How the 1999 Climate Shift Has Changed MJO Propagation Diversity

Abstract

Abstract An improved understanding of how the climate mean-state variation modulates MJO property is key to comprehending projected MJO’s future change. After 1999, the Pacific shows a mega La Niña–like cooling, enhancing mean precipitation and MJO variability over the Maritime Continent and the northern Indo-Pacific warm pool. However, how the 1999 shift changed MJO propagation has not been studied. This study has detected significant changes in MJO propagation diversity across the 1999 climate shift. From epoch period 1 (P1; 1979–98) to P2 (1999–2018), the number of standing oscillation (SO) events has doubled, but the fast eastward-propagating (fast) events have decreased by 42%, the slow eastward-propagating events have increased by 150%, and the zonal extents of the MJO propagation have shifted westward by 10°–20° of longitude. The SO (fast) events are often associated with a La Niña–like (El Niño–like) mean state. Therefore, the La Niña–like mean-state change associated with the 1999 climate shift has increased the chance for more SO but fewer fast events. The La Niña–like mean-state shift has also promoted the slow-propagating events from the Indian Ocean to the western Pacific (WP) by increasing moisture content, convective instability, low-level convergence, and upward transport moisture over the equatorial WP. The results imply that the future change of the MJO propagation would critically depend on how the anthropogenic forcing changes the Indo-Pacific sea surface temperature pattern, especially an El Niño–like or La Niña–like warming pattern. The existing uncertainties in projecting the Pacific warming pattern could pose a significant ambiguity in anticipating MJO’s future change.