What determines propagation speed of the Madden-Julian Oscillation?

The eastward propagating Madden-Julian Oscillation (MJO) events exhibit various speeds ranging from 2 to 9 m/s, but what controls the propagation speed remains elusive. By using observational and reanalysis data, this study reveals that the Kelvin wave response (KWR) induced by the MJO convection is a major factor controlling the MJO propagation speed, with stronger KWR corresponding to faster eastward propagation. A stronger KWR enhances the low-level pre-moistening process to the east of the MJO deep convection , which amplifies the shallow and congestus convection, pre-conditioning the lower-middle atmosphere and therefore accelerating the eastward development of the MJO deep convection. The moisture budget analysis also supports that the enhanced vertical moisture advection associated with the shallow and congestus convection is the major process intensifying the pre-conditioning process.

Further study reveals that the strength of the KWR is affected by the background sea surface temperature (SST). When the equatorial central Pacific SST warms, the zonal scale of the Indo-Pacific warm pool expands, which increases the zonal scale of the MJO, favoring enhancing the KWR. Using a simple theoretical model, we have demonstrated this effect of the zonal scale of the warm pool on the strength of the KWR.