Influence of three phases of El Niño–Southern Oscillation on daily precipitation regimes in China

In this study, we used daily values of climate Chinese surface stations compiled by the National Meteorological Center in China. This dataset (http://data.cma.cn/, last access: 25 June 2016) comprises detailed spatial coverage of precipitation across China, but only 400 stations were operational in the 1950s (Xu et al., 2011). Non-climatic noise can complicate the accuracy of the dataset analysis (Qu et al., 2016). Stations that experienced observation errors, missing values, or data homogeneity problems were omitted from analysis in this study, according to similar methods used by Qian and Lin (2005). Of the 819 meteorological stations across China, data from 713 were ultimately selected for analysis, which covered the time period 1960–2013 (Fig. 1). Precipitation indices were calculated based on daily observations at the stations (Table 1). Annual total precipitation, as well as intensity and frequency of daily precipitation events, were used to formulate precipitation characteristics. Four other indices were introduced (Zhang et al., 2011) and used to analyze precipitation extremes in this study (Table 1). Precipitation indices were calculated for ENSO developing and decaying years. Indices for precipitation anomalies (A_ij, the i precipitation index at the j meteorological station) were analyzed as follows:

where $\stackrel{\mathrm{‾}}{{\mathrm{PI}}_{ij}}$ is the average of the i precipitation index at the j meteorological station during a specific time period, and $\stackrel{\mathrm{‾}}{{\mathrm{PA}}_{ij}}$ is the average of the i precipitation index at the j station for a multi-year average (1971–2000).

To quantify the variability of the summer monsoon and ENSO impacts over China, the monsoon index proposed by Wang and Fan (1999) was used in this study. It is defined as the 850 hPa wind speed averaged over 5–15^∘ N, 100–130^∘ E, minus the wind speed averaged over 20–30^∘ N, 110–140^∘ E, and is frequently used to study interannual and decadal variability of summer monsoons over the western North Pacific and eastern Asian region (WNP-EA). For TC activity, the best-track dataset from the Joint Typhoon Warning Center was obtained at http://www.metoc.navy.mil/jtwc/jtwc.html (last access: 22 August 2018). Following the methods by Kim et al. (2011), the TC genesis and track density were generated for ENSO developing and decaying years for the period 1960–2013. Track density anomalies are defined as the annual average TC frequency during a specific type of ENSO event minus the long-term mean value in each $\mathrm{2}{}^{\circ }\times \mathrm{2}{}^{\circ }$ grid box.

Figure 1Distribution of the 713 meteorological stations used in this study. China is divided into four regions based on climatic type. The two non-monsoon regions are the continental climate zone (northwest – NW) and the highland climate zone (Tibetan Plateau – TP). The monsoon region is divided into two regions: the tropical and subtropical monsoon zone (south – S) and the temperate monsoon zone (north – N).

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Table 1Definitions of precipitation indices used in this study.

^∗ 95th percentile of multi-year daily precipitation is the 95th quantile of the daily precipitation distribution over 1971–2000 (percentiles near 100 represent extremely intense precipitation).

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In this study, two indices, created by Ren and Jin (2011) by transforming the traditionally used Niño 3 and Niño 4 indices, were used to distinguish between CP and EP El Niño phases. La Niña years were identified using the methods of McPhaden and Zhang (2009). The ENSO events (1960–2013) analyzed in this study are displayed in Table 2. As an EP El Niño evolves, positive SST anomalies expand latitudinally and negative signals expand eastward, reaching a maximum amplitude in autumn and winter (Feng et al., 2011). The first year was defined as the developing year of an EP El Niño in this study. Warm SST anomalies disappear and are replaced by cool anomalies in the eastern Pacific during summer of the next year (defined as the decaying year). Similarly, the emerging and vanishing years of CP El Niño and La Niña events are defined as the developing and decaying years, respectively.

Table 2ENSO emerging years from 1960 to 2013.

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The significance of ENSO-induced precipitation anomalies is tested using a Mann–Whitney U approach. The Mann–Whitney U test is a nonparametric test applied to the site data which do not conform to normality even after several transformations are performed (Teegavarapu et al., 2013). It tests whether two series are independent from each other. One series represents precipitation during an ENSO event phase (EP, CW, or LN), while the other series represents precipitation during average years. This test was applied to evaluate the significance of precipitation anomalies at a significance level of 5 %.

