1. Introduction
El Niño–Southern Oscillation (ENSO) is one of the most important factors that influence the world climate. Classical El Niño is associated with maximum warm anomalies in the eastern equatorial Pacific and is referred to as canonical or eastern Pacific (EP) El Niño. In recent years, a new type of tropical Pacific sea surface temperature (SST) warming pattern, with maximum warm anomalies in the central equatorial Pacific, has been discussed widely (e.g., Fu et al. 1986; Larkin and Harrison 2005; Ashok et al. 2007; Yu and Kao 2007). It is alternatively referred to as date line El Niño (Larkin and Harrison 2005), El Niño Modoki (Ashok et al. 2007; Weng et al. 2007, 2009), central Pacific (CP) El Niño (Kao and Yu 2009), and warm-pool El Niño (Kug et al. 2009). While it was rarely observed before the 1980s, the number of CP El Niños has almost doubled in the past three decades (Lee and McPhaden 2010), probably because of the recent weakened equatorial easterlies over the central Pacific (Ashok et al. 2007) or the projected global warming scenarios (Yeh et al. 2009). For example, seven El Niño events are identified over the equatorial eastern and central Pacific during the 1990s and the 2000s: 1991–92, 1994–95, 1997–98, 2002–03, 2004–05, 2006–07, and 2009–10. Except for the 1997–98 event as the strongest EP type, all the others are classified as CP El Niño (Ashok et al. 2007; Yeh et al. 2009; Lee and McPhaden 2010).
The different climate impacts of the two types of El Niño have received increased research interest in the recent years. It is documented that the frequencies of both Atlantic hurricanes and northwestern Pacific tropical cyclones are influenced differently by the two types of El Niño (Kim et al. 2009; Chen and Tam 2010). Most of the western United States experiences more rainfall in EP El Niño winter, but a seesaw pattern with less precipitation in the north and more precipitation in the south appears over the western United States during CP El Niño winter (Weng et al. 2009). In Australia, the austral spring and autumn rainfall is also more sensitive to CP El Niño than to EP El Niño (Wang and Hendon 2007; Taschetto and England 2009).
ENSO has also been recognized as a major factor of the year-to-year variability of the East Asian monsoon (EAM; e.g., Zhang et al. 1999; Chang et al. 2000; Zhou and Chan 2007; Zhou et al. 2009; Li and Yang 2010), but only limited studies have examined the different impacts of the two types of El Niño on EAM. For example, during the developing phase of CP El Niño, summer rainfall and temperature anomalies in China and Japan are different from those during EP El Niño (Weng et al. 2007, 2011). In the winter of CP El Niño, the drier conditions in Southeast Asia move more northward than those in EP El Niño (Weng et al. 2009; Feng et al. 2010b).
What then are the different impacts of the two types of El Niño on the East Asian climate in other seasons, especially in the next spring and summer after the peaks of EP El Niño and CP El Niño? As suggested by previous studies, more significant variations of EAM occur in the year after El Niño rather than during its developing year (e.g., Ye and Huang 1996). After El Niño, the summer rainfall over the northwestern Pacific tends to decrease (Wang et al. 2000), while the mei-yu in China and baiu in Japan tend to enhance (e.g., Chang et al. 2000). It is recently shown that southern China experiences increased rainfall during the decaying spring and summer of EP El Niño but reduced rainfall during these seasons of CP El Niño (Feng et al. 2010a; Feng and Li 2011). Therefore, to better understand the physical mechanisms for the year-to-year variations of EAM associated with the different types of El Niño, we investigate the seasonally dependent characteristics of monsoon anomalies during EP El Niño and CP El Niño. We will address the seasonal variations of climate anomalies related to ENSO and the features related to ENSO turnabout. Three questions will be particularly addressed in this paper. First, what are the different impacts of EP and CP El Niño on the precipitation over East Asia from El Niño developing year to El Niño decaying year? Second, is there any difference in the impacts of EP and CP El Niño between the developing summer and the decaying summer? Third, how do EP and CP El Niño events influence the temperature over East Asia?
In section 2, we describe the datasets and analysis methods applied in this study. In section 3, we first compare the evolutions of SST anomalies (SSTAs) between EP and CP El Niño, and then explore the seasonal variations of precipitation and air temperature from El Niño developing year to El Niño decaying year. Possible physical mechanisms responsible for the features being revealed are investigated in section 4, based mainly on anomalous low-level winds, the Walker and local Hadley circulations, and 500-hPa geopotential height. Section 5 provides a summary with further discussion.
2. Data and methods
Multiple datasets are used in this study, which include the monthly SST data from the Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST; Rayner et al. 2003), monthly atmospheric reanalysis data from the National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) (Kalnay et al. 1996), the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP; Xie and Arkin 1997), and the CPC Precipitation Reconstruction over Land (PRECL) data with 1° × 1° latitude–longitude resolution (Chen et al. 2002). We focus on the time period from January 1979 to December 2010 for the following reasons: 1) The quality of the NCEP–NCAR reanalysis over Asia prior to 1968 may be questionable (Yang et al. 2002; Inoue and Matsumoto 2004; Wu et al. 2004). 2) The influence of ENSO on EAM has exhibited a decadal shift since the late 1970s (e.g., Chang et al. 2000; Kinter et al. 2002; Miyakoda et al. 2003; Zhou et al. 2007; Wang et al. 2008; Xie et al. 2010). 3) CP El Niño, rarely observed before the 1980s, has occurred more frequently in the past three decades (Yeh et al. 2009; Feng et al. 2010b; Lee and McPhaden 2010).
Anomalies of all variables are obtained by removing the means of 1979–2010. Seasonal means, which are discussed throughout the paper, are constructed by averaging data for March–May (MAM), June–August (JJA), September–November (SON), and December–February (DJF). Here, DJF 1979 refers to December 1979 and January–February 1980. All these seasons mean boreal seasons; herein JJA (JJA1) indicates ENSO developing (decaying) summer, and the same format is used for the other seasons.
