Nonstationary El Niño teleconnection on the post-summer upwelling off Vietnam

Summer upwelling has often been observed off Vietnam in the South China Sea (SCS). Occasional disappearance of the upwelling has attracted much attention because it modulates the regional climate and harms surrounding fisheries. Fluctuations of the East Asian monsoon associated with El Niño are considered responsible for the weakened or abolished upwelling. However, analyses of observations performed in the present study were equivocal in terms of the dominant influence of El Niño. Based on long-term sea surface temperature data, we demonstrated that weak upwelling off Vietnam occurs more frequently during periods of accelerated global warming compared to warming hiatus periods. Warming signals in the Indian Ocean vanished relatively quickly during the hiatus period. The accompanying easterly anomalies south of the anomalous anticyclone (AAC) in the northwestern Pacific were also weakened, reducing the impact of the El Niño teleconnection on the SCS summer monsoon and thus preserving the regular post-summer upwelling off Vietnam during warming hiatus periods.

are short-term fluctuations below the interannual timescale. Eight weakened summer upwelling events were identified, including 1958, 1973, 1983, 1987, 1988, 1995, 1998, and 2015 (standard deviation > 1.5; red dots in Fig. 2a). IMFs 4-6 are low-frequency components that, when summed, show the decadal variability and longterm tendency of SCS upwelling off Vietnam. In addition to the global warming signal, a phase change over a decadal timescale is apparent (bold curve in Fig. 2a). Closer inspection indicates much more frequent weak SCS upwelling events during the warm phase than during the cold phase; this phenomenon merits further study.
During weak upwelling events, the SCS summer monsoon was weakened, with a decrease in maximum wind speed of around 30-50% (− 2 to − 3 m s −1 ; mean ~ 6 m s −1 ) over the upwelling region (vectors in Fig. 2b). Under the weakened monsoon, SST anomalies increase up to 1 °C (Fig. 2b). A dipole off Vietnam with positive and negative SSHs to the north and south, respectively, indicates an anomalous westward jet (contours in Fig. 2b and vectors in Fig. 2c). This anomalous jet results in unfavorable conditions for coastal upwelling, giving rise to negative chlorophyll-a anomalies off Vietnam (shading in Fig. 2c). Warming and low-nutrient water signals are visible, extending from the coast of Vietnam to the central SCS due to weak upwelling (Fig. 2b, c).
We found that most El Niño events do not weaken summer upwelling off Vietnam in the SCS (Table 1), although almost all of the weak upwelling events took place during a post-El Niño summer. This demonstrates that SCS upwelling off Vietnam is at least partially free from El Niño control. Thus, the reportedly inescapable impact of El Niño on upwelling off Vietnam suggested by most previous studies 2,13 should be reevaluated. The distinct evolution and dynamics of diverse El Niño events result in different climate influences on the SCS, impacting the post-summer upwelling off Vietnam.

Upwelling fluctuation in response to diverse El Niño events
Different El Niño events exert differential impacts on local atmospheric and oceanic circulation [15][16][17] . For example, regarding the impact of the 1994 and 1997 El Niño events on SCS circulation, the former enhanced the western North Pacific summer monsoon in the SCS, leading to stronger circulation in the summer of 1994. Table 1 shows that SCS upwelling may be weakened by Central Pacific (CP), Eastern Pacific (EP), or mixed-type El Niño events 18 Table 1). These two periods www.nature.com/scientificreports/ correspond to periods of accelerated global warming (Fig. 3a) and were characterized by generally stronger El Niños (Table 1). By contrast, few weak upwelling events were identified during El Niño events prior to 1977, or www.nature.com/scientificreports/ in the period 1999-2013; both of these periods were characterized by a warming hiatus (Fig. 3a). Global warming hiatuses are attributed to natural variability to some extent [19][20][21][22] , including fluctuations of Pacific Decadal Oscillation (PDO) 21 and/or Atlantic multidecadal oscillation (AMO) 20 (Fig. 3a). During the regime shift of the PDO/AMO since the late 1990s, warming over the North Atlantic triggers transbasin teleconnections that lead to stronger trade winds and cooling over the eastern and central Pacific (Fig. 3b) 20,23 . Those conditions contribute to the global warming hiatus and cause a La Niña-like decadal cooling 22 which do not favor the development of El Niño events. The strong association between El Niño and SCS upwelling off Vietnam has deteriorated sharply during the hiatus period (Table 1) because of weakened El Niño events. The dissimilar El Niño dynamics between global warming and hiatus periods leads to different atmospheric and oceanic anomalies in the northwestern Pacific, which exert an immediate impact on SCS upwelling off Vietnam. To further examine the influence of climate, the evolution of El Niño was compared between warming and hiatus periods (Fig. 4).

