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Interference of the East Asian Winter Monsoon in the Impact of ENSO on the East Asian Summer Monsoon in Decaying Phases


doi: 10.1007/s00376-013-3118-8

  • The variability of the East Asian winter monsoon (EAWM) can be divided into an ENSO-related part (EAWMEN) and an ENSO-unrelated part (EAWMres). The influence of EAWM res on the ENSO-East Asian summer monsoon (EASM) relationship in the decaying stages of ENSO is investigated in the present study. To achieve this, ENSO is divided into four groups based on the EAWMres: (1) weak EAWM res-El Nio (WEAWMres-EN); (2) strong EAWMres-El Nio (SEAWMres-EN); (3) weak EAWMres-La Nia (WEAWMres-LN); (4) strong EAWM res-La Nia (SEAWM res-LN). Composite results demonstrate that the EAWMres may enhance the atmospheric responses over East Asia to ENSO for WEAWMres-EN and SEAWMres-LN. The corresponding low-level anticyclonic (cyclonic) anomalies over the western North Pacific (WNP) associated with El Nio (La Nia) tend to be strong. Importantly, this feature may persist into the following summer, causing abundant rainfall in northern China for WEAWMres-EN cases and in southwestern China for SEAWMres-LN cases. In contrast, for the SEAWMres-EN and WEAWMres-LN groups, the EAWMres tends to weaken the atmospheric circulation anomalies associated with El Nio or La Nia. In these cases, the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes, which results in deficient summer rainfall in northern China for SEAWMres-EN and in southwestern China for WEAWMres-LN. Further study suggests that anomalous EAWM res may have an effect on the extra-tropical sea surface temperature anomaly, which persists into the ensuing summer and may interfere with the influences of ENSO.
    摘要: The variability of the East Asian winter monsoon (EAWM) can be divided into an ENSO-related part (EAWM EN) and an ENSO-unrelated part (EAWM res). The influence of EAWM res on the ENSO-East Asian summer monsoon (EASM) relationship in the decaying stages of ENSO is investigated in the present study. To achieve this, ENSO is divided into four groups based on the EAWM res: (1) weak EAWM res-El Ni?o (WEAWM res-EN); (2) strong EAWM res-El Ni?o (SEAWM res-EN); (3) weak EAWM res-La Ni?a (WEAWM res-LN); (4) strong EAWM res-La Ni?a (SEAWM res-LN). Composite results demonstrate that the EAWM res may enhance the atmospheric responses over East Asia to ENSO for WEAWM res-EN and SEAWM res-LN. The corresponding low-level anticyclonic (cyclonic) anomalies over the western North Pacific (WNP) associated with El Ni?o (La Ni?a) tend to be strong. Importantly, this feature may persist into the following summer, causing abundant rainfall in northern China for WEAWM res-EN cases and in southwestern China for SEAWM res-LN cases. In contrast, for the SEAWM res-EN and WEAWM res-LN groups, the EAWM res tends to weaken the atmospheric circulation anomalies associated with El Ni?o or La Ni?a. In these cases, the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes, which results in deficient summer rainfall in northern China for SEAWM res-EN and in southwestern China for WEAWM res-LN. Further study suggests that anomalous EAWM res may have an effect on the extra-tropical sea surface temperature anomaly, which persists into the ensuing summer and may interfere with the influences of ENSO.
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Manuscript received: 06 June 2013
Manuscript revised: 22 July 2013
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Interference of the East Asian Winter Monsoon in the Impact of ENSO on the East Asian Summer Monsoon in Decaying Phases

  • 1. Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
Fund Project:  This work was supported by the National Natural Science Foundation of China (Grant Nos. 41025017, 41230527 and 41205047).

