Impacts of two types of La Niña on the NAO during boreal winter
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The present work identifies two types of La Niña based on the spatial distribution of sea surface temperature (SST) anomaly. In contrast to the eastern Pacific (EP) La Niña event, a new type of La Niña (central Pacific, or CP La Niña) is featured by the SST cooling center over the CP. These two types of La Niña exhibit a fundamental difference in SST anomaly evolution: the EP La Niña shows a westward propagation feature while the CP La Niña exhibits a standing feature over the CP. The two types of La Niña can give rise to a significantly different teleconnection around the globe. As a response to the EP La Niña, the North Atlantic (NA)–Western European (WE) region experiences the atmospheric anomaly resembling a negative North Atlantic Oscillation (NAO) pattern accompanied by a weakening Atlantic jet. It leads to a cooler and drier than normal winter over Western Europe. However, the CP La Niña has a roughly opposing impact on the NA–WE climate. A positive NAO-like climate anomaly is observed with a strengthening Atlantic jet, and there appears a warmer and wetter than normal winter over Western Europe. Modeling experiments indicate that the above contrasting atmospheric anomalies are mainly attributed to the different SST cooling patterns for the two types of La Niña. Mixing up their signals would lead to difficulty in seasonal prediction of regional climate. Since the La Niña-related SST anomaly is clearly observed during the developing autumn, the associated winter climate anomalies over Western Europe could be predicted a season in advance.
KeywordsTwo types of La Nina Climate impacts The North Atlantic and Western Europe
The El Niño–Southern Oscillation (ENSO) represents a periodic fluctuation between warm (El Niño) and cold (La Niña) conditions in sea surface temperature (SST) over the central to eastern tropical Pacific (Philander 1990; McPhaden et al. 2006). As one of the most important coupled ocean–atmosphere phenomenon, the ENSO has received extensive public attention because of its profound global climate impacts (e.g., van Loon and Madden 1981; Ropelewski and Halpert 1987, 1996; Trenberth and Caron 2000). By now, the linkage between ENSO and the climate in the North Pacific and North America has been well understood and is usually referred to as the “Pacific–North America” (PNA) teleconnection (e.g., Wallace and Gutzler 1981; Branston and Livezey 1987). However, climate responses to ENSO over the North Atlantic (NA)–Western European (WE) sector are controversial.
In the 1980s and early 1990s, early studies showed that ENSO-related precipitation and temperature anomalies are almost absent over the NA–WE region (Ropelewski and Halpert 1987; Halpert and Ropelewski 1992). The viewpoint is supported by later studies that the climate signal of ENSO over the NA–WE sector is difficult to be detected because of the large inter-event variability (see an extensive review by Brönnimann 2007). This non-stationary behavior is possibly due to some modulating factors, such as the complexity of ENSO itself (Greatbatch et al. 2004), natural (or internal) variability in the extratropical circulation (Kumar and Hoerling 1998), tropical volcanic eruptions (Brönnimann et al. 2007a), and other climate signals independent of ENSO (e.g., Mathieu et al. 2004; Garfinkel and Hartmann 2010). Nevertheless, the argument of the absence of ENSO signal over the NA–WE region was challenged by numerous studies (e.g., Brönnimann et al. 2007b; Ineson and Scaife 2009; Li and Lau 2012). These studies argued that a significant ENSO signal is found over the region of Europe despite the large inter-event variability. A canonical El Niño response in late winter is suggested to be accompanied by a negative North Atlantic Oscillation (NAO)-like pattern with a colder and drier than normal weather, and the La Niña has a largely opposing impact (e.g., Gouirand and Moron 2003; Brönnimann et al. 2007b). In comparison, the NA atmospheric response to La Niña is found to be much more stable than that due to El Niño during winter (Pozo-Vázquez et al. 2005). Since the NA atmosphere shows higher predictability associated with the La Niña compared to the El Niño, our focus of this study is on the NA–WE atmospheric response associated with La Niña events.
Recent studies argued that a new type (or flavor) of El Niño, in addition to the conventional El Niño, occurs more frequently in the recent decades with its maximum center over the central equatorial Pacific rather than the eastern Pacific (EP) (Larkin and Harrison 2005; Ashok et al. 2007; Kao and Yu 2009; Kug et al. 2009; Yeh et al. 2009; Ren and Jin 2011; Wang and Wang 2013). In particular, the new type of El Niño becomes the dominant mode after the late 1990s (Xiang et al. 2013). For convenience, EP and CP El Niños are referred to as the conventional and the new type of El Niño herein, respectively. Many studies have reported the importance of the CP El Niño in terms of its distinctly different climate impacts from the EP El Niño (Weng et al. 2007; Taschetto and England 2009; Feng et al. 2010; Feng and Li 2011, 2013; Lee et al. 2010; Zhang et al. 2011, 2012, 2013; Xie et al. 2012; Yu et al. 2012; Afzaal et al. 2013).
