A large part of the variability in tropical cloudiness appears as propagating disturbances moving parallel to the equator. These organized systems of clouds constitute the predominant 'weather' of the tropics, and the latent heat release within them provides an important forcing for atmospheric circulations locally and remotely.
A comprehensive wavenumber-frequency spectral analysis of subseasonal tropical convective cloud variability is performed. The broad nature of the spectrum of the satellite-observed outgoing longwave radiation (OLR), a proxy for convectively generated cloudiness, is red in both zonal wavenumber and frequency, but there are a number of distinct spectral peaks. Many of these spectral peaks correspond to the dispersion relations of the equatorially-trapped wave modes of shallow-water theory, with implied equivalent depths in the range of 12 to 50 m. We call the waves contributing to these peaks the "convectively-coupled equatorial waves". Some of these waves are identified with those studied previously in observations, while others are apparently new.
Through cross-spectrum analysis with satellite-based microwave sounding unit (MSU) temperatures, the implied equivalent depths of the convectively-coupled equatorial waves are shown to be shallower than those typical of tropospheric equatorial waves that are uncoupled with convection. Such a small equivalent depth is thought to be a result of the interaction between convection and dynamics, for which there are at least two different current theories.
The statistically significant large-scale dynamical fields associated with the convectively-coupled equatorial waves are extracted from a global reanalysis dataset using a lagged regression technique. The horizontal structures of the dynamical fields are consistent with those that are theoretically expected, while the vertical structures of all except the equatorial Rossby wave show a typical "boomerang" shape in the temperature anomaly field. In the lower stratosphere, the vertically propagating signals show phase speeds and vertical wavelengths much like those often previously observed.
The observations in this paper provide a target for models of convectively-coupled waves to simulate. Indeed, wavenumber-frequency spectra of cloud-proxy fields from a sample of modern general circulation models show that waves of the correct scales are often not present.