Abstract
Abstract Mechanisms for extratropical influences on the initiation of the Madden–Julian oscillation (MJO) are investigated using numerical simulations and a global reanalysis product. Previous simulations by a tropical channel model captured the timing and gross features of the initiations of two MJO events and suggested that the initiations were due to influences from the extratropics. In this study, latitudinal transport of momentum from the extratropics is found to be crucial in generating the lower tropospheric westerlies in the tropics associated with the MJO initiation. The diagnoses of the zonal momentum budget for the MJO initiation region revealed that the advection by meridional winds could be important prior to the initiation of the MJO. The time evolution of the wave activity identifies its source over the southern Indian Ocean where it grows by extracting kinetic energy from the mean flow. The time scale of the lateral boundary conditions that is responsible for the MJO initiation is also investigated. The implications of the results and limitations of the approach are discussed.
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