1ST OBSERVATIONS OF INTRASEASONAL OSCILLATIONS IN THE EQUATORIAL MESOSPHERE AND LOWER THERMOSPHERE

We present the first evidence of strong intraseasonal (10-100day) variability of the equatorial mesosphere and lower thermosphere, drawn from spectral analysis of three years of radar wind measurements of the region at Christmas Island (2-degrees-N, 157-degrees-W). Oscillations in the zonal wind as large as +/- 20 m s-1 are observed with periods approximately 60 days at 85-95 km. Amplitudes attenuate near 80 km then reintensify near 75 km, where they are usually of longer period and are out of phase with the upper-level oscillations. Oscillations with periods approximately 35-40 days are also observed at 85-95 km. These oscillations are transient and usually appear when the background flow is westward. Similar signals are not observed in the meridional winds. These facts suggest that they could be Kelvin waves, similar to intraseasonal variability in the equatorial lower atmosphere associated with The Madden-Julian Oscillation. Somewhat different time-height activity at approximately 20 days could be related to the (1,3) Rossby normal mode or the 20-25 day tropospheric oscillation. As it is unlikely that any of these oscillations could propagate to these heights from the lower atmosphere, we postulate that they arise due to variations in gravity-wave activity and transmissivity induced by intraseasonal oscillations lower down, which is transferred into the mean flow when these gravity waves dissipate in the mesosphere. This may mean that this intraseasonal activity is a time-varying mean wind rather than a forced planetary wave.