Abstract
The characteristics and possible impact factors of the South China Sea summer monsoon (SCSSM) evolution from onset to withdrawal before and after 1993/94 are investigated using ERA-Interim, CPC rainfall, and OLR data. During the late-onset period of 1979–1993, the SCSSM was characterized by stronger onset intensity and a gradual withdrawal, resulting in a continuous, strong preflood season in Southern China and a slower rain-belt retreat from north to south China in September. In addition, the rain-belt in the Yangtze River basin persisted much longer during summer. However, during the early-onset period in 1994–2016, the SCSSM is associated with a weaker onset intensity and comparatively faster retreat. The advanced preflood season lasted intermittently throughout May and the whole eastern China precipitation lasted until October when it retreated rapidly, making the rain-belt in Southern China persist for an extended duration. Further analysis indicates that a strong modulation of SCS intraseasonal oscillation (ISO) on the SCSSM evolution is observed. There are two active low-frequency oscillations over the SCS in summer during the late-onset period but three during the early-onset period. The wet ISO in the Northwest Pacific propagating northwestward into the SCS and enhanced SCSSM ISO activity may contribute to the early onset and faster withdrawal after 1993/94. The effect of warm western Pacific sea surface temperatures (SST) on the SCSSM evolution is also discussed.
Highlights
Asian monsoons are one of the main energy and moisture sources of the global atmosphere [1]. e South China Sea summer monsoon (SCSSM) has a significant position in the Asian monsoon, forming a vital link between the East Asian and South Asian summer monsoons [2,3,4]
The SCSSM onset is generally accompanied by a continuous eastward retreat of the western North Pacific (WNP) subtropical high, convection enhancement, and the reversal of low-level zonal wind in the SCS [5, 6]. e synoptic-scale circulation systems, the disturbance of mid-latitude, and the intraseasonal oscillation (ISO) activities can trigger SCSSM onset [7,8,9,10,11,12,13,14,15]
Hu et al [21] found that SCSSM withdrawal mainly occurred due to the westward intrusion of the WNP subtropical high, which is accompanied by the retreat of the weakening low-level intertropical convergence zone (ITCZ) and rain-belt in the SCS-WNP. e factors contributing to the interannual variability of the SCSSM withdrawal may include tropical cyclones (TCs) and ISO [22]
Summary
Received 8 April 2020; Revised 21 August 2020; Accepted 3 September 2020; Published 16 September 2020. During the late-onset period of 1979–1993, the SCSSM was characterized by stronger onset intensity and a gradual withdrawal, resulting in a continuous, strong preflood season in Southern China and a slower rain-belt retreat from north to south China in September. During the early-onset period in 1994–2016, the SCSSM is associated with a weaker onset intensity and comparatively faster retreat. E advanced preflood season lasted intermittently throughout May and the whole eastern China precipitation lasted until October when it retreated rapidly, making the rain-belt in Southern China persist for an extended duration. Ere are two active low-frequency oscillations over the SCS in summer during the late-onset period but three during the early-onset period. E wet ISO in the Northwest Pacific propagating northwestward into the SCS and enhanced SCSSM ISO activity may contribute to the early onset and faster withdrawal after 1993/94. Further analysis indicates that a strong modulation of SCS intraseasonal oscillation (ISO) on the SCSSM evolution is observed. ere are two active low-frequency oscillations over the SCS in summer during the late-onset period but three during the early-onset period. e wet ISO in the Northwest Pacific propagating northwestward into the SCS and enhanced SCSSM ISO activity may contribute to the early onset and faster withdrawal after 1993/94. e effect of warm western Pacific sea surface temperatures (SST) on the SCSSM evolution is discussed
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