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Impacts of two types of La Niña on the NAO during boreal winter

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Abstract

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.
Impacts of two types of La Nin
˜a on the NAO during boreal winter
Wenjun Zhang Lei Wang Baoqiang Xiang
Li Qi Jinhai He
Received: 12 October 2013 / Accepted: 17 April 2014 / Published online: 4 May 2014
ÓThe Author(s) 2014. This article is published with open access at Springerlink.com
Abstract The present work identifies two types of La
Nin
˜a based on the spatial distribution of sea surface tem-
perature (SST) anomaly. In contrast to the eastern Pacific
(EP) La Nin
˜a event, a new type of La Nin
˜a (central Pacific,
or CP La Nin
˜a) is featured by the SST cooling center over
the CP. These two types of La Nin
˜a exhibit a fundamental
difference in SST anomaly evolution: the EP La Nin
˜a
shows a westward propagation feature while the CP La
Nin
˜a exhibits a standing feature over the CP. The two types
of La Nin
˜a can give rise to a significantly different tele-
connection around the globe. As a response to the EP La
Nin
˜a, 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 Nin
˜a 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 Nin
˜a. Mixing up their signals would lead to
difficulty in seasonal prediction of regional climate. Since
the La Nin
˜a-related SST anomaly is clearly observed dur-
ing the developing autumn, the associated winter climate
anomalies over Western Europe could be predicted a sea-
son in advance.
Keywords Two types of La Nina Climate impacts
The North Atlantic and Western Europe
1 Introduction
The El Nin
˜o–Southern Oscillation (ENSO) represents a
periodic fluctuation between warm (El Nin
˜o) and cold
(La Nin
˜a) 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
W. Zhang (&)L. Wang L. Qi J. He
Collaborative Innovation Center on Forecast and Evaluation of
Meteorological Disasters, Key Laboratory of Meteorological
Disaster of Ministry of Education, Nanjing University of
Information Science and Technology, Nanjing 210044, China
e-mail: zhangwj@nuist.edu.cn
W. Zhang
Key Laboratory of Numerical Modeling for Atmospheric
Sciences and Geophysical Fluid Dynamics, Institute of
Atmospheric Physics, Chinese Academy of Sciences,
Beijing 100029, China
B. Xiang
International Pacific Research Center, University of Hawaii
at Manoa, Honolulu, HI 96822, USA
123
Clim Dyn (2015) 44:1351–1366
DOI 10.1007/s00382-014-2155-z
almost absent over the NA–WE region (Ropelewski and
Halpert 1987; Halpert and Ropelewski 1992). The view-
point 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 exten-
sive review by Bro
¨nnimann 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 circu-
lation (Kumar and Hoerling 1998), tropical volcanic
eruptions (Bro
¨nnimann 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., Bro
¨nnimann
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 Nin
˜o 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 Nin
˜a has a largely
opposing impact (e.g., Gouirand and Moron 2003;
Bro
¨nnimann et al. 2007b). In comparison, the NA atmo-
spheric response to La Nin
˜a is found to be much more
stable than that due to El Nin
˜o during winter (Pozo-Va
´z-
quez et al. 2005). Since the NA atmosphere shows higher
predictability associated with the La Nin
˜a compared to the
El Nin
˜o, our focus of this study is on the NA–WE atmo-
spheric response associated with La Nin
˜a events.
Recent studies argued that a new type (or flavor) of El
Nin
˜o, in addition to the conventional El Nin
˜o, 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 Nin
˜o becomes the dominant mode after the
late 1990s (Xiang et al. 2013). For convenience, EP and CP
El Nin
˜os are referred to as the conventional and the new
type of El Nin
˜o herein, respectively. Many studies have
reported the importance of the CP El Nin
˜o in terms of its
distinctly different climate impacts from the EP El Nin
˜o
(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 Nin
˜a 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 Nin
˜o,
however, whether La Nin
˜a 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 Nin
˜a
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 Nin
˜a (e.g., Cai and Cowan
2009; Shinoda et al. 2013). For example, the CP La Nin
˜ais
argued to be clearly distinguished from the EP La Nin
˜a
events in terms of ocean surface currents through analyzing
recent satellite data (Shinoda et al. 2013). So far, the fun-
damental 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 telecon-
nections. 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
Nin
˜a. The result, on the one hand, will provide a possible
indirect evidence for the existence of different flavors of La
Nin
˜a. On the other hand, the necessity is emphasized to
separate the La Nin
˜a events into two types when analyzing
their associated extratropical climate impacts. Mixing up
their signals would increase difficulty in seasonal predic-
tion 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
Nin
˜a. In the remainder of the paper, Sect. 2describes data,
methodology, and model experiments. Section 3illustrates
SST anomaly patterns for the two types of La Nin
˜a and its
associated atmospheric responses over the tropical Pacific.
Section 4presents atmospheric responses over the NA–WE
region. In Sect. 5, we explore possible mechanisms for the
climate impacts on influencing the NA–WE climate asso-
ciated with the two types of La Nin
˜a. Section 6discusses
asymmetry in influences of ENSO on the NA–WE climate.
The major conclusions are summarized in Sect. 7.
2 Data and methodology
2.1 Observations
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
1352 W. Zhang et al.
123
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 vari-
ables 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 dif-
ferences in climatic impact associated with the two types of
La Nin
˜a, using Student’s two-tailed significance test.
2.2 Definition of two types of La Nin
˜a events
Unlike contrasting SST anomaly patterns associated with
the two types of El Nin
˜o, Ren and Jin (2011) suggested that
the La Nin
˜a events seem to be difficult to be clearly sep-
arated into two types based on their index. Based on the
DJF (December–February) mean ENSO and ENSO Mod-
oki indices (Ashok et al. 2007), half of events selected are
the same in the two types of La Nin
˜a (Tedeschi et al. 2012).
It is also expected that the two types of La Nin
˜a 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 Nin
˜a events, we
therefore identify the selection by an analysis of the spatial
distribution of SST anomaly patterns. First, 17 La Nin
˜a
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 Nin
˜o3.4 (5°S–5°N, 120°
170°W) SST anomaly. Then we identify seven EP La Nin
˜a
winters (1954/55, 1955/56, 1964/65, 1971/72, 1984/85,
1995/96, and 2005/06) and seven CP La Nin
˜a 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 Nin
˜a, are classified into the EP
(CP) La Nin
˜a winters. The longitude of 150°W is selected
because it is a boundary of Nin
˜o3 (5°S–5°N, 150°–90°W)
and Nin
˜o4 (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
Nin
˜a, because the large cooling SST anomaly covers both
the EP and CP during the mature phase. Their character-
istics will be further discussed in Sect. 3. The year listed
here corresponds to year(0)/year(1). The developing year
of the La Nin
˜a 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 Nin
˜a 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.
2.3 Simulations
All model simulations are performed using the National
Center for Atmospheric Research (NCAR) Community
Atmospheric Model Version 5 (CAM5) (Neale et al. 2010).
