The WSCS study region is located off the southeast Vietnam coast (Figure 1A). Southwesterly winds first appear in this study in May, extend throughout the entire SCS in July and August, and persist until late September (Shaw and Chao, 1994). The stirring effect of strong summer winds, low water temperatures, and high nutrients caused by the coastal upwelling provided favorable conditions for Chl-a, high concentrations of which first appear off southeast Vietnam in early June and persisted for about 4 months (Kuo et al., 2000; Xie et al., 2003; Tang et al., 2004, 2006; Feng et al., 2019; Shi et al., 2019). The high concentrations occurred in a long, narrow plume along with the strong offshore currents (Chen et al., 2014).

In this study, the months from June–September represented summer to investigate the entire process of plume formation and dissipation. The optical conditions in the study area were complex and affected by the terrestrial input, suspended material, sea–land interactions, sediments, and high phytoplankton primary productivity (Lu et al., 2010; Xing et al., 2019). The regions with water depth shallower than 200 m, where Chl-a is overestimated due to the influence of coastal water with high suspended matter concentration and yellow matter were excluded (Lin and Zhao, 2012; Shen et al., 2020). Time series data for Chl-a and other environmental variables were collated for the region from the box (109–111°E, 11–12.5°N) for Chl-a time series, the box (109–111°E, 11–14°N) as SST time series, 109–111°E, 10–12°N for SSW time series, 109–111°E, 11–12.5°N for AOT time series, 109–113°E, 12–15°N for SLA time series, 109–111°E, 10–11.5°N for ET time series, and the parallelogram with four vertexes (107°E, 11°N; 109.5°E, 12.5°N; 110°E, 11.5°N; and 107.5°E, 10°N) for EPV time series, where the variations of them were more significant, respectively.

The data of Chl-a and AOT of T865 derived from four different sensors of Sea WiFS, MODIS, MERIS, and VIIRS with GSM Model (Maritorena and Siegel, 2005; Maritorena et al., 2010) are generated from GlobColour database provided by HERMES. The merged daily/weekly/monthly L3 products of Chl-a and monthly AOT with a spatial resolution of 0.04° × 0.04° are obtained for the period from 1998 to 2020. The influence of dust indexed by AOT is investigated through the analysis of the relation between the aerosol depth (i.e., AOT) and the Chl-a in the WSCS.

The data of SST and SSW are obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) at 0.25° × 0.25° spatial resolution. The monthly products are used from 1998 to 2020. In addition, EPV and ET in the study region are investigated to clarify the change of Chl-a, and the possible roles of wind fields play in the WSCS.

The data of SLA and geostrophic velocity both with 0.25° × 0.25° spatial resolution are available from AVISO (the Archiving, Validation and Interpretation of Satellite Oceanographic dataset). The daily multi-mission merged products are obtained from 1998 to 2020.

The World Ocean Atlas 2018 (WOA18) is obtained from the Ocean Climate Laboratory of the National Oceanographic Data Center. The spatial resolution of monthly nitrate is 1° × 1°. Considering the fact that nitrate limitation occurs in the SCS, the nitrate data are used as an indicator of available nutrients in the study.

In situ data, such as Chl-a, temperature, and salinity that were derived from the work of Wang et al. (2016), during the period from September 2 to September 9, 2007, were used in the study. The detailed data information can be found in Wang et al. (2016).

The monthly data for Chl-a concentrations averaged over 23 years from 1998 to 2020 reveals a plume of high concentration in the WSCS during summer (Figure 2). The high concentrations (about 0.5 mg·m⁻³) typically appear in early June near the southeast coast of Vietnam, then extend into the open sea in a plume induced by the northeastward jet (Figure 1C), consistent with the previous research (Xie et al., 2003; Tang et al., 2004; Zhao and Tang, 2007); high concentrations persist for about 4 months, peaking in August and dissipating in September.

The scatter graphs (Figure 6) for Chl-a vs. SST, SSW, AOT, SLA, EPV, and ET, present a good tendency with each other. A correlation analysis indicates that Chl-a (Table 1) is positively correlated with SSW, AOT, ET, and EPV (Chl-a vs. SSW, Pearson's r = 0.5882, p < 0.01; Chl-a vs. AOT, r = 0.4002, p < 0.01; Chl-a vs. ET, r = 0.6171, p < 0.01; and Chl-a vs. EPV, r = 0.6115, p < 0.01), and negatively correlated with SLA and SST (Chl-a vs. SST, r = −0.7080, p < 0.01 and Chl-a vs. SLA, r = −0.4491, p < 0.01). Of these, SST and ET are most highly correlated with Chl-a, followed by EPV, SSW, and last SLA and AOT. These correlations indicate that Chl-a concentration is mainly affected by SST and ET.

