1. Introduction
El Niño–Southern Oscillation (ENSO) is one of the most important factors that influence the world’s 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., 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).
During the latest years, many studies have examined the different impacts of the two types of El Niño on Atlantic hurricane frequency (e.g., Kim et al. 2009), western North Pacific tropical cyclone frequency (e.g., Chen and Tam 2010), and precipitation patterns over the western United States during boreal winter (e.g., Weng et al. 2009). Previous studies have also investigated their impacts on the austral spring and autumn rainfall in Australia (Wang and Hendon 2007; Taschetto and England 2009) and the variations of precipitation and temperature over East and Southeast Asia (Weng et al. 2007, 2009, 2011; Feng et al. 2011, 2010; Feng and Li 2011; Yuan and Yang 2012).
The low-level anticyclone near the Philippines, the Philippine Sea anticyclone (PSAC), has been considered to be an important atmospheric system in the interaction between ENSO and the East Asian monsoon (Wang et al. 2000; Wu et al. 2003; Lau and Nath 2006; Chou et al. 2009). The PSAC is excited by the descending Rossby waves associated with the El Niño–induced subsidence over the western Pacific and the Maritime Continent (Wang and Zhang 2002). It forms in fall, about one season prior to the peak of El Niño, and persists from the mature phase to the ensuing summer of El Niño. Its persistence enhances the western Pacific subtropical high and brings abundant precipitation to southeastern China (e.g., Zhang et al. 1999; Wang and Zhang 2002; Wang et al. 2008). However, during CP El Niño, the location of the maximum warm SST anomaly (SSTA) shifts more westward compared with that during EP El Niño. Through anomalous Walker circulation, the anomalous subsidence over the western Pacific induced by CP El Niño also shifts westward (Feng et al. 2011; Yuan and Yang 2012). Then, is there any difference in the evolution and the strength of PSAC between EP El Niño and CP El Niño? This is one of several questions that will be addressed in this study.
On the other hand, EP El Niño is suggested to have a strong connection with the SSTA of tropical Indian Ocean (IO), while CP El Niño appears to be more strongly related to the southern IO (Kao and Yu 2009). If this is true, what are the different impacts of IO SST on the East Asian climate during EP El Niño and CP El Niño? Over the tropical IO, the first empirical orthogonal function mode of monthly SST during the recent 60 years is the IO basin-wide (IOBW) mode, which accounts for nearly 43% of the total variance (e.g., Yuan 2008). This mode may be forced by ENSO-induced heat flux anomalies in most of the basin (Klein et al. 1999), and ocean Rossby waves in the tropical southern Indian Ocean (Xie et al. 2002), with warming (cooling) IOBW pattern lagging the mature phase of El Niño (La Niña) (e.g., Klein et al. 1999; Lau and Nath 2003; S. Yang et al. 2007). More importantly, the ENSO–IO coupling is suggested to be different between El Niño and La Niña events (Kug et al. 2006) and depends on the timing of El Niño onset (Sooraj et al. 2009). For the cases of early El Niño onset, SSTA is larger in the developing summer and thus it can affect the tropical IO through the changes in the northwestern Pacific summer monsoon and related cross-equatorial flow over Sumatra (Li et al. 2002, 2006), the Indian summer monsoon and the Somali jet (Li et al. 2003), the convection over the Maritime Continent, and the Walker circulation over the IO. Recent studies have emphasized the important influence of the IOBW mode on the Asian monsoon through both observational and modeling analyses (Annamalai et al. 2005a; Yoo et al. 2006; Li et al. 2008). Although the role of IOBW in monsoon variations is considered to be associated with ENSO, the IOBW is proposed as an important “capacitor” during the decaying phases of ENSO, through modulating the convective heating and the Walker circulation, or inducing an eastward-propagating Kelvin wave (e.g., Wu and Kirtman 2004; Annamalai et al. 2005b; Kug et al. 2006; J. Yang et al. 2007; Yuan et al. 2008a; Xie et al. 2009; Wu et al. 2009, 2010). From a series of experiments with a moist linear model, Watanabe and Jin (2002) also found that modest IO warming, in addition to the strong warming in the central-eastern Pacific and the weak cooling in the western Pacific, was significant to strengthen the PSAC and it was one of the important factors to improve the predictability of East Asian climate during ENSO.
Besides the IOBW mode, another important SST mode over the tropical IO is the Indian Ocean dipole (IOD; Saji et al. 1999; Webster et al. 1999). It is characterized by opposite SSTAs in the western IO and the eastern IO, and shows strong phase-locking characteristics during summer and autumn. In the past decade, numerous studies have been conducted to understand its potential effects on the Asian monsoon (e.g., Li and Mu 2001; Behera and Yamagata 2003; Guan et al. 2003; Yuan et al. 2008b), the European and North American climate (Saji and Yamagata 2003), and the SSTA in the equatorial Pacific (Chang and Li 2001; Li et al. 2006). The IOD is strongly correlated with ENSO, especially after the 1970s (Yuan and Li 2008). It would influence the development of El Niño/La Niña mainly through modulating the strength of the Walker circulation (e.g., Yuan 2008; Izumo et al. 2010). Therefore, more questions shall be addressed in this study. 1) What are the different responses of IO SST to EP El Niño and CP El Niño? 2) What are the different impacts of IO SST on PSAC development during EP El Niño and CP El Niño? 3) What are the possible physical mechanisms responsible for these SST impacts?
In section 2, we describe the data and methods applied in this study. Responses of IO SST to EP El Niño and CP El Niño are examined in section 3. We analyze the different evolutions of the low-level anticyclone over the tropical northwestern Pacific during EP El Niño and CP El Niño in section 4, and further investigate the possible impacts of IO SST on the anticyclone development during EP El Niño and CP El Niño in section 5. Finally, section 6 provides a summary and a further discussion.
