Impacts of Two Types of El Niño on Atmospheric Circulation in the Southern Hemisphere

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1 ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 30, NO. 6, 2013, Impacts of Two Types of El Niño on Atmospheric Circulation in the Southern Hemisphere SUN Dan 1,2 ( ), XUE Feng 1 ( ), and ZHOU Tianjun 1 ( ) 1 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing Beijing Meteorological Bureau, Beijing (Received 16 November 2012; revised 4 March 2013; accepted 7 March 2013) ABSTRACT Based on NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data from 1979 to 2010, the impacts of two types of El Niño on atmospheric circulation in the Southern Hemisphere (SH) are analyzed. It is shown that, when a warming event occurs in the equatorial eastern Pacific (EP El Niño), there is a negative sea level pressure (SLP) anomaly in the eastern Pacific and a positive one in the western Pacific. Besides, there exists a negative anomaly between 40 S and 60 S and a positive anomaly to the south of 60 S. When a warming event in the central Pacific (CP El Niño) occurs, there appears a negative SLP anomaly in the central Pacific and a positive SLP anomaly in the eastern and western Pacific, but the SLP anomalies are not so evident in the SH extratropics. In particular, the Pacific South America (PSA) pattern induced by the CP El Niño is located more northwestward, with a weaker anomaly compared with the EP El Niño. This difference is directly related with the different position of heating centers associated with the two types of El Niño events. Because the SST anomaly associated with CP El Niño is located more westward than that associated with EP El Niño, the related heating center tends to move westward and the response of SH atmospheric circulation to the tropical heating changes accordingly, thus exciting a different position of the PSA pattern. It is also noted that the local meridional cell plays a role in the SH high latitudes during EP El Niño. The anomalous ascending motion due to the enhancement of convection over the eastern Pacific leads to an enhancement of the local Hadley cell and the meridional cell in the middle and high latitudes, which in turn induces an anomalous descending motion and the related positive anomaly of geopotential height over the Amundsen-Bellingshausen Sea. Key words: eastern Pacific El Niño, central Pacific El Niño, atmospheric circulation, Southern Hemisphere, Pacific South America pattern Citation: Sun, D., F. Xue, and T. J. Zhou, 2013: Impacts of two types of El Niño on atmospheric circulation in the Southern Hemisphere. Adv. Atmos. Sci., 30(6), , doi: /s Introduction ENSO (El Niño Southern Oscillation) refers to the large-scale SST anomalies over the equatorial central and eastern Pacific and the associated anomalous tropical atmospheric circulation. As the strongest interannual signal in the coupled ocean atmosphere system, ENSO plays a role in global climate anomalies through atmospheric teleconnection. In the Southern Hemisphere (SH), the teleconnection induced by ENSO is the Pacific South America (PSA) pattern, which is a large-scale cell formed by divergence in the upper troposphere due to tropical convection (Karoly, 1989; Revell et al., 2001). As the most significant signal found in the South Pacific, the PSA pattern extends poleward to the Drake Passage and Antarctic Peninsula. The geopotential height anomalies associated with the PSA pattern are shown as a wave-train with centers in New Zealand, the Southeast Pacific and South America (van Loon and Shea, 1987; Mo and Higgins, 1998). A similar wave-train is also evident at a synoptic timescale, such as the occurrence of blocking events over the Southeast Pacific, which are strongly modulated by the ENSO cycle (Renwick, 1998; Ren- Corresponding author: XUE Feng, fxue@lasg.iap.ac.cn China National Committee for International Association of Meteorology and Atmospheric Sciences (IAMAS), Institute of Atmospheric Physics (IAP) and Science Press and Springer-Verlag Berlin Heidelberg 2013

2 NO. 6 SUN ET AL wick and Revell, 1999). On a longer timescale, ENSO plays a role in the phase transition of Antarctic oscillation through the global tropical wave propagating eastward (Liu and Xue, 2010). Numerical modeling demonstrates that the interannual variability and predictability of the Antarctic oscillation is significantly forced by ENSO-related SST anomalies in the tropical ocean (Zhou and Yu, 2004). Over the Southeast Pacific, the strongest manifestation of ENSO appears in the Amundsen Sea area, comprising a pole of maximum variability in the SH sea level pressure (SLP) field (Connelley, 1997), and thus the Amundsen Sea low pressure tends to become shallower in the warm phase of ENSO (Kiladis and Mo, 1998). These studies indicate that ENSO plays an important role in the