Observations of atmospheric waves in the tropical Pacific with radars and radiosondes

Abstract

By an international research consortium between Japan, Indonesia and Australia, we have been operating a meteor wind radar (MWR) near Jakarta (6.4°S, 106.7°E) since November 1992, and a medium frequency (MF) radar in Pontianak (0.03°N, 109.3°E), west Kalimantan, from November 1995, respectively. We also conducted campaign observations with a balloon-borne radiosonde in LAPAN observatories in Watukosek (7.6°S, 112.7°E), Bandung (6.9°S, 107.6°E) and Pontianak since February 1990. In this paper, radar observations of wind fields in the mesosphere and lower thermosphere (MLT) region are used to study equatorial atmosphere dynamics. We first describe the long-term behavior of zonal wind at 70-100 km using the Jakarta MWR data during November 1992 and October 1997. The mesospheric semiannual oscillation (MSAO) is dominant in the equatorial MLT region, whose westward winds were much larger in March-April than in September-October. Moreover, the maximum amplitudes of MSAO showed a clear biennial periodicity. We also discuss characteristics of ultra-fast Kelvin (UFK) waves, having wave period of 3-4 days. A correlative study between the Jakarta MWR and MF radars at Pontianak and Christmas Island (2°N, 158°W) has indicated that the UFK wave propagates eastward at a zonal wavenumber one. We have further analyzed the long-term behavior of the UFK waves in Jakarta, and have found that the wave activity is characterized by strong enhancements twice a year, having a large value in the westward winds of MSAO.