In this study, the Niño-3 index and the El Niño Modoki index (EMI) are used to describe EP El Niño and CP El Niño, respectively. The Niño-3 index is defined by the SSTA averaged in the Niño-3 region (5°S–5°N, 90°–150°W, as marked in the left panels of Fig. 1). EMI is defined as (SSTA)C–0.5(SSTA)E–0.5(SSTA)W, where (SSTA)C, (SSTA)E, and (SSTA)W stand for the area-mean SSTA over the central (C: 10°S–10°N, 165°E–140°W), eastern (E: 15°S–5°N, 110°–70°W), and western (W: 10°S–20°N, 125°–145°E) Pacific, respectively (Ashok et al. 2007; Weng et al. 2007, 2009). These regions are also marked in the right panels of Fig. 1.
Partial correlations of seasonal SSTA with normalized DJF (left) Niño-3 and (right) EMI, for (a),(f) summer, (b),(g) autumn, (c),(h) winter, (d),(i) next spring, and (e),(j) next summer. Shadings indicate correlations above the 95% and 99% confidence levels, and black boxes are for definitions of the (left) Niño-3 index and (right) EMI.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of seasonal SSTA with normalized DJF (left) Niño-3 and (right) EMI, for (a),(f) summer, (b),(g) autumn, (c),(h) winter, (d),(i) next spring, and (e),(j) next summer. Shadings indicate correlations above the 95% and 99% confidence levels, and black boxes are for definitions of the (left) Niño-3 index and (right) EMI.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of seasonal SSTA with normalized DJF (left) Niño-3 and (right) EMI, for (a),(f) summer, (b),(g) autumn, (c),(h) winter, (d),(i) next spring, and (e),(j) next summer. Shadings indicate correlations above the 95% and 99% confidence levels, and black boxes are for definitions of the (left) Niño-3 index and (right) EMI.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
To investigate the possible impacts of EP and CP El Niño on EAM variations from El Niño developing year to the decaying year, lag correlations (mainly from JJA to JJA1) are employed based on normalized DJF Niño-3 index and EMI. Although the correlation between monthly Niño-3 and EMI is insignificant (R = 0.2), the correlation between the winter season means is higher (R = 0.39), significantly exceeding the 95% confidence level. Partial correlations 
3. Seasonal climate anomalies
In this section, we analyze the seasonal variations of anomalous SST, precipitation, and air temperature associated with EP and CP El Niño. We focus on the features that are associated with ENSO cycles.
a. SST
Figure 1 shows the lag partial correlations of seasonal SST with normalized DJF (left) Niño-3 and (right) EMI, respectively. Corresponding to the positive Niño-3 index, significant warming is always concentrated along the equatorial eastern Pacific from the developing summer to the decaying summer. However, negative SSTA dominates in the western Pacific to the east of the Philippines, forming a cold “boomerang” (Trenberth and Stepaniak 2001), and they extend both northeastward and southeastward (Figs. 1a–e). Such a dipole SSTA pattern over the tropical Pacific is an EP-type El Niño. Corresponding to positive EMI, on the other hand, the maximum warming is located persistently over the equatorial central Pacific from JJA to JJA1, with cold SSTA to the west and southeast of the warming center (Figs. 1f–j). This triple pattern along the tropical Pacific denotes a significant CP-type El Niño.
Three important features can be discerned from Fig. 1. First, EP El Niño seems to begin earlier than CP El Niño, since the warm SSTA in EP El Niño developing summer (Fig. 1a) is more apparent than that in the CP type (Fig. 1f). However, CP El Niño tends to end later than EP El Niño because of the longer-lasting significant warming over the equatorial central Pacific in JJA1 (Fig. 1j). Nevertheless, in the decaying summer of EP El Niño, almost no significant warming can be discerned in the Niño-3 region (Fig. 1e).
Second, although the boomerang pattern of cold SSTA is similar for both EP and CP El Niño, the strength and location of SSTA are different between the two types. The SSTA for CP El Niño is weaker and extends more westward compared with that for EP El Niño. During CP El Niño, cold SSTA dominates to both the west and the east of the Philippines (Figs. 1f–j). During EP El Niño, however, only the western Pacific to the east of the Philippines is covered by cold SSTA. To the west, significant positive SSTA develops in autumn (Fig. 1b) and winter (Fig. 1c), and it even extends eastward to the east of the Philippines in the following spring (Fig. 1d) and summer (Fig. 1e).
Third, over the tropical Indian Ocean (TIO), SSTA responds differently to EP and CP El Niño. During the early developing seasons of EP El Niño, a positive Indian Ocean dipole (IOD), with warm SSTA in the west and cold SSTA in the southeast, begins to develop in summer (Fig. 1a) and reaches maximum intensity in fall (Fig. 1b). During the mature phase (DJF) of EP El Niño, IOD dissipates. In the meantime, another important SSTA mode, the basinwide warming mode, dominates in the TIO (Fig. 1c). It fully develops in MAM1 (Fig. 1d) and is maintained to JJA1 (Fig. 1e). On the contrary, these two SSTA modes in the Indian Ocean cannot be found during the life cycle of CP El Niño (Figs. 1f–j).
b. Precipitation
For EP El Niño, the most significant features of anomalous precipitation include the wet conditions over the equatorial eastern-central Pacific and the dry conditions over the warm-pool region (roughly from the tropical eastern Indian Ocean to the western Pacific). There is another evident belt of anomalous rainfall over southeastern China and the northwestern Pacific. Thus, an anomalous positive–negative–positive (+/−/+) rainfall pattern forms from East Asia through the western Pacific to the equatorial eastern Pacific. This pattern develops during the onset year of EP El Niño, and persists to the decaying summer (Figs. 2a–e).
As in Fig. 1, but for precipitation anomalies of CMAP data.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 1, but for precipitation anomalies of CMAP data.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 1, but for precipitation anomalies of CMAP data.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
For CP El Niño, however, less precipitation now appears over the equatorial eastern Pacific and more precipitation over the equatorial central Pacific. In southern East Asia including southeastern China, southern Japan, the South China Sea (SCS), and the Philippines, dry conditions persist from the developing summer to the decaying summer. That is an anomalous −/+/− rainfall pattern over East Asia and the equatorial Pacific during the lifetime of CP El Niño (Figs. 2f–j).