2. Data and methods
The primary data used in this study are from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (Kalnay et al. 1996), including 850-hPa zonal and meridional winds, 500-hPa vertical p velocity, and sea level pressure (SLP). Monthly SST data from the Hadley Centre Global Sea Ice and Sea Surface Temperature dataset (HadISST; Rayner et al. 2003) is also applied. These datasets have long records for at least 50 years. However, we only focus on the time period from 1979 to 2010 for the following reasons: 1) the quality of the NCEP–NCAR reanalysis over some Asian regions prior to 1968 may be questionable (Yang et al. 2002; Wu et al. 2005), 2) the influence of ENSO on the East Asian monsoon has exhibited a decadal shift since the late 1970s (Chang et al. 2000; Kinter et al. 2002; Miyakoda et al. 2003; Zhou et al. 2006, 2007; Wang et al. 2008; Xie et al. 2010), and 3) CP El Niño has been rarely observed before the 1980s but has occurred more frequently in the past three decades (Kao and Yu 2009; Yeh et al. 2009; Lee and McPhaden 2010). Accordingly, anomalies of all variables are obtained by removing the means of 1979–2010.
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 SST averaged over the Niño-3 region (5°S–5°N, 150°–90°W, as marked in Fig. 1c). The 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 SST averaged 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) regions, respectively (Ashok et al. 2007; Weng et al. 2007, 2009). These regions are also marked in Fig. 1h. The correlation between EMI and CP index (CPI; defined by Kao and Yu 2009) is 0.93, significantly exceeding the 99% confidence level by a two-tailed Student’s t test. Thus, EMI is considered to be an appropriate index to represent the CP El Niño events (Ashok et al. 2007).
Partial correlations of seasonal SST (shadings) and 850-hPa winds (vectors) with normalized (a)–(e) DJF Niño-3 and (f)–(j) DJF EMI: (a),(f) summer; (b),(g) autumn; (c),(h) winter; (d),(i) the next spring; and (e),(j) the next summer. Shadings indicate correlations above the 95% and 99% confidence levels, and only the vectors that are significantly above the 95% confidence level (either zonal or meridional) are shown. The “C” (“A”) is for anomalous cyclone (anticyclone) and black boxes in (b),(c),(d), and (h) are for definitions of IOD, Niño-3, IOBW index, and EMI, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of seasonal SST (shadings) and 850-hPa winds (vectors) with normalized (a)–(e) DJF Niño-3 and (f)–(j) DJF EMI: (a),(f) summer; (b),(g) autumn; (c),(h) winter; (d),(i) the next spring; and (e),(j) the next summer. Shadings indicate correlations above the 95% and 99% confidence levels, and only the vectors that are significantly above the 95% confidence level (either zonal or meridional) are shown. The “C” (“A”) is for anomalous cyclone (anticyclone) and black boxes in (b),(c),(d), and (h) are for definitions of IOD, Niño-3, IOBW index, and EMI, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of seasonal SST (shadings) and 850-hPa winds (vectors) with normalized (a)–(e) DJF Niño-3 and (f)–(j) DJF EMI: (a),(f) summer; (b),(g) autumn; (c),(h) winter; (d),(i) the next spring; and (e),(j) the next summer. Shadings indicate correlations above the 95% and 99% confidence levels, and only the vectors that are significantly above the 95% confidence level (either zonal or meridional) are shown. The “C” (“A”) is for anomalous cyclone (anticyclone) and black boxes in (b),(c),(d), and (h) are for definitions of IOD, Niño-3, IOBW index, and EMI, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
In addition, the IOD index is defined as the SST difference between the western (10°S–10°N, 50°–70°E) and the eastern (10°S–0°, 90°–110°E, as marked in Fig. 1b) tropical IO (Saji et al. 1999). We also define an IOBW index as the SSTA averaged within 20°S–20°N, 40°–110°E (as marked in Fig. 1d) to describe the basin-wide SST mode over the tropical IO. Because of the phase-locking feature of all these phenomena, seasonal mean indices are used in the paper: December–January–February (DJF) Niño-3, DJF EMI, August–September–October (ASO) IOD, and March–April–May (MAM) IOBW.
Considering the significant correlation between DJF Niño-3 and DJF EMI (R = 0.39, above the 95% confidence level), partial correlation analysis 
All statistical significance tests for correlation analysis are performed using the two-tailed Student’s t test. For most correlations, the degree of freedom is 30 for a time series of 32 seasons (1979–2010). The correlation coefficients at the confidence levels of 90%, 95%, and 99% are 0.30, 0.35, and 0.45, respectively. However, for the monthly lag correlation between two indices, the effective number of the degree of freedom should be calculated differently because of the possible autocorrelation of each time series. We use the equation 
3. Responses of Indian Ocean SST to EP El Niño and CP El Niño
Figure 1 presents the seasonal SST variations (shading) for EP El Niño and CP El Niño, from the El Niño developing summer [June–July–August (JJA)] to the decaying summer (JJA1). The two types of El Niño show different spatial distributions. For EP El Niño, maximum warming is mainly confined to the equatorial eastern Pacific, mostly covering the Niño-3 and Niño-1–2 regions (Figs. 1a–e). For CP El Niño, however, a maximum warm center dominates persistently over the central equatorial Pacific (near the Niño-4 or Niño-3.4 region) from JJA to JJA1, with negative SSTA in the western Pacific and the southeastern Pacific (near the Niño-1–2 regions), respectively (Figs. 1f–j).
