Observational evidence for the generation of a 4-day oscillation in the low-latitude middle atmosphere associated with an anomalous stratospheric cooling

Altitude profiles of temperature in the stratosphere and lower mesosphere over Gadanki (13.5 N, 79.2 E), based on lidar observations during the Equatorial Wave Study campaign (29 February–2 April 2000), showed an anomalous cooling of the stratosphere, lasting for about 4– 5 days. Immediately following this stratospheric cooling, a prominent ∼4-day period oscillation is observed in the zonal, as well as meridional wind in the 30–50 km altitude region, obtained from simultaneous rocketsonde and balloon wind measurements. The amplitude of this wave is in the range of 5–15 ms−1 in this altitude region. The altitude profiles of phase of this 4-day oscillation show a constant phase with height indicating that the wave is not propagating vertically. The causative mechanism for the sudden cooling is investigated by examining the day-to-day variation in the total atmospheric column ozone. Zonal and meridional wind components of this oscillation are approximately in quadrature phase, with zonal wind leading the meridional wind. Strikingly, while the 4-day wave is clearly observable in the zonal and meridional winds, it is almost absent in the temperature.