The La Niña diversity is also concerned in its impact on extratropical atmosphere, such as over East Asia (e.g., Wang et al. 2012). At present, there appears a scientific consensus on the occurrence of the new type of El Niño, however, whether La Niña events can be separated into two types remains open to debate. Some studies suggested that the zonal location of the maximum SST anomaly center does not show apparent change for individual La Niña event (Kug et al. 2009; Kug and Ham 2011; Ren and Jin 2011). On the contrary, some other studies argued for the existence of two types of La Niña (e.g., Cai and Cowan 2009; Shinoda et al. 2013). For example, the CP La Niña is argued to be clearly distinguished from the EP La Niña events in terms of ocean surface currents through analyzing recent satellite data (Shinoda et al. 2013). So far, the fundamental dynamics is not well understood that is used to explain differences in the generation and maintenance of two types of ENSO. Given unclear dynamical mechanisms, one possible way to distinguishing them is to investigate the associated local circulation and extratropical teleconnections. The analyses performed in this paper show that the winter atmospheric anomalies over the NA–WE region are very different from each other for these two types of La Niña. The result, on the one hand, will provide a possible indirect evidence for the existence of different flavors of La Niña. On the other hand, the necessity is emphasized to separate the La Niña events into two types when analyzing their associated extratropical climate impacts. Mixing up their signals would increase difficulty in seasonal prediction of the climate particularly over the NA–WE sector.
The purpose of the study is to explore the different teleconnection patterns and their associated climate anomalies over the NA–WE sector for the two types of La Niña. In the remainder of the paper, Sect. 2 describes data, methodology, and model experiments. Section 3 illustrates SST anomaly patterns for the two types of La Niña and its associated atmospheric responses over the tropical Pacific. Section 4 presents atmospheric responses over the NA–WE region. In Sect. 5, we explore possible mechanisms for the climate impacts on influencing the NA–WE climate associated with the two types of La Niña. Section 6 discusses asymmetry in influences of ENSO on the NA–WE climate. The major conclusions are summarized in Sect. 7.
2 Data and methodology
The monthly SST datasets (1951–2009) used in this study are the global sea ice and SST analyses from the Hadley Centre (HadISST1) provided by the Met Office Hadley Centre (Rayner et al. 2003). Atmospheric circulations were examined based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data (Kalnay et al. 1996). The precipitation data are taken from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) (1979–2009) (Xie and Arkin 1996) and the Global Precipitation Climatology Centre (GPCC) (1951–2009) (Rudolf et al. 2005). The surface temperature anomalies over WE are investigated using the Climate Research Unit (CRU) air temperature anomalies version 4 (CRUTEM4) (1951–2009) (Jones et al. 2012). Anomalies for all variables were conducted as the deviation from the 30-year climatological mean (1961–1990). The 1971–2000 average can also be defined as the climate mean, which does not influence qualitative results. The average over the entire period (1979–2009) is taken as the climate mean for the CMAP precipitation because the data are available after 1979. In order to remove possible influence associated with the long-term trend, all anomalies are linearly detrended over the period 1951–2009, except for the CMAP data over the period 1979–2009. The non-detrended data are also examined and the results are almost the same. Composite and regression analyses are employed to investigate differences in climatic impact associated with the two types of La Niña, using Student’s two-tailed significance test.
2.2 Definition of two types of La Niña events
Unlike contrasting SST anomaly patterns associated with the two types of El Niño, Ren and Jin (2011) suggested that the La Niña events seem to be difficult to be clearly separated into two types based on their index. Based on the DJF (December–February) mean ENSO and ENSO Modoki indices (Ashok et al. 2007), half of events selected are the same in the two types of La Niña (Tedeschi et al. 2012). It is also expected that the two types of La Niña events cannot be well distinguished based on the index defined by Kao and Yu (2009), since the current indices of the two-type ENSO show high consistency (Ren and Jin 2013). Considering the fact that the present ENSO indices cannot effectively distinguish the two types of La Niña events, we therefore identify the selection by an analysis of the spatial distribution of SST anomaly patterns. First, 17 La Niña winters are defined by the Climate Prediction Center (CPC) over the period 1951–2009 based on a threshold of −0.5 °C for winter (DJF) mean Niño3.4 (5°S–5°N, 120°–170°W) SST anomaly. Then we identify seven EP La Niña winters (1954/55, 1955/56, 1964/65, 1971/72, 1984/85, 1995/96, and 2005/06) and seven CP La Niña winters (1973/74, 1974/75, 1975/76, 1983/84, 1988/89, 1998/99, and 2000/01). The winters, having larger SST anomaly in the EP (CP) east (west) of 150°W during the developing and mature phases of La Niña, are classified into the EP (CP) La Niña winters. The longitude of 150°W is selected because it is a boundary of Niño3 (5°S–5°N, 150°–90°W) and Niño4 (5°S–5°N, 160°E–150°W) areas, which are usually used to define the two-type ENSO events (e.g., Kim et al. 2009; Kug et al. 2009). Other three years (1970/71, 1999/00, and 2007/08) are defined as the mixed type of La Niña, because the large cooling SST anomaly covers both the EP and CP during the mature phase. Their characteristics will be further discussed in Sect. 3. The year listed here corresponds to year(0)/year(1). The developing year of the La Niña event and the following year is designated as year(0) and year(1), respectively. A typical ENSO tends to develop during the spring season and lasts for roughly a year. However, long-lasting La Niña events are often observed, such as the events for 1954–56 and 1973–76 selected in this study. After excluding these events, the qualitative difference influencing conclusions is not detected.
