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Gravity wave observations using all-sky CCD imager to measure the airglow OH, O2, and OI emissions were made at Cachoeira Paulista (22.7°S, 45°W) (CP), at the low-middle latitude in Brazil, from October 1998 to September 1999. Near the equator, at Tanjungsari observatory (6.9°S, 107.9°E) (TJS), Indonesia, another wide angle CCD imager measuring the OH airglow emission layer has been operated from September 2000 to September 2001. With these data sets, a reverse ray tracing method was used to study propagation of gravity waves through the middle atmosphere and to estimate the source region. The CIRA-86 reference zonal wind and temperature models and the GSWM-02 tidal wind model were used in the present analysis. Both observation sites showed apparently similar wave characteristics, except for the horizontal phase speed, which was much faster in the equatorial region. From the inverse ray tracing calculations, it was found that at CP, only about 15% of the wave events originated in the troposphere. Most of the events were located over the continent within an area of less than 400 km from the observation site. The identified source region in the troposphere showed a seasonal anisotropy, being mainly to the northwest of CP during summer and to the southeast and northwest in winter. This corresponds to the region of high tropospheric convection activity in the South American continent during summer and high meteorological front activity on the southwestern side during winter. At CP more than 70% of the waves showed that the ray tracing paths stopped in the mesosphere. At TJS the ray tracing indicates that 60% of the gravity wave sources might be located in the troposphere with an area of less than 200 km from the observation site, much closer than the case of CP. This corresponds to convective cloud activities on the north and northwest side of TJS. However, during southern hemisphere summer at both observation sites, most of the ray tracing paths stopped at the mesosphere owing to a condition of m 2 0 or critical level interactions.

[1] 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.

[1] From OH airglow imaging observation carried out over one year in Indonesia, 74 events of gravity waves in the MLT (Mesosphere and Lower Thermosphere) region were extracted. Observed period, horizontal wavelength and observed horizontal phase speeds of gravity waves were typically 5-13 min, 13-45 km and 37-75 m/s, respectively. Propagation directions were mostly southward except for the period between December and February, when eastward propagation was preferential. Spatial distributions of tropospheric clouds, estimated with GMS (Geostationary Meteorological Satellite) were consistent with the propagation direction of gravity waves, i.e., these clouds mainly existed in the opposite direction to the propagation direction of waves. This suggests that horizontal propagation characteristics of the short period gravity waves in the low latitude MLT region are mainly affected by the distribution of the wave sources in the troposphere, and the effect of the background mean wind in the middle atmosphere is smaller, as it is weaker (∼20 m/s) in the equatorial region.