CAM5 has been updated in many physical processes
compared to the previous version. The version has a finite
volume dynamic core with resolution of 1.9°longi-
tude 92.5°latitude and 30 vertical levels. In the control
run (CNTRL), CAM5 is driven by climatological (seasonal
varying) SST and the results were derived as a reference
state. A series of sensitive experiments listed in Table 1
were performed to compare the climate impacts of different
SST anomaly patterns associated with the two types of La
Nin
˜a. In the first simulation (EP cooling, EP_COOL), the
cold SST anomaly during the EP La Nin
˜a winter is
imposed on the monthly climatological SST from October
to February in the tropical Pacific (30°S–30°N, 120°E–
90°W). All anomalies outside of the region are set to zero.
The second experiment (CP cooling, CP_COOL) designed
is the same as the EP_COOL experiment, expect that the
SST anomaly is the composition during the CP La Nin
˜a
winter. The third experiment (CP warming, CP_WARM) is
Table 1 List of SST perturbation experiments conducted in this
study
Expt Description of SST perturbation
EP_COOL Cooling anomalies associated with EP La Nin
˜a
events imposed in the tropical Pacific (30°S–30°N,
120°E–90°W)
CP_COOL As in EP_COOL but for the CP La Nin
˜a events
CP_WARM As in CP_COOL but for warming anomalies in the
tropical Pacific (30°S–30°N, 120°E–120°W)
CP_CW CP_COOL cooling and CP_WARM warming
anomalies imposed together
AT_COOL As in CP_COOL but for cooling anomalies in the
northern tropical Atlantic Ocean (10°S–25°N, 0°
80°W)
CPAT_COOL CP_COOL and AT_COOL cooling anomalies
imposed together
Impacts of two types of La Nin
˜a 1353
123
also the same as the EP_COOL and CP_COOL experi-
ments, but the warming SST anomaly during the CP La
Nin
˜a winter is added to the seasonally varying monthly
climatological SST over the western tropical Pacific. In this
experiment, we consider the possible impacts of the
warming SST anomaly, since the positive SST anomaly
appears significantly over the western tropical Pacific
during the CP La Nin
˜a winter. In the fourth experiment (CP
cooling and warming, CP_CW), we conduct sensitivity
simulations where the CP_COOL and CP_WARM forcings
are imposed together to inspect the combined contribution
of the warming and cooling SST anomaly over the tropical
Pacific during the CP La Nin
˜a winter. We also study pos-
sible effects of the cooling SST anomalies in the northern
tropical Atlantic Ocean during the CP La Nin
˜a winter in the
fifth experiment (Atlantic cooling, AT_COOL), since the
significant cooling SST anomalies occur there. In the last
experiment (CP and Atlantic cooling, CPAT_COOL), the
CP_COOL and AT_COOL forcings are both imposed to
examine their combined impacts on atmosphere. All sim-
ulations are integrated for 15 years and the last 10 years’
integration was considered to exclude influence of the
initial condition and the internal variability.
3 SST anomaly pattern and its associated atmospheric
response over the tropical Pacific
Figure 1displays the seasonal evolution of the equatorial
(5°S–5°N) SST anomaly for the above mentioned three
types of La Nin
˜a. The evolution of the EP La Nin
˜ais
similar to the conventional La Nin
˜a event (Fig. 1a) with its
SST anomaly developing in the far EP and reaching its
largest amplitude during November(0) and December(0).
The evolution of the maximum centers indicated by
marked crosses shows that the EP La Nin
˜a–SST anomaly
propagates westward at a certain speed. During the
developing and mature phase, the EP La Nin
˜a is manifested
by larger cooling SST anomaly mainly confined in the
eastern equatorial Pacific east of 150°W. In contrast to the
EP La Nin
˜a, the CP La Nin
˜a exhibits a fundamental dif-
ference in the SST anomaly structure and its evolution
(Fig. 1b). Firstly, its action center is shifted westward into
the central equatorial Pacific. Secondly, the SST anomaly
almost does not propagate for the CP La Nin
˜a, representing
a standing feature. The CP La Nin
˜a seems to be mainly
associated with the local air–sea interaction that develops
and decays in situ over the CP. As the CP La Nin
˜a
develops, its SST anomaly also extends eastward and
westward from the CP. Contrasting features of SST
anomaly evolution associated with the EP and CP La Nin
˜a
suggest that their underlying dynamics should be different,
which provides a possible evidence for the existence of
different types of La Nin
˜a. Such discussion has been given,
for example, previous studies argued that the thermocline
dynamics, as the most important dynamics for traditional
(or EP) ENSO, seems to play a less important role on the
CP ENSO (e.g., Kao and Yu 2009). For the mixed type of
La Nin
˜a (Fig. 1c), the SST anomaly center starts in the EP,
and shifts slightly eastward in the developing phase. Dur-
ing the mature phase, the SST anomaly center propagates
rapidly from the EP to the CP. The large negative SST
anomaly stretches across the EP and CP during the mixed
type of La Nin
˜a winter, mixing the SST anomalies asso-
ciated with the EP and CP La Nin
˜a (not shown).
To further investigate the phase locking of La Nin
˜a, we
use Nin
˜o3, Nin
˜o4, and Nin
˜o3.4 SST anomaly to denote the
EP, CP, and mixed type, respectively. As shown in Fig. 2,
the EP and CP La Nin
˜a events exhibit an approximate
feature during the developing phase. Both types of events
reach the maximum around December(0) with almost the
same intensity. After December(0), the CP La Nin
˜a enters a
slower decay phase than the EP La Nin
˜a. For the mixed
type of La Nin
˜a, it is characterized by much larger
amplitude and the delayed occurrence of the mature phase
by a month compared to the EP and CP La Nin
˜a. Again, the
mixed type of La Nin
˜a exhibits its distinct feature and it
seems necessary to classify them into a single group. Since
the action center of the mixed-type La Nin
˜a covers the EP
and CP as shown in Fig. 1, it may mix signals of the EP
and CP La Nin
˜a in terms of the extratropical atmospheric
response. In this paper, the contrasting climate impacts of
the EP and CP La Nin
˜a over the NA–WE sector are our
focus, so the mixed type of La Nin
˜a will not be discussed in
the remainder of the paper.
Figure 3shows the composite SST and surface wind
anomalies during EP and CP La Nin
˜a winters. During
boreal winter, the SST anomaly for the EP La Nin
˜a covers
the CP and EP with the maximum center occurring in the
EP. Almost no significant warming SST anomaly is found
over other domain of the tropical Pacific. As a Rossby
wave response (Gill 1980) to the cooling SST anomaly, a
pair of anticyclone anomalies resides at each side of the
central equatorial Pacific accompanying by easterly wind
anomaly at the equator. In contrast, the cooling center of
SST anomaly associated with the CP La Nin
˜a is displaced
westward into the central equatorial Pacific west of 150°W
with a weak SST anomaly in the far eastern equatorial
Pacific. As shown in Fig. 3, the amplitude of the CP La
Nin
˜a appears to be stronger than that of the EP La Nin
˜a,
with significant warming SST anomaly occurring in the
northwestern and southwestern tropical Pacific. The
weaker amplitude of the EP La Nin
˜a is possibly associated
with the faster decaying rate in its SST anomaly (Fig. 2). In
comparison with the surface wind response to the EP La
Nin
˜a, the pair of anticyclone anomalies occurring at both
1354 W. Zhang et al.
123
sides of the equator also shifts relatively westward into the
western tropical Pacific (Fig. 3b). It is notable that westerly
anomaly occurring over the far eastern equatorial Pacific
that is not observed in the EP La Nin
˜a composition could
inhibit the cooling upwelling and thus weaken the SST
anomaly there.