Abstract: The variability of the East Asian winter monsoon (EAWM) can be divided into an ENSO-related part (EAWMEN) and an ENSO-unrelated part (EAWMres). The influence of EAWM res on the ENSO-East Asian summer monsoon (EASM) relationship in the decaying stages of ENSO is investigated in the present study. To achieve this, ENSO is divided into four groups based on the EAWMres: (1) weak EAWM res-El Nio (WEAWMres-EN); (2) strong EAWMres-El Nio (SEAWMres-EN); (3) weak EAWMres-La Nia (WEAWMres-LN); (4) strong EAWM res-La Nia (SEAWM res-LN). Composite results demonstrate that the EAWMres may enhance the atmospheric responses over East Asia to ENSO for WEAWMres-EN and SEAWMres-LN. The corresponding low-level anticyclonic (cyclonic) anomalies over the western North Pacific (WNP) associated with El Nio (La Nia) tend to be strong. Importantly, this feature may persist into the following summer, causing abundant rainfall in northern China for WEAWMres-EN cases and in southwestern China for SEAWMres-LN cases. In contrast, for the SEAWMres-EN and WEAWMres-LN groups, the EAWMres tends to weaken the atmospheric circulation anomalies associated with El Nio or La Nia. In these cases, the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes, which results in deficient summer rainfall in northern China for SEAWMres-EN and in southwestern China for WEAWMres-LN. Further study suggests that anomalous EAWM res may have an effect on the extra-tropical sea surface temperature anomaly, which persists into the ensuing summer and may interfere with the influences of ENSO.

1. Introduction
  • The East Asian summer monsoon (EASM) is an important subsystem of the Asian summer monsoon (Tao and Chen, 1987; Ding, 1992; Chang et al., 2000a, 2000b; Huang et al., 2003). The year-to-year variability of the EASM tends to cause severe droughts and floods in East Asian regions, resulting in human casualties and economic losses (Huang et al., 2003). The El Niño-Southern Oscillation (ENSO) is widely considered as the most important external forcing acting upon the variation of the EASM (Zhang et al., 1999; Wu and Wang, 2002; Huang et al., 2004; Chan and Zhou, 2005; Zhou et al., 2006; Zhou and Chan, 2007), and their relationship exhibits distinct features during the different stages of ENSO. During a developing El Niño summer, negative rainfall anomalies appear in northern China, while during a decaying El Niño summer, rainfall anomalies show a different distribution, with negative anomalies in northern and southern China, and positive anomalies over the Yangtze River Valley (Huang and Wu, 1989; Liu and Ding, 1992; Chen, 2002; Wu et al., 2003). For example, the extremely severe floods in the Yangtze River Valley in 1998 happened during the decaying summer of the extraordinarily strong 1997/98 El Niño (Lau and Weng, 2001).

    Despite knowledge of the relationship, prediction of the EASM based on ENSO is not always accurate. One reason may be that there are many factors impacting the EASM, such as soil moisture, ice cover and regional SST anomalies (e.g., Lau, 1992; Huang et al., 2003; Wu and Kirtman, 2007; Ding et al., 2010; Zhang et al., 2011; Wu et al., 2012a), while another important reason may be the instability of the ENSO-EASM relationship (Wang, 2002). This instability may be caused by changes in ENSO behavior. For example, El Niño exerts different impacts on the following EASM when the onset time of El Niño occurs during different seasons (Li and Shou, 2000). (Xue and Liu, 2008) suggested that a strong ENSO tends to have a robust impact on the EASM, while a moderate ENSO has a weak impact. The El Niño-EASM relationship also depends on the location of warm SST anomalies (Lin and Yu, 1993; Weng et al., 2007; Feng et al., 2011). When these are located in the tropical eastern Pacific (EP)-named EP-type El Niño, or conventional El Niño-the EASM is characterized by positive rainfall anomalies over the Yangtze River Valley during the decaying summer of El Niño. In contrast, when the warm SST anomalies appear in the tropical central Pacific (CP)-named CP-type El Niño, or El Niño Modoki-the positive rainfall belt moves northward to the Huaihe-Yellow River, and the Yangtze River region experiences negative rainfall anomalies (Feng et al., 2011).