When comparing precipitation variations during EP and CP El Niño, we emphasize two significant characteristics. First, a rainfall belt appears from southeastern China to southern Japan during EP El Niño, a feature has been investigated by numerous studies (e.g., Zhang et al. 1999; Chang et al. 2000; Wang et al. 2008). However, this feature almost disappears and is replaced by dry or near-normal condition during CP El Niño. Secondly, during the developing summer, CP El Niño might have a stronger impact on East Asia than EP El Niño. While during the decaying summer, a more significant influence on East Asia seems to be from EP El Niño. This feature may not be particularly clear in the CMAP data (Fig. 2), but it becomes more evident in the PRECL data (Fig. 3), which is the gauge-based precipitation over land. In JJA of EP El Niño, except for the anomalous less rainfall in most Maritime Continent and part of central China, most of the East Asian landmass shows no significant correlation with the Niño-3 index (Fig. 3a). During CP El Niño, however, an anomalous −/+/−/+ rainfall pattern can be seen clearly from southern to northern East Asia, with dry conditions in the Maritime Continent, wet conditions in the eastern Indo-China Peninsula and northern Philippines, dry conditions in the middle reach of the Yangtze River valley, southeastern China, and southern Japan, and wet conditions over parts of northeast China (Fig. 3b). The features shown in the composite maps of several recent CP El Niño events (not shown) are consistent with the results of correlation analysis, further confirming the conclusion.
As in Fig. 2, but only for the (a),(b) developing summer and (c),(d) decaying summer. The PRECL dataset is used.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 2, but only for the (a),(b) developing summer and (c),(d) decaying summer. The PRECL dataset is used.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 2, but only for the (a),(b) developing summer and (c),(d) decaying summer. The PRECL dataset is used.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
During the next summer (JJA1) of EP El Niño, anomalous more rainfall occurs along the Yangtze River valley, while less rainfall occurs in the southeastern coast of China, the northern Indo-China Peninsula, and the northern Philippines (Fig. 3c). For CP El Niño, however, no significant correlation can be discerned over the East Asian land area (Fig. 3d). Note that EP El Niño has almost disappeared by JJA1, while SST over the TIO still shows significant warming (Fig. 1e). Thus, it can be inferred that the stronger impact of EP El Niño on East Asia might result from the long-lasting warm SSTA over the TIO in JJA1 (Fig. 1e), a “capacitor effect” as proposed by Xie et al. (2009).
c. Air temperature
Figure 4 presents the different impacts between EP and CP El Niño on 2-m air temperature anomalies. Over the tropical Indian and Pacific Oceans, regions of significant positive and negative correlations with Niño-3 and EMI are quite similar to those for SSTA (see Fig. 1) and thus will not be repeated in our discussion. Over the East and Southeast Asian land, especially the Korean Peninsula, Japan, the Indo-China Peninsula, and the Maritime Continent, anomalous warm conditions appear in the winter of EP El Niño (Fig. 4c), intensify in the next spring (Fig. 4d), and persist to the next summer (Fig. 4e). However, anomalous cold conditions dominate over parts of North China, the Korean Peninsula, southern Japan, and the Philippines from the developing summer to the next spring in CP El Niño (Figs. 4f–i).
As in Fig. 1, but for 2-m air temperature anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 1, but for 2-m air temperature anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 1, but for 2-m air temperature anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
4. Physical explanations
From the discussion in section 3, several features can be identified. 1) EP El Niño may induce an anomalous +/−/+ rainfall pattern over East Asia and the equatorial Pacific, whereas CP El Niño may result in an anomalous −/+/− rainfall pattern over these regions. 2) During El Niño developing summer, the impact of CP El Niño on East Asian precipitation seems to be more significant than the impact of EP El Niño. Opposite features are found for the decaying summer. 3) The air temperature over East and Southeast Asia and the western Pacific tend to increase during EP El Niño but decrease during CP El Niño.
In this section, we explore the possible physical mechanisms for the above features, based on analyses of low-level winds, the Walker and Hadley circulations, the East Asian winter monsoon (EAWM), and 500-hPa geopotential height (H500).
a. For anomalous precipitation patterns
Figure 5 shows the lag partial correlations of seasonal 850-hPa winds from JJA to JJA1 with normalized DJF (left) Niño-3 and (right) EMI. Among the most significant differences between EP and CP El Niño are the low-level zonal wind anomalies along the equator. For EP El Niño, low-level westerly anomalies begin to develop over the equatorial western-central Pacific in summer (Fig. 5a) and intensify and move eastward in autumn (Fig. 5b). During the mature phase (winter), westerly anomalies control most of the equatorial central-eastern Pacific (Fig. 5c), persisting to the next spring (Fig. 5d) and decaying in the next summer (Fig. 5e). Over the TIO, significant easterly anomalies prevail across the equator from the El Niño developing autumn to the decaying spring. Thus, anomalous low-level divergence occurs over the equatorial western Pacific with easterly anomalies to the west and westerly anomalies to the east, resulting in anomalous subsidence over the equatorial western Pacific (100°–150°E), while over the equatorial eastern Pacific and the western Indian Ocean, anomalous rising motions dominate (left panels in Fig. 6). Two anomalous cells therefore form over the tropical Indian–Pacific Ocean, a strong one over the equatorial Pacific and a relatively weaker one over the equatorial Indian Ocean. The two cells persist from EP El Niño developing summer to the decaying summer, consistent with the underlying SSTA pattern (Fig. 1). Hence, the anomalous rising motion over the equatorial eastern Pacific increases convection and leads to more precipitation, whereas the anomalous sinking motion over the equatorial western Pacific induces dry condition, as displayed in the left panels of Fig. 2.
