Abstract
The South China Sea (SCS) summer monsoon (SCSSM) onset signifies the commencement of large-scale summer monsoon over East Asia and the western North Pacific (WNP). Previous studies on the influencing factors of the SCSSM onset mainly focus on the tropical systems, such as El Niño-Southern Oscillation (ENSO). This study reveals that the wave train along the Asian jet could act as an extratropical factor to modulate the SCSSM onset, and it is largely independent of ENSO. The SCSSM onset tends to be earlier during the positive phase of the wave train (featured by northerly anomalies over Central Iran plateau and eastern China, southerly anomalies over Arabian Peninsula, eastern Indian subcontinent, and eastern Bonin islands). The wave train affects the SCSSM onset mainly via modulating the WNP subtropical high. The wave train during the positive phase can induce negative geopotential height anomalies in the mid-troposphere and anomalous cyclones in the lower-troposphere over the SCS and the Philippine Sea, leading to the weakening of the WNP subtropical high. Specifically, the anomalous ascending motions associated with the low-level cyclone are favorable for the increased rainfall over the SCS, and the anomalous westerly on the south of the anomalous cyclone is conducive to the transition of the zonal wind (from easterly to westerly). The above circulation anomalies associated with the positive phase of the wave train provide a favorable environment for the advanced SCSSM onset.
Highlights
The South China Sea (SCS) is located in the center of the Asian-Australian monsoon, connecting four different monsoon sub-systems: the Indian monsoon, the East Asian monsoon, the Australian monsoon, and the western North Pacific (WNP) monsoon [1]
This study reveals that the wave train along the Asian jet could act as an extratropical winds and pentad mean 850–500 hPa omega regressed onto the normalized time series of PC1
This study reveals that the wave train along the Asian jet could act as an extratropical factor affecting South China Sea summer monsoon (SCSSM) onset, which is largely independent of El Niño-Southern Oscillation (ENSO)
Summary
The South China Sea (SCS) is located in the center of the Asian-Australian monsoon, connecting four different monsoon sub-systems: the Indian monsoon, the East Asian monsoon, the Australian monsoon, and the western North Pacific (WNP) monsoon [1]. The earlier or later SCSSM onset has essential impacts on summer climate anomalies in East Asia. Previous studies on the impact factors of the SCSSM onset mainly focus on the tropical systems; the extratropical systems affecting the monsoon onset are rarely investigated except for AO and cold fronts. The wave train has important impacts on the Asian summer monsoon circulation system, such as the South Asia high [26] and the WNP subtropical high [27,28]. The interdecadal change of the summer precipitation from a meridional triple pattern to a dipole pattern (south-flood-north-drought) over East China around the late-1990s is related to the wave train [29].
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