Figure 2Anomalies of annual precipitation in developing years (a) and decaying years (b) of EP, CP, and LN phases. Stations experiencing significant anomalies are represented by large points. The percentage of stations experiencing increases or decreases in the number of rainfall anomalies are shown in (c), with significant increase (blue), increase (light blue), decrease (light red), and significant decrease (red). Y1 and Y2 represent developing years and decaying years, respectively.

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Summer monsoons over eastern Asia (EA) consist of staged progressions of zonally oriented rain belts as fronts advance and retreat. Huang and Wu's (1989) study first revealed that these summer monsoon rain belts are closely linked with ENSO cycle phases. Figure 7 shows the mean WNP-EA monsoon index and its significant difference from average conditions (1971–2000).

Figure 7The western North Pacific and eastern Asian (WNP-EA) summer monsoon index during different ENSO phases (average for 1971–2000 is −0.007). The asterisk indicates 95 % significance.

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Results reveal that the WNP-EA monsoons tend to be weak during EP decaying years (Fig. 7). Wang et al. (2001) showed that a weak WNP-EA monsoon usually features enhanced rainfall along the monsoon's front over eastern Asia. As for the mechanism, anomalous anticyclones in the subtropical WNP are the key systems linking the ENSO and the eastern Asian climate (Feng et al., 2011; Wang and Chan, 2002; Wang et al., 2001; Yuan et al., 2012). An anomalous WNP anticyclone during a weak WNP-EA monsoon brings plentiful moisture to southern China; meanwhile, it can also shift the ridge of the subtropical high westward (Feng et al., 2011). When summer monsoons advance and retreat along WNP anticyclone fronts, heavy and continuous rainfall typically develops along the monsoon fronts (Chang, 2004). In this study, our examination of variations in precipitation anomalies reveals that rainfall is largely enhanced over the NW and N region during EP decaying years (Fig. 2).

Weak WNP-EA summer monsoons also tend to occur during CP decaying and LN developing years (Fig. 7). Typically, anomalous WNP anticyclones originate and develop during the previous autumn in the El Niño developing year and persist until the following spring and summer before intensities decrease (Wang et al., 2003). Yuan et al. (2012) found that WNP anticyclones display distinct location, intensity, and lifetime evolutions in the CP and EP El Niños due to the different anomalous SSTs in the equatorial Pacific. The EP El Niño tends to create stronger, wider, and longer-lived WNP anticyclones than the CP El Niño (Shi and Qian, 2018). In terms of rainfall pattern, the CP El Niño induced asymmetric anomalies that do not follow the patterns seen in the EP El Niño (Fig. 2). For example, during CP decaying years, the S region experienced a negative annual precipitation anomaly. Anomalous WNP anticyclones may explain this incongruity between the influences of the EP and CP El Niño phases, reflecting the potential for changes in atmospheric diabatic forcing over the tropics. In contrast, weak WNP-EA summer monsoons during LN developing years possibly correlate with the disappearance of EP during decaying years, when WNP anticyclones tend to reinvigorate and extend northwestward and inland (Feng et al., 2011). Precipitation anomalies in China also reveal a marked consistency between EP decaying years and LN developing years (Fig. 2).

Figure 7 further reveals that strong WNP-EA summer monsoons occur during EP or CP developing years, although not significantly. However, only the EP developing stage induces a negative rainfall anomaly over China (Fig. 2). Similar results were observed by Wu et al. (2003), who documented seasonal rainfall anomalies in eastern Asia, finding that the rainfall correlation distribution displayed pronounced differences between developing and decaying ENSO years. A reverse monsoon signal between developing and decaying years suggests that WNP anticyclones respond in terms of location and intensity to the evolution of SST anomalies over the tropical Pacific (Chang, 2004).

Using a climate model, Chou et al. (2012) found that changes in precipitation frequency and intensity are closely associated with changes in atmospheric water vapor and vertical motion. As demonstrated by Chou et al. (2012), an increase in water vapor reduces the magnitude of the vertical motion required to generate the same strength of precipitation, resulting in an increase in precipitation frequency and intensity. Therefore, large amounts of water vapor transported during EP and CP decaying years, or during LN developing years, when WNP-EA summer monsoons are relatively weak, may enhance both precipitation frequency and intensity. On the other hand, atmospheric vertical motion also tends to be intense during these periods, as summer monsoons over China feature strong southerly winds (Chen et al., 2013). This leads to further anomalous R_95 p, $R_{x_{\mathrm{1}\mathrm{d}}}$, and CWDs, resulting in increased flood risk during these years (Table 3). However, a reduction in water vapor availability and vertical motion may occur during EP and CP developing years, as WNP-EA summer monsoons tend to be strong (Fig. 7), resulting in a negative anomaly in frequency and intensity of precipitation (Figs. 4 and 5).

