Effect of the MJO on East Asian Winter Rainfall as Revealed by an SVD Analysis

This paper studies the influences of the Madden Julian oscillation (MJO) on East Asian (EA) winter rainfall using the singular value decomposition (SVD) approach. This method uses two-dimensional instead of latitudinally averaged variables in the commonly used real-Time multivariate MJO (RMM) index. A comparison of the two approaches is made using the same OLR and zonal wind data over 37 boreal winter seasons of December March. The SVD composite reveals a more conspicuous and coherent variation throughout the MJO cycle, while the RMM composite is more ambiguous. In particular, the SVD analysis identifies the convection anomalies over the Maritime Continent and the subtropical western Pacific (MCWP) as amajor cause of enhanced rainfall in EAatRMMphases 8 and 1. This is at least one-eighth of a cycle earlier than the phases of convection development over the IndianOcean (IO) that were emphasized by previous studies. A linearized global baroclinic model is used to demonstrate the mechanism of MJO forcing on EA rainfall during various phases, with a focus on the MCWP cooling. The result shows that the anomalous MCWP cooling and the resultant low-level anticyclonic flow interact with the East Asian jet, leading to an overall weakened EA winter monsoon circulation. The associated anomalous overturning circulation, with ascending motion and low-level horizontal moisture convergence in EA, contributes to the enhanced rainfall. This model result supports the interpretation of the SVD analysis, in that theMCWP cooling induced anomalousmeridional circulation is a more direct cause of enhanced EA rainfall than the IO heating (or the IO MCWP heating dipole) induced Rossby wave teleconnection. ? The Author(s), 2021.