List of SST perturbation experiments conducted in this study
Description of SST perturbation
Cooling anomalies associated with EP La Niña events imposed in the tropical Pacific (30°S–30°N, 120°E–90°W)
As in EP_COOL but for the CP La Niña events
As in CP_COOL but for warming anomalies in the tropical Pacific (30°S–30°N, 120°E–120°W)
CP_COOL cooling and CP_WARM warming anomalies imposed together
As in CP_COOL but for cooling anomalies in the northern tropical Atlantic Ocean (10°S–25°N, 0°–80°W)
CP_COOL and AT_COOL cooling anomalies imposed together
3 SST anomaly pattern and its associated atmospheric response over the tropical Pacific
4 Atmospheric responses over NA and WE
As indicated by the composite geopotential height at 300 hPa (Fig. 6c, d), the similar anomaly pattern in the lower troposphere can also be detected in the upper troposphere over the North Pacific and NA–WE regions. The barotropic features are shown in the atmospheric response to the two types of La Niña over the mid-latitude regions. The result is consistent with the previous study (Ting 1996), in which it is pointed out that the tropical heating can cause a barotropic response in the atmospheric circulation over the extratropics.
5 Mechanisms for the contrasting impacts over NA–WE of two types of La Niña
Previous studies (e.g., Li et al. 2006) have discussed importance of the western Pacific warming. Here, the CP_WARM experiment is conducted to inspect possible impacts of the western Pacific warming on the North Atlantic atmosphere. As shown in Figs. 9c and 10c, circumglobal wave train is displayed, suggesting that the CP_WARM has a minor impact on the positive NAO-like atmospheric response for the CP La Niña. We also consider the impacts of both cooling and warming SST anomalies over the tropical Pacific in the CP_CW simulation. Their responses appear to be a mixture of the CP_COOL and CP_WARM responses (Figs. 9d, 10d), which are largely the same as those of the CP_COOL forcing.
In addition, many studies reported that the tropical Pacific heating have effects on the tropical Atlantic SST anomaly (Wolter 1987; Curtis and Hastenrath 1995; Gallego et al. 2001; Alexander et al. 2002; Huang et al. 2002), which is argued to affect the North Atlantic atmosphere (e.g., Watanabe and Kimoto 1999; Robertson et al. 2000). Therefore, the tropical Atlantic SST may serve as a mediator to link the tropical Pacific SST anomaly and the NA atmosphere. In order to examine the possible effects of the tropical Atlantic SST, Fig. 3 also presents the SST and surface wind anomalies over the Atlantic during the two types of La Niña winters. For the EP La Niña, almost no significant SST anomalies are observed over the tropical Atlantic but the SST warming over the eastern NA is robust (Fig. 3a). The warm SST anomaly is arguably due to the cyclonic circulation and the associated easterly wind anomalies, which could weaken the strong background westerlies and thus the local evaporation. In contrast to the EP La Niña, there appear significantly cold SST anomalies over the northern tropical Atlantic and warming SST anomalies over the western NA during the CP La Niña winters (Fig. 3b). In accordance with the SST anomaly pattern, an unusually anticyclonic circulation occurs over the NA. Over the western mid-latitude Atlantic, the anomalous southeasterlies can possibly pile up the surface warm water and lead to increase in the SST there (Fig. 3b). The SST cooling in the tropical Atlantic could also be regarded to be a consequence due to the strengthened easterlies and thus evaporation through wind–evaporation–SST feedback.
