Modulation of MJO propagation speed by the fluctuation of large-scale background zonal circulation

Eastward progression of the convectively active region is one of the distinguishing characteristics of the Madden-Julian Oscillation (MJO). However, understanding the mechanisms of their eastward propagation and what determines their propagation speeds is still limited. Taking this into consideration, this study investigated how the boreal winter MJO propagation speed is modulated by the background environment and sought for an intrinsic relationship of the MJO with the background atmospheric states.

MJO events in boreal winter (November to April) between 1982-2016 were identified by application of the real-time-multivariate MJO index (RMM; Wheeler and Hendon 2004) following the detection method of Suematsu and Miura (2018). To evaluate MJO propagation speed a method to track MJO convection was constructed. The propagation speed for each of the detected events was calculated with this tracking method and was checked for consistency with the angular phase speed on the RMM phase space.

With the evaluated MJO propagation speed, we classified the detected events into fast and slow MJO events. Comparison of the two groups indicated that the intensity of convective activities was stronger and spanned over a larger region for the slower events. Then, building onto our recent finding of MJO enhancement with background zonal SST gradient, we examined how MJO propagation speed was influenced by the background SST. The analysis revealed that there is a tendency of the MJO to propagate slower under low-frequency SST distribution with a larger zonal gradient that peaks over the western Pacific. In contrast, there was little dependence of MJO propagation speed on the high-frequency SST distribution. How such background SST distribution influence the MJO was further investigated by analyzing the relationship between MJO propagation speed and strength of the large-scale zonal circulation. Correlation analysis between MJO propagation speed and strength of the two zonal circulation cells of the Walker-Circulation indicated that MJO tends to propagate slower when the strength of the two cells are stronger. In other words, MJO tends to propagate slower when there are stronger background low-level westerlies over the region where MJO convection is active.

This study counterintuitively indicated that eastward propagation of the MJO is slower with stronger background westerlies. Our results demonstrated that MJO is not Doppler-shifted by the background circulation and therefore it is unlikely that advective processes are predominant for the MJO propagation. Rather, the new finding points to a view that MJO is an integral part of the large-scale zonal circulation induced by the zonal SST gradient over the tropics, and that slower and stronger MJO events manifest with the intensification of the large-scale zonal circulation.