As in Fig. 1, but for 850-hPa wind anomalies. Black boxes are for the definition of EAWM index IChW.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 1, but for 850-hPa wind anomalies. Black boxes are for the definition of EAWM index IChW.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 1, but for 850-hPa wind anomalies. Black boxes are for the definition of EAWM index IChW.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of anomalous Walker circulation averaged in 5°S–5°N with normalized DJF (left) Niño-3 and (right) EMI, for (a),(f) summer, (b),(g) autumn, (c),(h) winter, (d),(i) next spring, and (e),(j) next summer. Shadings indicate correlations above the 95% and 99% confidence levels.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of anomalous Walker circulation averaged in 5°S–5°N with normalized DJF (left) Niño-3 and (right) EMI, for (a),(f) summer, (b),(g) autumn, (c),(h) winter, (d),(i) next spring, and (e),(j) next summer. Shadings indicate correlations above the 95% and 99% confidence levels.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of anomalous Walker circulation averaged in 5°S–5°N with normalized DJF (left) Niño-3 and (right) EMI, for (a),(f) summer, (b),(g) autumn, (c),(h) winter, (d),(i) next spring, and (e),(j) next summer. Shadings indicate correlations above the 95% and 99% confidence levels.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
For CP El Niño, the low-level westerly anomalies over the equatorial Pacific are much weaker than those for EP El Niño. They mainly cover the equatorial western and central Pacific during the lifetime of CP El Niño, compared to those over the equatorial eastern Pacific during EP El Niño. While over the equatorial eastern Pacific, anomalous easterlies are significant from the developing autumn to the next summer (Figs. 5g–j). Therefore, anomalous low-level convergence occurs over the equatorial central Pacific, causing anomalous rising motion around the date line, while over the equatorial eastern Pacific, anomalous subsidence persists from the developing summer to the decaying summer (right panels in Fig. 6). Such anomalous Walker circulation causes anomalously more precipitation over the equatorial central Pacific and less precipitation over the eastern Pacific (right panels in Fig. 2), resulting in a reversed precipitation pattern over the equatorial Pacific as compared to that for EP El Niño.
Another prominent difference of the low-level wind anomalies between EP and CP El Niño is the location and evolution of the anomalous anticyclone around the Philippines [the Philippine Sea anticyclone (PSAC)], an important system conveying the impact of El Niño to East Asia (Zhang et al. 1999; Wang et al. 2000; Wang and Zhang 2002). During the developing summer of EP El Niño, the PSAC appears over the southern SCS (Fig. 5a). It moves to the Philippines in autumn (Fig. 5b), intensifies and shifts eastward to the northeast of the Philippines in winter (Fig. 5c), and persists during the following spring (Fig. 5d) and summer (Fig. 5e). The significant southwesterly anomalies to the northwestern flank of the PSAC prevail over the southeastern coast of China and southern Japan, favoring moisture transport from the equatorial western Pacific and the SCS to East Asia, leading to an anomalous rainfall belt over southeastern China and the northwestern Pacific (left panels in Fig. 2). On the other hand, the anomalous subsidence over the equatorial western Pacific, as part of the anomalous Walker circulation associated with EP El Niño (left panels in Fig. 6), would excite an anomalous local Hadley circulation over East Asia within 100°–140°E. As shown in the left panels of Fig. 7, the anomalous Hadley circulation with anomalous sinking motion over the equator and anomalous rising motion over subtropical region around 20°N develops in summer (Fig. 7a), intensifies in winter (Fig. 7c) and spring (Fig. 7d), and then moves northward in the following summer (Fig. 7e). The anomalous rising motion around 20°N provides an important dynamic mechanism for the above-normal rainfall belt over southeastern China and the northwestern Pacific from EP El Niño developing autumn to the decaying summer (left panels in Fig. 2).
As in Fig. 6, but for the anomalous Hadley circulation averaged over 100°–140°E.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 6, but for the anomalous Hadley circulation averaged over 100°–140°E.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 6, but for the anomalous Hadley circulation averaged over 100°–140°E.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
On the contrary, both the PSAC and the regional Hadley circulation over East Asia show different characteristics during CP El Niño. In the developing summer (Fig. 5f) and autumn (Fig. 5g), an anomalous low-level cyclone occupies the western Pacific to the east of the Philippines. The PSAC does not form until winter (Fig. 5h), about two seasons later than that for EP El Niño. More importantly, it is located over the SCS to the west of the Philippines, instead of the east of the country as in the winter of EP El Niño. The PSAC stays over the SCS and intensifies in the ensuing spring (Fig. 5i), but weakens and moves northward to southern Japan in the decaying summer of CP El Niño (Fig. 5j). The associated significant southwesterly anomalies to the northwestern flank of PSAC continuously control southeastern China from winter to the next summer. Comparing with those during EP El Niño, they extend more northwestward and are thus unfavorable for water vapor transport from the SCS or the western Pacific to East Asia. Therefore, anomalous dry conditions dominate over southern East Asia and the northwestern Pacific from winter to the decaying summer (Figs. 2h–j). Patterns of moisture flux integrated vertically from 1000 to 300 hPa also show anomalous divergence of moisture flux over the southeastern East Asia during CP El Niño but convergence of moisture flux over the same region during EP El Niño (not shown).
In addition, the anomalous vertical circulation averaged over 100°–140°E shows that anomalous subsidence controls the subtropical region from JJA to MAM1 (Figs. 7f–i), thus providing a dynamical condition for anomalous less precipitation in southern East Asia and the northwestern Pacific (right panels in Fig. 2). However, on both sides of the anomalous subsidence, almost no significant rising motion can be identified in any season, implying that the mechanism for the subsidence may not be the regional Hadley circulation as in the case for EP El Niño (left panels in Fig. 2). To explore the formation of this anomalous subsidence around 20°N over East Asia, we show the partial correlations of the wintertime vertical p-velocity anomalies at different levels with DJF Niño-3 and DJF EMI, respectively (Fig. 8). It can be inferred that, during the EP El Niño winter, the anomalous rising motion over southeastern China and the northwestern Pacific should be caused by the anomalous sinking motion over the equatorial western Pacific through the anomalies of the regional Hadley circulation (left panels in Fig. 8). The sinking motion over the western Pacific is a branch of the anomalous Walker circulation over the equatorial Pacific associated with the EP type of SSTA. On the contrary, during CP El Niño winter, the anomalous subsidence over East Asia should directly result from the anomalous rising motion over the equatorial central Pacific (right panels in Fig. 8), which is also a branch of the anomalous Walker circulation but is caused by the CP type of underlying SSTA. Partial correlations of the vertical p velocity at different levels during other seasons also show similar results (not shown).
