Convectively Coupled Equatorial Waves: Analysis of Clouds and Temperature in the Wavenumber–Frequency Domain

Abstract

A wavenumber-frequency spectrum analysis is performed for all longitudes in the domain 15°S?15°N using a long (?18 years) twice-daily record of satellite-observed outgoing longwave radiation (OLR), a good proxy for deep tropical convection. The broad nature of the spectrum is red in both zonal wavenumber and frequency. By removing an estimated background spectrum, numerous statistically significant spectral peaks are isolated. Some of the peaks correspond quite well to the dispersion relations of the equatorially trapped wave modes of shallow water theory with implied equivalent depths in the range of 12?50 m. Cross-spectrum analysis with the satellite-based microwave sounding unit deep-layer temperature data shows that these spectral peaks in the OLR are ?coupled? with this dynamical field. The equivalent depths of the convectively coupled waves are shallower than those typical of equatorial waves uncoupled with convection. Such a small equivalent depth is thought to be a result of the interaction between convection and the dynamics. The convectively coupled equatorial waves identified correspond to the Kelvin, n = 1 equatorial Rossby, mixed Rossby-gravity, n = 0 eastward inertio-gravity, n = 1 westward inertio-gravity (WIG), and n = 2 WIG waves. Additionally, the Madden?Julian oscillation and tropical depression-type disturbances are present in the OLR spectra. These latter two features are unlike the convectively coupled equatorial waves due to their location away from the equatorial wave dispersion curves in the wavenumber-frequency domain. Extraction of the different convectively coupled disturbances in the time?longitude domain is performed by filtering the OLR dataset for very specific zonal wavenumbers and frequencies. The geographical distribution of the variance of these filtered data gives further evidence that some of the spectral peaks correspond to particular equatorial wave modes. The results have implications for the cumulus parameterization problem, for the excitation of equatorial waves in the lower stratosphere, and for extended-range forecasting in the Tropics.