Figure 8Anomalies in TC genesis longitude (a), latitude (b), and number (c) during JASO over the WNP. The dashed line indicates the average value for 1971–2000. The asterisk indicates 95 % significance.

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In addition, the relative stability of the atmosphere tends to reduce the frequency and intensity of precipitation by reducing vertical motion (Chou et al., 2012). The WNP subtropical high is a prime circulation system over the WNP-EA, and anomalies of location and intensity largely affect summer monsoon activities in eastern Asia (Wang et al., 2013). Huang and Wu (1989) found that when the location of a subtropical high is shifted unusually northward, hot and dry weather occurs in eastern China due to the dominance of the stable atmosphere. The location and intensity of subtropical highs are also closely associated with the development of WNP anticyclones, and the northward shift usually coincides with strong WNP-EA summer monsoons (Wang et al., 2001). Therefore, anomalous WNP subtropical highs possibly exacerbate negative precipitation frequency anomalies and positive DS anomalies in the S region during LN decaying years (Fig. 4 and Table 3). This may also explain the strong reductions in precipitation frequency in the S region during CP decaying years (Fig. 4), because WNP anticyclones display different anomalies than EP phases.

ENSO is one of the most important factors affecting TC activity over the WNP (Wu et al., 2012). In this study, the modulation of TC activity by ENSO was analyzed during developing and decaying phases for the period 1960–2013. Figure 8 shows ENSO-induced anomalies in terms of TC number and location of formation. Track density anomalies are shown in Fig. 9.

Figure 9Track density anomalies during JASO in developing years (a) and decaying years (b) of EP, CP, and LN phases.

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Although the total number of TCs formed in the WNP did not vary significantly from year to year, TCs tended to form further east and south during EP developing years (Fig. 8). This shift in the location of the TC genesis constrained TCs westward propagation into eastern Asia (Wang and Chan, 2002). Therefore, track density was largely reduced over the coast of China (Fig. 9). This suggests that the EP developing stage induces a smaller impact by TC on rainfall in China. Conversely, during LN developing years, TCs tend to form further north (Fig. 8), and the track density shows a remarkable increase in the South China Sea (Fig. 8). The enhancement of TC activity tends to induce heavier rainfall events, leading to positive anomalies of precipitation intensity, $R_{x_{\mathrm{1}\mathrm{d}}}$, R_95 p, and CWD during such years (Figs. 4 and  5; Table 3). TCs during CP developing years also form at higher latitudes than during EP phases, but the average latitude is lower than during the LN phase (Fig. 8). In contrast, the CP developing stage increases track density from the central–western Pacific to the eastern China coast and decreases it over the South China Sea (Fig. 9). Kim et al. (2011) revealed that a shift in the TC genesis location during CP years is closely associated with anomalous westerly winds induced by the westward shift in ocean heating and that this shift further provides more favorable conditions for westward TC propagation. Zhang et al. (2012) also claimed that TCs during CP summers are more likely to make landfall over eastern Asia because of a westward shift in subtropical highs and a northward shift in the TC genesis. However, TCs during CP developing years do not exert significant rainfall anomalies over China. The anomalous WNP-EA summer monsoons induced by ENSO may further explain this discrepancy. As discussed above, the strong WNP-EA monsoons during CP developing years do not induce negative rainfall anomalies over China (Fig. 7). This suggests that enhanced TC activity may cause a reduction in rainfall along monsoon fronts, resulting in neutral conditions over China. However, further studies are needed to examine how the CP developing stage influences rainfall over China.

In contrast, no significant shifts in the locations of the TC genesis occurred during decaying years (Fig. 8). This suggests that the impact of ENSO on TC formation may decrease after ENSO maturation. However, nearly opposite TC track density patterns occur over the WNP during developing and decaying years of an EP or CP El Niño (Fig. 9). For example, in EP decaying years, TC activity increased in the South China Sea and decreased from the western Pacific to the eastern China coast. This shift in track density affected water vapor transport and contributed to a reversed pattern of rainfall anomalies between developing and decaying years.

All the data used in our study were downloaded from websites indicated in Sect. 2.

AL conceived the study. AL and BQ performed the data analysis and wrote the paper. AL, SJ and WZ read and edited the manuscript. All authors reviewed and approved the manuscript.

The authors declare that they have no conflict of interest.