Longitudinal variations in planetary wave activity in the equatorial mesosphere

Zonal and meridional winds in the equatorial mesosphere and lower thermosph ere (65-98 km) measured at two sites separated by 94 degrees in longitude a re used to study the zonal structure of planetary-scale waves. The data wer e obtained with MF radars located at Pontianak (0 degrees N, 109 degrees E) and Christmas Island (2 degrees N, 157 degrees W). The data at Christmas Is land were collected from January 1990 to December 1997 and the observations at Pontianak were made from November 1995 to July 1997. Power spectral tec hniques are used to study the amplitude and frequency variations of long-pe riod oscillations as a function of height and time. A mean climatology of t hese variations taken from years 1990-1997 is presented. Strong peaks in zo nal and meridional winds are found at tidal periods and for the quasi 2-day wave. Zonal spectra exhibit considerable power at periods of 3-10 days, wi th transient oscillations with periods near 3.5 day and 6.5 days being espe cially prominent. The 6.5-day wave is particularly strong during April and September. Examination of the phase differences obtained from cross-spectra between the two stations show that the 6.5-day wave is westward propagatin g with zonal wavenumber 1, while the 3.5 day wave is eastward propagating w ith wavenumber 1. The 6.5-day wave is identified as a manifestation of an u nstable mode, while the 3.5-day wave is identified as an ultrafast Kelvin w ave. There are significant longitudinal variations in the amplitudes and in ferred momentum fluxes of the 3.5-day wave, amplitudes being larger in the Asian region than in the central Pacific.