A multiple correlation analysis is used to further analyze the influence of SST, SSW, AOT, SLA, ET, and EPV on Chl-a concentrations. A strong correlation between Chl-a concentration and ocean conditions (Figure 6; Table 2) indicates that these factors largely explain increased Chl-a. The coefficient of multiple correlation (R) (Table 2) between Chl-a and the total environmental conditions is significant (R = 0.798, p < 0.01). Additionally, partial coefficients between Chl-a concentration and SST, SSW, AOT, SLA, ET, and EPV are −0.481, −0.132, 0.094, −0.187, 0.218, and 0.144, respectively (Table 2). The partial correlation between AOT and Chl-a is the poorest and is the first to be eliminated. The variables with the lowest partial correlation coefficients are then sequentially eliminated from the remaining environmental variables. EPV also has a non-negligible role on Chl-a based on Table 2.

Relationships between Chl-a and SST, SSW, AOT, SLA, ET, and EPV are examined using multiple stepwise linear regression analysis (Table 3). AOT is first removed from the regression equation because its p-value is the greatest. After removing AOT, the SSW p-value is then, the greatest, so it is also removed in the next step, and so on, until the remaining SST and ET in the regression equation cannot be removed, revealing that these two variables most significantly impact Chl-a.

Considerable variation in high Chl-a concentration appears off the southeast coast of Vietnam in summer (Figure 2). The high Chl-a levels typically first appear in early June and persist for about 4 months. The SST, SSW, AOT, SLA, ET, and EPV values all vary from June to September (Figure 3). Based on a simple correlation analysis (Figure 6; Table 1), six environmental factors are highly correlated with Chl-a. The multiple correlation coefficient (Table 2) between Chl-a and all the environmental factors (R = 0.798, p < 0.01) indicates that each of all factors impacts summer phytoplankton blooms in the WSCS. However, in offshore waters, ET and EPV have greater effects on summer phytoplankton blooms than any other factor, consistent with the previous studies (Liu et al., 2012; Gao et al., 2013; Tang et al., 2014; Liang et al., 2018; Zhao et al., 2018).

Analysis of multiple and partial correlations (Table 2) suggests that the influence of AOT on surface Chl-a is low, as the partial correlation (r = 0.094) is insignificant. The AOT inputs are considered major forces in regional oceanic ecosystems (Wong et al., 2007). Although AOT can significantly enhance the phytoplankton blooms in the SCS (Wang et al., 2012b), and impact primary production and ocean biogeochemical cycles (Duce, 1986; Martin et al., 1994; Migon and Sandroni, 1999; Jo et al., 2007), aerosol precipitation plays a more obvious role in the basin (open sea) (Lin et al., 2007). Lin et al. (2009) reported that with increased distance from the coastline, the potential role of atmospheric aerosols increased, and the correlation increased between Chl-a and AOT. Due to other nutrient sources (upwelling and convective-overturn), the effect of AOT is only weakly correlated with Chl-a.

Illumination conditions (such as temperature and light) and nutrients are the main limiting factors of phytoplankton growth (Wu et al., 2003; Zhao and Tang, 2007). The optimum temperatures for growth of most phytoplankton range 20–30°C (Lin et al., 2009; Li and Lin, 2019). The SCS in the tropical monsoon region has strong light penetration throughout the year (Liu et al., 2002; Zhao and Tang, 2007). Summer SST is relatively uniform at 28–30°C, and its influence on the phytoplankton distribution is deemed to be insignificant. Therefore, we believe that nutrients are the main limiting factor of Chl-a concentrations in summer throughout most of the SCS (Tac and Du, 1988). Low SST weakens the stratification and enhances the vertical mixing (Phillips, 1966; Zhao et al., 2005b), further promoting dispersal of deep nutrients into upper ocean layers, thereby contributing to phytoplankton growth. As low SST is caused by upwelling, what we see in the SCS is essentially the product of indirect stimulation of phytoplankton growth by upwelling.