Partial correlations of anomalous vertical p velocity (omega) at different levels in winter with normalized DJF (left) Niño-3 and (right) EMI, for (a),(d) 300, (b),(e) 500, and (c),(f) 850 hPa.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of anomalous vertical p velocity (omega) at different levels in winter with normalized DJF (left) Niño-3 and (right) EMI, for (a),(d) 300, (b),(e) 500, and (c),(f) 850 hPa.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of anomalous vertical p velocity (omega) at different levels in winter with normalized DJF (left) Niño-3 and (right) EMI, for (a),(d) 300, (b),(e) 500, and (c),(f) 850 hPa.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
To conclude, during the developing and decaying year of EP El Niño, the anomalous more (less) precipitation over the equatorial central–eastern (western) Pacific may be caused by the anomalous upward (downward) motion over the equatorial eastern (western) Pacific through the anomalous Walker circulation, consistent with results from previous studies (Ashok et al. 2007; Weng et al. 2007). Also, the anomalous rainfall belt over southeastern China and the northwestern Pacific during EP El Niño is due to the anomalous PSAC to the east of the Philippines and the anomalous rising motion related to the anomalous sinking motion over the equatorial western Pacific through the regional Hadley circulation. For CP El Niño, however, the anomalous wet (dry) conditions over the equatorial central (eastern) Pacific are induced by the anomalous rising (sinking) motion in the equatorial central (eastern) Pacific through the anomalous Walker circulation, which is associated with the maximum warming in the central Pacific. The anomalous rising motion over the equatorial central Pacific also leads to an anomalous sinking branch to its west, which extends more northwestward than that for EP El Niño. Therefore, the anomalous subsidence over southeastern China and the northwestern Pacific, accompanied by persistent anomalous PSAC over the SCS, gives rise to anomalous dry conditions over southern East Asia and the northwestern Pacific.
b. For precipitation anomalies focusing on JJA0 and JJA1
In this section, we will explore the possible dynamics for the different precipitation features between El Niño developing summer and decaying summer. During the developing summer, negative correlation appears over the Maritime Continent, implying anomalous dry conditions associated with EP El Niño (Fig. 3a). These dry conditions should be caused by the anomalous low-level anticyclone over the southern SCS (Fig. 5a), the significant positive anomalies of H500 over the northern Indian Ocean and the Maritime Continent (Fig. 9a), and the anomalous sinking motion over the Maritime Continent as part of anomalous Walker circulation (Fig. 6a). However, over most of East Asia, the correlations of 850-hPa winds and H500 with Niño-3 index are overall small for JJA0, suggesting that the impact of EP El Niño on the East Asian land is weak during the developing summer.
As in Fig. 6, but for 500-hPa geopotential height anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 6, but for 500-hPa geopotential height anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 6, but for 500-hPa geopotential height anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
During CP El Niño summer (JJA), the low-level wind anomalies show a cyclone over the equatorial western Pacific, an anticyclone over southern China and southern Japan, and a cyclone over northeast China (Fig. 5f). The 500-hPa geopotential height also consistently displays negative anomalies over the SCS, positive anomalies over southern Japan, and negative anomalies over northeast China (Fig. 9f). These meridionally oriented anomalies indicate a Pacific–Japan wavelike pattern (Nitta 1987), which can be seen from the anomalous Hadley circulation averaged over 100°–140°E (Fig. 7f) with anomalous rising (sinking) motion over 10°–15°N (25°–30°N). Meanwhile, partial correlations of 200-hPa zonal wind anomalies with EMI also show a significant positive correlation center (above the 90% confidence level) over the northern Korean Peninsula and northern Japan, indicating an enhanced East Asian jet stream during the CP El Niño developing summer (not shown). The enhanced jet stream intensifies the meridional anomalies of 500-hPa atmospheric circulations (Yang et al. 2002), leading to a −/+/− wave pattern over the mid-high latitudes of Eurasia (Fig. 9f). The wave pattern suggests an intensified ridge around the Lake Baikal and two enhanced troughs over the Urals and East Asia, respectively. Therefore, both low-level and high-level circulation patterns are favorable for the anomalous −/+/−/+ rainfall pattern from the Maritime Continent through eastern China to Northeast China during the developing summer of CP El Niño (Fig. 3b).
The above comparison indicates that the impact of CP El Niño on East Asian climate is more evident than EP El Niño during the developing summer, consistent with the result by Weng et al. (2007). However, nearly the opposite is observed for the decaying summer.
By the decaying summer, EP El Niño has almost disappeared (Fig. 1e), but CP El Niño still shows significant warming over the equatorial central Pacific (Fig. 1j), in spite of the reduced strength compared with previous seasons. However, the influence of EP El Niño on the precipitation over East and Southeast Asia tends to be stronger than the influence of CP El Niño (Figs. 3c,d). During the decaying summer of EP El Niño, the anomalous PSAC still significantly controls the northwestern Pacific, the SCS, and southeastern China (Fig. 5e), increasing the 500-hPa geopotential height over southern East Asia (Fig. 9e). Meanwhile, the anomalous regional Hadley circulation also moves northward compared to previous seasons, leading to anomalous rising motion around 30°N and sinking motion over 15°–25°N (Fig. 7e). Accordingly, the Yangtze River valley tends to have more rainfall, whereas southeastern China, the northern Indo-China Peninsula, and the northern Philippines experience less rainfall (Fig. 3c).
During the decaying summer of CP El Niño, an anomalous low-level anticyclone is located to the south of Japan (Fig. 5j), accompanied by an increase in H500 (Fig. 9j). However, the correlations at both 850 and 500 hPa largely decrease compared with those during the previous seasons, and are much weaker than those during EP El Niño for respective seasons. The anomalous regional Hadley circulation even shows insignificant correlation with EMI (Fig. 7j). Therefore, CP El Niño exerts a weak impact on the precipitation over East Asia during the decaying summer (Fig. 3d), compared to EP El Niño.
