QBO–MJO Connections: Physical Mechanisms and Seasonality

The Quasi-Biennial Oscillation (QBO) is stratified by zonal wind direction and stratospheric height into four phase pairs (easterly mid-stratospheric winds, easterly lower-stratospheric winds, westerly mid-stratospheric winds, and westerly lower-stratospheric winds) using an empirical orthogonal function analysis of daily stratospheric (100 hPa to 10 hPa) zonal wind data from 1980-2017. In addition, Madden-Julian Oscillation (MJO) events in which the MJO convective envelope moved eastward across the Maritime Continent (MC) from 1980-2017 are identified using the Real-time MJO Multivariate (RMM) index and the Outgoing Longwave Radiation (OLR) MJO Index (OMI). Comparisons of the RMM and OMI amplitude changes over the MC (MJO RMM phases 4 and 5) during QBO phases reveal several patterns. Boreal winter MJO events have the strongest amplitudes when mid-stratospheric winds are easterly (QBOEM) and the weakest amplitudes when mid-stratospheric winds are westerly (QBOWM). In addition, boreal winter MJO events strengthen while moving over the MC during QBOEM and weaken over the MC during QBOWM. This change in strength is consistent with QBO-driven changes in lower-stratospheric static stability, as temperatures are in thermal wind balance with stratospheric zonal wind shear. However, this relationship reverses during boreal spring, when MJO events weaken significantly more during QBOEM than during any other of the three QBO phases. This boreal spring QBO-MJO relationship is also present in Boreal Summer Intraseasonal Oscillation (BSISO) events propagating northeastward across the region. Finally, no MJO-QBO relationship is found for boreal autumn.