More importantly, the IO SST shows different features between EP El Niño and CP El Niño. During the developing phase of EP El Niño, a positive IOD mode, with warm anomalies in the western and cold anomalies in the southeastern tropical IO, appears in summer (Fig. 1a) and reaches its maximum in fall [September–October–November (SON); Fig. 1b]. During the mature phase (DJF) of EP El Niño, the IOD dissipates. Meanwhile, another IO SST mode, the IOBW, begins to develop (Fig. 1c). It reaches the strongest in the El Niño decaying spring (Fig. 1d) and weakens in summer (Fig. 1e).
Nevertheless, from the developing summer to the decaying summer of CP El Niño, the SST over tropical IO shows negligible correlation with the DJF EMI (Figs. 1f–j). Neither IOD nor IOBW can be discerned during the entire cycle of CP El Niño. While in the decaying spring (MAM1), a subtropical southern dipole mode (e.g., Behera and Yamagata 2001; Qian et al. 2002) appears, with weak warm anomalies in the southern central IO and cold anomalies in the southeastern IO (Fig. 1i). This SST mode becomes more evident in the ensuing summer of CP El Niño (Fig. 1j), supporting the previous suggestion that CP El Niño appears to be more strongly related with the southern IO (Kao and Yu 2009). However, because of the weakness of this subtropical SST mode, and because its development occurs in the decaying phase of CP El Niño, we will not emphasize its relationship with the East Asian monsoon circulation.
To better illustrate the different correlations of the two IO SST modes with EP El Niño and CP El Niño, we demonstrate the monthly lag correlations between IOD and Niño-3, IOD and EMI, IOBW and Niño-3, and IOBW and EMI, respectively (Fig. 2). The IOD leads EP El Niño for about 2–3 months, with significant correlation above the 95% confidence level occurring about 5 months earlier than the peak of EP El Niño. The highest correlation above 0.4 appears about 2 months before the peak phase (Fig. 2a). In contrast, the IOBW mode lags EP El Niño, with the highest correlation above 95% confidence level appearing about 4 months later than the peak of EP El Niño (Fig. 2b). For CP El Niño, however, neither of the two IO SST modes exhibits a significant correlation with the EMI from the leading 12 months to the lagging 12 months (Figs. 2c,d).
Lag correlations: (a) between Niño-3 and IOD, (b) between Niño-3 and IOBW, (c) between EMI and IOD, and (d) between EMI and IOBW. The “0” in the x axis means simultaneous correlation, and “−n” (“+n”) indicates that IOD or IOBW leads (lags) Niño-3 or EMI for n months. The black long-dashed line is for the 95% confidence level.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Lag correlations: (a) between Niño-3 and IOD, (b) between Niño-3 and IOBW, (c) between EMI and IOD, and (d) between EMI and IOBW. The “0” in the x axis means simultaneous correlation, and “−n” (“+n”) indicates that IOD or IOBW leads (lags) Niño-3 or EMI for n months. The black long-dashed line is for the 95% confidence level.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Lag correlations: (a) between Niño-3 and IOD, (b) between Niño-3 and IOBW, (c) between EMI and IOD, and (d) between EMI and IOBW. The “0” in the x axis means simultaneous correlation, and “−n” (“+n”) indicates that IOD or IOBW leads (lags) Niño-3 or EMI for n months. The black long-dashed line is for the 95% confidence level.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Thus, the lag correlation analysis further confirms that the IO SST is more strongly related to EP El Niño than to CP El Niño. Positive IOD tends to occur during the developing year of EP El Niño, and warming IOBW pattern happens during the EP El Niño decaying year. However, none of the two SST modes can be identified when CP El Niño develops over the equatorial central Pacific.
4. Different evolutions of PSAC during EP El Niño and CP El Niño
a. Low-level circulation
Figure 1 also shows the 850-hPa winds that are significantly correlated with EP El Niño and CP El Niño, respectively. One of the most important features is the anomalous zonal wind along the equatorial Pacific. Westerly anomalies associated with EP El Niño dominate over most of the equatorial central-eastern Pacific from the developing autumn to the decaying spring (Figs. 1b–d). During CP El Niño, however, westerly anomalies prevail over the equatorial western Pacific persistently from the developing summer to the decaying summer (Figs. 1f–j). While over the equatorial eastern Pacific, easterly anomalies are significant, especially during winter (Fig. 1h) and the next spring (Fig. 1i). These low-level zonal wind anomalies along the equatorial Pacific are respectively consistent with the underlying EP and CP types of SSTA.
Another prominent feature is the different responses of the low-level circulation over the tropical IO to the different types of El Niño. During EP El Niño, significant southeasterly anomalies begin to develop over the southeastern IO in the developing summer (Fig. 1a) and become more significant in autumn (Fig. 1b). Meanwhile, easterly anomalies prevail over the central northern IO. Such an anomalous circulation pattern is consistent with the underlying positive IOD mode, with anomalous warm (cold) SSTA over the western (southeastern) tropical IO. During EP El Niño winter, easterly anomalies cover the equatorial IO, with an anomalous anticyclone to each side of the equator (Fig. 1c). In the next spring, the anomalous anticyclone over the northern IO moves eastward to the Indo-China peninsula, and seems to be combined with the low-level anomalous circulation over the western equatorial Pacific (Fig. 1d). During CP El Niño, however, no significant circulation anomaly can be observed over IO (right panels in Fig. 1), except for some features in the decaying spring (Fig. 1i).
Therefore, considering the different responses of IO SST to EP El Niño and CP El Niño described in section 3, these results further confirm that EP El Niño is significantly correlated with not only the SST but also the atmospheric circulation over the tropical IO. However, no significant correlation can be found during CP El Niño.
Important features are also found in the location and the evolution of PSAC, which is considered to be an essential system conveying El Niño impact to East Asia (e.g., Zhang et al. 1999; Wang et al. 2000; Wang and Zhang 2002). During the developing summer of EP El Niño, the PSAC is established over the southern South China Sea (SCS; Fig. 1a). It moves to the Philippines in autumn (Fig. 1b), intensifies and moves to the northeast of the Philippines in winter (Fig. 1c), and persists to the following spring (Fig. 1d) and summer (Fig. 1e).
