Slow and ultraslow equatorial kelvin waves: The UARS‐CLAES view

Abstract

AbstractTemperature retrievals from the Cryogenic Limb Array Etalon Spectrophotometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS) are used to determine the behaviour of the slower Kelvin‐wave modes in the lower and middle stratosphere. the CLAES dataset permits their study during the easterly phase of the quasi‐biennial oscillation and the onset and descent of the westerly phase.Given the sampling and operational characteristics of the CLAES, which result in breaks in the time series, it was necessary to implement unequally spaced data series spectral methods, as an intermediate step in the application of the Hayashi method for bi‐dimensional spectral analysis. Comparisons with results for fast and ultrafast Kelvin waves obtained from retrievals by other sensors on board the UARS are very good, confirming the validity of this approach, which does not require interpolations of any kind to fill in the missing data before the use of the spectral methods.The wide‐band reconstruction of the time series allowed the observation of slow Kelvin waves (approximately 15‐day periods, zonal wave numbers 1 and 2) and ‘ultraslow’ Kelvin waves (approximately 25 to 30‐day periods, zonal wave number 1) in the lower stratosphere, for the period January 1992‐May 1993. Slow wave‐number‐2 events occur at the same time as ‘ultraslow’ wave events, in a way similar to that previously observed between fast and ultrafast Kelvin waves in the middle and upper stratosphere. It was possible to determine the behaviour of ‘ultraslow’ Kelvin waves in the CLAES observation period. These waves had only been previously reported in radiosonde data.Results for the slow Kelvin waves are in good agreement with previous observations, mostly obtained from radiosonde data. the effect of the shears, both vertical and meridional, upon the vertical propagation of the observed Kelvin waves is discussed.