Mechanism
SST and surface wind anomalies indicate that the mature-phase El Niño during the hiatus period is substantially weaker in the eastern and central tropical Pacific compared to that during the warming period. This is illustrated in Fig. 4a,d: SST anomalies over the tropical Pacific are reduced from 1.5-2 °C to 0.5-1 °C, and maximum westerly anomalies decrease from ~ 2.5 to ~ 1 m s −1 . Notably, high-pressure anomalies, such as the AAC (marked by an "A" in Fig. 4a), which are usually seen in the northwestern Pacific during the mature phase of El Niño 13 vanish during the hiatus phase (Fig. 4d). SST and westerly anomalies in the tropical Pacific are diminished during the post-El Niño spring in both warming and hiatus periods (Fig. 4b,e). In particular, the signatures of El Niño events, including SST, surface wind, and sea level pressure anomalies, are largely absent in the Pacific and Indian Oceans during the post-El Niño spring in the hiatus period (Fig. 4e), indicating that the lifetime of El Niño events during the hiatus period is much shorter than that during the warming period.
In the post-El Niño summer, a noteworthy feature distinguishing warming and hiatus periods is the AAC over the northwestern Pacific (Fig. 4c,f). The AAC is apparent only during the warming period El Niño event. This mature AAC is associated with easterly anomalies on its southern border that decrease the southwesterly monsoon in the SCS, thus weakening upwelling in the summer. The easterly anomalies to the south of the AAC are maintained by the El Niño teleconnection that occurs due to warming of the Indian Ocean 13 , which stores the warming signals from the previous El Niño. Regarding the underlying dynamics, El Niño triggers oceanic Rossby waves in the eastern Indian Ocean, which propagate warming signals westward in winter, leading to warming in the Indian Ocean during the post-El Niño spring and, in turn, easterly anomalies south of the AAC in the summer 13 . By contrast, the warming signals in the Indian Ocean vanish quickly during the El Niño in the hiatus period (Fig. 4d-f). The accompanying easterly anomalies south of the AAC are further weakened, removing any influence of El Niño on the SCS summer monsoon and the associated upwelling off Vietnam during the post-El Niño summer. www.nature.com/scientificreports/ In summary, the comparatively weak and short El Niño events during the hiatus period do not induce warming over the Indian Ocean during the post-El Niño summer. The reduced warming of the Indian Ocean weakens the AAC over the northwestern Pacific, thus decreasing the impact of El Niño on the SCS summer monsoon and the associated upwelling.

concluding remarks
In this study, we compared the effects of El Niño events on SCS upwelling off Vietnam between warming and hiatus periods. We identified weak SCS upwelling events based on 68 years of observations. Most El Niño events do not weaken summer SCS upwelling off Vietnam, although almost all of the weak upwelling events occurred during a post-El Niño summer. The distinct evolution and dynamics of different El Niño events lead to differential impacts on the SCS, including on summer upwelling. Furthermore, SCS upwelling off Vietnam shows   www.nature.com/scientificreports/ are given by National Aeronautics and Space Administration (NASA)/Ocean Color (https ://ocean color .gsfc. nasa.gov/). The definition of El Niño diversity in Table 1 is based on Yu and Kim 18 and Paek et al. 27 . The PDO and AMO indices are adopted from the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) and the NOAA/ Earth System Research Laboratory (ESRL), respectively.