Dry Signals of the MJO

This study presents signals in zonal wind, temperature, geopotential height and pressure associated with MJO and non-MJO large scale convective events using more than 10 years of sounding observations at Manus Island and global (re)analysis data. While rainfall anomalies in both types of events is comparable during their peak at Manus, MJO signals present much stronger anomalies in zonal wind and geopotential height with opposite signs in the low and mid-upper troposphere and positive temperature anomalies extending through most of the troposphere compared to those of non-MJO events. The MJO signals in these fields also demonstrate a global evolution. Anomalies in upper-tropospheric geopotential height and temperature start over the equatorial Indian Ocean before convective initiation of the MJO, amplify and propagate eastward into the western Pacific in tandem with MJO convection, and continue their propagation over the central and eastern Pacific at slow speeds (~ 3 m s-1) after MJO convection ceases to exist. These results lead to a hypothesis that there is an intrinsic structure of the MJO that is governed by dry dynamics. Different processes such as moist convection, lateral energy and momentum forcing from the higher latitude, circumnavigating perturbations, etc., may act to energize this structure.