    Meanwhile, some recent studies have pointed out that the instability of the ENSO-EASM relationship could be caused by extra-tropical influences (Chan and Zhou, 2005; Rong et al., 2010; Yoon and Yeh, 2010; Wu et al., 2012b). (Yoon and Yeh, 2010) reported that the Pacific Decadal Oscillation (PDO) disrupts the linkage between El Niño and the following Northeast Asian summer monsoon (NEASM) through inducing the Eurasian pattern in the mid-high latitudes. (Wu et al., 2012b) suggested that the spring North Atlantic Oscillation (NAO) could strengthen the ENSO-EASM relationship. The spring NAO tends to trigger a tripole SST anomaly in the North Atlantic region, which can persist into the following summer and induce a Rossby wave in the mid-high latitudes and a Gill-Matsuno wave over the low latitudes. These two waves enhance the ENSO-EASM relationship.

    The East Asian winter monsoon (EAWM) is another important climate system, which is characterized by strong northeasterlies along the eastern edge of the Siberian High and the coast of East Asia (e.g., Ding, 1994; Chen et al., 2005; Wang and Chen, 2010). During the EAWM season, ENSO generally reaches its mature phase and has the most prominent impact on the climate. Hence, a warm ENSO tends to lead to a weak EAWM through inducing southwesterly winds over the coast of East Asia, while a cool ENSO causes a reversed EAWM condition (Zhang et al., 1996; Chen, 2002). The vast majority of studies have concentrated on the impacts of ENSO on the EAWM (Tomita and Yasunari, 1996; Zhang et al., 1996; Chen et al., 2000; Chen, 2002; Wang et al., 2008; Zhou and Wu, 2010). However, the effect of modulation of the EAWM on the impact of ENSO has rarely been examined. (Chen et al., 2013) divided the EAWM into two components: an ENSO-related part-named EAWM EN, arising from the tropical process-and an ENSO-unrelated part, named EAWM res, arising from the extra-tropical process. They found that the EAWM EN explains only 35% of the total EAWM variation, while the variation explained by EAWM res reaches up to 65%, demonstrating that the EAWM res accounts for most of the EAWM variation. Thus, we wonder whether EAWM res has a modulation effect on the ENSO-EASM relationship; and hence, in the present study, the impact of EAWM res on the ENSO-EASM relationship during the decaying phase of ENSO is investigated-the aim being to aid our understanding of the reasons behind the instability of the ENSO-EASM relationship.

    The structure of the paper is as follows. The data and methods employed are introduced in section 2. Section 3 reveals the modulation effect of the EAWM res on the impact of ENSO on summer rainfall anomalies in China. The corresponding atmospheric circulation anomalies responsible for the rainfall anomalies are then analyzed in section 4. Section 5 discusses the possible mechanisms through which the EAWM res modulates the ENSO-EASM relationship; and finally, section 6 summarizes the key findings of the study.

2. Data and methods
  • Monthly mean ERA-40 reanalysis data, including winds and geopotential heights, derived from the European Centre of Medium-Range Weather Forecasts (ECMWF), are used in the present study (Uppala et al., 2005). The dataset covers the period from September 1957 to August 2002, and has a horizontal resolution of 2.5°× 2.5° and a 17-level vertical resolution from 1000 hPa to 1 hPa. Also used are rainfall data from 160 stations across China, obtained from the Chinese Meteorological Data Center. This dataset is available from 1951 to the present day. In addition, Hadley Center Global Sea Ice and Sea Surface Temperature (HadISST) monthly SST data are employed, spanning the period from 1870 to the present day, with a horizontal resolution of 1°× 1° (Rayner et al., 2003). For consistency, the data period from September 1957 to August 2002 is considered in this study. The interdecadal variation with a period over seven years has been removed from the original data using the Lancos filter in order to highlight the interannual variation (Duchon, 1979).

