Convectively coupled gravity waves in the tropics: Why do most cloud systems travel westward?

Tropical deep convection is known to interact strongly with tropospheric gravity waves, giving rise to convectively coupled gravity waves that generally move parallel to the equator. While the basic mechanisms of the coupling are by now well understood, a question that remains open is why most of these waves are observed to move predominantly westward, rather than having say comparable numbers of westward- vs. eastward-moving waves. To address this issue, we have a performed a series of explicit simulations of convection on an equatorial beta-plane. A novel aspect of these runs is that the coriolis force is allowed to act on only perturbation winds about the zonal mean, so that the background zonal winds can be externally specified and maintained through nudging. Results show that the observed bias towards westward propagation can be captured qualitatively, even when the background zonal winds are nudged to zero, provided that the ITCZ is narrow enough. Importantly this westward bias disappears when the coriolis force is turned off, so the effects of planetary rotation must somehow be involved. To get a better handle on this issue, we have performed a detailed analysis of the structures of the simulated waves and have also considered the linear forced response to a moving heat source centered on the equator.