A series of modelling experiments discussed above suggest that the two types of La Niña have different impacts on the NA–WE atmosphere through the atmospheric teleconnection. The tropical Atlantic SST anomalies associated with the CP La Niña also have effects on NA atmospheric anomalies. Although the simulated experiments suggest that the contrasting atmospheric anomalies in the NA are mainly attributed to different cooling SST anomaly patterns for the two types of La Niña, dynamical mechanism addressing how the tropical SST influences the NA–WE atmosphere is still an open question. The atmosphere over the North Pacific is usually argued to be linked to the tropical Pacific heating and the NA–WE atmosphere anomalies (e.g., Wu and Hsieh 2004; Li and Lau 2012). The North Pacific anomalies could modify local mean flow and standing waves, which possibly propagate downstream to the North Atlantic and leads to different NAO-like atmospheric responses. There exhibits a nonlinear relationship between the atmospheric anomalies over the North Pacific and NA. For example, Castanheira and Graf (2003) demonstrated that a significantly negative correlation could be detected between the SLP over the North Pacific and the NA only when the polar vortex is strong enough. Recently, the subtropical jet is also emphasized to act as an “atmospheric bridge” to connect the tropical Pacific heating and NA–WE atmospheric anomalies (Graf and Zanchettin 2012). Further studies are required to understand the mechanisms behind the contrasting atmospheric anomalies over the Atlantic Ocean with these two types of La Niña.
6 Discussion: Asymmetry in influences of ENSO on climate over the NA–WE sector
An investigation discussed above shows that the two types of La Niña have roughly opposing impacts on the atmosphere over the NA–WE sector. A significantly negative (positive) NAO-like pattern is observed during the EP (CP) La Niña winters. The previous study (Graf and Zanchettin 2012) have compared the climate impacts associated with the EP and CP El Niño events and suggested that the two types of El Niño lead to distinctly different atmospheric responses over the NA–WE region. It is found that a significantly negative NAO-like pattern occurs over the NA–WE region during the CP El Niño winter, whereas no apparent signal is found there during the EP El Niño winter.
As shown in Fig. 12a, a negative NAO index appears in most of the EP La Niña winters. Their composite NAO index reaches −2.1, which is statistically significant at the 95 % confidence level. However, four out of six EP El Niño events are in favor of occurrence of a negative NAO-like pattern, and another two events correspond to a positive NAO-like pattern (Fig. 12a). Their composite result shows a weak negative NAO index, which is not significant at the 95 % confidence level. It is consistent with the previous study (Graf and Zanchettin 2012) suggesting that the atmospheric response to the EP El Niño over the NA–WE region seems to be originated by chance. As a consequence, the EP El Niño effect on NAO seems to be asymmetric to the EP La Niña effect during winter.
Similar to the El Niño phenomena discussed previously, this study has shown that the La Niña should be classified into two types (i.e., the EP and CP La Niña) considering their distinctly different climate impacts. The EP La Niña is characterized by the cooling SST anomaly center confined to the EP east of 150°W and relatively weak SST anomaly observed over the CP. By contrast, the SST anomaly center associated with the CP La Niña is shifted westward into the CP west of 150°W and small cooling SST anomaly is found over the EP. The two types of La Niña exhibit very different features in the SST anomaly evolution. For the EP La Niña, the SST anomaly starts in the EP and propagates westward during the developing and mature phase, while the CP La Niña shows a standing feature with its SST anomaly developing and decaying in situ over the CP. These differences in zonal location of SST anomaly and their evolutions suggest the possibility in different underlying dynamics.
Although the two types of La Niña can produce a similar response in the atmosphere over the north Pacific, distinctly different teleconnection patterns are found over the NA–WE sector. For the EP La Niña, the NA–WE region experiences the climate anomalies resembling a negative NAO pattern accompanied by a weakening Atlantic jet. This weakening jet tends to inhibit a strong transportation of warm and moist air from the Atlantic sea and cause a cooler and drier than normal winter over the WE region. However, roughly opposing atmospheric anomalies appear over the NA–WE sector during the CP La Niña winter, which seems like a positive NAO phase with strengthening Atlantic jet. The strong jet tends to bring more warm and moist air from the sea to the WE area and results into a warmer and wetter than normal winter there. A series of modeling experiments indicate that the contrasting NAO-like patterns are mainly attributed to different cooling SST patterns for two types of La Niña. The analyses provided here have shown that it is necessary to separate the La Niña into two types considering their different SST anomaly location and evolution, and especially, very different climate impacts over the extratropics.
This work is supported by the National Basic Research Program “973” (2012CB417403), the National Nature Science Foundation of China (41005049), the Special Fund for Public Welfare Industry (Meteorology) (GYHY201206016), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). BX is supported by APEC Climate Center. BX also acknowledges partial support from International Pacific Research Center which is sponsored by the JAMSTEC, NASA and NOAA.
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