Influence of two types of El Niños on the East Asian climate during boreal summer: a numerical study
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The sea surface temperature anomaly pattern differs between the central Pacific (CP) and eastern Pacific (EP) El Niños during boreal summer. It is expected that the respective atmospheric response will be different. In order to identify differences in the responses to these two phenomena, we examine the Community Atmosphere Model Version 4 simulations forced with observed monthly sea surface temperature during 1979–2010 and compare with the corresponding observations. For CP El Niño, a triple precipitation anomaly pattern appears over East Asia. During EP El Niño, the triple pattern is not as significant as and shifts eastward and southward compared to CP El Niño. We also examine the influence of CP La Niña and EP La Niña on East Asia. In general, the impact of CP (EP) La Niña on tropics and East Asia seems to be opposite to that of CP (EP) El Niño. However, the impacts between the two types of La Niña are less independent compared to the two types of warm events. Both types of El Niño (La Niña) correspond to a stronger (weaker) western North Pacific summer monsoon. The sensitivity experiments support this result. But the CP El Niño (La Niña) may have more significant influence on East Asia summer climate than EP El Niño (La Niña), as the associated low-level anomalous wind pattern is more distinct and closer to the Asian continent compared to EP El Niño (La Niña).
KeywordsCP El Niño CP La Niña CAM4 AMIP
El Niño-Southern Oscillation (ENSO) is one of the largest sources of interannual variability in the tropical troposphere. Usually, ENSO events can be classified according to their onset time, propagation characteristics and zonal sea surface temperature (SST) distribution (e.g. Fu et al. 1986; Enfield and Luis 1991; Wang 1995; Horii and Hanawa 2004; Larkin and Harrison 2005; Wang and Fiedler 2006; Kao and Yu 2009). The positive phase of ENSO corresponds to El Niño. According to zonal SST distribution, the typical El Niño is associated with maximum warm SST anomaly in the eastern Pacific and cold SST anomaly in the western Pacific (e.g. Rasmusson and Carpenter 1982). In recent studies, a different type of El Niño featuring maximum warm SST anomaly in the central Pacific has been identified. It is alternatively named dateline El Niño (Larkin and Harrison 2005), El Niño Modoki (Ashok et al. 2007; Weng et al. 2007), central Pacific El Niño (Yu and Kao, 2007; Kao and Yu, 2009; Yeh et al. 2009) and warm pool El Niño (Kug et al. 2009). In this study, these two types of El Niño events are denoted as central Pacific El Niño (CP El Niño) and eastern Pacific El Niño (EP El Niño), respectively.
Kao and Yu (2009) indicated that the CP El Niño has most of its surface wind, SST, and subsurface anomalies confined in the central Pacific and tends to originate, develop and disappear in the tropical central Pacific in situ and is not necessarily followed by the negative phase. Kug et al. (2009) demonstrated that zonal advective feedback plays a crucial role during CP El Niño, while thermocline feedback is a key process during EP El Niño. And it is reported that CP El Niño is associated with two cells of anomalous Walker circulation with ascent over the equatorial central Pacific, instead of a single cell associated with EP El Niño. For CP El Niño, the teleconnections, such as the positive phase of Pacific-Japan (PJ) pattern and the Pacific North American (PNA) pattern, are different from those during EP El Niño (e.g. Ashok et al. 2007; Weng et al. 2007). Thus, the two types of El Niños may exert different influences on regional climate. Feng and Li (2011) reported that CP El Niño events are accompanied by a significant reduction in spring rainfall over southern China, while there is enhanced spring rainfall associated with EP El Niño. It is shown that the Yangtze River Valley suffers more rain and lower temperature during the boreal summer of CP El Niño, but it suffers the opposite situation during the EP El Niño scenario (Weng et al. 2007, 2011). Yuan and Yang’s study (2012) pointed out the impact of CP El Niño on East Asian climate is more significant than that of EP El Niño during the developing summer, and, however, EP El Niño exerts a stronger influence on East Asia during the decaying summer. Besides, some studies report that there are different impacts of two types of El Niños on northwestern Pacific tropical cyclones activities (Chen and Tam 2010; Wang et al. 2012).
Most of the aforementioned studies rely on linear statistical methods, such as partial correlation and regression. It is reported that CP El Niño is rarely observed before the 1980s, and its frequency increases in the past three decades (Ashok et al. 2007; Yeh et al. 2009; Lee and McPhaden 2010). Some studies attribute the recent increase in CP El Niño frequency to anthropogenic and natural climate variability (Ashok et al. 2007; Yeh et al. 2009). Besides, Yeh et al. (2009) concluded that the ratio of occurrence of the CP El Niños to the EP El Niños increases in the global-warming scenario as compared to the corresponding twentieth-century climate simulation by analyzing the Couple Model Intercomparison Project phase three multi-model dataset (Meehl et al. 2007). However, the number of CP El Niños observed during boreal summer is still small. As will be mentioned later, only 3 years can be identified as CP El Niños during boreal summer in our study period. As such, the robustness of the obtained results is an issue that needs to be addressed. The present study performs numerical experiments to identify the impact of two types of El Niños on East Asia climate during boreal summer.
