Physical Mechanisms Associated with 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, MJO events were identified and classified according to their success in propagating through the Indian Ocean, Maritime Continent (MC), and western Pacific Ocean from 1980-2016. A similar classification system was employed for the BSISO over the MC using the Kikuchi bimodal Intraseasonal (ISO) index, also from 1980-2016. MJO events that both entered and exited all three regions as active featured greater lower- and middle-tropospheric specific humidity than MJO events that entered active and exited inactive. Those moisture anomalies are likely indicators of sustained deep convection, which would generate a Kelvin wave in the upper troposphere that supports rising motion to the east of the MJO active convective envelope and favors its eastward propagation. Furthermore, positive anomalies of lower- and middle tropospheric specific humidity were seen to the east of the MJO as many as 8 days prior to it entering the MC, but a similar lower tropospheric moisture anomaly was not seen over the IO or WP. That difference suggests that moistening to the east of the MJO convective envelope prior to its reaching the MC is important to successful MJO propagation across the MC. Finally, BSISO propagation was analyzed using similar methods to determine the extent to which differences in tropospheric moistening within the BSISO active envelope are important to its propagation.