    The interannual variability of the EAWM is estimated by the index (EAWMI) proposed by (Yang et al., 2002), which is defined by the meridional 850-hPa winds averaged over the region (20°-40°N, 100°-140°E). To investigate the roles of the EAWM res in the impact of ENSO on the following EASM, the ENSO-related EAWMI is removed from the EAWM through the linear regression method (Chen et al., 2013) (Fig.1). The obtained EAWM part is denoted as EAWM res and the index as EAWMI res.A positive (negative) EAWMI res indicates a weak (strong) EAWM, and strong (weak) EAWM winters are selected according to EAWMI res>0 (<0). ENSO events are identified by the normalized winter mean (Dec-Jan-Feb) Niño3 (5°S-5°N, 150°-90°W) index being greater than 0.5, and the maximum SST anomaly in the central-eastern Pacific exceeding 1°. The ENSO events are then further sorted into four groups: (1) weak EAWM res with El Niño (WEAWM res-EN); (2) strong EAWM res with El Niño (SEAWM res-EN); (3) weak EAWM res with La Niña (WEAWM res-LN); (4) strong EAWM res with La Niña (SEAWM res-LN). The cases for the four groups are shown in Table 1.

    Figure 1.  Time series of the normalized EAWMI (solid line) and the ENSO-unrelated EAWMI (dashed line). "E" and "L" denote El Niño and La Niña years, respectively.

    Figure 2.  Composite summer mean (JJA) rainfall anomalies during the decaying phases of the (a) El Niño and (b) La Niña events. The shading indicate the 90% confidence level according to a two-tailed Student's t-test.

    Figure 3.  Composite summer mean (+JJA) rainfall anomalies during the (a) weak EAWM res-El Niño; (b) strong EAWM res-El Niño; (c) weak EAWM res-La Niña; and (d) strong EAWM res-La Niña. (e) The difference between (a) and (b). (f) The difference between (c) and (d). The contour interval is 10 mm month-1 for (a)-(d), while it is 15 mm mon-1 for (e) and (f). The shading indicate the 90% confidence level according to a two-tailed Student's t-test.

3. Modulation effect of EAWM res on the impact of ENSO on summer rainfall in China
  • The general lagged influence of ENSO on summer rainfall in China is depicted in Fig. 2, which shows the composite rainfall anomalies during the decaying summer of the El Niño and La Niña cases. The rainfall anomalies in eastern China present alternative positive and negative values in the north-south direction for both the El Niño and La Niña events. During El Niño decaying summers, above-normal rainfall is located in northern China and over the Yangtze River valley, while below-normal rainfall is seen over the Huaihe River valley (Fig. 2a). In contrast, the rainfall anomaly distribution shows an opposite feature during decaying summers of La Niña, with wet conditions over the Huaihe River valley and dry conditions in northern China and over the Yangtze River valley (Fig. 2b). These rainfall anomaly distributions are consistent with previous results (Huang and Wu, 1989; Chen, 2002).

    The ENSO-EASM relationship is re-investigated in Fig. 3 based on the classification of ENSO into the four different EAWM res groups, in which the rainfall anomalies in the following summer for WEAWM res-EN, SEAWM res-EN, WEAWM res-LN and SEAWM res-LN are shown. Under El Niño conditions, the rainfall anomalies in China display different spatial patterns between the weak EAWM res and strong EAWM res (Figs. 3a and b), with the most pronounced difference occurring in northern China (Fig. 3e). When the EAWMres is in a weak state, northern China experiences significantly positive rainfall anomalies (Fig. 3a). Conversely, when the EAWM res is in a strong state, northern China is covered by negative rainfall anomalies (Fig. 3b). For the decaying summer of the two groups of La Niña cases, the significant difference in summer rainfall anomalies is dominantly seen in southwestern China (Fig. 3f). Severe droughts happen there in the WEAWM res-LN cases (Fig. 3c), whereas floods occur in the SEAWM res-LN cases (Fig. 3d). Comparing Fig. 2 to Fig. 3, one can see that the ENSO-EASM relationship changes under EAWM res conditions. Hence, the EAWM res may play an active role in the influence of ENSO on the following EASM.