The tropical convection anomalies associated with the
two types of La Nin
˜a are expected to be distinct due to their
differing SST anomaly patterns (Fig. 4). Because precipi-
tation data over the tropical ocean are available since the
late 1970s, the divergence of water vapor (integrated from
surface to 300 hPa) is examined here to roughly reflect the
precipitation anomaly based on the balance equation of the
atmospheric water vapor (Yanai et al. 1973). Correspond-
ing to the EP La Nin
˜a, the associated convective anomaly
center emerges mainly over the CP to the west of the
negative SST anomaly center (Fig. 4a). Enhanced precip-
itation indicated by the convergence of water vapor appears
to the south and the north of the positive center and over
the Philippine Sea. During the CP La Nin
˜a winter, the
center of the negative precipitation is also located over the
central tropical Pacific but shifted slightly westward com-
pared to that during the EP La Nin
˜a winter (Fig. 4b). The
intensity of precipitation response to the CP La Nin
˜ais
obviously stronger than that to the EP La Nin
˜a, which is
associated with the stronger SST anomaly for the CP La
Nin
˜a. Another possible reason is related to the different
location of SST anomaly. Compared to the EP, the con-
vection over the CP is much more sensitive to the SST
anomaly because of a higher background SST (e.g., Kug
et al. 2009). As such, the SST anomaly for the CP La Nin
˜a
can induce stronger atmospheric response than that for the
(a) (b) (c)
Fig. 1 Time-longitude diagram of SST anomaly (°C) composites in
the equatorial Pacific (5°S–5°N) for aEP La Nin
˜a, bCP La Nin
˜a, and
cmixed La Nin
˜a. The ordinate presents an 11-month period from July
of year 0 to May of year 1. Contour lines indicate values that are
significant at the 95 % confidence level. Red crosses mark the
longitudes of the maximum SST anomalies which are smoothed
spatially using a 3-point running mean
Fig. 2 Composite monthly evolution of the Nin
˜o3 SST anomalies
(°C) for the EP La Nin
˜a(red curve), the Nin
˜o4 SST anomalies for the
CP La Nin
˜a(blue curve), and Nin
˜o3.4 SST anomalies for the mixed
La Nin
˜a(black curve). The abscissa indicates a 13-month period from
July of year 0 to July of year 1
Impacts of two types of La Nin
˜a 1355
123
EP La Nin
˜a. Another difference in the moist convergence
anomaly is that a more precipitation belt elongates north-
eastward from the Philippine Sea to the CP during the CP
La Nin
˜a episode. The CMAP precipitation is also examined
to investigate the convection anomalies for these two types
of La Nin
˜a compositions after 1979 and their difference is
similar to that indicated by the divergence of water vapor
except for the region of the northwestern tropical Pacific
(Fig. 4).
4 Atmospheric responses over NA and WE
The contrast in the tropical atmosphere anomalies, in
association with different SST anomaly patterns of the two
types of La Nin
˜a, may result in large differences in the
extratropical circulation and thus regional climate. In this
paper, we focus on the climate response over the NA–WE
sector, in particular on their potential impacts on the NAO
associated with these two types of La Nin
˜a since it is the
dominant climate variability mode over the NA–WE sector.
In general, ENSO events reach their peaks during late
autumn and winter, however, the associated climate impacts
over the NA and WE region are found to be significant
during late winter (Gouirand and Moron 2003; Knippertz
et al. 2003; Bro
¨nnimann et al. 2007b). To illustrate the
seasonality of the ENSO signal, the NAO index is defined as
the difference in the normalized monthly sea level pressure
(SLP) regionally zonal-averaged over the NA–WE sector
from 80°Wto30°E between 35°N and 65°N (Li and Wang
2003). This simple NAO index is demonstrated to well
describe the spatial–temporal characteristics associated
with NAO (Li and Wang 2003). For the EP La Nin
˜a, the
NAO index appears to be at a normal state in ND(0)
(Fig. 5). During the JFM(1) period, a negative value cor-
responds to a negative NAO-like pattern indicative of a high
pressure anomaly in the mid-latitude and a low pressure
anomaly in the subtropics. This configuration is reversed
from AM(1). For the CP La Nin
˜a, the atmospheric response
in N(0) is characterized by a weak negative NAO-like
pattern. The revised sign of the NAO index in following
3 months is manifested by a positive NAO-like pattern,
demonstrating a low pressure anomaly to the north and a
high pressure anomaly to the south (Fig. 5). As shown in
Fig. 5, an opposite sign of the atmospheric response is
observed in winter for the two types of La Nin
˜a, and the
difference is most evident in JF(1). To detect the robust
signal, we shall hereafter define ‘‘winter’’ as the JF period
when investigating the atmospheric responses over the NA–
WE sector to the two types of La Nin
˜a. The ‘‘winter’’ can
also be defined by D(0)JF(1), and even D(0)JFM(1) or
JFM(1), and the qualitative conclusion is unchanged.
One prominent teleconnection associated with ENSO
events has been referred to as the PNA pattern
(a)
(b)
Fig. 3 Composites of SST
anomalies (shading and
contours in °C) and surface
wind anomalies (vectors in m/s)
during DJF for aEP La Nin
˜a,
and bCP La Nin
˜a. The SST
anomalies that are not
significant at the 95 % level are
not shown. Contour interval is
0.5 °C and zero contours are
omitted. Only values above
0.6 m/s are shown for surface
wind anomalies
1356 W. Zhang et al.
123
accompanied by an intensified Aleutian low during ENSO
warm phase. Corresponding to the ENSO cold phase, a
positive SLP anomaly covers the North Pacific indicating
a weakened Aleutian Low during both types of La Nin
˜a
events (Fig. 6a, b). Compared to the EP La Nin
˜a
composite, the significantly positive SLP anomaly is
slightly shifted southeastward for the CP La Nin
˜a com-
posite. However, their differences in SLP are pronounced
over the NA–WE sector. During winter for the EP La
Nin
˜a, a significantly high pressure anomaly to the north
and a significantly low pressure anomaly to the south
elongate zonally from the central North Atlantic to Eur-
ope (Fig. 6a). This SLP anomaly response resembles the
negative NAO-like pattern. Nevertheless, the signal of the
CP La Nin
˜a is roughly opposite to that of the EP La
Nin
˜a. A significant low (high) pressure anomaly to the
north (south) extends zonally from the western to eastern
NA (Fig. 6b), which corresponds to a positive NAO-like
pattern.
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 tro-
posphere over the North Pacific and NA–WE regions. The
barotropic features are shown in the atmospheric response
to the two types of La Nin
˜a 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
(a)
(b)
Fig. 4 Composites of vertically
integrated moisture divergence
(shading and black contours in
mm/day) and CMAP
precipitation (pink contours in
mm/day) during DJF for aEP
La Nin
˜a, and bCP La Nin
˜a.
Shading presents values
exceeding the 90 and 95 %
confidence level. Black and pink
contour intervals are 1 and
2 mm/day, respectively. Zero
contours are omitted
Fig. 5 Composite monthly evolution of the NAO index for EP La
Nin
˜a(solid line) and CP La Nin
˜a(dashed line). The abscissa indicates
a 7-month period from November of year 0 to May of year 1
Impacts of two types of La Nin
˜a 1357
123
can cause a barotropic response in the atmospheric circu-
lation over the extratropics.
