The Unique Characteristics And Potential Mechanisms Of The Mjo-Qbo Relationship

Previous studies have shown that Madden-Julian oscillation (MJO) convective activity increases when quasi-biennial oscillation (QBO) easterlies appear in the lower stratosphere, and that this relationship is seen only during boreal winter. However, the physical mechanism behind this relationship is not fully understood. Building upon previous literature on the MJO-QBO relationship, this study first tests whether the relationship between the QBO and MJO extends to other modes of organized tropical convection. Our analysis shows that the QBO does not have any significant relationship with other modes of variability such as convectively coupled equatorial waves (CCEWs). An extended analysis for periods prior to 1979 also shows that the boreal winter QBO-MJO relationship substantially weakens prior to 1979. These results imply that, in addition to the seasonal dependence, any proposed mechanisms for the relationship between the QBO and MJO must be able to explain the uniqueness of this relationship with respect to other CCEWs, as well as the reason for its strengthening after 1979. Among the analyzed modes of tropical convection, the MJO is the most sensitive to the modulation of high cloud fraction and associated radiative feedback by the QBO and this sensitivity may weaken prior to 1979. These results indicate that the strong top-heaviness of vertical motion and cloud-radiative feedback associated with the MJO are important to explain its unique relationship with the QBO.