However, it is worth noting that during the decaying summer of EP El Niño, warming in the TIO and the SCS seems to be more evident than that in the eastern Pacific (Fig. 1e). Although the basinwide warming in the TIO has been widely considered as a response to El Niño (e.g., Klein et al. 1999; Lau and Nath 2000), it is recently emphasized as a “capacitor” to extend the El Niño impact on East Asia especially when El Niño is decaying (Yoo et al. 2006; Yang et al. 2007; Yuan et al. 2008; Xie et al. 2009). Therefore, using the method proposed by Clark et al. (2000), we remove the Indian Ocean basinwide warming (IOBW) signal from the variables examined. For example, UWND2 = UWND1 − r[σ(UWND1)/σ(IOBW)]IOBW, where UWND1 is the original zonal wind and r is the correlation between the zonal wind in each grid and MAM1 IOBW index (defined as the seasonal mean SSTA averaged over 20°S–20°N, 40°–110°E during the decaying spring of El Niño). Also, 
Partial correlations of anomalous (a) precipitation using PRECL data, (b) 850-hPa wind, (c) Hadley circulation average over 100°–140°E, and (d) 500-hPa geopotential height, with normalized DJF Niño-3 index (not considering EMI effects). All variables are in the decaying summer of El Niño, and the effect of Indian Ocean basinwide warming has been removed.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of anomalous (a) precipitation using PRECL data, (b) 850-hPa wind, (c) Hadley circulation average over 100°–140°E, and (d) 500-hPa geopotential height, with normalized DJF Niño-3 index (not considering EMI effects). All variables are in the decaying summer of El Niño, and the effect of Indian Ocean basinwide warming has been removed.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Partial correlations of anomalous (a) precipitation using PRECL data, (b) 850-hPa wind, (c) Hadley circulation average over 100°–140°E, and (d) 500-hPa geopotential height, with normalized DJF Niño-3 index (not considering EMI effects). All variables are in the decaying summer of El Niño, and the effect of Indian Ocean basinwide warming has been removed.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
c. For temperature anomalies over East Asia
Over East Asia, one of the most prominent surface features of the winter monsoon is the strong northeasterly along the eastern flank of the Siberian high and the coastal regions (Chen et al. 2000). During the winter of El Niño, the atmospheric flow pattern in East Asia is unfavorable for the southward outbreaks of cold air and the winter monsoon is thus weaker. The East Asian trough is weaker than normal, but the westerly disturbances to the south are more frequent than normal, causing more precipitation in South China (Tao and Zhang 1998; Chen 2002).
Here, we investigate physical mechanisms for the different air temperature anomalies over East Asia and the western Pacific between EP and CP El Niño. As temperature anomalies are largely influenced by the EAWM, we choose an index to represent the variability of the monsoon, which is the 850-hPa meridional wind averaged over 25°–40°N, 120°–140°E and 10°–25°N, 110°–130°E, defined by Chen et al. (2000); this index will be referred to as IChW. This index has been considered as one of the best winter monsoon indices to reflect the variations of the winter monsoon circulation and air temperature over East Asia (Wang and Chen 2010). Table 1 lists the partial correlations of the seasonal IChW in autumn, winter, and the next spring with normalized DJF Niño-3 and DJF EMI, respectively. Since positive (negative) IChW indicates weaker (stronger) EAWM, it can be seen that EP El Niño would induce a weaker EAWM during winter and the following spring, while CP El Niño may lead to a stronger EAWM from its developing autumn to the decaying spring (Table 1).
Partial correlations of normalized IChW of SON, DJF, and MAM1 with normalized DJF Niño-3 and DJF EMI. Boldface numbers indicate the correlations that are significant above the 90% confidence level.
Partial correlations of low-level wind anomalies with Niño-3 index also show that during the EP El Niño developing autumn, winter, and the decaying spring, anomalous southerly or southwesterly wind prevails from the SCS along the eastern coast of China to Japan and northeast China (Figs. 5b–d). However, the partial correlations with EMI indicate that anomalous northerly or northeasterly wind dominates over parts of East Asia from autumn to the next spring, corresponding to CP El Niño (Figs. 5g–i). Note that the significant meridional wind anomalies caused by CP El Niño are mainly located over the southern box in the IChW definition, suggesting that CP El Niño affects the EAWM over southern East Asia more significantly than the northern portion. Therefore, the strengthened EAWM associated with CP El Niño gives rise to anomalous cold temperature over the northwestern Pacific and parts of East Asia from autumn to the next spring (right panels in Fig. 4), whereas the weakened EAWM during EP El Niño leads to anomalous warm seasons over these regions (left panels in Fig. 4).
The above discussed different influences of EP and CP El Niño on EAWM can also be seen in the variability of 500-hPa geopotential height (Fig. 9). During the winter and following spring, significant positive correlations with Niño-3 index over the tropics extend northward to southern Japan, Korea, and eastern China (Figs. 9c,d). This feature indicates positive H500 anomalies over East Asia during EP El Niño, which would largely reduce the climatological East Asian trough along the eastern coast of China, resulting in a weakened EAWM. During the autumn and winter of CP El Niño, however, anomalous negative H500 dominates over the western Pacific and southeastern China (Figs. 9g,h), suggesting an intensified East Asian trough and hence a stronger EAWM.
To further demonstrate the influences of EP and CP El Niño on EAWM, we focus on the El Niño decaying spring as an example, and analyze the variations of air temperature over East Asia by comparing the composite patterns for EP and CP El Niño years. Based on one standard deviation of normalized DJF Niño-3 and DJF EMI, there are three EP El Niño events during 1979–2010 (1982, 1991, and 1997) and nine CP El Niño events (1979, 1986, 1990, 1991, 1992, 1994, 2002, 2004, and 2009). Note that during the winter of 1991/92, both Niño-3 index and EMI are above one standard deviation. It has been categorized as an EP El Niño in some studies (e.g., Feng and Li 2011) but as a CP El Niño in other studies (e.g., Ashok et al. 2007). Some studies even classified it as a mixed El Niño (e.g., Kug et al. 2009). Thus, the 1991/92 El Niño may have the characteristics of both EP and CP El Niño.
During the decaying spring of EP El Niño, positive temperature anomalies appear over most of East Asia, especially in northeast and northern China, southeastern China, Korea, Japan, the Indo-China Peninsula, and the northwestern Pacific (Fig. 11a). During the decaying spring of CP El Niño, however, temperature anomalies tend to be negative over most of the above regions (Fig. 11b). The composite maps are quite consistent with the partial correlations with DJF Niño-3 (Fig. 4d) and DJF EMI (Fig. 4i).