Few studies have been done on the impacts of negative phase of ENSO Modoki (La Nina Modoki) during boreal summer. Kao and Yu (2009) argued that the warm and cold phases of CP-ENSO tend to have similar physical characteristic and similar patterns. And it is expected that the influence of La Nina Modoki, characterized by anomalous cooling in the central Pacific and flanked by anomalous warming in the eastern and western Pacific (Ashok and Yamagata 2009), may be opposite to that of El Niño Modoki (Diaz et al. 2001). Feng and Li (2011) indicated that there is a strong asymmetry in the relationship between South China rainfall, typical ENSO and ENSO Modoki events during boreal spring. Although South China spring rainfall is influenced by typical ENSO and ENSO Modoki events, their relationships are only statistically significant during the positive events. Kug and Ham (2011) explored the existence of two types of La Niña events. They found that the SST and precipitation patterns between the two types of La Niña are much less distinctive or less independent compared to the two types of warm events and there is a strong asymmetric character between warm and cold events. Thus, is there an asymmetric influence on East Asian between warm and cold events during summertime?
The purpose of this work is to extend the analysis of Weng et al. (2007) using numerical modeling results. In our study, we will define the two types of El Niños during boreal summer (see Sect. 2). We will make use of 6 ensemble simulations of the Community Atmosphere Model version 4 (CAM4, Neale et al. 2013), run with monthly observed SST (Hurrell et al. 2008) during 1979–2010. Besides, we have designed some experiments to further explore the simultaneous influence of equatorial Pacific SST anomaly pattern on East Asia climate during boreal summer. In addition, the impacts of negative phase of ENSO (CP and EP La Niña) will also be investigated.
The remainder of this manuscript is organized as follows: Sect. 2 presents the data sets and methods used in this study, and Sect. 3 shows the anomalies of two types of ENSO. Section 4 examines the asymmetry influences of CP ENSO. Finally, discussions and concluding remarks are provided in Sect. 5.
2 Data and methods
2.1 Observational data
Multiple datasets are used in this study for the period from January 1979 to December 2010. These include monthly SST from the Hadley Centre of the U.K. Met Office (HadISST, Rayner et al. 2003), monthly atmospheric field from the ECMWF reanalysis data (ERA-Interim, Dee et al. 2011), and precipitation from the Global Precipitation Climatology Project (GPCP, Adler et al. 2003). Some previous studies have explored the Pacific Decadal Oscillation (PDO, Mantua et al. 1997; Zhang et al. 1997, 1998) related SST anomalies (e.g. Zhu and Yang 2003a, b) and their impact on East Asian Climate (Zhu and Yang 2003a, b; Yang et al. 2005; Xu et al. 2005). Besides the direct impact, the PDO may provide a background modulating the interannual ENSO’s impact on East Asian Climate (Zhu and Yang 2003a, b; Zhu et al. 2007; Yang and Zhu 2008). The present study focuses on the period 1979–2010 corresponding to the warm phase of PDO.
The definitions of CP and EP El Niño (La Niña) and the respective selected years during 1979–2010
Niño4 SST index >0.5
1994, 2002, 2004
Niño4 SST index >Niño3 SST index
Niño3 SST index >0.5
1982, 1983, 1987, 1991, 1997, 2009
Niño3 SST index >Niño4 SST index
Niño4 SST index <−0.5
1989, 1998, 1999, 2008
Niño4 SST index <Niño3 SST index
Niño3 SST index <−0.5
1984, 1985, 1988, 2000, 2007, 2010
Niño3 SST index <Niño4 SST index
2.2 Model data
CAM4 is the seventh generation atmospheric general circulation model (AGCM) developed with significant community collaboration at the National Center for Atmospheric Research (Neale et al. 2013). It is part of the Community Climate System Model (CCSM4, Gent et al. 2011). CAM4 exhibits significant changes and improvements in climate simulation compared to CAM3 (Collins et al. 2006) due to the moderate changes in model configuration. The finite volume dynamical core is the default option in CAM4, and the default horizontal resolution is 0.9° latitude by 1.25° longitude, and the default number of levels is 26 (Neale et al. 2013).
3 Anomalies of two types of ENSO
3.1 Atmospheric response to two types of El Niños
As mentioned earlier, the spatial feature of SSTA differs between the CP El Niño and EP El Niño during boreal summer. It is natural to expect that the respective atmospheric response will be different. In order to identify the differences in responses to these two phenomena, we will examine CAM4 AMIP simulations and compare with the corresponding observations. In this section, we will mainly analyze the precipitation and low-level wind response to these two phenomena during the boreal summer.
