The Lack of a QBO‐MJO Connection in Climate Models With a Nudged Stratosphere.

The observed stratospheric quasi‐biennial oscillation (QBO) and the tropospheric Madden‐Julian oscillation (MJO) are strongly connected in boreal winter, with stronger MJO activity when lower‐stratospheric winds are easterly. However, the current generation of climate models with internally generated representations of the QBO and MJO do not simulate the observed QBO‐MJO connection, for reasons that remain unclear. This study builds on prior work exploring the QBO‐MJO link in climate models whose stratospheric winds are relaxed toward reanalysis, reducing stratospheric biases in the model and imposing a realistic QBO. A series of ensemble experiments are performed using four state‐of‐the‐art climate models capable of representing the MJO over the period 1980–2015, each with similar nudging in the stratosphere. In these four models, nudging leads to a good representation of QBO wind and temperature signals, however no model simulates the observed QBO‐MJO relationship. Biases in MJO vertical structure and cloud‐radiative feedbacks are investigated, but no conclusive model bias or mechanism is identified that explains the lack of a QBO‐MJO connection. Plain Language Summary: Observations show a strong link between the stratospheric quasi‐biennial oscillation (QBO)—the alternation of tropical stratospheric zonal winds between easterly and westerly phases—and the Madden‐Julian oscillation (MJO), an eastward propagating phenomenon in the tropical troposphere in which the circulation and convection are coupled. Stronger MJO activity is observed when lower‐stratospheric winds are easterly. This coupling is intriguing for many reasons, but most practically because it suggests that the stratosphere can potentially enhance surface weather and inform subseasonal climate prediction. However, current climate models do not show this observed connection. One reason may be related to biases in how models simulate stratospheric winds, which can be corrected for in an artificial way by relaxing the model simulated winds to better match observationally constrained data sets. One recent study, however, showed that correcting for this bias using this approach in one climate model still fails to produce credible QBO‐MJO coupling. Here we expand that analysis to include four climate models and find that no model produces a robust QBO‐MJO relationship like that seen in observations. Our results show that properly representing the QBO winds and temperatures via nudging is therefore not sufficient for reproducing the observed relationship. Furthermore, while biases in how models represent cloud processes may still be a likely culprit, any definitive model bias or missing mechanism remains elusive. Key Points: The link between the quasi‐biennial oscillation (QBO) and Madden‐Julian oscillation (MJO) is explored in four climate models with nudged stratospheric winds and free‐evolving tropospheresNo model shows as strong of a QBO‐MJO connection as in observationsModel biases in cloud‐radiative feedbacks and MJO vertical velocity are diagnosed, but neither conclusively explains the lack of a QBO‐MJO connection [ABSTRACT FROM AUTHOR]