Moist tropical waves and the basic state

Convectively coupled waves in the tropics are found to exhibit dramatic dependencies on the basic state, as evidenced by their large geographic and seasonal variability. A relevant example is the MJO, whose convective signals are confined almost exclusively over the Indo-Pacific warm pool region, where the basic state zonal flow is typically characterized by strong upper-level easterlies over nominal low-level westerlies. In contrast, convectively coupled Kelvin waves are seen to occur throughout the tropics, but are generally strongest over the Western Hemisphere, where the basic state flow is typified by upper-level westerlies over low-level easterlies. At the previous AOFD meeting, we showed how these observed relationships between the basic state and the dominant types of large-scale, eastward-moving tropical waves (i.e., MJO vs Kelvin wave) could be captured using a full-physics numerical model with a superparameterization for unresolved convective-scale processes. Here we extend these results by showing how similar sorts of behavior are obtained even in the context of dry numerical models run under idealized (Held-Suarez-type) forcing. A mechanistic framework for interpreting these findings is currently under development.