It is suggested that the subtropical jet is of importance in
bridging the ENSO and NAO teleconnection (Graf and
Zanchettin 2012). Here, the composite zonal wind anom-
alies at 200 hPa are displayed to investigate the change in
jet stream for the two types of La Nin
˜a (Fig. 7). In asso-
ciation with the EP La Nin
˜a, the zonal wind anomalies at
200 hPa over the North Pacific exhibit a tripolar structure
and tilt slightly southeastward (Fig. 7a). In the mid-latitude
of the North Pacific, significantly negative anomalies
suggest a weakening East Asia subtropical jet. These
anomalies elongate zonally from the western Pacific and
stay west of 60°W. Almost opposite anomaly structure in
200 hPa zonal wind emerges over the NA. The Atlantic jet
is significantly weakened indicated by a negative anomaly
in zonal wind at 200 hPa (Fig. 7a), corresponding to a
negative NAO-like atmospheric response (Fig. 6a, c).
During the CP La Nin
˜a, a similarly tripolar structure in
200 hPa zonal wind anomalies appears over the North
Pacific, however, the location is displaced equatorward.
The anomalies elongate zonally from the North Pacific and
extend far eastward to the NA. The Atlantic jet is
significantly strengthened and extends farther eastward
reaching WE, which corresponds to the positive NAO-like
atmospheric anomalies as shown in Fig. 6b, d. It can be
seen that the two types of La Nin
˜a could lead to a roughly
opposing response in the Atlantic jet, consistent with an
opposing NAO-like pattern over the NA (Figs. 5,6).
Many studies have demonstrated that the NAO con-
tributes significantly to surface temperature and precipita-
tion over the WE during winter (see the review of Jones
et al. 2003). The approximately opposing atmospheric
responses to the two types of La Nin
˜a over the NA may
result in diametric climate anomalies over WE. As
expected, the anomalies in surface air temperature and
precipitation show very different patterns (Fig. 8). During
the EP La Nin
˜a winter, the weakened Atlantic jet associ-
ated with the negative NAO phase tends to transport
unusually cold and dry air to WE. Thus a winter occurs
over WE that is colder than normal, where the anomalous
surface air temperature can reach -2°C (Fig. 8). Simul-
taneously, the precipitation is reduced in most regions of
WE, whereas the southwestern region including Spain and
Portugal receives excessive precipitation, which is likely
associated with the strengthened zonal wind to the south of
(a) (c)
(b) (d)
Fig. 6 Composites of JF SLP anomalies (hPa) for aEP La Nin
˜a and
bCP La Nin
˜a, and JF geopotential height anomalies (m) at 300 hPa
for cEP La Nin
˜a and dCP La Nin
˜a. Shading indicates values
exceeding the 90 and 95 % confidence level. Contour intervals in (a,
b) and (c,d) are 2 hPa and 30 m, respectively
1358 W. Zhang et al.
123
(a)
(b)
Fig. 7 Composites of JF zonal
wind anomalies (m/s) at
200 hPa for aEP La Nin
˜a and
bCP La Nin
˜a. Shading indicates
values exceeding the 90 and
95 % confidence level. Contour
intervals are 2 m/s
(a) (c)
(b) (d)
Fig. 8 Composites of JF surface air temperature anomalies (contours
in °C) for aEP La Nin
˜a and bCP La Nin
˜a, and JF GPCC precipitation
anomalies (contours in mm/day) for cEP La Nin
˜a and dCP La Nin
˜a.
Shading indicates values exceeding the 90 and 95 % confidence level.
Contour intervals in (a,b) and (c,d) are 0.5 °C and 0.2 mm/day,
respectively
Impacts of two types of La Nin
˜a 1359
123
the Atlantic jet (Fig. 7a). In comparison, WE experiences a
warmer than normal winter during the CP La Nin
˜a winter.
This is because that the enhanced Atlantic jet across the
NA related to the positive NAO phase tends to transport
relatively warm and moist air to WE. The precipitation
anomalies are characterized by a dipolar structure with
increasing (decreasing) over northern (southern) WE. As
demonstrated in Fig. 8, WE experiences a very different
climate anomalies corresponding to the two different types
of La Nin
˜a. Therefore, it is necessary to consider the two
types of La Nin
˜a events when understanding their climate
impacts.
5 Mechanisms for the contrasting impacts
over NA–WE of two types of La Nin
˜a
According to the observed analyses above, different SST
anomaly patterns during the two types of La Nin
˜a are
possibly responsible for the approximately opposing NAO-
like atmospheric anomalies. To verify it, four experiments
were designed and performed, which has been described in
Sect. 2. Figure 9shows the SLP responses to the
EP_COOL, CP_COOL, CP_WARM, and CP_CW forcings
relative to the CNTRL run. In the EP_COOL simulations,
the imposed tropical SST cooling induces a weakened
Aleutian Low and a negative NAO-like atmospheric
anomaly indicated by a high SLP anomaly to the north and
a low SLP anomaly to the south of the North Atlantic
(Fig. 9a). These anomalies closely resemble the observed
EP La Nin
˜a composition (Fig. 6a). Under the CP_COOL
forcing, a positive SLP anomaly appears over the North
Pacific and a positive NAO-like atmospheric response
occurs over the North Atlantic (Fig. 9b). These features
agree well with the observed anomaly patterns during the
CP La Nin
˜a winter (Fig. 6b). Similarly anomalous patterns
are also simulated in the upper troposphere over the North
Pacific and North Atlantic (Fig. 10a, b). Consistent with
the observations, different cooling SST anomaly patterns
for the two types of La Nin
˜a can cause similar responses of
the Aleutian Low, but they trigger roughly opposing NAO-
like atmospheric responses.
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, cir-
cumglobal wave train is displayed, suggesting that the
CP_WARM has a minor impact on the positive NAO-like
atmospheric response for the CP La Nin
˜a. We also consider
(a) (c)
(b) (d)
Fig. 9 The ensemble mean JF SLP response (hPa) to aEP_COOL, bCP_COOL, cCP_WARM, and dCP_CW forcings. Contour intervals are
2 hPa
1360 W. Zhang et al.
123
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; Gal-
lego 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. 3also presents the SST and
surface wind anomalies over the Atlantic during the two
types of La Nin
˜a winters. For the EP La Nin
˜a, 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 Nin
˜a, there appear significantly cold SST anomalies
over the northern tropical Atlantic and warming SST
anomalies over the western NA during the CP La Nin
˜a
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.
