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Seasonal activity of the Madden-Julian Oscillation (MJO) is observed to be greater during austral summer when the lower stratospheric winds are in the easterly phase of the quasi-biennial oscillation (QBO). Using initialized predictions from two coupled model seasonal prediction systems, we show a systematic impact of the QBO on the amplitude of the MJO during the first few days of the forecast before model biases become too large. In both models as for the observed, the difference in MJO amplitude between easterly phases and westerly phases of the QBO increases with lead time, despite having similar initial amplitudes. Enhanced destabilization of the tropopause is argued to be the key mechanism that promotes stronger MJO convection during easterly phase of QBO. Caveats for the inability of the models to reproduce the observed strength of the MJO-QBO relationship are discussed.

Plain Language Summary The austral summertime Madden-Julian Oscillation (MJO) is observed to be stronger and more predictable when stratospheric winds are in the easterly phase of the quasi-biennial oscillation (QBO). The observed relationship between stratospheric QBO and MJO is examined in the multiweek forecasts from two ocean-atmosphere coupled models. A systematic impact of the QBO on the amplitude of the MJO is detected during the first few days of forecast. The predicted MJO with similar initial amplitude maintains its amplitude longer in the easterly phase of the QBO, while it weakens faster in the westerly phase of the QBO. We show that more in-phase reduction of tropopause static stability with the MJO convective anomalies during easterly phase of QBO promotes stronger MJO events, especially over the eastern side of the Maritime Continent. The predicted MJO during easterly phase of QBO is found to be stronger and propagates farther into the western Pacific.