During CP El Niño, however, the anomalous low-level anticyclone shows different evolution features. In the developing summer (Fig. 1f) and autumn (Fig. 1g), an anomalous low-level cyclone, instead of anticyclone, occupies the western Pacific to the east of the Philippines. Anticyclonic circulation does not appear until winter (Fig. 1h), about two seasons later than the PSAC during EP El Niño. More importantly, it is located over the SCS, instead of the east of the Philippines as in the winter of EP El Niño. The anticyclone stays over the SCS and intensifies in the ensuing spring (Fig. 1i), but decreases and moves northward to southern Japan in the decaying summer (Fig. 1j). In short, the anticyclone during CP El Niño is weaker and has a shorter lifetime than that during EP El Niño.
In the following sections, we further investigate the evolution of anticyclonic pattern and the possible effect of IO SST.
b. SLP
To better illustrate the different evolution features of the low-level anticyclone between EP El Niño and CP El Niño, we analyze SLP field to reflect the strength of the anticyclone as referred to Wang and Zhang (2002). During the developing summer of EP El Niño, significant positive anomalies of SLP dominate over the eastern IO and the Maritime Continent (Fig. 3a), which increase and extend to the equatorial western Pacific in autumn (Fig. 3b). The maximum positive anomalies move more eastward to the east of the Philippines during the mature phase of EP El Niño (Fig. 3c), persist to the next spring (Fig. 3d), and decrease in summer (Fig. 3e). The monthly correlation averaged in 5°–20°N demonstrates the SLP variation associated with EP El Niño more clearly (Fig. 4a). Significant positive anomalies of SLP first exist over the equatorial northern IO in early spring. They intensify and gradually move eastward during summer and autumn. Maximum SLP anomalies persist over the equatorial western Pacific, mostly to the east of 130°E, in winter and the next spring, and then decay in late summer. As indicated by the black arrow in Fig. 4a, the PSAC moves eastward from IO to the western Pacific during EP El Niño. These features are very similar to those shown in the Fig. 2 in Wang and Zhang (2002), which is a composite map of six major El Niño events during the past 50 years.
Partial correlations of SLP with normalized (a)–(e) DJF Niño-3 and (f)–(j) DJF EMI. Only the values above the 90% confidence level are shown. Black boxes are for definitions of the low-level anticyclone for (a)–(e) EP El Niño and(f)–(j) CP El Niño, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of SLP with normalized (a)–(e) DJF Niño-3 and (f)–(j) DJF EMI. Only the values above the 90% confidence level are shown. Black boxes are for definitions of the low-level anticyclone for (a)–(e) EP El Niño and(f)–(j) CP El Niño, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of SLP with normalized (a)–(e) DJF Niño-3 and (f)–(j) DJF EMI. Only the values above the 90% confidence level are shown. Black boxes are for definitions of the low-level anticyclone for (a)–(e) EP El Niño and(f)–(j) CP El Niño, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Time–longitude profile of partial correlations of SLP with normalized (a) DJF Niño-3 and (b) DJF EMI. Correlations are averaged in 5°–20°N, and vary from January in El Niño developing year (“0”) to December in the decaying year (“1”). Shadings indicate the significant correlations above the 90%, 95%, and 99% confidence levels.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Time–longitude profile of partial correlations of SLP with normalized (a) DJF Niño-3 and (b) DJF EMI. Correlations are averaged in 5°–20°N, and vary from January in El Niño developing year (“0”) to December in the decaying year (“1”). Shadings indicate the significant correlations above the 90%, 95%, and 99% confidence levels.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Time–longitude profile of partial correlations of SLP with normalized (a) DJF Niño-3 and (b) DJF EMI. Correlations are averaged in 5°–20°N, and vary from January in El Niño developing year (“0”) to December in the decaying year (“1”). Shadings indicate the significant correlations above the 90%, 95%, and 99% confidence levels.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
During the developing summer of CP El Niño, however, significant negative anomalies, instead of positive anomalies, of SLP are identified from the equatorial western Pacific to the east of the Philippines (Fig. 3f). They become more intense in autumn (Fig. 3g) and move to the central Pacific in winter (Fig. 3h). Significant positive SLP anomalies develop around the Philippines in winter (Fig. 3h), increase in the following spring (Fig. 3i), but dissipate in summer (Fig. 3j). The maximum positive SLP anomalies persistently dominate from the SCS to the west of the Philippines during winter and the next spring. However, the intensity is much weaker than that during EP El Niño. The time–longitude profile of the monthly correlation averaged in 5°–20°N confirms the distinct variation of SLP during CP El Niño (Fig. 4b). Unlike the eastward movement of the positive SLP anomalies during EP El Niño, the location of SLP anomalies during CP El Niño mostly remains unchanged, with maximum anomalies over the regions from SCS to the west of 130°E. The significant correlation of EMI with SLP is consistent with the correlation of EMI with the low-level circulation over SCS and the western Pacific. The result further substantiates the different characteristics of the anticyclone during CP El Niño: a shorter lifetime, weaker strength, more westward location, and lack of eastward movement, compared to those during EP El Niño.
Note that the eastward movement of the low-level anticyclone has been discussed in previous studies (e.g., Chou 2004; Chen et al. 2007; Wu et al. 2009). However, this analysis suggests that the eastward movement of the anticyclone around the Philippines exists during EP El Niño but disappears during CP El Niño.
