Modeling Evidence of QBO‐MJO Connection: A Case Study

The boreal winter Madden‐Julian Oscillation (MJO) is modulated by the Quasi‐Biennial Oscillation (QBO). The MJO becomes relatively strong during the easterly QBO (EQBO) winters but weak during the westerly QBO (WQBO) winters. To better understand their relationship, a set of WRF model experiments is conducted with varying lateral boundary conditions. The MJO event in December 2007, during EQBO winter, is chosen as a reference case. The control experiment qualitatively reproduces the observed MJO. When the lateral boundary conditions are switched with those of WQBO or strong WQBO winters, the MJO becomes weak over the Maritime Continent. All eight ensemble members exhibit enhanced outgoing longwave radiation and reduced precipitation from EQBO to WQBO, and to strong WQBO conditions, although the magnitude of changes is smaller than observations. This result, one of the first mesoscale modeling evidences of the QBO‐MJO connection, suggests that the MJO is at least partly modulated by the QBO. Plain Language Summary The boreal winter Madden‐Julian Oscillation (MJO), which is the dominant mode of tropical intraseasonal variability, is strongly modulated by the Quasi‐Biennial Oscillation (QBO), that is the dominant mode of interannual variability in the tropical stratosphere. Despite observational evidence, such QBO‐MJO connection is not well reproduced by climate models. Here we show that the regional Weather Research and Forecasting (WRF) model qualitatively simulates the MJO event in December 2007. The WRF model also reproduces the QBO modulation of the MJO convection over the Maritime Continent although weaker than observations. With these results, the mechanisms underlying the QBO‐MJO connection can be further analyzed to improve its representation in climate models. Key Points The WRF model qualitatively simulates an MJO event in December 2007 Although weak, the model reproduces the QBO‐MJO connection over the Maritime Continent The QBO‐MJO connection is likely induced by the thermal structure change in response to the QBO