Dependence of moist tropical waves on the basic state

The tropical atmosphere supports a broad spectrum of zonally-propagating wavelike disturbances in cloudiness and circulation, ranging from relatively high-frequency Kelvin and inertia-gravity waves to the much lower frequency Madden-Julian Oscillation (MJO). Climatologies of these waves show dramatic dependencies on both longitude and season, suggesting an important role of the basic state. Here, the influence of the basic state is explored through a set of aqauplanet simulations with convection parameterized using an embedded two-dimensional cloud-resolving model - the so-called "superparameterization" approach. Results show that the simulated tropical wave spectra are strongly sensitive to the background zonal flow, with fast-moving Kelvin waves being supported by upper-level westerlies, while slower-moving MJO-like disturbances are supported by upper-level easterlies together with weak zonal flow near the surface. The mechanisms responsible for these background-flow dependencies are not entirely clear, but appear to be at least partly due to changes in the nature of the interactions between the tropics and extratropics.