The Leading Mode and Factors for Coherent Variations among the Subsystems of Tropical Asian Summer Monsoon Onset

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AbstractPrevious studies on the Asian summer monsoon (ASM) onset mainly focused on each monsoon subsystem. Mainly based on the monthly mean rainfall and low-level winds in May, this study investigated the dominant onset mode from the perspective of the entire tropical ASM region, which reveals the coherent features among the regional-scale onsets. The results of multivariate empirical orthogonal function (MV-EOF) analysis indicate that the MV-EOF1 presents reduced rainfall and anomalous low-level easterly winds at 850 hPa over the tropical ASM region in May during its positive phase. The corresponding principal component (PC1) is highly correlated with the local monsoon onset dates over the Arabian Sea, the Bay of Bengal, the Indo-China Peninsula, and the South China Sea, where the mean monsoon onsets occur in May. The only exception is the Indian subcontinent, where the mean monsoon onsets occur in June. The results indicate that the leading mode captures the synchronized variation of monsoon onset over most Asian monsoon subsystems, which exhibits remarkable interannual and interdecadal changes. The factors that modulate the coherent variation of the tropical ASM onset are further examined. The simultaneously delayed ASM onset tends to occur during the easterly phase of the 30–80-day oscillation, the decaying phase of El Niño, and the positive phase of Pacific decadal oscillation (PDO). The 30–80-day oscillation serves as a background condition for the synchronized delayed or advanced ASM onset. El Niño–related sea surface temperature anomalies modulate the tropical ASM onset mode by modulating the tropical Walker circulation and inducing an atmospheric Rossby wave response. The PDO affects the tropical ASM onset mode mainly via the equatorial Rossby wave response and the extratropical Rossby wave train.