interannual variability of SH circulation. Recent studies have noted that El Niño events can be divided into two types, i.e., the eastern Pacific El Niño event (EP El Niño) and central Pacific El Niño event (CP El Niño), the latter also referred to as El Niño Modoki in some studies (Ashok et al., 2007; Ashok and Yamagata, 2009). Distinct differences exist between the two types of El Niño events, such as the spatial pattern and time evolution. With its significant SST anomaly centered in the eastern equatorial Pacific off the coast of South America, the EP El Niño is accompanied by large-scale variations in thermocline, surface pressure and wind. By comparison, with the anomalies confined to the central Pacific, the CP El Niño is generally manifested as a local phenomenon (Kao and Yu, 2009). In addition, the mechanisms for the two types of El Niño events are different. Owing to the spatial scale of the SST anomaly, the efficiency of the discharge process of the equatorial heat content in the CP El Niño is not enough to induce a large-scale air sea interaction as in the EP El Niño (Kug et al., 2009). Moreover, the low-level anticyclone over the tropical western Pacific is more influenced by Indian Ocean SST in the EP El Niño than in the CP El Niño (Yuan et al., 2012a). It is also noted that the influence of the CP El Niño is different from that of the EP El Niño. Weng et al. (2007, 2009) noted that the climate anomaly around the Pacific countries such as Japan, New Zealand and the west coast of the United States in the CP El Niño is generally opposite to that in the EP El Niño. Kumar et al. (2006) showed that CP El Niño events are more effective in forcing drought-producing subsidence over India. In southeastern Asia, a different spatial distribution of rainfall associated with the two types of El Niño events can also be seen (Feng et al., 2010). From El Niño developing summer to El Niño decaying summer, the impacts of EP El Niño and CP El Niño on East Asian climate are different, especially on rainfall over China in their decaying phases (Feng and Li, 2011; Feng et al., 2011; Yuan and Yang, 2012; Yuan et al., 2012b). Besides, it has been shown that rainfall anomalies over Australia associated with CP El Niño events are significantly different from those associated with EP El Niño events (Wang and Hendon, 2007; Cai and Cowan, 2009). While a reduction in rainfall over northeastern and southeastern Australia is found in the EP El Niño, the CP El Niño plays a role in driving a large-scale decrease in rainfall over northwestern and northern Australia (Taschetto and England, 2009). The aforementioned studies are generally focused on regional impacts, especially rainfall. The responses of SH circulation to the two types of El Niño events are not fully understood. In particular, the physical mechanism has not been fully elucidated. Based on NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data and other data from 1979 to 2010, the present study attempts to reveal the commonalities and contrasts of their impacts on SH circulation and the related mechanisms. The remainder of the paper is organized as follows. After a short description of the data and methods in section 2, the different impacts on SH circulation are compared in section 3, and the related physical mechanisms are discussed in section 4. Finally, a summary is given in section Data and methods Several datasets were used in this study. The primary dataset used was the NCEP/NCAR reanalysis products, with a resolution of (Kalnay et al., 1996). The data of outgoing long-wave radiation (OLR) used to infer the tropical convection were also provided by NCEP/NCAR (Liebmann and Smith, 1996). The monthly SST data used to calculate the EP El Niño and CP El Niño index were obtained from the Hadley Center global sea ice and sea surface temperature dataset (HadISST), with a resolutions of 1 1 (Rayner et al., 2003). The precipitation data were taken from the Global Precipitation Climatology Project (GPCP), with a resolution of (Huffman et al., 1997). The period from 1979 to 2010 was taken for consistency. Because the El Niño signal and its impact on SH circulation are strongest in boreal winter (austral summer), the focus is on the seasonal means in boreal winter, which were constructed from the monthly mean of December, and January and February of the succeeding year (DJF). For instance, the DJF of 1979 refers to the mean of December 1979, and January and Febru-