Considering the different locations of maximum correlation of SLP with Niño-3 index and EMI, we define two anticyclone indices: one is the partial correlation of SLP with the Niño-3 index averaged in the western Pacific (5°–20°N, 130°E–180°; as marked in Figs. 3c–e), and the other is the partial correlation of SLP with EMI averaged around the SCS (5°–20°N, 100°–130°E; marked in Figs. 3h,i). It can be inferred from Fig. 5a that the PSAC excited by EP El Niño (solid line) develops around November, becomes stronger during winter and the next spring, and then begins to decay in late spring. It shows another weak increase in the next summer. The characteristics of a sudden establishment and a quick decay of the PSAC described by Wang and Zhang (2002) can be identified clearly during EP El Niño.
In contrast, the anticyclone around the SCS induced by CP El Niño (long-dashed line in Fig. 5a) establishes in early winter, intensifies in late winter and the following spring, and then dissipates in summer. It is much weaker, has a shorter life cycle, and does not show a sudden establishment or a second increase as seen during EP El Niño.
(a) Monthly variation of partial correlations of SLP with normalized DJF Niño-3 index average in 5°–20°N, 130°E–180° (solid line with closed circles), and with DJF EMI averaged in 5°–20°N, 100°–130°E (long-dashed line with open circles). (b) The black solid line with closed circles is the same as the one in (a). The blue (red) line is the same as the black solid line, but with ASO IOD (MAM1 IOBW) signal removed. Dots are for the zero line and the 95% confidence level, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
(a) Monthly variation of partial correlations of SLP with normalized DJF Niño-3 index average in 5°–20°N, 130°E–180° (solid line with closed circles), and with DJF EMI averaged in 5°–20°N, 100°–130°E (long-dashed line with open circles). (b) The black solid line with closed circles is the same as the one in (a). The blue (red) line is the same as the black solid line, but with ASO IOD (MAM1 IOBW) signal removed. Dots are for the zero line and the 95% confidence level, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
(a) Monthly variation of partial correlations of SLP with normalized DJF Niño-3 index average in 5°–20°N, 130°E–180° (solid line with closed circles), and with DJF EMI averaged in 5°–20°N, 100°–130°E (long-dashed line with open circles). (b) The black solid line with closed circles is the same as the one in (a). The blue (red) line is the same as the black solid line, but with ASO IOD (MAM1 IOBW) signal removed. Dots are for the zero line and the 95% confidence level, respectively.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
5. Possible effects of Indian Ocean SST on PSAC development
Previous studies have emphasized the eastward movement of the PSAC from northern Indian Ocean during El Niño events, and suggested that its eastward movement is driven by an east–west asymmetry of moisture and temperature anomalies (Chou 2004), as well as the divergent flow and associated atmospheric vertical motion (Chen et al. 2007; Wu et al. 2009). Considering the significant correlation of SST and low-level circulation over the tropical IO with EP El Niño, in this section, we further explore the possible impact of IO SST on the development of PSAC during EP El Niño.
a. Possible effects of IOD and IOBW modes
We first remove the fall (ASO) IOD signal from the SLP data (method described in section 2) during the El Niño developing summer, autumn, and winter, and then recalculate the partial correlation with Niño-3 index and EMI, respectively. Comparison of Figs. 6a–c with Figs. 3a–c indicates that the original positive correlation between SLP and Niño-3 covers relatively larger areas, mainly from the eastern IO through the Maritime Continent to the equatorial western Pacific, with the most significant correlation above 0.7 (Figs. 3a–c). After the IOD signal is removed, however, the correlation largely decreases from the eastern IO to the western Pacific. Significant changes can be identified especially during summer (Fig. 6a) and autumn (Fig. 6b), but not in winter (Fig. 6c). This feature is due probably to the phase-locking characteristic of the IOD mode, which usually develops in summer and autumn but decays in winter. The result suggests a possible effect of the IOD mode on the early development of PSAC during EP El Niño.
(a)–(c) As in Figs. 3a–c, but with ASO IOD signal being removed from the SLP data. (d)–(f) As in Figs. 3c–e, but with MAM1 IOBW signal being removed from SLP. Black boxes are for definitions of the PSAC during EP El Niño.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
(a)–(c) As in Figs. 3a–c, but with ASO IOD signal being removed from the SLP data. (d)–(f) As in Figs. 3c–e, but with MAM1 IOBW signal being removed from SLP. Black boxes are for definitions of the PSAC during EP El Niño.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
(a)–(c) As in Figs. 3a–c, but with ASO IOD signal being removed from the SLP data. (d)–(f) As in Figs. 3c–e, but with MAM1 IOBW signal being removed from SLP. Black boxes are for definitions of the PSAC during EP El Niño.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Similarly, we remove the spring (MAM) IOBW signal from the SLP anomalies in winter and the decaying spring and summer of El Niño, and then recalculate the partial correlation with Niño-3 index and EMI, respectively. It can be discerned by comparing Figs. 6d–f with Figs. 3c–e that when the IOBW signal is removed, the originally significant correlation over the northern IO and South Asia in winter and the next spring almost disappears. The correlation over the equatorial western Pacific is also reduced drastically (Figs. 6d,e), especially within the domain for defining the PSAC index during EP El Niño (black box). In summer, there is no significant correlation between SLP and the Niño-3 index (Fig. 6f), indicating that the IO basin-wide warming SST after the peak of EP El Niño may play an important role in the large persistence of PSAC during the El Niño decaying year.
However, partial correlation between the residual SLP (with IOD and IOBW signals removed) and EMI shows no evident feature (figure not shown). Since the correlation of CP El Niño with the SST and low-level atmospheric circulation over the tropical IO is negligible, we may claim that the evolution of the low-level anticyclone during CP El Niño is not strongly influenced by the IO SST.
