Abstract
Motivated by observed structure of Madden–Julian oscillation (MJO), a general theoretical model framework is advanced for understanding fundamental aspects of MJO dynamics. The model extends the Matsuno–Gill theory by incorporating (a) moisture feedback to precipitation, (b) a trio-interaction among equatorial waves, boundary layer (BL) dynamics, and precipitation, and (c) a simplified Betts–Miller (B–M) cumulus parameterization. The general model with B–M scheme yields a frictionally coupled dynamic moisture mode, which produces an equatorial planetary-scale, unstable system moving eastward slowly with coupled Kelvin–Rossby wave structure and BL moisture convergence leading major convection. The moisture feedback in B–M scheme reinforces the coupling between precipitation heating and Rossby waves and enhances the Rossby wave component in the MJO mode, thereby slowing down eastward propagation and resulting in a more realistic horizontal structure. It is, however, the BL frictional convergence feedback that couples equatorial Kelvin and Rossby waves with convective heating and selects a preferred eastward propagation. The eastward propagation speed in the model is inversely related to the relative intensity of the equatorial “Rossby” westerly versus “Kelvin” easterly associated with the MJO. The cumulus parameterization scheme may affect propagation speed through changing MJO horizontal structure. The SST or basic-state moist static energy has a fundamental control on MJO propagation speed and intensification/decay. Model sensitivity to BL and cumulus scheme parameters and ramifications of the model results to general circulation modeling are discussed.
Highlights
Notable progress has been made in development of the general circulation models (GCMs), but by far the Madden– Julian oscillation (MJO) remains poorly simulated in many GCMs
Why does the moisture feedback in the B–M simulation substantially slow down the eastward propagation of the MJO mode? Surprisingly, we found that the MJO eastward propagation speed decreases when the Rossby wave component is enhanced
How does the B–M scheme enhance the Rossby wave component? We found that the B–M scheme produces a stronger coupling of the Rossby wave and convective heating than the Kuo scheme
Summary
Notable progress has been made in development of the general circulation models (GCMs), but by far the MJO remains poorly simulated in many GCMs. The dynamical models’ prediction skill for the MJO has been rapidly improved, yet remains limited compared to its predictability estimate (Neena et al 2014; Lee et al 2015) These indicate the necessity to improve our understanding of the fundamental physics of the MJO. A number of critical issues regarding the MJO dynamics remain, : (a) Why does the MJO possess a coupled Kelvin–Rossby wave structure and how could Kelvin and Rossby waves, which propagate in opposite directions, couple together with convection and select eastward propagation? The last section summarizes the results and discusses the model sensitivity to some critical parameters as well as the ramifications of theoretical results
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