The Madden-Julian Oscillation (MJO) is a major mode of the tropical intraseasonal (20-100-day) variability. Reliable simulation and accurate forecasts of the MJO are key for the subseasonal-to-seasonal (S2S) prediction of extreme weather and climate events around the globe. Despite diverse mechanisms used to explain the MJO initiation and propagation, the potential upscale feedbacks of atmospheric synoptic-scale eddies (e.g., the mixed Rossby-Gravity wave, the Rossby wave, and tropical depression-type disturbances) to the MJO remain less investigated. Here, the authors using a novel conditional nonlinear optimal perturbation (CNOP) method reveal that the westward-propagating Rossby-like eddies can be regarded as useful precursor signals of primary MJO initiation from both inside and outside the Indian Ocean. Besides, the tropical depression (TD)-type disturbances are found to be key ingredients of the MJO eastward propagation over the western Pacific during boreal summer. The El Nino-Southern Oscillation (ENSO) can modulate the moisture vertical structure of the boreal summer MJO by affecting the eddy feedback associated with the TD-type disturbances in the middle troposphere. These results highlight the crucial importance of synoptic-intraseasonal-interannual multiscale interactions in tropical climate dynamics.

Madden-Julian Oscillation,El Nino,Atmospheric eddy,Upscale feedback,Convective initiation