This study focuses on the physics of a low wavenumber - low frequency variability simulated in the two-dimensional (2D) version of an aqua-planet equatorial global circulation model (GCM) with detailed parameterizations of cumulus convection, clouds and radiation. We investigate especially the role cloud-radiative and moisture-convective feedbacks play in the organization of the tropical disturbances.

In the absence of cloud-radiative interactions this model spontaneously generates eastward propagating planetary scale waves with a period of 13-14 days. Although the dominance of the spectral power in wave number 1 could be indicative of the Madden-Julian Oscillation (MJO), the primary mode of large-scale interseasonal variability in the tropics, these waves are too fast as compared to the 30-60 day period pertinent to the MJO. When clouds are allowed to affect the radian stream of energy, the eastward propagating waves of the Kelvin-like type are slowed down and a higher selectivity of wave number 1 is observed. In the westward part of the spectrum, waves advected with the imposed mean flow are detected as well. An experiment with constant cloud-radiative forcing which results in the similar mean state was, however, not able to slow down the fast Kelvin waves indicating the importance of time-varying interactions between clouds and radiation. The amount of upper-tropospheric clouds appears to affect the propagation and the selectivity of planetary scale waves as well. In an experiment with larger cloud cover the spectral peak of eastward moving planetary wave is shifted further towards lower frequency and has a period of about 30 days. The small-scale westward moving waves are slowed down too.

We further find evidence that the tropical wave dynamics is very sensitive to how convection interacts with the environmental humidity and that the wave propagation speed decreases in case of enhanced moisture-convection feedback. In fact, when cloud-radiative interactions are accounted for in conjunction with the enhanced moisture-convection feedback the time-space spectral analysis shows a very strong peak in the wave number 1 along with the lower powers for wave numbers 2-6. All of the waves are however aligned along the phase speed line of 20m/s, characteristic of moist Kelvin waves. In a similar experiment with larger cloud cover we notice that the enhanced cloud-radiative forcing leads to an increased selectivity of the dominating wave number 1, but does not lead to any significant change in the wave frequency. The contamination by the waves advected along with the imposed mean flow is again apparent.