Boreal summer is the rainy season for China. During early summer (May-June-July, MJJ), warm and moist southwesterly winds, originated from the Indian Ocean, the South China Sea, and the Western Pacific, supply abundant water vapor to central China (Fig. 1a; Sampe and Xie, 2010), one of the most populated regions in China, ranged from 106.25°E-121.25°E, 26.25°N-33.75°N in this study (the red box in Fig. 1a). The rainfall in central China has strong interannual variability (Fig. 1b) and is greatly influenced by the El Niño-Southern Oscillation (ENSO) (Huang and Sun 1992, Wu et al., 2003; Wang et al., 2013; Chen et al., 2017; 2019). For example, during the decay phase of an El Niño event, a significant low-level anticyclone maintains over the western North Pacific (WNPAC), indicating the western Pacific Subtropical High (WPSH) is more intense and southward compared to its climatology (Lu and Dong, 2001; Gao et al., 2019). More water vapor is transported into central China due to the anomalous southwesterly winds in the northwestern flank of the WNPAC, thus increasing the rainfall in central China (Chen 2002; Ding 1992; Ding and Wang 2008; Zhou and Yu, 2005). The previous study revealed that a basin-wide sea surface temperature (SST) warming in the tropical Indian Ocean (TIO) always follows an El Niño event and peaks in the boreal spring (e.g., Klein 1999; Alexander et al., 2002; Chiang and Sobel, 2002; Du et al., 2009; Xie et al., 2016). The TIO warming plays an important role in sustaining the WNPAC when the El Niño turns to the decay phase (e.g., Yang et al., 2007; Xie et al., 2016; Chen et al., 2018).
The anomalous rainfall in central China can largely affect socioeconomic activity and influence millions of people. Zhou et al. (2021) reported that the June-July rainfall in 2020 was twice the mean of 1981–2010. It caused severe floods lashing Yangtze River Valley and Huai River Valley (Ding et al., 2021). By 22 July 2020, more than 45.5 million people were affected, and 142 people were dead or missing, with a direct economic loss of 116 billion yuan (16.6 billion US dollars) (Wei et al., 2020). Figure 1b shows the time series of rainfall anomalies in central China during early boreal summer. Four significant rainfall events, which exceed one standard deviation of rainfall anomaly, occurred in 1983, 1998, 2016, and 2020. The 2020 rainfall event was extremely strong, with its rainfall anomaly exceeded three standard deviations, making a historical record since 1979 (Fig. 1b) and even 1961 (Ding et al., 2021). Many factors may contribute to the 2020 extreme rainfall in central China (e.g., Takaya et al., 2020; Ding et al., 2021; Zhou et al., 2021; Zheng and Wang, 2021). Ding et al. (2021) reported that the East Asian monsoon circulation system, including the high-level westerly jet, the low-level southwesterly jet, and WPSH, featured a quasi-biweekly oscillation, all contributing to this extreme event. Besides, the atmospheric circulation pattern in the middle to high latitudes facilitated the cold air intrusions to the central China region, making an extraordinary contribution (Ding et al., 2021). Takaya et al. (2020) proposed that the Indian Ocean warming enhanced the 2020 summer rainfall in East Asia and regarded the extreme Indian Ocean Dipole (IOD) in 2019 as the cause. Zhou et al. (2021) further highlighted the role of 2019 IOD in the historic Yangtze flooding in 2020.
Water vapor is the basis of local precipitation, influenced by the atmospheric circulation through advection and vertical transport (Zhou et al. 2005; Sherwood et al. 2010; Zhu et al. 2011). Regional water vapor supply arises mainly from two sources: local evaporation and external moisture advection. On the continent, Benton et al. (1950) showed that even local evaporation is significant, the external moisture advection contributes the majority of rainfall. As the abnormal water vapor continuously enters the sink region, heavy rainfall can be formed (Trenberth et al., 2003). Water vapor transport is vital for determining the rainfall pattern of China (Simmonds et al. 1999; Ninomiya and Kobayashi 1998; Ding and Sun 2001). For four significant rainfall events in central China in 1983, 1998, 2016, and 2020, the origin of the water vapor and the cause of such water vapor transport patterns are not always the same. This motivates us to distinguish the difference, especially for the 2020 event, from the aspect of water vapor transport. This study found that not like the other three events, no strong El Niño conditions occurred in the central to eastern equatorial Pacific in the previous winter of 2020. Only a weak warm state appeared in the central-western tropical Pacific. Our research investigates the 2020 case and demonstrates the greatly important role of TIO SST warming even without ensuring El Niño in the Pacific.
The rest of the study is organized as follows. Section 2 describes the dataset and methods utilized in this work. Section 3 analyzes the water vapor transport associated with four significant rainfall events. Section 4 explores the direct impact of WNAPC. Section 5 discusses the IPOC effect on extreme rainfall in central China during early summer 2020 in detail. Section 6 summarizes major points and provides further discussions.