To further illustrate the above features, we remove IOD and IOBW signals, respectively, from the PSAC index for the entire cycle from the developing year to the decaying year of EP El Niño (Fig. 5b). After removing the IOD signal, the strength of PSAC is largely reduced during the early development stage (blue line), with its establishment (measured by the correlation that is significantly above the 95% confidence level) about two months later than the usual condition shown by the original data. However, during the peak and decaying phases of PSAC, no significant variation can be observed, due to the phase-locking feature of IOD in summer and autumn. When the IOBW signal is removed, however, a significant decrease occurs in the mature and decaying phases of PSAC (red line). The PSAC weakens dramatically in early spring, about three months earlier than the usual condition. More importantly, the previously mentioned second small increase of PSAC during the decaying summer of EP El Niño (black line) also disappears when the IOBW signal is removed. During the developing year, however, no significant difference can be found, because the IOBW mode mainly develops during the decaying year of EP El Niño.
From the above discussion, we may conclude that the IO SST may exert an impact on the evolution of PSAC during EP El Niño (but not during CP El Niño), with the IOD mode influencing the early development of PSAC during the El Niño onset year and the IOBW mode influencing the intensity and persistence of PSAC during the El Niño decaying year. Then, how does the IO SST influence the evolution of PSAC during EP El Niño?
b. Possible dynamics
We further discuss the respective influences of IOD and IOBW modes on atmospheric circulation by separating the SST modes from EP El Niño based on partial correlation. Since the IOD usually occurs during the late summer and autumn of the EP El Niño developing year and the IOBW matures in the El Niño decaying spring, we calculate the partial correlations of seasonal SST in JJA and SON with DJF Niño-3 and ASO IOD, and the partial correlations of seasonal SST in DJF, MAM1, and JJA1 with DJF Niño-3 and MAM1 IOBW. As a result, the IOD and IOBW SST modes are successfully separated from EP El Niño (Fig. 7).
Partial correlations of seasonal SST with normalized (a),(b) DJF Niño-3 and (f),(g) ASO IOD and with normalized (c),(e) DJF Niño-3 and (h),(j) MAM1 IOBW: (a),(f) summer; (b),(g) autumn; (c),(h) winter; (d),(i) the next spring; and (e),(j) the next summer. Shadings indicate the significant correlations above the 95% and 99% confidence levels.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of seasonal SST with normalized (a),(b) DJF Niño-3 and (f),(g) ASO IOD and with normalized (c),(e) DJF Niño-3 and (h),(j) MAM1 IOBW: (a),(f) summer; (b),(g) autumn; (c),(h) winter; (d),(i) the next spring; and (e),(j) the next summer. Shadings indicate the significant correlations above the 95% and 99% confidence levels.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of seasonal SST with normalized (a),(b) DJF Niño-3 and (f),(g) ASO IOD and with normalized (c),(e) DJF Niño-3 and (h),(j) MAM1 IOBW: (a),(f) summer; (b),(g) autumn; (c),(h) winter; (d),(i) the next spring; and (e),(j) the next summer. Shadings indicate the significant correlations above the 95% and 99% confidence levels.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
The left panels in Fig. 7 show a typical development of EP El Niño, with maximum warming over the equatorial eastern Pacific and anomalous cooling to its west, forming a horseshoe pattern. The overall feature of the development of seasonal SSTA over the Pacific is very similar to that in the left panels of Fig. 1. However, a prominent difference is observed in the IO SST. Unlike the left panels of Fig. 1, no positive IOD mode exists during the developing year of EP El Niño, and no warming IOBW mode can be found during the decaying year of EP El Niño (left panels in Fig. 7). Therefore, both the IOD and IOBW modes are extracted from the life cycle of EP El Niño.
On the other hand, the partial correlation of seasonal SST with the IOD index exhibits a positive IOD pattern, with anomalous warm (cold) values in the western (southeastern) tropical IO, which develops in summer (Fig. 7f) and matures in autumn (Fig. 7g). The partial correlation of seasonal SST with the IOBW index shows a warming IOBW mode that develops in winter (Fig. 7h), intensifies in the following spring (Fig. 7i), and persists in summer (Fig. 7j). The SST evolution is also very similar to that in the left panels of Fig. 1, but without the coexistence of equatorial central-eastern Pacific signals during EP El Niño. Therefore, partial correlation seems to be a good method to separate the IO SST modes from EP El Niño.
It is also worth mentioning that during the development of the warming IOBW mode from winter to the ensuing summer (Figs. 7h–j), significant warming extends to the northwestern and southwestern Pacific as well, forming a horseshoe pattern that is similar to the pattern of negative SSTA over the equatorial western Pacific during EP El Niño (left panels in Fig. 7). However, compared with the left panels of Fig. 1, the extent of the warming over the northwestern and southwestern Pacific is overestimated. In the method of partial correlation, when the correlation between SST and the Niño-3 index is removed, the negative SSTA over the tropical western Pacific during EP El Niño may be overturned because of its significant negative correlation with the Niño-3 index. Therefore, excessive warming tends to dominate over the northwestern and southwestern Pacific, accompanied by the warming IOBW mode when EP El Niño is excluded (Figs. 7h–j).
