Physical Mechanisms Associated with the Propagation of the Madden–Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO)

In this study, propagation characteristics of both the Madden-Julian Oscillation (MJO) and Boreal Summer Intraseasonal Oscillation (BSISO) were examined. Using the Real-time Multivariate MJO (RMM) index, 199 MJO events were identified and classified according to their success in traversing the maritime continent (MC) from 1980-2016. A similar classification system was employed for the BSISO using the Kikuchi bimodal Intraseasonal (ISO) index. MJO events that both entered and exited the MC as active featured greater specific humidity than the events that entered the MC as active and left as inactive. These differences in specific humidity were noted in the lower- and middle tropical (15S to 15N) troposphere of the Indian Ocean and MC as many as 8 days prior to the MJO event entering the MC, and primarily were associated with the MJO event itself and not necessarily antecedent conditions to the east of it. Furthermore, MJO events that entered the MC inactive but left active featured greater specific humidity than the events that entered and exited the MC as inactive, differences which began to appear nearly 15 days prior to the MJO reaching the MC. These differences in specific humidity and geopotential height were seen despite both sets of MJO events entering the MC with similar RMM amplitudes. Similar results were found for BSISO propagation. These results suggest that differences in tropospheric moistening within the MJO and BSISO active envelope may be more important to their propagation through the maritime continent than the intensity of the convective or circulation anomalies to the east of the oscillations.