4. Anomalous atmospheric circulations responsible for the abnormal rainfall in China
  • The low-level anomalous anticyclone (cyclone) over the western North Pacific (WNP) is the main anomalous atmospheric circulation that links ENSO to the following summer's rainfall in East Asia (Zhang et al., 1999; Wang et al., 2000). Figures 4 and 5 present the composite evolutions of the 850-hPa wind anomalies for the WEAWM res-EN, SEAWM res-EN, WEAWM res-LN and SEAWM res-LN cases, respectively. The results show that the anomalous WNP anticyclone is very strong and significant during WEAWM res-EN winters (Fig. 4a), but very weak and insignificant for SEAWM res-EN winters (Fig. 4b). This difference can be attributed to the different circulation anomalies induced by the EAWM res. A weak EAWM res winter is characterized by anomalous anticyclonic flow over the WNP (Chen et al., 2013, Fig. 5), and a strong EAWM res winter has the reverse features. Consequently, when an El Niño is combined with a weak EAWM res, the anomalous WNP anticyclone associated with El Niño tends to be strengthened and covers a larger domain due to the negative vorticity arising from a weak EAWM res. In contrast, when an El Niño occurs together with a strong EAWM res, the anomalous WNP anticyclone tends to be substantially weakened. For the La Niña cases, similar mechanisms can be detected in Figs. 5a and d. Thus, the anomalous WNP anticyclone (cyclone) associated with ENSO may be enhanced (weakened) in winter by an anomalous EAWM res. This effect of the EAWM res on the atmospheric responses to ENSO may further influence the atmospheric circulations in the following spring and summer.

    Figure 4.  Composite 850-hPa wind anomalies in the (a, d) DJF; (b, e) MAM (+); and (c, f) JJA (+). The left (right) column is for the WEAWM res-EN (SEAWM res-EN) groups. The shading indicates the 90% confidence level according to a two-tailed Student's t-test.

    Figure 5.  The same as Fig. 4, but for the WEAWM res-LN and SEAWM res-LN cases.

    Figure 6.  Temporal evolutions of composite meridional wind anomalies averaged from 110° to 130°E during the (a) weak EAWN res-El Niño; (b) strong EAWN res-El Niño; (c) weak EAWM res-La Niña; and (d) strong EAWM res-La Niña. The contour interval is 0.3 m s-1. Shading is applied for highlighting.

    Figure 7.  The same as Fig. 4, but for the 500-hPa geopotential heights. The contour interval is 4 gpm. The solid (dashed) lines denote positive (negative) anomalies. The shading indicates the 90% confidence level, according to a two-tailed Student's t-test.

    Figure 8.  Composite two-monthly average SST anomalies for the WEAWM res-EN (left column) and SEAWM res-EN (right column) groups. The contour interval is 0.2°C. The solid (dashed) lines denote positive (negative) anomalies. The shading indicates the 90% confidence level according to a two-tailed Student's t-test.

    During the ensuing spring and summer of WEAWM res-EN events, the wintertime enhanced WNP anticyclonic anomalies still cover a large domain and extend to the high latitudes (Figs. 4b and c), which favors the transportation of more water vapor from the South China Sea to northern China and in turn leads to more substantial rainfall there (Fig. 3a). However, for SEAWM res-EN events, the wintertime suppressed WNP anticyclone is located in the low latitudes during the following spring and summer (Figs. 4d-f), thereby confining the water vapor to the low latitudes and then causing less summer rainfall in northern China (Fig. 3b).

    Under La Niña conditions, the evolution of the WNP cyclone has a similar feature to that under El Niño conditions. During the SEAWM res-LN cases, the anomalous WNP cyclone strengthened by the strong EAWM res in the winter covers a larger region in East Asia during the subsequent summer (Fig. 5f). The anomalous northeasterly winds on the western side of this cyclone enhance the convergence in southwestern China. In addition, weak anomalous southwesterly winds are observed in southwestern China. Hence, more substantial rainfall is caused in southwestern China (Fig. 3d). However, during the WEAWM res-LN cases (Figs. 5a-c), the weakened anomalous WNP cyclone in the winter is still suppressed and confined to lower latitudes in the following spring and summer, transporting the water vapor from the East China Sea to the South China Sea. In such cases, less rainfall is observed in southwestern China.