As proposed in previous studies, the anomalous low-level PSAC is instigated by the El Niño–induced Indonesian subsidence that generates a low-level anticyclone over South Asia as a response to the descending Rossby waves (Wang and Zhang 2002). Its persistence is attributed to the positive thermodynamic feedback between the atmospheric descending Rossby waves and the underlying cooling over the equatorial western Pacific (Wang et al. 2000). Here, we further focus on the anomalous 500-hPa vertical p velocity (ω500) and 850-hPa streamfunction (Ψ850) and investigate the dynamics of the possible impact of IO SST on PSAC. The partial correlations of ω500 (shading) and Ψ850 (contour) with ASO IOD and DJF Niño-3 indices are shown in the left and right panels of Fig. 8, respectively. During the mature phase of positive IOD, significant anomalous sinking motion prevails over the southeastern tropical IO and anomalous rising motion dominates over the equatorial western IO from August to November (Figs. 8a–d), consistent with the underlying warming in the western IO and the cooling in the eastern IO (Figs. 7f,g). Meanwhile, there are significant negative anomalies of Ψ850 over the Bay of Bengal and southern SCS and positive anomalies of Ψ850 over the southern-central IO. These features indicate two anomalous anticyclones symmetrically located to both sides of the equator, excited by the anomalous subsidence over the eastern IO through anomalous descending Rossby waves. The anomalous anticyclone to the north of the equator exists over South Asia as early as May, accompanied by anomalous sinking motion over the eastern IO (figure not shown). However, the one to the south of the equator does not fully develop until August. The two anticyclones intensify with the maturing positive IOD in September (Fig. 8b) and October (Fig. 8c). They are inspired by the IOD mode, without much effect from El Niño in the Pacific. In November, the two anomalous anticyclones weaken, with another anomalous anticyclone developing over the equatorial western Pacific (Fig. 8d). Therefore, the anomalous anticyclone over the northern IO excited by positive IOD may be recognized as a precursory signal of the PSAC during EP El Niño.
Partial correlations of 500-hPa vertical p velocity (shadings) and 850-hPa streamfunction (contours) with normalized (a)–(d) ASO IOD and (e)–(h) DJF Niño-3: (a),(e) August; (b),(f) September; (c),(g) October; and (d),(h) November. Shadings indicate the significant correlations above the 90%, 95%, and 99% confidence levels, and only the contours that are significantly above the 90% confidence level are shown.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of 500-hPa vertical p velocity (shadings) and 850-hPa streamfunction (contours) with normalized (a)–(d) ASO IOD and (e)–(h) DJF Niño-3: (a),(e) August; (b),(f) September; (c),(g) October; and (d),(h) November. Shadings indicate the significant correlations above the 90%, 95%, and 99% confidence levels, and only the contours that are significantly above the 90% confidence level are shown.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
Partial correlations of 500-hPa vertical p velocity (shadings) and 850-hPa streamfunction (contours) with normalized (a)–(d) ASO IOD and (e)–(h) DJF Niño-3: (a),(e) August; (b),(f) September; (c),(g) October; and (d),(h) November. Shadings indicate the significant correlations above the 90%, 95%, and 99% confidence levels, and only the contours that are significantly above the 90% confidence level are shown.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
The partial correlations of ω500 and Ψ850 with the Niño-3 index show more significant signals over the Pacific (right panels in Fig. 8). During the developing phase of EP El Niño, there is anomalous rising motion over the equatorial eastern Pacific and sinking motion over the Maritime Continent. Correspondingly, two anomalous low-level cyclones (positive correlation of Ψ850 in the north and negative correlation of Ψ850 in the south) dominate over the northern Pacific and the southern Pacific, reflecting a Rossby wave response to the anomalous warming in the equatorial eastern Pacific. Anomalous cyclones are also identified over South and East Asia in August (Fig. 8e) and over the northern central IO in September (Fig. 8f). In October, however, two anomalous anticyclones exist in South Asia and the southern IO (Fig. 8g). The northern one moves to the southern SCS in November (Fig. 8h) and intensifies in December (figure not shown). Therefore, without the effect of IO SST, PSAC forms in late autumn during the developing year of EP El Niño, about one season later than the development revealed in the original data (Fig. 1a).
Figure 9 shows the partial correlations of ω500 (shading) and Ψ850 (contour) with MAM1 IOBW and DJF Niño-3 for the decaying year of EP El Niño. Significant negative anomalies of Ψ850 with IOBW appear over the northwestern Pacific and positive anomalies dominate over the southern central Pacific from January to August (contours in Figs. 9a–d), implying two anomalous anticyclones to both sides of the equator. It is therefore inferred that even without the effect of El Niño, the warming IOBW mode is still able to maintain the anomalous anticyclone to the east of the Philippines. The PSAC is sustained through an anomalous Kelvin wave response from the effect of the IOBW warming mode (Xie et al. 2009; Wu et al. 2009, 2010).
As in Fig. 8, but for the partial correlations with (a)–(d) MAM1 IOBW and (e)–(h) DJF Niño-3 during El Niño decaying year: (a),(e) averages in January and February; (b),(f) averages in March and April; (c),(g) averages in May and June; and (d),(h) averages in July and August.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
As in Fig. 8, but for the partial correlations with (a)–(d) MAM1 IOBW and (e)–(h) DJF Niño-3 during El Niño decaying year: (a),(e) averages in January and February; (b),(f) averages in March and April; (c),(g) averages in May and June; and (d),(h) averages in July and August.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
As in Fig. 8, but for the partial correlations with (a)–(d) MAM1 IOBW and (e)–(h) DJF Niño-3 during El Niño decaying year: (a),(e) averages in January and February; (b),(f) averages in March and April; (c),(g) averages in May and June; and (d),(h) averages in July and August.
Citation: Journal of Climate 25, 22; 10.1175/JCLI-D-12-00004.1
The partial correlations of ω500 and Ψ850 with the Niño-3 index in the decaying year of EP El Niño show an anomalous cyclone over the Pacific to each side of the equator, accompanied by anomalous rising (sinking) motion over the equatorial eastern (western) Pacific (Figs. 9e–h). This pattern of Rossby wave response to EP El Niño can last to the early summer of the decaying year, but it is much weaker compared to that during the developing year (right panels in Fig. 8). The PSAC can only be observed in January and February, as indicated by the significant negative anomalies of Ψ850 over the northern IO and the SCS (Fig. 9e). Afterward, it seems to disappear (Figs. 9f–h). This feature is different from that showing the long-lasting PSAC during the decaying spring and summer of EP El Niño displayed in Figs. 1d,e and 3d,e. Thus, without the effect of warming IOBW mode, the PSAC may not be able to persist to the decaying spring and summer of EP El Niño, consistent with the previous work by Xie et al. (2009).
