Kelvin waves in the equatorial lower stratosphere as revealed by cryogenic limb array etalon spectrometer temperature data

Space-time variations of atmospheric temperature in the equatorial lower stratosphere are investigated using temperature data derived from the cryogenic limb array etalon spectrometer (CLAES) instrument onboard the Upper Atmosphere Research Satellite (UARS) for January 1992 to May 1993. During this period, clear evidence is observed for eastward progression of zonal wavenumber 1 temperature anomalies. This variation has a period of about 2 weeks and a maximum amplitude at the equator. The vertical scale of this variation is about 10 km and has an eastward phase tilt with increasing height. Consequently, the phase structure propagates downward with time. From these observed features it is concluded that the temperature variation seen in the equatorial lower stratosphere is due to so-called ''slow'' Kelvin waves. This observational study using satellite measurements clearly shows much more detail on the global structure of ''slow'' Kelvin waves than previous studies. Space-time spectral analysis of temperature variations for the entire CLAES observation period were also performed. During the period January 1992 to May 1993 the background zonal winds (derived from radiosonde observations at Singapore) change from easterlies to westerlies in agreement with the quasi-biennial oscillation (QBO) in the equatorial lower stratosphere. The slow Kelvin wave activity (measured by the power spectral density averaged over a period of about 2 weeks) is vigorous during the easterly regime but rapidly decreases in the westerly regime of the QBO. This clearly indicates the different transmission characteristics of Kelvin waves in the two QBO wind regimes.