A 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 theoretically expected, while the vertical structures of all except the equatorial Rossby wave show a typical slanted shape in the temperature anomaly field. In the lower stratosphere, the vertically propagating signals show phase speeds and vertical wavelengths much like those previously observed.
The observations in this thesis provide a target for models of convectively-coupled waves to simulate. Indeed, wavenumber-frequency spectra of cloud-proxy fields from several general circulation models show that waves of the correct scales are often not present.
This figure is a presentation of the wavenumber-frequency power spectral peaks of the "convectively-coupled equatorial waves". The power spectra were calculated from a long (~18 years) twice-daily dataset of outgoing longwave radiation (OLR) between the latitudes of 15°S and 15°N. The power s pectra have then been divided by a red-noise background spectrum to give the contours as shown above. (a) is for the antisymmetric component (w.r.t. the equator), and (b) is for the symmetric component. Contour interval of this power ratio is 0.1, and the shading begins at a value of 1.1, for which the spectral signatures are statistically significantly above the background at the 95% level (based on 500 d.o.f.). Superimposed are the dispersion curves of the respectively even and odd meridional-numbered equatorial waves as in Matsuno (1966) fo r the three equ ivalent depths of h=12, 25, and 50m.