Observed propagation and structure of the 33-h atmospheric Kelvin wave

The structure of the 33-hour Kelvin wave, a normal mode of the atmosphere, is examined in 6-hourly station and NCEP--NCAR reanalysis data. Cross-spectral analysis of 6 years (1993-98) of tropical station pressure data shows a peak in coherence in a narrow frequency band centered near 0.74 cycles per day, corresponding to a period of approximately 33 hours. The phase angles are consistent with an eastward-propagating zonal-wavenumber-one structure, implying an equatorial phase speed of approximately 340 m s-1. The global structure of the mode is revealed by empirical orthogonal function and regression analysis of 31 years (1968--98) of reanalysis data. The horizontal structure shows a zonal-wavenumber-one equatorial Kelvin wave with an equatorial trapping scale of approximately 34 degrees latitude. The vertical structure has zero phase change. The amplitude of the wave is approximately constant in the troposphere with an equatorial geopotential height perturbation of 0.9 m, and then increases exponentially with height in the stratosphere. Cross-spectral analysis between the station and reanalysis data shows that the results from the two data sets are consistent. No evidence can be found for forcing of the wave by deep tropical convection, which is is examined using a twice-daily outgoing longwave radiation data set.