Here, another series of experiments (AT_COOL) are
performed to inspect the possible effect of the cooling SST
anomaly over the northern tropical Atlantic during the CP
La Nin
˜a winter. Figure 11a, c show the atmospheric
responses at the lower and upper troposphere in the
AT_COOL simulations relative to the control run. The
AT_COOL forcing can trigger positive NAO-like atmo-
spheric anomalies, suggesting that the cooling SST
anomaly at the northern tropical Atlantic Ocean has con-
tribution to the NA atmospheric anomaly during the CP La
Nin
˜a event. However, the Aleutian Low is strengthened
under the forcing of the AT SST anomaly, implying the
dominant forcing effect from the tropical Pacific rather
than the local SST. Furthermore, we conducted another
experiment (CPAT_COOL), in which the CP_COOL and
AT_COOL forcings are imposed together. As shown in
Fig. 11b, d, the Aleutian Low is weakened and positive
NAO-like atmospheric anomalies occur under the
CPAT_COOL forcing. The atmospheric responses
(a)
(b)
(c)
(d)
Fig. 10 The ensemble mean JF
geopotential height response
(m) at 300 hPa (hPa) to
aEP_COOL, bCP_COOL,
cCP_WARM, and dCP_CW
forcings. Contour intervals are
20 m
Impacts of two types of La Nin
˜a 1361
123
resemble closely those of the CP_COOL and CP_CW
forcings, but with a slight improvement in the NA–WE
region compared to the observed pattern (Figs. 6b, d, 9b, d,
10b, d, 11b, d). It can be seen that the AT_COOL forcing
has some contribution on the local atmospheric anomalies.
A series of modelling experiments discussed above
suggest that the two types of La Nin
˜a have different
impacts on the NA–WE atmosphere through the atmo-
spheric teleconnection. The tropical Atlantic SST anoma-
lies associated with the CP La Nin
˜a 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 Nin
˜a,
dynamical mechanism addressing how the tropical SST
influences the NA–WE atmosphere is still an open ques-
tion. 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 pos-
sibly 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 signifi-
cantly 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 con-
trasting atmospheric anomalies over the Atlantic Ocean
with these two types of La Nin
˜a.
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 Nin
˜a have roughly opposing impacts on the atmo-
sphere over the NA–WE sector. A significantly negative
(positive) NAO-like pattern is observed during the EP (CP)
La Nin
˜a winters. The previous study (Graf and Zanchettin
2012) have compared the climate impacts associated with
the EP and CP El Nin
˜o events and suggested that the two
types of El Nin
˜o lead to distinctly different atmospheric
responses over the NA–WE region. It is found that a sig-
nificantly negative NAO-like pattern occurs over the
(a)
(b)
(c)
(d)
Fig. 11 The ensemble mean JF SLP response to aAT_COOL and bCPAT_COOL forcings. Contour intervals are 2 hPa. The ensemble mean JF
geopotential height response (m) at 300 hPa (hPa) to cAT_COOL and dCPAT_COOL forcings. Contour intervals are 20 m
1362 W. Zhang et al.
123
NA–WE region during the CP El Nin
˜o winter, whereas no
apparent signal is found there during the EP El Nin
˜o
winter.
To clearly inspect the symmetry between the warm and
cold phases of ENSO, Fig. 12a, b display the relationship
between NAO and the two types of ENSO, respectively.
Following the study of Graf and Zanchettin (2012), EP El
Nin
˜o events identified are 1951, 1957, 1965, 1972, 1976,
and 1997; and CP El Nin
˜o events selected are 1968, 1977,
1986, 1994, 2002, and 2009. Some El Nin
˜o events are
excluded in their definition because concurrent volcanic
eruptions also have an important impact on the mid-latitude
climate (Graf and Zanchettin 2012). The SST anomaly over
the Nin
˜o3 region is used to denote the EP ENSO events
since the dominant SST anomaly is confined to the eastern
equatorial Pacific. Similarly, we referred to the Nin
˜o4
index as the CP ENSO events considering their SST
anomaly occurring mainly over the central equatorial
Pacific.
As shown in Fig. 12a, a negative NAO index appears in
most of the EP La Nin
˜a winters. Their composite NAO
index reaches -2.1, which is statistically significant at the
95 % confidence level. However, four out of six EP El
Nin
˜o events are in favor of occurrence of a negative NAO-
like pattern, and another two events correspond to a posi-
tive 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 Nin
˜o over the NA–
WE region seems to be originated by chance. As a con-
sequence, the EP El Nin
˜o effect on NAO seems to be
asymmetric to the EP La Nin
˜a effect during winter.
Unlike the EP ENSO events, the CP ENSO events
exhibit linearity in their climate impacts over the NA–WE
sector (Fig. 12b). Six out of seven CP La Nin
˜a events tend
to result into a positive NAO-like atmospheric anomaly,
whereas five out of six CP El Nin
˜o events are in favor of
occurrence of a negative NAO index. Their compositions
are both significant at the 95 % confidence level, indicating
that the NA–WE atmospheric anomalies during the CP
ENSO winters are most likely not due to chance. It can be
seen that the NA–WE atmospheric response to the CP SST
anomaly is very different from that to the EP SST anomaly.
Figure 13 displays the relationship between the NAO index
and tropical SST anomaly indicated by the linear correla-
tion. As shown in Fig. 13, the NAO index is significantly
correlated with the CP SST anomaly near 150°–180°W. It
(a) (b)
Fig. 12 a Scatter plot of the
DJF Nino3 index and JF
(January–February) NAO index
for EP El Nin
˜o(red triangle)
and EP La Nin
˜a(blue triangle).
Red and blue solid circles are
the composites of EP El Nin
˜o
and EP La Nin
˜a, respectively.
bSame as (a), but for CP El
Nin
˜o and CP La Nin
˜a
Fig. 13 DJF SST anomaly
(contours in °C) regressed upon
JF NAO index from 1950/1951
to 2009/2010. Light (dark)
shading indicates regression
exceeding the 90 % (95 %)
confidence level. Contour
intervals are 0.1 °C
Impacts of two types of La Nin
˜a 1363
123
suggests that a positive SST anomaly over the CP is usually
accompanied by a negative NAO-like atmospheric anom-
aly, and vice versa. However, there is no significant cor-
relation between the NAO index and the EP SST anomaly.
Only the linear part of the relationship can be examined in
correlation analysis. The signal of the EP La Nin
˜a over the
NA–WE region is easily overlooked using a correlation
analysis. It is a possible reason that the traditional (or EP)
ENSO signal is difficult to be detected in the North Atlantic
and its adjacent land. Other methods such as composition
need to be used to detect the nonlinear relationship.
However, the physical mechanisms are not clear for the
asymmetry and deserve study in future.
7 Conclusions
Similar to the El Nin
˜o phenomena discussed previously,
this study has shown that the La Nin
˜a should be classified
into two types (i.e., the EP and CP La Nin
˜a) considering
their distinctly different climate impacts. The EP La Nin
˜a
is characterized by the cooling SST anomaly center con-
fined 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 Nin
˜a 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
Nin
˜a exhibit very different features in the SST anomaly
evolution. For the EP La Nin
˜a, the SST anomaly starts in
the EP and propagates westward during the developing and
mature phase, while the CP La Nin
˜a 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 Nin
˜a 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 Nin
˜a, 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 Nin
˜a 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 Nin
˜a. The analyses provided
here have shown that it is necessary to separate the La Nin
˜a
(a)
(b)
Fig. 14 Composites of SST
anomalies (shading and
contours in °C) during autumn
(SON) for aEP La Nin
˜a, and
bCP La Nin
˜a. The SST
anomalies that are not
significant at the 95 % level are
not shown. Contour intervals
are 0.5 °C and zero contours are
omitted
1364 W. Zhang et al.
123
into two types considering their different SST anomaly
location and evolution, and especially, very different cli-
mate impacts over the extratropics.
