Zonal-Scale of the Madden-Julian Oscillation and Its Propagation Speed on the Interannual Time-Scale

Abstract

While no significant long-term trend in the propagation speed of the Madden-Julian Oscillation (MJO) in boreal winter is found during the past decades, pronounced year-to-year variability of the MJO phase speed is illustrated by analyzing a century-long record data set. During the winters when fast MJO propagation is observed, the MJO exhibits a much larger zonal-scale than that during the winters with slow propagation. A broader extension in MJO circulation effectively induces stronger and broader lower-tropospheric moistening (drying) to the east (west) of MJO through horizontal moisture advection, prompting a faster MJO phase speed. The larger MJO zonal-scale during the fast MJO propagation winters is coincident with anomalously increased background sea surface temperatures and precipitable water over both the western Indian Ocean and central/eastern Pacific, reminiscent of an expansion of the Indo-Pacific warm pool. A fundamental question remains open regarding the key processes that determine the zonal-scale of MJO organization.