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Yun-Lan Chen
,
Chung-Hsiung Sui
,
Chih-Pei Chang
, and
Kai-Chih Tseng

Abstract

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 a major cause of enhanced rainfall in EA at RMM phases 8 and 1. This is at least one-eighth of a cycle earlier than the phases of convection development over the Indian Ocean (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 the MCWP cooling induced anomalous meridional circulation is a more direct cause of enhanced EA rainfall than the IO heating (or the IO–MCWP heating dipole) induced Rossby wave teleconnection.

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See Yee Lim
,
Charline Marzin
,
Prince Xavier
,
Chih-Pei Chang
, and
Bertrand Timbal

Abstract

TRMM rainfall data from 1998–2012 are used to study the impacts and interactions of cold surges (CSs) and the Madden–Julian oscillation (MJO) on rainfall over Southeast Asia during the boreal winter season from November to February. CSs are identified using a new large-scale index. The frequencies of occurrences of these two large-scale events are comparable (about 20% of the days each), but the spatial pattern of impacts show differences resulting from the interactions of the general flow with the complex orography of the region. The largest impact of CSs occurs in and around the southern South China Sea as a result of increased low-level convergence on the windward side of the terrain and increased shear vorticity off Borneo that enhances the Borneo vortex. The largest impact of the MJO is in the eastern equatorial Indian Ocean, sheltered from CSs by Sumatra. In general CSs are significantly more likely to trigger extreme rainfall. When both systems are present, the rainfall pattern is mainly controlled by the CSs. However, the MJO makes the environment more favorable for convection by moistening the atmosphere and facilitating conditional instability, resulting in a significant increased rainfall response compared to CSs alone. In addition to the interactions of the two systems in convection, this study confirms a previously identified mechanism in which the MJO may reduce CS frequency through opposing dynamic structures.

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