Although ENSO events usually reach their mature phase in
winter, the associated strong SST anomaly pattern is clearly
observed during the preceding autumn. As sho wn in Fig. 2,the
La Nin
˜a events tend to intensively develop from August to
December and decay in the following months, which has also
been mentioned by previous studies (e.g., Larkin and Harrison
2001). Figure 14 displays the composite SST anomaly corre-
sponding to the two types of La Nin
˜a during the developing
autumn (September–November). The SST anomaly structure
and intensity in the autumn is similar to those in the winter,
indicating that the related SST anomaly signal in the mature
phase can be obviously observed in the autumn. If the SST
signal of La Nin
˜a is observed over the equatorial Pacific during
the autumn, we easily expect that the La Nin
˜aeventwould
persist into winter. It provides a potential predictability source
for predicting the NA–WE climate anomalies at least a season
in advance based on the strong cooling SST anomaly at the
equator.
Acknowledgments 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 Insti-
tutions (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.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
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... Besides the atmospheric circulation and teleconnections, the Eurasian SATAs can also be affected by remote forcing, such as the El Niño-Southern Oscillation (ENSO) (Graf and Zanchettin 2012;Zhang et al., 2015;Feng et al., 2017;García-Serrano et al., 2017), sea surface temperature (SST) anomalies (SSTAs) over the North Atlantic (Liu et al., 2014;Wang et al., 2019;Chen et al., 2020), sea ice (Mori et al., 2014;Chen H. W. et al., 2016;Chen et al., 2019;Cohen et al., 2019), and snow cover Saito et al., 2001;Chen S. et al., 2016). Zhang et al. (2015) and Feng et al. (2017) found that the central Pacific (CP) El Niño (La Niña) can lead to negative (positive) AO/NAO-like atmospheric responses with negative (positive) geopotential height anomalies over the subtropical Atlantic and Eurasia as well as a cooler (warmer) winter, and part of the mechanism can be explained by the tropospheric bridge according to Graf and Zanchettin (2012). ...
... Besides the atmospheric circulation and teleconnections, the Eurasian SATAs can also be affected by remote forcing, such as the El Niño-Southern Oscillation (ENSO) (Graf and Zanchettin 2012;Zhang et al., 2015;Feng et al., 2017;García-Serrano et al., 2017), sea surface temperature (SST) anomalies (SSTAs) over the North Atlantic (Liu et al., 2014;Wang et al., 2019;Chen et al., 2020), sea ice (Mori et al., 2014;Chen H. W. et al., 2016;Chen et al., 2019;Cohen et al., 2019), and snow cover Saito et al., 2001;Chen S. et al., 2016). Zhang et al. (2015) and Feng et al. (2017) found that the central Pacific (CP) El Niño (La Niña) can lead to negative (positive) AO/NAO-like atmospheric responses with negative (positive) geopotential height anomalies over the subtropical Atlantic and Eurasia as well as a cooler (warmer) winter, and part of the mechanism can be explained by the tropospheric bridge according to Graf and Zanchettin (2012). ...
... The Gill-type response is the stationary Rossby wave response to the tropical SST anomalies (Gill, 1980). The Gilltype response corresponding to the cooling SST anomalies of La Niña winters can be characterized by a pair of low-level anticyclonic anomalies residing at the northern and southern sides of the equator, along with low-level easterly wind anomalies and cyclonic anomalies symmetrical about the equator at the upper level of the troposphere (Yuan and Yan, 2013;Zhang et al., 2015). In Figure 7, the Gill-type response to the CTP SSTAs is remarkable: on the one hand, anomalous easterlies and Figures 7A,B), and the easterly anomalies are relatively stronger in January than in December; whilst on the other hand, in both December and January, a pair of cyclonic anomalies appears over the CTP at 300 hPa, and these Frontiers in Earth Science | www.frontiersin.org ...
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The reversal of winter surface air temperature anomalies (SATAs) over Central Asia (CA) between December and January is investigated in this study and found to be closely related to the sea surface temperature anomalies (SSTAs) over the central tropical Pacific (CTP). The cold CTP SSTAs can lead to positive (negative) SATAs over CA in December (January). The different responses of SATAs over CA to the SSTAs are attributed to different Rossby wave propagations. In December, a wave train from the North Pacific directly reaches CA, while in January it mainly propagates in the meridional direction and cannot reach CA. The January SATAs of CA are influenced by a wave train from the North Atlantic, which is induced by CTP SSTAs indirectly. The wave trains from the North Pacific are mainly driven by the Gill-type response to the cold CTP SSTA in both December and January. In January, since the climatological subtropical jet stream over the North Pacific is stronger and situated more towards the equator, a stronger Gill-type response is excited and causes the meridional propagation of the Rossby waves. Then, this stronger Gill-type response can cause strong zonal wind anomalies over the East Pacific. Local anomalies of the synoptic-scale transient eddy can be further caused by the zonal wind anomalies and travel eastward to the North Atlantic. The eddy-induced geopotential anomalies over the North Atlantic can further trigger Rossby waves and cause the negative SATAs over CA. Numerical simulations reproduce these mechanisms.
... Indeed, there are several methods for defining the CP ENSO events. In our study, we firstly use the three most commonly used definitions to identify the CP La Niña events (Ashok et al. 2007;Ren and Jin 2011;Zhang et al. 2015;Ding et al. 2017) and CP El Niño events (Ashok et al. 2007;Kao and Yu 2009;Ren and Jin 2011) events. Following these methods, the related results are shown in Fig. 10b. ...
... In this paper, the winter zonal contrasting thermal conditions in the SCS during 1982-2018 are firstly studied by using the SODA3.4.2 dataset and other satellite remote sensing data. Since the characteristics of the EOF2 of the winter SCS SSTa is similar to the winter zonal contrasting thermal conditions, EOF2 could represent the interannual variability in the zonal contrasting thermal conditions and their related Zhang et al. (2015), Ren and Jin (2011) and Kao and Yu (2009). (c) The relationship between east-west contrasting tempera-ture pattern (EWCTP) and CP ENSO events. ...
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The distinct winter temperature difference between the eastern South China Sea (ESCS) and western South China Sea (WSCS) has a crucial impact on regional air–sea interactions. By utilizing satellite and reanalysis data, the zonal contrasting winter thermal conditions and their formation mechanisms are investigated. The second empirical orthogonal function (EOF) mode of winter sea surface temperature (SST) anomalies is responsible for this east–west contrasting temperature pattern (EWCTP), with warming (cooling) in the ESCS (WSCS) and cooling (warming) in the WSCS (ESCS) during the positive (negative) phase events. A mixed layer heat budget analysis reveals that the net heat flux plays a primary role in the WSCS. In the ESCS, the temperature variation is instead mainly dominated by the horizontal heat advection term. In the positive phase events, an anomalous cyclonic circulation promotes an eastern boundary current (EBC) anomaly, which enhances the northward heat transport and thus warms the ESCS. In contrast, an anomalous anticyclonic circulation pattern weakens the heat transport by the southward EBC anomaly and cools the ESCS in the negative phase events. The water exchange through the Mindoro Strait and the vertical entrainment term also contribute to the ESCS SST anomalies. Further analyses show that although there are many EWCTP events that co-occur with the central Pacific El Niño-Southern Oscillation (CP ENSO) events, they have a complex relationship. The EWCTP could appear without CP ENSO events and some CP ENSO events do not lead to the EWCTP. It is because of the different temperature state in the WSCS and ESCS during October–December months and the different contributions of net heat flux and ocean processes to the temperature changes during October–February months.
