizontal winds at latitudes 70°S?70°N and altitudes 90?120 km is obtained from Wind-Imaging Interferometer (WINDII) green line measurements in December?January and March?April of 1991?96. The observed solstitial SPW fields are relatively stronger and dominated by zonal wavenumber-1 variations. In contrast, the equinoctial SPW fields are weaker and characterized by zonal wavenumber-2 variations. The zonal amplitude maxima of 10?25 m s?1 are generally centered at the midlatitudes of 35°?40° in both hemispheres around 96 km, with the eastward perturbation velocity maxima around 90°E for wavenumber 1 and 60° and 240°E for wavenumber 2. The meridional amplitude maxima are about 5?15 m s?1 and show more variabilities in their latitude?height distributions. The meridional phases indicated that Eliassen?Palm (EP) fluxes were downward?poleward for the winter maxima, vertically varying poleward for the summer maxima, and more variable during March?April. The hemispheric?seasonal?interannual variations in amplitude and phase are of 10 m s?1 and 30°, respectively. In particular, a distinguishable local summer maximum with an amplitude of 10?20 m s?1 is found to exist in the wavenumber-1 variation of zonal wind component. The hemispheric asymmetry is also characterized by the nodal phase (or phase jump) lines shifted toward the winter hemisphere by 10°?30°. Wave penetrations across the equator are observed with amplitudes of 5 m s?1 at 97?100 km. While the summer maximum of the wavenumber-1 component persisted during the four years, large variability is found in the winter hemisphere where the wavenumber-2 component became significant at the 90?105-km region during December 1992?January 1993 and December 1993?January 1994 and at the 105?120-km region during December 1991?January 1992. The excitation due to in situ forcing of azonal gravity wave drag, which varies longitudinally, is thought to be largely responsible for the observed SPW, particularly for the summer maximum, while the leakage of upward propagating SPW from the lower to the higher atmosphere also plays a role, especially in the winter and the equinoctial periods.