    As is known, the meridional winds along the coast of East Asia are an important feature of the EASM. Figure 6 presents the temporal evolutions of composite meridional wind anomalies averaged from 110°E to 130°E for the four groups as indicated in Table 1. In winter, the anomalous WNP anticyclone induced by El Niño generates anomalous southerlies over East Asia. In the case of WEAWM res-EN, the southerly anomalies are enhanced by the weak EAWM res and extend to middle and high latitudes from winter to the following summer (Fig.6a). In contrast, in the case of SEAWM res-EN, the southerly anomalies stretch only to the mid-latitudes and are then broken off by the northerly anomalies associated with strong EAWM res (Fig.6b). Therefore, El Niño combined with weak EAWM res generally gives rise to stronger meridional winds and causes more rainfall in northern China. In the La Niña events, the anomalous northerly winds associated with La Niña are weakened in a weak EAWM res winter and enhanced in a strong EAWM res winter. Subsequently, the weak northerly anomalies are broken off in WEAWM res-LN cases, but in SEAWM res-LN cases the strong northerly anomalies are able to persist into the following summer (Figs. 6c and d).

    Figure 7 presents the evolutions of the anomalous 500-hPa geopotential heights for the WEAWM res-EN and SEAWM res-EN groups. When El Niño occurs together with a weak EAWM res, the anomalous wintertime atmospheric circulation exhibits a western Pacific (WP) teleconnection pattern over the North Pacific region, with the positive pole centered west of Japan and the negative pole centered over the Kamchatka Peninsula (Fig. 7a). When El Niño is combined with a strong EAWM res, the anomalous atmospheric teleconnection in the winter exhibits a different pattern from that during the weak EAWM res. The negative pole moves eastward to the North Pacific and the positive pole shifts to the north-central Pacific (Fig. 7d). Hence, the Pacific North American (PNA) pattern is formed during the SEAWM res-EN events (Fig. 7d). In such a case, the positive geopotential height anomaly around the East Asian region is suppressed more extremely compared to that during WEAWM res-EN events (Fig. 7a), which is consistent with the results shown in Fig. 4. These different atmospheric responses in the winter impart very distinct signals to the subsequent seasons. The strong (weak) and large-scale (small-scale) positive geopotential height anomalies around the WNP persist to the following spring and summer for WEAWM res-EN (SEAWM res-EN) cases, consistent with the strong (weak) WNP anticyclone at the low level in Fig. 4.

    Importantly, the 500-hPa geopotential height anomalies during decaying summers of El Niño are closely related to the anomalous western Pacific subtropical high (WPSH), which is another salient factor responsible for the abnormal East Asian summer rainfall. During decaying summers of El Niño with a weak EAWM res (Fig.7c), East Asia is covered by largely positive geopotential height anomalies. These positive anomalies facilitate the WPSH to shift northward, causing substantial rainfall on the northwestern edge of this WPSH. In contrast, during decaying summers of the SEAWM res-EN cases, the positive geopotential height anomaly over the WNP is confined to lower latitudes (Fig. 7f), which anchors the WPSH to the southward region. Thus, dry conditions are caused in northern China due to this southward location of the WPSH. In addition, the features of the 500-hPa geopotential height anomalies for La Niña are similar to those for El Niño (not shown).