... This arrangement is characterized by violent disturbances at mid-latitudes and rainfall at transit latitudes including Morocco. This expresses the negative index of the North Atlantic Oscillation (NAO) with a weakening of the Atlantic jet (Zhang et al. 2015). Regarding La Niña phenomenon, there is no justification for its effect on rainfall fluctuations in the LSB as it predominates in the months most affected by the fluctuations (December and March) (Figs. 4,9). ...
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Rainfall in north-western Morocco has shown significant monthly fluctuations with dramatic socio-economic impacts over the past decades. Several studies have suggested that variability is related to fluctuations in large-scale circulation patterns. In recent years, several promising centennial reanalysis datasets have become available, paving the way for new dynamic studies that facilitate an understanding of observed rainfall variability. Here, a statistical analysis of the rainfall time series of three stations in the Lower Sebou Basin (LSB) covering the period 1948–2017, as well as other global reanalysis datasets is used to explore teleconnections based on extensive statistical analysis and empirical orthogonal functions (EOFs). The Mann–Kendall test (MK) was conducted to identify significant monthly rainfall trends. The notable trend among significant trends is selected using the Theil Sen’s slope (TS) where downward trends were identified in early winter (December) and early spring (March) with rainfall decreases of 0.5 mm/year and 0.42 mm/year, respectively. Changes in significant trends found under the influence of different teleconnection patterns using their indices such as the North Atlantic Oscillation Index (NAOI), the Mediterranean Oscillation Index (MOI) and the Southern Oscillation Index (SOI) are determined by the partial Mann–Kendall test (P-MK). Overall, the results obtained in this study suggest that teleconnections affect the amounts of rainfall in the LSB. Negative and statistically significant correlations are observed between the NAOI and MOI and the amounts of rainfall falling in the LSB. Spring rainfall shows significantly negative (positive) correlations in the Eastern (Western) Pacific with sea surface temperature (SST), again highlighting the negative impact of El Nino (negative phase of SOI) on rainfall in Northwestern Morocco. These results are useful for monthly rainfall forecasting and water resource planning.
... The CP ENSO is also known as the "ENSO Modoki" (Ashok et al. 2007) or Warm Pool ENSO Yeh et al. 2009), and has the largest SSTAs variability in the central Pacific (Ashok et al. 2007; Kao and Yu 2009;Kug et al. 2009;Yu and Kao 2007;Yu et al. 2010;Zheng et al. 2014). Compared with the canonical ENSO, ENSO Modoki can generates different teleconnections (Taschetto and England 2009;Zhang et al. 2015), and has different effects on the precipitation (Ashok et al. 2009;Feng and Li 2011;Feng et al. 2016a;Jiang et al. 2019;Zhang et al. 2013Zhang et al. , 2014, the Hadley circulation (Feng and Li 2013), the stratosphere (Xie et al 2012(Xie et al , 2014a, aerosol concentrations (Feng et al. 2016b, and tropical cyclone activity (Kim et al. 2009;Wang et al. 2013;Magee et al. 2017). As the ENSO has a worldwide climatic effect, the prediction of ENSO can provide a predictability source for short range climate prediction. ...
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In this study, we evaluated the predictability of the two flavors of the El Niño Southern Oscillation (ENSO) based on a long-term retrospective prediction from 1881 to 2017 with the Community Earth System Model. Specifically, the Central-Pacific (CP) ENSO has a more obvious Spring Predictability Barrier and lower deterministic prediction skill than the Eastern-Pacific (EP) ENSO. The potential predictability declines with lead time for both the two flavors of ENSO, and the EP ENSO has a higher upper limit of the prediction skill as compared with the CP ENSO. The predictability of the two flavors of ENSO shows distinct interdecadal variation for both actual skill and potential predictability; however, their trends in the predictability are not synchronized. The signal component controls the seasonal and interdecadal variations of predictability for the two flavors of ENSO, and has larger contribution to the CP ENSO than the EP ENSO. There is significant scope for improvement in predicting the two flavors of ENSO, especially for the CP ENSO.
... In general, El Niño events in the Eastern Pacific (EP) and Central Pacific (CP) are often accompanied by the atmospheric response of NAO − . CP-type and EP-type ENSOs have different effects on NAO due to their unique regional climatic influences [28]. Among them, NAO − events are more likely to be induced during El Niño, while NAO + events are more likely to occur during La Niña [29]. ...
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Machine learning methods have now become an optional technique in Earth science research, and such data-driven solutions have also made tremendous progress in weather forecasting and climate prediction in recent years. Since climate data are typically time series, the neural network layers, which can identify the intrinsic connections between the points of the sequence and features in two-dimensional data, perform particularly well for climate prediction. The North Atlantic Oscillation (NAO) is a prominent atmospherical mode in the northern hemisphere, with the frequency change characteristic of sea level pressure (SLP) in the North Atlantic sector. One of the reasons why NAO prediction is still challenging is that NAO is also proven to be influenced by other climate circulations, the most significant of which is the interaction between El Niño-Southern Oscillation (ENSO) and NAO. Therefore, sea surface temperature (SST) in the Pacific Ocean used to characterize ENSO is also one of the factors that contribute to the evolution of NAO and can be used as an input factor to predict the NAO. In this paper, the seasonal lag correlation between ENSO and NAO is explored and analyzed. The interaction has been considered in both short-term forecasting and midterm prediction of the NAO variability. The monthly NAO index (NAOI) fluctuation is predicted using the Niño indices based on the RF-Var model, and the accuracy achieves 68% when the lead time is about three months. In addition, integrating multiple physical variables directly related to the NAO and Pacific SST, the short-term NAO forecasting is conducted using a multi-channel neural network named AccNet with trajectory gated recursive unit (TrajGRU) layer. AccNet has the ability to identify the mechanism of the high-frequency variation in several days, and the NAO variability is indicated by SLP. The loss function of AccNet is set to anomaly correlation coefficient (ACC), which is the indicator that verifies spatial correlation in geoscience. Forecasting extreme events of NAO between 2010 and 2021, AccNet presents higher flexibility compared against other structures that can capture spatial-temporal features.
... The variables are from the 20CRv2c dataset, while the AMO index is calculated based on the ERSSTv5 dataset. Areas with significant values of RWS and streamfunction exceeding 95% confidence level are dotted [Colour figure can be viewed at wileyonlinelibrary.com] as a waveguide facilitating Rossby wave energy propagation into the downstream areas (Ambrizzi and Hoskins, 1997;Zhang et al., 2015;Wang et al., 2020a;. The mid-latitude Rossby wave splits into two branches over the East European Plain (approximately 30 -60 E). ...
