Longitudinal variations in planetary wave activity in the equatorial mesosphere

Abstract

Zonal and meridional winds in the equatorial mesosphere and lower thermosphere (65-98 km) measured at two sites separated by 94°in longitude are used to study the zonal structure of planetary-scale waves. The data were obtained with MF radars located at Pontianak (0°N, 109°E) and Christmas Island (2°N, 157°W). The data at Christmas Island were collected from January 1990 to December 1997 and the observations at Pontianak were made from November 1995 to July 1997. Power spectral techniques are used to study the amplitude and frequency variations of long-period oscillations as a function of height and time. A mean climatology of these variations taken from years 1990-1997 is presented. Strong peaks in zonal 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, with transient oscillations with periods near 3.5 day and 6.5 days being especially 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 propagating with zonal wavenumber 1, while the 3.5 day wave is eastward propagating with wavenumber 1. The 6.5-day wave is identified as a manifestation of an unstable mode, while the 3.5-day wave is identified as an ultrafast Kelvin wave. There are significant longitudinal variations in the amplitudes and inferred momentum fluxes of the 3.5-day wave, amplitudes being larger in the Asian region than in the central Pacific.