Influence of the Basic State Zonal Flow on Convectively Coupled Equatorial Waves

Observational data are used to test the hypothesis that the basic state modulates the dispersion properties of convectively coupled equatorial waves (CCEWs). This hypothesis is based on shallow water theory, which predicts that the zonal speed of propagation of equatorial modes is altered by the equivalent depth and the basic zonal flow. Typical diagnostics of space-time power spectra of cloudiness data reflect the mean behavior of CCEWs in space and time. Here, localized space-time spectra are calculated to investigate how the global spectral peaks vary across the tropics, and how they are affected by the substantial variations in zonal flow observed geographically and by season. The strength of some convectively coupled mode signals are seen to vary widely across the globe, while others show much less dependence on location. For example, Kelvin waves are observed in all sectors, while mixed Rossby-gravity waves only exist with appreciable amplitude over the western and central Pacific. Doppler shifting of the phase speed of CCEWs by the barotropic component of the wind is readily detectable due to both the mean flow and temporally varying extremes in this flow. However, once the Doppler effect is taken into account, the equivalent depths of CCEWs inferred from global power-spectra are surprisingly uniform, both geographically and temporally. There does not seem to be a unique steering level for CCEWs. For instance, the phase speed of Kelvin waves appear to be more influenced by the upper tropopspheric zonal flow, while mixed Rossby-gravity waves respond more to lower tropospheric flow. There are also detectable phase speed and equivalent depth shifts that are consistent with changes in the zonal flow vertical shear. This is particularly evident for equatorial Rossby modes.