Factors regulating the propagation speed of the Madden-Julian Oscillation on the interannual time scale

Factors regulating the year-to-year variability of the propagation speed of the Madden-Julian Oscillation (MJO) during boreal winter are investigated in this study by analyzing a long record dataset. During the winters that are characterized by fast MJO propagation, the MJO exhibits a larger zonal scale, which induces more eastward extended moistening to the east of MJO convection through horizontal moisture advection, thus promoting the fast propagation of MJO convection. It is further illustrated that the larger spatial scale of the MJO during the fast propagation winters is coincident with anomalous warm seasonal mean SSTs over both the western Indian Ocean and the central/eastern Pacific, reminiscent of expansion of the Indo-Pacific warm pool under an El Niño-like condition. These characteristics in the large-scale mean state and the MJO structure associated with the fast MJO propagation speed found on the interannual time scales are in accord with several recent studies based on composite analyses of individual MJO events and modeling studies on MJO propagation characteristics under a warming climate.