Intraseasonal oscillations of the zonal wind near the mesopause observed with medium‐frequency and meteor radars in the tropics

Abstract

We studied the behavior of intraseasonal oscillations (ISO) of the zonal wind velocity in the equatorial mesosphere and lower thermosphere (MLT) using simultaneous observations during 500 days from 1 January 1996 with a meteor radar at Jakarta (6°S, 107°E) and two medium‐frequency radars at Pontianak (0°N, 109°E) and Christmas Island (2°N, 157°W). Cross‐correlation analysis of the ISO at 88 km indicated that the phase difference of the ISO between Pontianak and Christmas Island is, on average, small in spite of the longitudinal difference of ∼90°. Therefore the ISO in the equatorial MLT region seems to be a variation of zonal mean flow. At Jakarta the amplitude of the ISO of the zonal wind at 88 km was somewhat smaller than that observed at the other two equatorial sites. The peak of the ISO amplitude appears at ∼88 km at both Pontianak and Christmas Island, but the peak seemed to be slightly lower at Jakarta. We also investigated long‐term variation of the ISO of the zonal wind at 84 km using Jakarta meteor radar data observed from January 1993 to October 1999. Biennial variations are dominant in the ISO amplitude of the mesospheric zonal winds and the zonal amplitude variations of the diurnal tide. A relation between the ISO of the zonal wind and tidal amplitude is suggested. However, gravity wave activity in the MLT region did not correlate with the ISO in the zonal wind. We employed the outgoing longwave radiation (OLR) in the tropical troposphere as an index for excitation intensity of the atmospheric waves. The long‐term variations of the ISO components in the OLR over the western Pacific were characterized by the tropospheric biennial oscillations, which seems to correlate with the ISO of the mean zonal winds and diurnal tides in the mesosphere.