Lateral Influences on Madden-Julian Oscillation (MJO) Initiations

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The Madden Julian oscillation (MJO) is the leading mode of month-to-month variability in the tropical atmosphere. Its influences extend to extratropical weather and the initiation of tropical cyclones. Despite its importance, many aspects of the MJO are not well understood, and it is poorly simulated in most global models. The research is motivated by Zhang's two preliminary findings: first, he noted that, while in nature the MJO variability of the wind is closely tied to that in thunderstorm activity, winds and thunderstorms often behave incoherently in MJOs in models. Secondly, he found that the initiation of MJO events in a tropical model matched observations and that the timing of the events was determined by the lateral boundaries of the model rather than initial conditions in the interior tropics. These results raise several questions that are to be addressed in this project: what features in the lateral boundary conditions initiate MJO events, what are the mechanisms for this initiation, how does lateral boundary initiation interact with other mechanisms of MJO initiation, and how important, overall, is lateral boundary initiation of the MJO?A tropical mesoscale atmospheric model in a channel configuration, developed under prior NSF support, will be used. Model experiments will isolate those features in the boundary conditions that are essential for the initiation of MJO. Many MJO events will be simulated, with and without time-dependent lateral boundary conditions, in order to determine how often MJO events are initiated from the boundaries. Additional experiments will determine the sensitivity of MJO initiation to the boundary conditions and to explore interactions between heating and boundary conditions. A final set of experiments will test whether such a boundary condition is truly a necessary and sufficient condition for MJO initiation.Broader impacts of this project are in the importance of the MJO for intraseasonal prediction. A graduate student and a post-doctoral investigator will be trained.