Impact of Rossby and Kelvin Wave Components on MJO Eastward Propagation

There are contrasting views concerning the impact of Rossby wave component of MJO flow on its eastward propagation. One view (“drag effect”) argues that because Rossby waves propagate westward, a stronger Rossby wave component slows down the eastward propagation. The other view (“acceleration effect”) argues that a stronger Rossby wave enhances east-west asymmetry of moist static energy (MSE) tendency and thus favors the eastward propagation. This study aims to resolve this issue through diagnosis of both idealized aqua-planet simulations and 26 MJOTF/GASS models. In the aqua-planet experiments, three sets of zonally uniform, equatorially symmetric SST distributions are specified. The MJO phase speed is faster in the presence of a narrower SST meridional profile, in which both the Rossby and the Kelvin wave components are stronger and the east-west asymmetry of MSE tendency is larger. A further analysis of the 26 general circulation models reveals that the MJO propagation skill and phase speed are positively correlated to both the Rossby wave and the Kelvin wave strength in lower free atmosphere (above 800 hPa). Models that have a stronger Rossby and Kelvin wave component tend to simulate realistic and faster eastward propagation. Therefore, both the aqua-planet and the multi-model simulations support the Rossby wave “acceleration effect” hypothesis.