3 1734 EL NIÑO IMPACTS ON SOUTHERN HEMISPHERE CIRCULATION VOL. 30 (a) EP El Nino (b) CP El Nino Fig. 1. Partial correlation coefficients of SST with the normalized DJF I Niño3 (a) and I EM (b) during (regions used to calculate the indices are indicated by the rectangles). Correlation coefficients at the confidence levels of 90%, 95%, and 99% are 0.296, and 0.449, respectively. ary Based on previous studies, the EP El Niño and CP El Niño were defined by the Niño3 index (I Niño3 ) and El Niño Modoki index (I EM ) in DJF, respectively (Ashok et al., 2007). The definitions of the two indices are I Niño3 = T a,ep, (1) where T a,ep is the average of SST anomalies over the equatorial eastern Pacific (5 S 5 N, W), and I EM = T a,c 0.5T a,e 0.5T a,w, (2) where T a,c, T a,e, T a,w stand for the average of SST anomalies over the central Pacific (10 S 10 N, 165 E 140 W), eastern Pacific (15 S 5 N, W) and western Pacific (10 S 20 N, E), respectively. The above regions used to define these indices are illustrated in Fig. 1. Figure 2 shows I Niño3 and I EM during DJF from 1979 to It is noted that, owing to the SST anomaly in some El Niño years extending to the central Pacific, the two types of El Niño events occur simultaneously in some years, especially in La Niña years such as 1988, 1998 and This fact indicates that the two indices are not fully independent. However, the major events with large positive anomalies can be distinguished clearly. If 0.8 C is used as a critical value, for example, four EP El Niño events (1982, 1986, 1991 and 1997) and six CP El Niño events (1979, Fig. 2. Time series of I Niño3 and I EM in boreal winter (DJF) during (units: C). 1990, 1994, 2002, 2004 and 2009) can be identified from 1979 to In order to exclude the interference of the two indices, the partial correlation method was used to analyze the correlation between the indices and some atmospheric variables (Ashok et al., 2007). The partial correlation coefficients were calculated as follows: r xy,z = r xy r xz r yz (1 rxz r xz )(1 r yz r yz ), (3) where r xy,z is the partial correlation coefficient between the variables x and y, r xy the correlation coefficient between the variables x and y, r xz the correlation coefficient between the variables x and z, and

4 NO. 6 SUN ET AL r yz the correlation coefficient between the variables y and z. The two-tailed Student s t-test was used to test the statistical significance for the correlations. Since the degrees of freedom are 30 for a time series with 31 seasons ( ), the correlation coefficients at the confidence levels of 90%, 95%, and 99% were and 0.449, respectively. The apparent heat source (Q 1 ) was used to investigate the effect of diabatic heating. The formula is as follows (Nigam, 1994): Q 1 = T ( ) (R/cp) p t + V T + ω θ p 0 p + ( p p 0 ) (R/cp) [ V θ + (ω θ ) p ], (4) where V, ω, T, p and θ are horizontal velocity, vertical velocity, temperature, atmospheric pressure and potential temperature, respectively; R and c p are the gas constant and the specific heat at constant pressure of dry air, respectively; and p 0 = 1000 hpa. The over-bar denotes the monthly average and the double prime represents the departure of 6-h reanalysis data from the monthly average. 3. Influences of the two types of El Niño on Southern Hemisphere circulation Figure 1 shows the partial correlations of the SST anomalies with two indices in DJF during , and the regions used to define the indices are indicated by the rectangles. The EP El Niño is characterized by a horseshoe distribution, i.e., a positive anomaly in the eastern tropical Pacific and negative anomalies in the western tropical Pacific and mid-latitude Pacific (Fig. 1a). By contrast, a triple-pole distribution is evident for the CP El Niño, i.e., a positive anomaly in the central Pacific is sandwiched by a negative one in the eastern and western Pacific, respectively (Fig. 1b). It is also noted that the positive SST anomaly for the EP El Niño is much stronger than that for the CP El Niño. A typical Southern Oscillation (SO) pattern dominates the whole tropics during the EP El Niño, i.e., a positive SLP anomaly in the western Pacific and a negative one in the eastern Pacific separated by the dateline, with the centers in northern Australia and Tahiti, respectively (Fig. 3a). Outside of the tropics, there is a well-defined belt with a negative anomaly in the middle latitudes between 40 S and 60 S. More poleward to the south of 60 S, a more pronounced positive anomaly is found with the center over the Amundsen- Bellingshausen Sea. Similar to SLP, the wind field at 850-hPa also exhibits a significant anomaly. While there is a westerly anomaly in most of the equatorial Pacific, there is an easterly anomaly from the western Pacific to the Indian Ocean. Corresponding to the SO pattern, a cyclonic anomaly appears over Tahiti and an anticyclonic anomaly over Australia. In the high latitudes, there is a significant easterly anomaly to the south of 50 S along with an anticyclonic anomaly cen- Fig. 3. Partial correlation coefficients of SLP (shaded) and 850-hPa wind (vector) with the normalized DJF I Niño3 (a) and I EM (b) during , (a) EP El Niño, (b) CP El Niño. Vectors and regions above the 90% confidence level are shown for clarity.