Ekman pumping velocity and ET greatly influence the phytoplankton blooms (Figure 6; Tables 1–3), with correlation coefficients Chl-a of 0.6172 and 0.6115 (Figure 6; Table 1), respectively; partial correlation coefficients also suggest that both EPV and ET play important roles in the formation of phytoplankton blooms. Although the influence of EPV on surface Chl-a is less than that of ET, a significant change in EPV (Figure 3F) is more consistent with that of Chl-a (Figure 2). Figure 3F indicates a possible prevailing tendency for upwelling in this region.

The regions of low SST in Figure 3A, low temperature from 50–75 m in Figure 5D, and high nitrate concentration from 30–50 m in Figure 4, are all similar, and coincide with that of ET-induced upwelling (Figure 3). This might indicate that ET-induced upwelling provides a continuous source of cold and nutrient-rich water to surface layers, leading to the phytoplankton blooms (Chereskin and Price, 2001; Zhao et al., 2012). SLA (Figure 3C) reveals the occurrence of both cold and warm eddies in the WSCS in summer. The areas of high Chl-a appear at the junction of two eddies, where current velocity is high. The high Chl-a distribution areas are consistent with the current direction, demonstrating that current velocity and direction affect surface layer Chl-a distribution.

Nutrients regulate phytoplankton growth in most WSCS areas (Zhao et al., 2005a; Zhao and Tang, 2007). Stable stratification during summer in euphotic layers inhibits nutrient transport from deeper waters (Zhao et al., 2018). Vertical transport is an important source of euphotic nutrients in oligotrophic seas (Xie et al., 2003; Jin et al., 2009; Zhao et al., 2018). In summer with conditions of high SST, the growth of phytoplankton gradually migrates to the subsurface layer to adapt to the growth environment. Therefore, maximum subsurface chlorophyll concentrations appear between 25 and 75 m depth (Figure 5). High temperature and low salinity (where precipitation exceeds evaporation) in upper layers, and low temperature and high salinity in deeper layers, are general characteristics of the SCS. Therefore, the horizontal movement of relatively low-salinity high-temperature water may represent either water mass sinking or mixing (Figures 5D,E); relatively high-salinity and low-temperature water may suggest an upward trend. EPV and ET are strong (Figures 3E,F) during summer, suggesting upwelling. Additionally, the center of high nitrate concentrations in the 30–50 m layer (Figure 4) is closer to the center of ET (Figure 3E), which is indicative of upwelling. Therefore, we consider that upwelling has a major impact on the summer phytoplankton blooms in this region.

A strong northward-flowing coastal current exists from 7.5°N to the turning point of the Vietnamese coastline at about 11°N (Figure 1C). This current flows north along the southeast coast of Vietnam, and when it flows into the sea near 11°N, it turns northeastward to form a northeastward jet (Figure 1C) (Fang et al., 2012). We refer to these two currents as the “northward current” and “northeastward jet” (Figure 1C). The centers of low SST (Figure 5D) and high Chl-a (Figure 2) do not overlap; areas of abnormally SLA also do not overlap with high Chl-a concentration. These differences are probably due to the northward current (Figure 1C), which may influence the centers of phytoplankton blooms and upwelling in summer. According to WOA18 mean nitrate distributions (Figure 4), high nitrate concentrations at 30 m occur further north than that in 50 m transported northward about 100 km (Figure 4). Additionally, relatively high Chl-a concentrations occur at 50 m centered around 111.5°E, 11.5°N, but high Chl-a concentrations at 25 m are centered around 111.5°E, 12.5°N, possibly indicating northerly movement (Figure 5). These changes indicate that the upwelling and phytoplankton blooms are likely to be affected by the northward current. The position of this current and where it jets northeastward from the coast of Vietnam varies between 10 and 14°N (Shaw and Chao, 1994; Chao et al., 1996; Takano et al., 1998), possibly because of changes in the direction of the Vietnamese coastline (Shaw and Chao, 1994). The coastal waters are influenced by marine, river, and terrestrial inputs (Gao et al., 2013). In our study region, high Chl-a near the Mekong River Estuary are contributed by discharge from the Mekong River all year along flowing northeastward far (Gao et al., 2013). Although this study excludes regions with water depth shallower than 200 m where Chl-a is overestimated due to the influence of coastal water, there is still a few nutrients input from the Mekong River that may be transported to the studied region by the background circulation (northeastward jet). In summer, the strong wind drives a northeastward jet of water along the Vietnamese coast. The northeastward jet will carry a few nutrients input from the Mekong River transported northeastward, which will enhance the growth of phytoplankton and cause the distribution of high Chl-a in the WSCS to be plume-shaped (Figure 2).