Article
Based on multiple long-term observational and reanalysis datasets, this study investigated the characteristics and physical mechanisms of the interdecadal variations in late spring (i.e. May) precipitation (LSP) over the southeastern extension of the Tibetan Plateau (SETP) since 1900. It was revealed that, by and large, LSP over the SETP experienced interdecadal decrease during the period preceding 1927, 1962–1988, and 2004 onwards, but saw an increase during the periods of 1928–1961 and 1989–2003. The atmospheric circulations responsible for interdecadal variations in LSP over the SETP were also analysed. These analyses identified significant synergistic impacts of decreased mid-latitude upstream westerlies and increased low-latitude monsoonal southerlies over the Central North Bay of Bengal (CNBOB) on interdecadal variations in precipitation, suggesting striking interactions between extratropical eastward cold air and tropical northward warm/humid air. Further observational and modelling evidence suggested that Atlantic Multidecadal Oscillation (AMO) was likely to be a salient oceanic driver for the interdecadal synergy between upstream westerlies and CNBOB monsoonal southerlies. The elevated sea surface temperature anomalies associated with the warm phase of the AMO could spark favourable local atmospheric anomalies, forcing an upper-tropospheric, planetary-scale teleconnection emanating from the east of the North Atlantic sector, which may serve as an effective bridge linking the remote AMO signal and the synergy between westerlies and monsoonal southerlies around the SETP on interdecadal timescales. Our findings provided new insights into the understanding of the synergistic roles of westerlies and monsoons in the modulation of interdecadal LSP over the SETP, prior to the peak Asian summer monsoon season.
... This study focuses on changes in the frequency and hydroclimate expression of CP and EP El Niño events between the pre-industrial period (1000-1850 C.E.) and the 20th century (1900-2000 C.E.). Given that multiple studies demonstrate that CP and EP La Niña events are not easy to partition (Kug & Ham, 2011;Ren & Jin, 2011;Zhang et al., 2015), and the fact that many of the methods described in this study focus solely on El Niño events, we opt to examine CP and EP El Niño events alone. We investigate the impacts of CP and EP El Niño on the hydroclimate over North America in both data products, with specific attention to three key questions: (a) Are teleconnections during CP and EP El Niño events self-consistent across different definitions classifying CP and EP El Niño? ...
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El Niño‐Southern Oscillation (ENSO) variability affects year‐to‐year changes in North American hydroclimate. Extra‐tropical teleconnections are not always consistent between El Niño events due to stochastic atmospheric variability and diverse sea surface temperature anomalies, making it difficult to quantify teleconnections using only instrumentally‐based records. Here we use two paleoclimate data assimilation (DA) products spanning the Last Millennium (LM) to compare changes in amplitudes and frequencies of diverse El Niño events during the pre‐industrial period and 20th century, and to assess the stationarity of their North American hydroclimate impacts on multi‐decadal to centennial timescales. Using several definitions for Central Pacific (CP) and Eastern Pacific (EP) El Niño, we find a marked increase in 20th century EP El Niño intensity, but no significant changes in CP or EP El Niño frequencies in response to anthropogenic forcing. The associated hydroclimate anomalies indicate (a) dry conditions across the eastern‐central and northwestern U.S. during CP El Niño and wetter conditions in the same regions during EP El Niño; (b) wet conditions over the southwestern U.S. for both El Niño types. The magnitude of regional hydroclimate teleconnections also shows large natural variability on multi‐decadal to centennial timescales. However, when the entire LM is considered, mean hydroclimate anomalies in North America during CP or EP El Niño are consistent in terms of sign (wet vs. dry). Results are sensitive to proxy data and model priors used in DA products. Inconsistencies between El Niño classification methods underscore the need for improved ENSO diversity classification when assessing precipitation teleconnections.
... And the atmospheric response to the ENSO over the Arctic and Europe is not stable due to internal atmospheric variability (Deser et al. 2017), which makes it difficult to address the ENSO impacts on the cold events over East Asia in numerical models. Besides, due to simplicity of the linear model and the diversity of ENSO (Zhang et al. 2015(Zhang et al. , 2019, the model result (Fig. 11) displays discrepancy from the observations (Fig. 7c and f). Furthermore, the anticyclonic anomalies over the Barents Sea that lead to the sea ice loss are contributed by other factors such as local baroclinic growth and transient eddy forcing, which also contribute to the discrepancy between the model result and the observational analysis. ...
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The occurrence of cold events over East Asia is modulated by the background mean state associated with the El Niño-Southern Oscillation (ENSO). The present study compares the processes of occurrence of cold events over East Asia between El Niño and La Niña winters. It is found that the difference in atmospheric mean state in El Niño and La Niña winters leads to different processes of occurrence of cold events over East Asia during boreal winter. The cold events during El Niño winters are closely related to negative Arctic Oscillation (AO) and those during La Niña winters are contributed by the Ural blocking. Both negative AO in El Niño winters and the Ural blocking in La Niña winters are followed by a Rossby wave train over Eurasia that strengthens the Siberian high and the East Asian trough, leading to cold anomalies over East Asia. The negative AO during El Niño winters is related to a tropospheric teleconnection that weakens the polar vortex, which is confirmed by numerical experiment with a linear baroclinic model. The Ural blocking is associated with the Arctic sea ice loss in the Barents Sea during La Niña winters through the reduced meridional temperature gradient and decreased zonal mean winds. The Arctic sea ice loss over the Barents Sea during La Niña winters is contributed by the development of anticyclonic anomalies induced by the enhanced stationary waves.
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Including significant warming trend, Arctic climate changes also exhibit strong interannual variations in various fields, which is suggested to be related to El Niño and Southern Oscillation (ENSO) events. Previous studies have demonstrated the different impacts on the Arctic of central Pacific (CP) and eastern Pacific (EP) ENSO events, and suggested these impacts are largely of opposite sign for ENSO warm and cold phases. Our results illustrate asymmetrical changes for the cold and warm ENSO events, especially for the La Niña events. Compared to the past frequent basin-wide cooling La Niña events, since the 1980s the cooling center for the La Niña event has strengthened and moved westward along with the increasing frequency for the canonical and CP La Niña events. Contrary to the basin-wide cooling and canonical La Niña events, the frequent CP La Niña events induce significant warming from the Beaufort Sea to Greenland via the convection center moving northward over the western Pacific. Observation analysis and numerical experiments both suggest that the changes in La Niña type may also accelerate Arctic warming.
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To evaluate the performance of a high-resolution atmospheric model (HiRAM) and to improve our understanding of the climatic impacts of ENSO forcing and associated teleconnections, we analyzed AMIP style HiRAM simulations conducted effectively at 25 km grid spacing. To better assess HiRAM response to ENSO climate variability; we categorized it into strong and weak El Niño/La Niña episodes. The HiRAM model reproduced the impacts of strong ENSO over global scale very well, however, it underestimated ENSO teleconnection patterns and associated changes over regional scale (e.g., MENA and South Asia), especially following weak ENSO that could be attributed to model weak response to circulation changes such as Pacific North American (PNA) and North Atlantic Oscillation (NAO). Moreover, our results emphasize that ENSO impacts are relatively stronger over the Inter-Tropical Convergence Zone (ITCZ) compared to extra-tropics and high-latitude regions. The positive phase of ENSO causes weakening in rainfall over the African tropical rain-belt, parts of South and Southeast Asia. Both the reanalysis and HiRAM results reveal that ENSO-induced negative (positive) NAO-like response and associated changes over Southern Europe and North Africa vary significantly following the increased intensity of El Niño (La Niña). We further found that the ENSO magnitude significantly impacts Hadley and Walker circulations. The El Niño phase of ENSO overall strengthens the Hadley Cell, and the reverse is true for the La Niña phase. This ENSO-induced strengthening and weakening of Hadley Cell induce significant impact over South Asian and African convective regions through modification of the ITCZ circulation system.
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