5 1736 EL NIÑO IMPACTS ON SOUTHERN HEMISPHERE CIRCULATION VOL. 30 Fig. 4. Partial correlation coefficients of 500-hPa geopotential height with the normalized DJF I Niño3 (a) and I EM (b) during : (a) EP El Niño, (b) CP El Niño. Regions above the 90% confidence level are shaded. tered over the Amundsen-Bellingshausen Sea. The anomalous circulation during the CP El Niño is much different from that during the EP El Niño (Fig. 3b). In the tropics, there is a negative SLP anomaly over the central Pacific and a positive one over the Indian Ocean and Maritime Continent. Instead of a large-scale anomaly during the EP El Niño, small-scale anomalous centers with a weaker intensity are only found over the southeastern Pacific off the coast of Chile and over the Weddell Sea. At 850-hPa in the tropics, there is a westerly anomaly over the western and central Pacific and an easterly anomaly over the eastern Pacific. Compared with the EP El Niño, the cyclonic anomaly corresponding to the negative SLP anomaly over the tropical Pacific is located more westward in the CP El Niño. In the middle and high latitudes, a cyclonic anomaly over the southeastern Pacific off the coast of Chile and an anticyclonic anomaly over the Weddell Sea correspond to a local negative and positive SLP anomaly, respectively. Figure 4 shows the partial correlations of 500-hPa geopotential height. During the EP El Niño event, the distribution is generally characterized by well-defined belts with positive anomalies over the tropics and high latitudes, and a negative anomaly over the middle latitudes between 40 and 60 S. The positive centers are located over the tropical Indian and Pacific Oceans and Amundsen-Bellingshausen Sea, while the negative centers are located over the South Atlantic Ocean and the South Pacific to the southeast of New Zealand. Compared with the EP El Niño, the distribution during the CP El Niño event is dominated by small-scale centers similar to those found in Fig. 3. Previous studies have shown that, as a response to the anomalous convective heating in the tropics, the atmospheric teleconnection pattern may transfer the tropical signal to SH high latitudes through stationary Rossby waves (Karoly, 1989; Mo and Higgins, 1998; Garreaud and Battisti, 1999), and the pattern is often referred to as the Pacific South American pattern (Kidson, 1999). Figure 4 clearly illustrates the differences of PSA pattern between the EP El Niño and the CP El Niño. During the EP El Niño event, the anomalous centers of geopotential height are located over the western Pacific off Australia (positive), the South Pacific to the southeast of New Zealand (negative), the Amundsen Sea (positive), and the South Atlantic Ocean (negative). During the CP El Niño event, the anomalous centers are located over eastern Australia (positive), New Zealand (negative), the middle latitudes of the South Pacific (positive), the southeastern Pacific off Chile (negative), and the Weddell Sea (positive). Compared with the EP El Niño, the PSA pattern for the CP El Niño is located more northwestward with a much weaker intensity. 4. The physical mechanism 4.1 Response to the tropical heat source It was proposed by Mastsuno (1966) and Gill (1980) that, a heat source symmetric to the Equator may excite a twin cyclonic anomaly on both sides of the Equator, while a heat source north of the Equator may excite a cyclonic anomaly northwest of the heat source. Based on the Matsuno Gill theory, the physi-