Composite patterns of 2-m air temperature anomalies for (a) EP El Niño (1982, 1991, and 1997) and (b) CP El Niño (1979, 1986, 1990, 1991, 1992, 1994, 2002, 2004, and 2009) during the decaying spring.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Composite patterns of 2-m air temperature anomalies for (a) EP El Niño (1982, 1991, and 1997) and (b) CP El Niño (1979, 1986, 1990, 1991, 1992, 1994, 2002, 2004, and 2009) during the decaying spring.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Composite patterns of 2-m air temperature anomalies for (a) EP El Niño (1982, 1991, and 1997) and (b) CP El Niño (1979, 1986, 1990, 1991, 1992, 1994, 2002, 2004, and 2009) during the decaying spring.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
At the lower troposphere, anomalous winds for EP El Niño show southerly and southwesterly components from the northern SCS along the eastern coast of China to southern Japan (Fig. 12a). The southwesterly anomalies are rightly located within the boxes for defining IChW, indicating a weakened winter monsoon circulation caused by EP El Niño. During the decaying spring of CP El Niño, however, an anomalous low-level anticyclone dominates over the SCS and the northern Philippines (Fig. 12b). Although southwesterly anomalies also cover southeastern China and southern Japan, they are not as strong as those for EP El Niño, and anomalous northeasterly flow can be identified over the southern box in the IChW definition, suggesting a relatively stronger winter monsoon induced by CP El Niño. The differences in low-level winds between CP and EP El Niño (Fig. 12c) further confirm that CP El Niño would increase the EAWM, while EP El Niño may decrease the winter monsoon.
Composite patterns of 850-hPa wind anomalies for (a) EP and (b) CP El Niño during the decaying spring. (c) The difference between (b) and (a). Black boxes are for the definition of EAWM index IChW.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Composite patterns of 850-hPa wind anomalies for (a) EP and (b) CP El Niño during the decaying spring. (c) The difference between (b) and (a). Black boxes are for the definition of EAWM index IChW.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
Composite patterns of 850-hPa wind anomalies for (a) EP and (b) CP El Niño during the decaying spring. (c) The difference between (b) and (a). Black boxes are for the definition of EAWM index IChW.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
During EP El Niño, positive H500 anomalies control northeast China, Korea, and Japan, while negative anomalies occur over the Ural and the northeastern Pacific during the decaying spring (Fig. 13a). Therefore, the climatological high ridge over the Urals, the trough over East Asia, and the ridge over the western coast of North America become weaker, leading to a weaker EAWM. The tilt of the East Asian trough is larger than normal (not shown), and the EAWM would prefer an eastern pathway (Wang et al. 2009). Therefore, less cold air would move southward into East Asia, leading to anomalous higher than normal temperature over most of east China and Japan (Fig. 11a). During CP El Niño, however, positive anomalies appear around the Urals and negative anomalies emerge over the Sea of Okhotsk, indicating an intensified ridge over the Urals and a deeper trough over East Asia (Fig. 13b). The tilt of the East Asian trough is smaller than normal (not shown), and the EAWM would prefer a southern pathway (Wang et al. 2009). This circulation pattern means an enhanced EAWM and is favorable for southward outbreaks of cold air into China. Therefore, temperature tends to be lower than normal over most of northern and eastern China (Fig. 11b). The composite circulation anomalies at both 850- and 500-hPa levels are consistent with the correlation maps with Niño-3 (Figs. 5d and 9d) and EMI (Figs. 5i and 9i), respectively, further confirming the previous result that EP (CP) El Niño would induce a weakened (strengthened) EAWM.
As in Fig. 11, but for 500-hPa geopotential height anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 11, but for 500-hPa geopotential height anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
As in Fig. 11, but for 500-hPa geopotential height anomalies.
Citation: Journal of Climate 25, 21; 10.1175/JCLI-D-11-00576.1
It should be pointed out that in the composite patterns shown in Figs. 11–13 there are nine CP El Niño events but only three EP El Niño events. To better balance these sample numbers, we randomly reduce the number of CP El Niño to six events (1979, 1990, 1991, 1994, 2004, and 2009). It is found that the composite patterns based on the new samples yield features similar to those shown in the figures.
5. Summary and discussion
In this study, we have first compared the seasonal SSTA variations from the developing summer to the decaying summer of eastern Pacific El Niño and central Pacific El Niño. Besides the different locations of maximum warming between EP and CP El Niño, SSTA over the tropical Indian Ocean responds differently to the two different types of El Niño. A positive IOD mode matures during EP El Niño developing autumn, and a warming IOBW mode peaks in the decaying spring. However, these features cannot be observed during the entire life cycle of CP El Niño.
We have also examined the seasonal variations of anomalous precipitation and temperature during the developing and decaying years of EP and CP El Niño. We have only focused on the interannual time scale and have not addressed interdecadal variability, which requires much longer data records. The EP (CP) type of El Niño is linked to an anomalous +/−/+ (−/+/−) rainfall pattern over East Asia and the equatorial Pacific. The nearly opposite rainfall patterns over the tropical Pacific between EP and CP El Niño are likely induced by the nearly reversed Walker circulation anomalies associated with the underlying SSTA distribution. A significant rainfall belt appears over southeastern China and the northwestern Pacific during EP El Niño years, as widely analyzed in many previous studies. However, it disappears and is replaced by near-normal or dry conditions when CP El Niño occurs. The low-level anomalous PSAC exists about two seasons later than that during EP El Niño, and its location moves more westward to the northern SCS. The anomalous anticyclone thereby dominates southeastern China, which is unfavorable for moisture transport from the western Pacific or the SCS to East Asia. Meanwhile, anomalous sinking motion, as part of the anomalous Walker circulation associated with CP El Niño, controls southeastern China from the developing summer to the next spring. Therefore, during CP El Niño, southeastern China and the northwestern Pacific tend to experience less precipitation. Because of a Rossby wave response to the suppressed convective heating around the Philippines, the anomalous anticyclone as well as the anomalous sinking motion over the southeastern East Asia should be promoted by both the anomalous warming over the central Pacific and the air–sea interaction over the northwestern Pacific (Wang et al. 2000).