For the EP El Niño, however, the spatial feature of precipitation is different from the CP El Niño situation. The equatorial central to eastern Pacific experiences wet conditions and the Maritime Continent experiences dry conditions. There is also a triple precipitation anomaly pattern in East Asia area: more rainfall over the lower reaches of the Yangtze River valley and east of Philippines, and less rainfall around the Taiwan Island (Fig. 5b). But this pattern is not as significant as CP El Niño situation and it shifts eastward and southward compared to CP El Niño. This distinct result implies that the influence of EP El Niño on East Asia climate during boreal summer is less significant than CP El Niño, due to an eastward and southward shift in its significant influent zone to the Pacific Ocean compared to CP El Niño.
The JJA precipitation anomaly composites are shown in Fig. 5c, d using the CAM4 AMIP runs data from January 1979 to December 2010. Although there are some differences between the observations and CAM4 AMIP runs, patterns of the observed precipitation anomalies in CP and EP El Niño can be captured by CAM4 AMIP run. And the triple precipitation anomaly pattern in EP El Niño is rather distinct. Figure 5e, f show the CAM4-CPEL and CAM4-EPEL sensitivity modeling results, the precipitation anomalies feature are similar to those in CAM4 AMIP runs.
However, for EP El Niño (Fig. 6b), there are westerly wind anomalies across the whole equatorial Pacific, which implies that there is one cell of anomalous Walker circulation with the ascending branch in the equatorial eastern Pacific and descending branch in the equatorial western Pacific. The cyclone-anticyclone-cyclone wind anomaly pattern can be seen although it is not as significant as CP El Niño. In comparison, the anomaly pattern shifts eastward and southward to the Pacific Ocean. The wind anomaly pattern appears to explain the corresponding anomalous precipitation pattern quite well. Chen and Tam (2010) reported that less TCs occur in the north part of WNP while more TCs form in the south-eastern part of WNP during EP El Niño. And such TCs activities over the WNP region can be attributed to an anomalous anticyclonic circulation in the subtropics and a cyclonic shear associated with the equatorial westerly anomalies in the southeast part of the WNP.
The above mentioned 850 hPa anomalous wind pattern can be generally captured by CAM4 AMIP run results, especially in the tropics and subtropics. But for CP El Niño, the EAP pattern is not distinct in the CAM4 AMIP run, as only the cyclonic wind anomalies in subtropical western Pacific can be well simulated. Furthermore, the subtropical western North Pacific cyclone wind anomalies in CP El Niño sensitivity experiment tend to be northward compared to the observations and AMIP runs. Sun and Yang (2005) conducted AGCM simulations to identify the impact of East Asian Climate to the conventional ENSO events. They found during the summer when an El Niño event develops, there is an anomalously negative geopotential height center over Northeast China, the Korean peninsula and the Sea of Japan. The subtropical high over the Western Pacific is weaker and shifts eastward. In this study, the CAM4 AMIP EP El Niño composite results seem to be consistent with those of Sun and Yang (2005). But the 850 hPa anticyclonic wind anomalies over subtropical western Pacific and cyclonic wind anomalies near Japan seem to be slightly eastward compared to Sun and Yang (2005).
3.2 Atmospheric response to two types of La Niñas
For EP La Niña, the equatorial central to eastern Pacific experiences dry conditions and the Maritime Continent experiences wet conditions (Fig. 8b). There is also a triple precipitation anomaly pattern in East Asia area: less rainfall over Japan and east of Philippines, and more rainfall around the Taiwan Island (Fig. 8b). This negative-positive–negative anomalous rainfall pattern during EP La Niña is similar but with opposite sign compared to EP El Niño scenario (Fig. 5b). And this pattern is not as significant as CP La Niña situation and shifts eastward compared to CP La Niña. This result further confirms that the influence of EP ENSO on East Asia climate during the boreal summer is less significant than CP ENSO, due to eastward and southward shift of the influence zone compared to CP ENSO. Similarly, the wind anomalies (Fig. 9b) of EP La Niña over tropics and East Asia also seem to be opposite to those in EP El Niño (Fig. 6b). And an asymmetry influence on south China can also be found between the EP El Niño and La Niña. The observed 850 hPa wind anomaly pattern in tropics can be generally simulated by CAM4 AMIP run (Fig. 9d) and CAM4 EPLA (Fig. 9f) experiment. Only the CAM4-EPLA can roughly capture the wind anomaly feature in East Asia and this feature is slightly northward compared to observations (Fig. 9b).