6 NO. 6 SUN ET AL Fig. 5. The same as Fig. 3, but for the vertically integrated apparent heat source Q 1 (shaded) and 850-hPa wind field (vector): (a) EP El Niño, (b) CP El Niño. cal mechanism for the influence of the two types of El Niño events on SH circulation is analyzed in this section. As shown in Fig. 5a, corresponding to the SST anomaly of the EP El Niño, there is a strong positive Q 1 anomaly over the central and eastern Pacific and a relatively weak negative Q 1 anomaly over the western Pacific. In agreement with the above theory, a cyclonic anomaly excited by the positive heat source appears over the subtropical South Pacific, while there is an anticyclonic anomaly over the subtropical Indian Ocean. During CP El Niño events, a positive Q 1 anomaly is located over the central Pacific, while a negative Q 1 anomaly is located over the eastern and western Pacific, respectively. Compared with the EP El Niño, the positive heat source is located more westward. As a result, the excited cyclonic anomaly is located between 150 E and 170 W. Besides, there is a much weaker anticyclonic anomaly to the east and west side of the cyclonic anomaly, respectively. Since the PSA pattern is considered as a response to the tropical heat source and the propagation of Rossby waves in the SH (Hoskins and Karoly, 1981; Mo and Higgins, 1998), it can be expected that the PSA pattern in the CP El Niño is located more westward than that in the EP El Niño. A warmer SST in the tropics can enhance local precipitation by convective processes through boundary layer instability, and it plays an important role in the anomalous heat source. Figure 1 has shown that warmer SST in the EP El Niño occupies a large area of the central and eastern Pacific, while warmer SST in the CP El Niño is confined to the central Pacific. In Fig. 6, the negative OLR and positive precipitation anomalies generally correspond to the warmer SST in Fig. 1 for both the EP and CP El Niño events, indicating that the anomalous heat source in the tropical atmosphere is caused by the warmer SST. 4.2 Role of the local meridional cell Besides the lower-level circulation, there is a significant discrepancy in the vertical structure between the two types of El Niño events (Feng et al., 2010, 2011). As shown in Fig. 7a, during the EP El Niño, an anomalous divergence over the eastern Pacific and an anomalous convergence over the western Pacific are separated by the dateline at 200-hPa. The distribution at 850-hPa is generally opposite to that at 200-hPa except that the divergence extends eastward to 160 W. This configuration indicates that the anomalous ascent is located over the eastern Pacific, while the anomalous descent is located over the western Pacific, leading to a more eastward displacement of Walker circulation. During CP El Niño events, corresponding to an anomalous convergence over the western and eastern Pacific, there is a strong anomalous divergence over the

7 1738 EL NIÑO IMPACTS ON SOUTHERN HEMISPHERE CIRCULATION VOL. 30 Fig. 6. The same as Fig. 2, but for (a, b) outgoing long-wave radiation and (c, d) precipitation. 60N 30N 0 30S 60N 30N 0 30S 60S 60S 0 60W 0 60W 60N 30N 0 30S 60S 0 60N 30N 0 30S 60S 60W 0 60W Fig. 7. The same as Fig. 3, but for divergent wind (vector) and velocity potential (contour) at (a, c) 200 hpa and (b, d) 850 hpa. Regions above the 95% confidence level are shaded. central Pacific at 200-hPa. In this case, the anomalous ascent is mainly confined to the central Pacific. Figure 8 shows more clearly the displacement of Walker circulation during the two types of El Niño events. During EP El Niño events, the Walker circulation moves eastward with an anomalous ascent over the central and eastern Pacific, as compared with the climatology in Fig. 8a. By contrast, the anomalous ascent during the CP El Niño is only confined to the central Pacific, near the dateline (Fig. 8c). The most significant difference between the two types of El Niño events is found in the eastern Pacific, where there is an anomalous ascent during the EP El Niño but an anomalous descent during the CP El Niño, as described by Yuan and Yang (2012). To further explore the differences in latitudinal sector of the eastern Pacific caused by the local meridional cell, Fig. 9 shows the latitude height cross section of vertical velocity and divergent wind during the two types of El Niño events, together with the climatology for comparison. During the EP El Niño, instead of a descent in the climatology (Fig. 9a), there appears an anomalous ascent over the SH tropics due to the enhanced convection excited by the warmer SST in the equatorial eastern Pacific (Fig. 9b). In turn, the local meridional cell changes, and there is a sig-