5. Discussion: causes of the modulation effect of EAWM res on the ENSO-EASM relationship
  • The EAWM res tends to modulate the impact of the ENSO on the following EASM. One question relating to this is how does the EAWM res disrupt the ENSO-EASM relationship during the following summer when the winter monsoon has ended? Figure 8 presents the composite two-monthly averaged SST anomalies for the WEAWM res-EN and SEAWM res-EN groups. Comparing the SST anomalies between these two groups, one can see that the SST anomalies in the tropical region exhibit a similar spatial pattern, but the extra-tropical SST anomalies display a totally different distribution. This difference may arise from the effect of the EAWM res. When El Niño is combined with a weak EAWM res (Fig. 8, left column), the extra-tropical SST anomalies are characterized by a tripole pattern with the warming signals in the North Pacific straddled by the cooling signals on its northern and southern sides. This pattern is significantly robust from winter to the following summer, although it is slightly weakened in late summer (Figs. 8a-d). In contrast, when El Niño is combined with a strong EAWM res (Fig. 8, right column), the North Pacific is mainly covered by negative SST anomalies persisting from winter to the following summer. (Vimont et al., 2001) suggested that this extra-tropical tripole SST anomaly in the summer could generate negative vorticity anomalies over the western Pacific. This may favor the formation of the strong and large-scale WNP anticyclone during the WEAWM res-EN cases, as shown in Fig. 4c and 7c. During the SEAWM res-EN events, the negative SST anomalies in the North Pacific do not have this effect, thereby inducing a weak WNP anticyclone. The SST anomalies during the La Niña events have similar features to those for the El Niño events. Specifically, the extra-tropical SST anomaly presents a tripole pattern during the SEAWM res-LN cases, with the negative SST anomaly in the North Pacific flanked by the positive SST anomaly on its northern and southern sides (not shown). According to the findings proposed by (Vimont et al., 2001), this tripole SST anomaly pattern in the North Pacific favors a strengthening the WNP cyclone associated with La Niña during SEAWM res-LN events. Therefore, the anomalous EAWM res tends to have an effect on the extra-tropical SST anomaly, which can persist into the next summer. Such extra-tropical SST anomalies may interfere with the influences of ENSO.

6. Summary
  • The modulation effect of the EAWM res on the ENSO-EASM relationship has been investigated based on observational data. ENSO events were divided into four groups according to EAWM res conditions: WEAWM res-EN, SEAWM res-EN, WEAWM res-LN and SEAWM res-LN. The composite results demonstrated that ENSO exerts a different influence on the following EASM under different EAWM res conditions. The anomalous summer rainfall differences occur most strikingly in northern China for El Niño events and in southwestern China for La Niña events. Northern China experiences excessive rainfall for the WEAWM res-EN group, but deficient rainfall for the SEAWM res-EN group. Meanwhile, southwestern China experiences less rainfall for the WEAWM res-LN group and more rainfall for the SEAWM res-LN group.

    The low-level anomalous WNP anticyclone/cyclone is a key atmospheric system linking ENSO to the following summer's rainfall in East Asia. For the WEAWM res-EN group, the weak EAWM res strengthens the anomalous WNP anticyclone associated with El Niño through inducing negative vorticity over the WNP. This strengthened WNP anticyclone occupies a large domain extending from the tropical western Pacific to mid-high latitudes, which can persist to the following summer. With the strong WNP anticyclone, the summertime WPSH tends to move northward and in turn favors abundant rainfall in northern China. In contrast, when El Niño happens together with a strong EAWM res, the anomalous WNP anticyclone in winter is suppressed by the positive vorticity associated with a strong EAWM res. This weakened WNP anticyclone is confined to the low latitudes and maintains from winter to the following summer, which anchors the WPSH to the southward region. With this southward location of the WPSH, less water vapor can be transported to the high latitudes, thereby leading to drought conditions in northern China. In the case of La Niña events, the anomalous WNP cyclone experiences a similar process under different EAWM res conditions. The WNP cyclone is enhanced during SEAWM res-LN events and then causes a large amount of rainfall in southwestern China, whereas it is suppressed during WEAWM res-LN events and results in drought conditions in southwestern China.

    The EAWM res can modulate the wintertime atmospheric responses to ENSO. Thus, the question was raised as to how this modulation persists from winter to the following summer when the winter monsoon has ended. It was hypothesized that the features of the anomalous SST distribution may shed light on this question, and were thus examined. It was found that the anomalous EAWM res imparts its effect on the extra-tropical SST anomaly. An anomalous tripole SST pattern in the North Pacific can be observed from winter to the following summer during WEAWM res-EN (SEAWM res-LN) cases, which tends to generate anticyclonic (cyclonic) anomalies in the summer over the western Pacific. Therefore, the anomalous WNP anticyclone (cyclone) associated with El Niño (La Niña) is enhanced for WEAWM res-EN (SEAWM res-LN) cases. However, the extra-tropical tripole SST pattern does not occur during SEAWM res-EN (WEAWM res-LN) cases, being replaced by negative (positive) SST anomalies. Accordingly, the WNP anticyclone (cyclone) is much weaker in such cases.

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