4 Asymmetry influence of CP ENSO
As mentioned earlier, CP El Niño mainly occurs since the late 1970s, only 3 CP El Niño years (1994, 2002 and 2004) and CP La Niña years (1989, 1998, 1999 and 2008) during boreal summer are defined in this study. Surprisingly, these CP El Niño years are all associated with enhanced western North Pacific summer monsoon, while 2 out of the 4 CP La Niña years (1998 and 2008) are associated with weak western North Pacific summer monsoon and the other two are associated with slightly positive WNPMI. It seems that the positive phase of CP ENSO (i.e. CP El Niño) may have more robust influence on East Asia compared to the negative phase of CP ENSO (i.e. CP La Niña). However, the CAM4 AMIP ensemble mean result presents that the western North Pacific summer monsoon is stronger (weaker) than normal during the selected CP El Niño (La Niña) years.
5 Discussions and concluding remarks
In this study, we calculate the boreal summer (JJA) Niño 3 and Niño 4 SST index and define two types of El Niño (i.e. CP and EP El Niño) and La Niña (i.e. CP and EL La Niña) by using monthly HadISST dataset. To further identify the different impacts of two types of El Niños (La Niñas), we make use of CAM4 AMIP runs and compare with the corresponding observations. Besides, we conduct 5 additional CAM4 simulations including four sensitivity experiments designed according to the corresponding composite SSTA pattern and one control experiment forced with the monthly evolving climatological SST.
For CP El Niño, there is a triple precipitation anomaly pattern in East Asia area: the South China Sea, southern China and north of Japan suffer more precipitation, but the Yangtze River Valley and southern Japan suffer less precipitation. In addition, the low-level wind anomalies exhibit a cyclone over subtropical western North Pacific, an anticyclone over Japan and a cyclone over Northeast Asia. The triple precipitation anomaly pattern can be explained by low-level cyclone-anticyclone wind anomaly pattern. But this triple pattern in both precipitation and wind anomalies during EP El Niño is not as significant as CP El Niño situation and it shifts eastward and southward compared to CP El Niño. In this study, we have also examined the influence of CP La Niña on East Asian climate, which is comparatively less explored. For CP La Niña, the South China Sea and northeast Asia experience dry condition while the Yangtze River Valley experiences wet condition. Similarly, an anticyclone-cyclone-anticyclone wind anomaly pattern is distinct in East Asia area. The low level anticyclone over WNP may enhance the moisture transport from the western tropical Pacific into subtropical frontal region, and thus more summer rainfall is expected over the subtropical monsoon front, the Yangtze River Valley region of China. The impact of CP (EP) La Niña on tropics and East Asia seems to be opposite to that in CP (EP) El Niño. Those spatial features can be generally captured by CAM4 simulations. Thus, we conclude that the CP El Niño (La Niña) may have more significant influence on East Asia summer climate than EP El Niño (La Niña), as the associated low-level anomalous wind pattern is more distinct and closer to the Asian continent compared to EP El Niño (La Niña).
From the corresponding composite results, there seem to be an asymmetry influence on south China between the CP (EP) El Niño and CP (EP) La Niño. For example, although south China may experiences more rain condition during CP El Niño, normal rain condition rather than less rain condition appears during CP La Niña. The physical reasons for the asymmetry of the influences between the CP (EP) El Niño and CP (EP) La Niño need further investigation.
To quantify the difference in the influence between CP El Niño (La Niña) and EP El Niño (La Niña), the western North Pacific summer monsoon index defined by Wang and Fan (1999) is used in this work. Both types of El Niño (La Niña) correspond to a stronger (weaker) western north Pacific summer monsoon. But the influence of EP El Niño (La Niña) on East Asia climate during boreal summer is less significant than CP El Niño (La Niña), in association with the eastward and southward shift in its significant influence zone to the Pacific Ocean compared to CP El Niño (La Niña). The impact between the two types of La Nina seem to be less independent compared to the two types of warm events, there is an asymmetric influence on East Asian between warm and cold events during summertime. Besides, the impact of CP El Niño is likely to be more robust than that of CP La Niña. The CAM4 sensitivity experiments support this hypothesis.
Kug et al. (2009) indicated that although two types of La Niña can be defined based on some criteria, their zonal SSTA distribution slightly changed and it is quite difficult to well separate them into two groups during boreal winter. In this study, we have analyzed the impacts of CP and EP La Niñas during boreal summer. It seems that there is different influence on East Asia between the two types of La Niña. Furthermore, the results based on observational composites may not be robust due to the small sample. Numerical studies may help to clarify this issue, but only one atmospheric model (i.e. CAM4) is used in this study. Much more investigation is needed in the future.
This research is jointly supported by the National Natural Science Foundation of China (Grant No. 41175076) and the National Key Basic Research Program of China (Grant No. 2009CB421404). RW acknowledges the support of a Hong Kong Research Grants Council grant (CUHK403612) and the National Natural Science Foundation of China (41275081). ZC acknowledges the support of the high-performance grid computing platform of Sun Yat-sen University.
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