8 NO. 6 SUN ET AL (a) Climatological mean (b) EP El Nino (c) CP El Nino Fig. 8. Longitude height cross section of vertical velocity (shaded; 10 2 m s 1 ) and divergent wind (vector; m s 1 ) averaged over 10 S 10 N in DJF during : (a) climatological mean; (b) partial correlations with I Niño3 ; (c) partial correlations with I EM. Correlation coefficients at the confidence levels of 90%, 95% and 99% are 0.296, and 0.449, respectively. nificantly anomalous descent near 60 S along with a relatively weaker anomalous ascent between 40 S and 50 S. Compared with the EP El Niño, except a narrow anomalous ascent between 0 and 10 S, the anomalies in the SH are generally weak during the CP El Niño (Fig. 9c). As a result, the geopotential height anomaly over the Amundsen-Bellingshausen Sea is not as significant as that in the EP El Niño (Fig. 4). The above analysis indicates that the local meridional cell plays a role in the anomalous circulation of the SH. In particular, the positive anomaly of geopotential height over the Amundsen-Bellingshausen Sea tends to be intensified significantly due to the anomalous descent near 60 S during the EP El Niño, together with an impact of PSA teleconnection pattern. By contrast, during the CP El Niño, there is no distinct anomaly in the above region because the vertical ascent in the SH high latitudes is relatively weak and the PSA pattern is located more westward. 5. Summary Based on partial correlation analysis, we have studied the influences of two types of El Niño events on SH

9 1740 EL NIÑO IMPACTS ON SOUTHERN HEMISPHERE CIRCULATION VOL. 30 Fig. 9. The same as Fig. 8, but for latitude height cross section averaged between W. circulation during boreal winter. When an EP El Niño occurs, there appears an anomalous SLP pattern in the tropical Pacific with a positive anomaly in the west and a negative anomaly in the east. Outside of the tropics, a well-defined belt with a negative anomaly is found in latitudes of 40 to 60 S. To the south of 60 S, a positive anomaly is significant, with a center in the Amundsen-Bellingshausen Sea. By contrast, there exists a negative anomaly in the central Pacific and a negative anomaly in the Indian Ocean corresponding to a CP El Niño event. Different from the EP El Niño, however, small-scale anomalies are found in the SH middle and high latitudes. It is also noted that similar features are evident in 850-hPa wind and 500- hpa geopotential height. As a strong response in the SH to the ENSO cycle, the PSA teleconnection pattern exhibits a significant difference between the two types of El Niño events. The pattern for the CP El Niño is located more northwestward with a weaker intensity than that for the EP El Niño. Because the anomalous heat source in the equatorial Pacific for the CP El Niño is located more westward than that for the EP El Niño, the anomalous cyclone due to the response of the SH circulation to the heat source is accordingly located more westward, resulting in a westward PSA pattern. Besides the PSA pattern, the local meridional cell also plays a role in SH circulation, especially in the region between 150 and 90 W. During EP El Niño events, the local Hadley cell is intensified and shifts

10 NO. 6 SUN ET AL more poleward due to the equatorial heating, leading to a change in the local meridional cell in the middle and high latitudes. As a result, the anomalous descending motion induces a positive anomaly of geopotential height over the Amundsen-Bellingshausen Sea. It should be noted that a further study based on numerical experiments is necessary to evaluate the relative importance of the two proposed mechanisms to SH circulation. Also of note is that the precipitation distribution exhibits a different pattern due to the different impacts of the two types of El Niño events on SH circulation, especially over the Pacific and adjacent regions (Fig. 6). During the EP El Niño, there is more rainfall over the tropical central and eastern Pacific and less rainfall over the