Multiscale waves in an MJO background and convective momentum transport feedback

Abstract

The authors use linear analysis for a simple model to study the evolution of convectively coupled waves (CCWs) in a background shear and background moisture mimicking the observed structure of the Madden- Julian oscillation (MJO). This is motivated by the observation, in an idealized setting, of intraseasonal twoway interactions between CCWs and a background wind. It is found here that profiles with a bottom-heavy moisture content are more favorable to the development of mesoscale/squall line-like waves whereas synoptic-scale CCWs are typically more sensitive to the shear strength. The MJO envelope is thus divided into three regions, in terms of the types of CCWs that are favored: an onset region in front that is favorable to Kelvin waves, a mature or active region in the middle in which squall lines are prominent, and the stratiform and decay phase region in the back that is favorable to westward inertia-gravity (WIG) waves. A plausible convective momentum transport (CMT) feedback is then provided according to the results of the idealized two-way interaction model. The active region, in particular, coincides with the westerly wind burst where both Kelvin waves and squall lines are believed to play a significant role in both the deceleration of low-/high-level easterly/westerly winds and the acceleration of low-/high-level westerly/easterly winds. The WIG waves in the wake could be a precursor for a subsequent MJO event through the acceleration of low-/high-level easterly/ westerly winds, which in turn favor Kelvin waves, and the cycle repeats. These results open interesting directions for future studies using observations and/or detailed numerical simulations using the full primitive equation. © 2012 American Meteorological Society.