Our analysis confirms that, during the developing summer, the impact of CP El Niño on East Asian climate seems to be more significant than that of EP El Niño, consistent with the conclusion by Weng et al. (2007). However, during the decaying summer, EP El Niño demonstrates more significant impact on East Asia than does CP El Niño. This is due probably to the capacitor effect of the long-lasting warming mode of the TIO. Several recent studies have proposed the crucial role of SSTA over the TIO, especially when El Niño events are decaying (Yoo et al. 2006; Yang et al. 2007; Yuan et al. 2008; Xie et al. 2009). Xie et al. (2009) defined the capacitor effect to mean that the warming over the TIO acts like a capacitor anchoring atmospheric anomalies over the Indo-western Pacific Oceans. The tropospheric temperature anomaly pattern resembles the Matsuno–Gill response to a localized heat source over the TIO, offering additional support for the capacitor mechanism in which the El Niño–induced TIO warming persists through the summer and sustains atmospheric anomalies after El Niño is terminated. In another paper, we discuss the respective effects of Indian Ocean SSTs on East Asian climate during the EP El Niño and CP El Niño (Yuan et al. 2012).
EP and CP El Niño events also exert different impacts on the air temperature over East Asia. During the developing autumn and winter and the decaying spring, eastern East Asia and the western Pacific tend to experience increased temperature during EP El Niño but decreased temperature during CP El Niño. One possible mechanism is the weakened EAWM with shallower East Asian trough at 500 hPa during EP El Niño, while intensified winter monsoon with deeper East Asian trough occurs during CP El Niño. Composite analyses of the recent EP and CP El Niño events during the decaying spring show consistent result with the partial correlation analysis, further confirming the above conclusions.
Partial correlations of H500 with Niño-3 and EMI indices also show remarkable differences over the tropics and the Pacific–North American (PNA) region. The primary difference between the left and right panels of Fig. 9 is probably the much stronger influence of EP ENSO on the tropics. The large belt of significant positive correlations with Niño-3 index covers almost the entire tropics (left panels in Fig. 9), indicating a persistent increase in H500 over the tropics during the EP El Niño cycle. On the contrary, the correlations with EMI are much weaker, with significant positive correlations only over the tropical central Pacific during winter (Fig. 9h), and the next spring (Fig. 9i) and summer (Fig. 9j).
Both correlations with Niño-3 index and with EMI show a significant PNA pattern, especially during the mature phase (winter) of ENSO. However, the PNA pattern for EP ENSO seems to tilt more northeastward than that for CP ENSO. Positive anomalies of H500 cover the eastern North America during EP El Niño (Fig. 9c) but dominate the western North America during CP El Niño (Fig. 9h). Such distinct PNA pattern is linked to different precipitation anomalies over North America (e.g., Weng et al. 2009). In addition, the PNA pattern during EP El Niño seems more intense than that during CP El Niño. However, it appears much earlier for CP El Niño than for EP El Niño. By the developing summer of CP El Niño, the PNA pattern has exhibited significant signals, with negative H500 anomalies over the northern central Pacific and positive H500 anomalies over the northwestern North America (Fig. 9f). In contrast, the PNA pattern does not become evident until the developing autumn during EP El Niño (Fig. 9b). Nevertheless, it can last to the decaying spring for EP El Niño (Fig. 9d), but only to winter (one season earlier) for CP El Niño (Fig. 9h). Therefore, the impacts of EP and CP El Niño on North American climate are significantly different from the developing year to the decaying year.
It should be pointed out that we choose IChW to reflect the strength of EAWM in this study. Although the partial correlations of IChW in SON, DJF, and MAM1 with normalized DJF EMI are significant (Table 1), the related low-level wind anomalies, especially the meridional flow, are only significant in the southern box (10°–25°N, 110°–130°E) that is used in defining the index. We also calculate the correlations between the normalized DJF EMI and another winter monsoon index, which is defined as the 850-hPa meridional wind anomalies averaged over 20°–40°N, 100°–140°E (Yang et al. 2002). As expected, the correlations in SON, DJF, and MAM1 are only −0.15, −0.12, and −0.08, respectively, because the box used for this index is much more similar to the northern box of the IChW definition. This result further confirms that the most prominent impact of CP El Niño on EAWM may mainly occur to southern East Asia, due probably to the westward shift of the low-level anomalous PSAC during CP El Niño, as compared with the conditions during EP El Niño.
Previous studies have revealed a connection between the winter monsoon and the summer monsoon over East Asia. After a strong (weak) East Asian winter monsoon, the subtropical western Pacific high tends to shift northward (southward) in the next summer, causing a stronger (weaker) East Asian summer monsoon and less (more) summer precipitation over the Yangtze River and the Huaihe River valleys (Sun and Sun 1995; Chen and Sun 1999; Chen et al. 2000). The SST anomaly over the SCS and the TIO was a possible medium for climate anomalies persisting from winter to the next summer (Chen et al. 2000; Yan et al. 2011). The connection between winter monsoon and summer monsoon over East Asia associated with EP and CP El Niño also follows the above physical mechanism. During the winter of EP El Niño, the winter monsoon is weaker over East Asia. In the next summer, the southward subtropical western Pacific high (Fig. 9e) and heavy summer precipitation limited to the Yangtze River valley (Fig. 2e) indicate a weak and less extensive summer monsoon. Meanwhile, SSTs over the SCS and the TIO are warmer than normal from winter to the next summer (Figs. 4c–e). During the winter of CP El Niño, however, the East Asian winter monsoon is stronger, and the summer monsoon is also stronger in the next summer, with more northward subtropical western Pacific high (Fig. 9j) and heavy summer precipitation (Fig. 2j). In the meantime, the SST over the SCS is colder than normal from the developing autumn to the next spring (Figs. 4g–i).
Acknowledgments
The authors thank the three anonymous reviewers who provided constructive comments on an early version of the manuscript, which were helpful for improving the overall quality of the paper. This study was supported by the Young Scientists Fund of the National Natural Science Foundation of China (41005038), the Chinese Public Sector (Meteorology) Research and Special Project (GYHY200906016), the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2009BAC51B01), and the NOAA–China Meteorological Administration Bilateral Program.
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