Maritime Continent and the subtropical Pacific to the east of Australia. Compared with the EP El Niño, the anomalous rainfall pattern during the CP El Niño tends to move westward as the PSA pattern, with less rainfall over the eastern Pacific and more rainfall over the central Pacific. In particular, Australia seems to be more significantly influenced by the CP El Niño, during which Australia receives more rainfall, as noted by some previous studies (Wang and Hendon, 2007; Taschetto and England, 2009). Previous studies have shown that EP El Niño is characterized by interannual variability, while the signal of CP El Niño is more significant at the interdecadal timescale. In particular, the intensity of CP El Niño has been doubled since 1979 (Weng et al., 2007; Lee and McPhaden, 2010). Therefore, the influence of CP El Niño on SH circulation before 1979 was perhaps not so significant. The influence of CP El Niño on SH circulation at the decadal timescale deserves further study. Acknowledgements. We would like to thank Dr. Bohua HUANG for his help, and the comments and suggestions from the two anonymous reviewers are also greatly appreciated. This study was jointly supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA ) and the Development and Validation of High Resolution Climate System Model of the National Basic Research Program of China (Grant No. 2010CB951901). REFERENCES Ashok, K., and T. Yamagata, 2009: The El Niño with a difference. Nature, 461, Ashok, K., S. Behera, S. A. Rao, H. Weng, and T. Yamagata, 2007: El Niño Modoki and its teleconnection. J. Geophys. Res., 112, C11, doi: /2006JC Cai, W., and T. Cowan, 2009: La Niña Modoki impacts Australia autumn rainfall variability. Geophys. Res. Lett., 36, L12805, doi: /2009GL Connolley, W. M., 1997: Variability in annual mean circulation in southern high latitudes. Climate Dyn., 13, Feng, J., and J. Li, 2011: Influence of El Niño Modoki on spring rainfall over south China. J. Geophys. Res., 116, D13102, doi: /2010JD Feng, J., L. Wang, W. Chen, S. K. Fong, and K. C. Leong, 2010: Different impacts of two types of Pacific Ocean warming on southeast Asian rainfall during boreal winter. J. Geophys. Res., 115, D24112, doi: /2010JD Feng, J., W. Chen, C.-Y. Tam, and W. Zhou, 2011: Different impacts of El Niño and El Niño Modoki on China rainfall in the decaying phases. Int. J. Climatol., 31, Garreaud, R. D., and D. S. Battisti, 1999: Interannual (ENSO) and interdecadal (ENSO-like) variability in the Southern Hemisphere tropospheric circulation. J. Climate, 12, Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, Hoskins, B. J., and D. J. Karoly, 1981: The steady linear response of a spherical atmosphere to thermal and orographic forcing. J. Atmos. Sci., 38, Huffman, G. J., and Coauthors, 1997: The Global Precipitation Climatology Project (GPCP) combined precipitation dataset. Bull. Amer. Meteor. Soc., 78, Kalnay, E. M., and Coauthors, 1996: The NCEP/NCAR40 year reanalysis project. Bull. Amer. Meteor. Soc., 77, Kao, H. Y., and J. Y. Yu, 2009: Contrasting eastern- Pacific and central-pacific types of ENSO. J. Climate, 22, Karoly, D. J., 1989: Southern Hemisphere circulation features associated with El Niño Southern Oscillation events. J. Climate, 2, Kidson, J. W., 1999: Principal modes of Southern Hemisphere low-frequency variability obtained from NCEP NCAR reanalyses. J. Climate, 12, Kiladis, G. N., and K. C. Mo, 1998: Interannual and intraseasonal variability in the Southern Hemisphere. Chapter 8, Meteorology of the Southern Hemisphere, Karoly and Vincent, Eds., American Meteorological Society Monograph, No. 27, Boston, Massachussetts, Kug, J., F. Jin, and S. An, 2009: Two types of El Niño events: Cold tongue El Niño and warm pool El Niño. J. Climate, 22, Kumar, K., B. Rajagopalan, M. Hoerling, G. Bates, and M. Cane, 2006: Unraveling the mystery of Indian monsoon failure during El Niño. Science, 314, Lee, T., and M. J. McPhaden, 2010: Increasing intensity of El Niño in the central-equatorial Pacific. Geophys.

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