Interannual variability of the onset of the South China Sea summer monsoon

Abstract

This article investigates the year-to-year variability of the onset of the South China Sea summer monsoon (SCSSM) and the possible influences exerted by the surface temperature anomalies over land and sea. Early and late monsoon onsets are related to the temperature anomalies in different regions. It is found that an early onset follows negative sea surface temperature (SST) anomalies in the central tropical Pacific (CP) Ocean during the preceding winter and spring, corresponding to a CP La Niña. In contrast, a late onset is preceded by the negative surface air temperature anomalies over land in the central Asian continent. Negative SST anomalies in the central-eastern equatorial Pacific Ocean and the associated warming in the western Pacific induce an anomalously enhanced Walker circulation. This anomalous Walker cell leads to an increase in convection, causing more latent heat release and a subsequent decrease of surface pressure. The anomalous Walker cell and the enhanced latent heat release weaken the Western North Pacific subtropical high and the Philippine Sea anticyclone, favouring a westerly flow from the Indian Ocean, resulting in an early SCSSM onset. On the other hand, negative land surface temperature anomalies cool the atmosphere over land, and locally modify the Hadley circulation, accompanied by the anomalous divergence in the low-level atmosphere over the western equatorial Pacific. This divergence anomaly reduces the latent heat release and strengthens the anticyclone in the Philippine Sea, thus preventing the westward extension of the westerlies from the Indian Ocean and causing a late SCSSM onset.

Objective determination of the onset and withdrawal of the South China Sea summer monsoon

“…El Niño–Southern Oscillation (ENSO) is one of the major factor affecting the variabilities of Asian monsoons (Huang et al ., ; Yuan and Yang, ; Chen et al ., ). For example, previous studies show that the SCSSM onset tends to occur later (earlier) during El Niño (La Niño) decaying summer (Zhou and Chan, ; Ding et al ., ; Luo et al ., ). Besides the monsoon onset, the monsoon withdrawal is also significant to societies since it signals the termination of the summer rainy season (Luo, ; Stolbova et al ., ).…”

“…The onset and withdrawal timing of the monsoon season should be correctly determined and predicted. As one important component of the Asian summer monsoon (Tao and Chen, ), the onset of the South China Sea summer monsoon (SCSSM) indicates the end of dry season and beginning of the summer rainy season over East Asia (Wang et al ., ; Ding and Chan, ; Luo et al ., ), whereas its withdrawal indicates the end of the rainy season.…”

“…Moreover, previous studies on monsoon timing mainly focused on monsoon onset (Zhou and Chan, ; Kajikawa and Wang, ; Luo et al ., ; Zhu and Li, ), with a few on the monsoon withdrawal, particularly its variability (Syroka and Toumi, ; Li and Zhang, ; Stolbova et al ., ). El Niño–Southern Oscillation (ENSO) is one of the major factor affecting the variabilities of Asian monsoons (Huang et al ., ; Yuan and Yang, ; Chen et al ., ).…”

Summer extreme precipitation in eastern China: Mechanisms and impacts

“…Moreover, the strength of the EASM and the associated rainfall are subject to the western North Pacific subtropical high (WNPSH) [ Lin and Wang , ]. Therefore, many efforts have been devoted to study the behaviors of WNPSH [ Chan , ; Ling et al ., ] and its possible impacts on Asian climate [e.g., Hong et al ., ; Luo et al ., ; Zhu et al ., ]. The change of WNPSH exerts considerable impacts on the spatiotemporal variations of precipitation in southeastern China [ Wang and Chen , ] and eastern China [ Ding et al ., ; Zhu et al ., ; Lu et al ., ].…”

The North Pacific Gyre Oscillation and East Asian summer precipitation

“…The variability of precipitation over East Asia is considerably modulated by the variation of sea surface temperature (SST). Previous work suggested that El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are specifically the dominant causal factors at the interannual scale (Chang et al , ; Wang et al , ; Ding and Chan, ; Luo et al , , ). For instance, an El Niño‐like SST pattern in June 2015 intensified the western Pacific subtropical high and shifted it southwestward, thus increasing the occurrence of heavy precipitation in eastern China (Wang and Gu, ).…”