A Multi-Scale Interaction Model for the Madden–Julian Oscillation Bin Wang and Fei Liu International Pacific Research Center and Department of Meteorology, University of Hawaii at Manoa, Honolulu ABSTRACT The nature and role of multi-scale interaction (MSI) in the Madden-Julian oscillation (MJO) is one of the elusive aspects of the MJO dynamics. Here a theoretical model is formulated to advance our understanding of the MSI in MJO dynamics. The model integrates three essential physical elements: a) large scale equatorial wave dynamics driven by boundary layer frictional convergence instability (FCI), b) effects of the upscale eddy momentum transfer (EMT) by vertically tilted synoptic systems resulted from boundary layer convergence and multi-cloud heating, and c) interaction between planetary scale wave motion and synoptic scale systems (the eastward propagating supper cloud clusters and westward propagating 2-day waves). We show that the EMT mechanism tends to yield a stationary mode with a quadrupole-vortex structure (enhanced Rossby wave component); whereas the FCI yields a relatively fast eastward-moving and rearward tilted Gill-like-pattern (enhanced Kelvin wave response). The MSI instability stems from corporative FCI or EMT mechanisms and its property is a mixture of FCI and EMT modes depending on the ratio of the deep convection versus stratiform/congestus cloud amount. With increasing stratiform/congestus heating, the EMT becomes more effective. A growing MSI mode has a horizontal quadrupole and rearward tilted structure and prefers slow eastward propagation, which resemble the observed MJO. The FCI sets the eastward propagation, while the EMT slows down the propagation speed. The theoretical results presented here point to the need to observe multi-cloud structure and vertical heating profiles within the MJO convective complex and to improve general circulation models' capability in reproducing correct partitioning of cloud amounts between deep convective and stratiform/congestus clouds. The limitation and future work are also discussed.