6. Summary and discussion
In this study, we have investigated the different evolution features of the anomalous low-level anticyclone over the tropical northwestern Pacific during EP El Niño and CP El Niño. We have particularly addressed the following three questions. 1) Is there any difference in the evolution and the strength of the low-level anticyclone during EP El Niño and CP El Niño? During EP El Niño, the PSAC exists over the southern SCS in summer, intensifies and moves eastward to the west of the Philippines in autumn, remains in the tropical western Pacific during winter and the ensuing spring, and finally decays in summer. During CP El Niño, however, the anticyclone develops in winter and dissipates in the next summer. It is much weaker than that during EP El Niño, and is always confined to the west of the Philippines. Moreover, the PSAC during EP El Niño shows a sudden establishment in the developing autumn and a second relatively weaker enhancement in the decaying summer. This feature cannot be observed for the anticyclone during CP El Niño.
2) What are the different responses of IO SST to EP El Niño and CP El Niño? The IO SST is more strongly related with EP El Niño than with CP El Niño. A positive IOD mode occurs during the developing summer and autumn and a warming IOBW mode develops during winter and the ensuing spring of EP El Niño. However, these two IO SST modes cannot be identified during CP El Niño. The anomalous low-level circulation over the tropical IO also exhibits a stronger connection with EP El Niño than with CP El Niño.
3) What is the possible impact of the IO SST on the low-level anticyclone development during EP El Niño and CP El Niño and what are the possible physical mechanisms responsible for this influence? Given the significant correlation of IO SST and low-level circulation with EP El Niño, the IO SST seems to be strongly linked to the development and persistence of the PSAC during EP El Niño, but not during CP El Niño. In the developing year of EP El Niño, a positive IOD appears in summer and matures in autumn. Correspondingly, anomalous sinking (rising) motion dominates over the eastern (western) IO, exciting two anomalous low-level anticyclones located symmetrically to both sides of the equator through the Rossby wave response. The northern anticyclone is suggested to contribute to the development of PSAC during EP El Niño. Without the effect of IOD, however, the PSAC exists about one season later and is much weaker. During the decaying year of EP El Niño, a warming IOBW mode matures in spring and persists in summer. Through an anomalous Kelvin wave response, it also instigates anomalous anticyclone twins over the Pacific to both sides of the equator, which can persist from spring to summer. Without the effect of IOBW, however, the PSAC disappears quickly after winter. Therefore, the IOD may favor an earlier development of the PSAC and the IOBW would contribute to a larger persistence of the PSAC during EP El Niño.
The low-level PSAC is widely considered as an important system conveying the ENSO impact to East Asia (Zhang et al. 1999; Wang et al. 2000; Wang and Zhang 2002). However, during different types of El Niño events, the low-level anticyclone shows different evolution features. One possible reason is the different intensity of El Niño events (Wang and Zhang 2002). Because CP El Niño is usually weaker than EP El Niño, the PSAC excited by CP El Niño also tends to be weaker and has a shorter life cycle. Another possible reason proposed in this paper is the coexistence of IO SSTA and Pacific SSTA during EP El Niño, which is not the case during CP El Niño. Since a positive IOD appearing in the developing year of EP El Niño may cause an earlier development of PSAC and a warming IOBW developing in the EP El Niño decaying year may contribute to larger intensity and persistence of PSAC, the anticyclone during EP El Niño is much stronger and has a longer lifetime than that during CP El Niño. However, in addition to the Rossby and Kelvin wave responses discussed in this paper, the PSAC is also excited by the combined effects of other factors such as the cooling in northeastern Asia land surface, the deepened East Asian trough, the Asian monsoon westerly flow, and intraseasonal oscillations (Wang and Zhang 2002). Apparently, more investigations are needed to further explore the mechanisms for PSAC variability during EP El Niño and CP El Niño.
Issues about the low-level PSAC are always interesting topics, especially for the decaying year of El Niño. Although El Niño dissipates in the next year, its impact on the East Asian climate still persists and may even be more significant than that during the developing year (e.g., Zhang et al. 1999; Wang and Zhang 2002) because of the large-persistent low-level anticyclone around the Philippines. Wang et al. (2000) proposed that the persistence of PSAC to the next summer of El Niño was attributed to the in situ air–sea interaction over the tropical western Pacific. Through a series of moist model experiments, Watanabe and Jin (2002) argued that a modest warming over the tropical IO could play a more important role than the strong warming in the central-eastern Pacific and the weak cooling in the western Pacific. Xie et al. (2009) confirmed this argument based on both observation and an atmospheric general circulation model. Wu et al. (2009, 2010) further suggested that both remote and local SSTA forcings are important to the PSAC development, with the contribution of positive (negative) SSTA in the tropical IO (northwestern Pacific) gradually enhancing (weakening) from June to August. In our study, we only focus on the possible effect from IO SST, because the air–sea interaction over the tropical IO shows different developing features between EP El Niño and CP El Niño. Our results are consistent with the previous conclusion that the IO SST contributes to the PSAC development, but only for EP El Niño, not for CP El Niño. On the other hand, we do not exclude the important role played by the local SST over the tropical northwestern Pacific. It seems from Figs. 9a–d that the IOBW-related anticyclonic circulation is too far to the north (Fig. 9b) or too far to the east (Fig. 9d), which might imply a possible effect of local SSTA forcing caused by the overestimated warming over the northwestern Pacific (Figs. 7h–j). Further studies are still needed to better understand this aspect.
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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