Captured QBO‐MJO Connection in a Subseasonal Prediction System
Using the CESM2 subseasonal prediction system, we are able to capture the QBO-MJO connection in two MJO hindcasts. We find that the observed QBO-MJO connection where the MJO is stronger in the QBOE phase is only captured when the QBO temperature anomalies are imposed on the model stratosphere, but the stratospheric zonal wind is left free. This indicates a critical role of the wave-mean flow interaction in the QBO-MJO connection. The tropopause instability theory alone is not enough to explain the captured connection.
The Madden-Julian Oscillation (MJO) provides the dominant predictability on subseasonal timescales by exciting teleconnection patterns at extra-tropics. The mean MJO activity in the boreal winter season is strongly regulated by the stratospheric Quasi-Biennial Oscillation (QBO) in the observations where the MJO is stronger in the QBOE phase than QBOW phase. However, such QBO-MJO connection is missing in all climate models and therefore hinders the harvest of the extra sub-seasonal predictability provided by it. We demonstrate that the observed QBO-MJO connection can be captured by the CESM2 subseasonal prediction system in the hindcasts of two selected MJO cases. The tropopause instability theory, as the most accepted mechanism for the QBO-MJO connection, is proved insufficient to explain the captured connection. The wave-mean flow interactions are also important for the QBO-MJO connection.
We explore the observed QBO-MJO connection by using the CESM2 subseasonal prediction system to carry out two MJO case hindcasts. We found that the long-missing QBO-MJO connection in the climate models can be captured in these MJO case hindcasts when the QBO temperature anomalies are imposed to the model stratosphere. The free-evolving stratospheric zonal wind is also key to capturing the connection. The most accepted QBO-MJO connection mechanism, the tropopause instability theory, is demonstrated insufficient to explain the captured connection in the case hindcasts. The failure of capturing the QBO-MJO connection in the hindcasts with the prescribed stratospheric zonal wind indicates an important role of the wave-mean flow interaction in the QBO-MJO connection.