An unusual coupled mode in the tropical Pacific during 2004
Karumuri Ashok1, Swadhin K. Behera1, Suryachandra A. Rao1, Hengyi Weng1,
and
Toshio Yamagata1,2,*
1
FRCGC/JAMSTEC, 3173-25, Showamachi, Kanazawa-Ku, Yokohama, Kanagawa,
236-0001, Japan
2
Also at Department of Earth & Planetary Science, Graduate School of Science,
University of Tokyo, Japan
*Email: [email protected]
ABSTRACT
The sea surface temperature anomalies (SSTA) of tropical Pacific in the boreal
summer of 2004 show a tripolar pattern with warm SSTA in the central tropical Pacific
flanked on both sides by cold SSTA. It is found that the phenomenon involves equatorial
coupled ocean-atmosphere dynamics that are different from the conventional El Niño; the
peak warming of the SSTA is confined to the central equatorial Pacific until boreal winter
instead of propagating to the east. Anomalous twin Walker circulation cells associated
with the SSTA give rise to rainfall anomalies that also form a tripole. Because of this
unique nature, we distinguish this condition as El Niño Modoki (Pseudo El Niño) - a new
coupled phenomenon. Similar strong events are also observed in 1986, 1990, 1991, 1992,
1994 and 2002 since 1979.
1
1. Introduction
Several parts of the globe experienced anomalous climate conditions during the
summer (June-August; JJA) of 2004 (Figure 1). Many areas of Japan and the Far East
region experienced severe drought and heat waves whereas Philippine Islands received
surplus rainfall due to active cumulus activities. The Maritime Continent, and southern
Mexico and Ecuador suffered from drought whereas central tropical Pacific received
surplus rainfall. Peninsular India experienced weaker than normal monsoon conditions
and Queensland in Australia also received less than normal austral winter rainfall. These
anomalous summer conditions are seen to be associated with an unusual distribution of
sea surface temperature anomalies (SSTA), which is composed of anomalous warming in
the tropical central Pacific flanked by colder than normal SST in east and west (Figure
2a). Interestingly, the peak warming did not migrate to the eastern tropical Pacific in
course of time as in case of El Niño events. In fact, the central warming maximum
persisted through the following winter. The evidence of similar anomalous SSTA is also
seen in studies such as Donguy and Dessier [1983] and McPhaden [2004]. Interestingly,
some earlier EOF analyses also hint such non-ENSO type SSTA patterns in the tropical
Pacific [see EOF3 pattern of Meyers et al. 1999; also Weare at al. 1976].
Because of the unique behavior of this phenomenon that is to be discussed in
section 3, we classify the tropical Pacific SSTA pattern during 2004 as part of a new
phenomenon and we named it as the El Niño Modoki (Pseudo-El Niño). “Modoki” is a
classical Japanese word which means “similar but different”. This terminology was
introduced by the corresponding author in 2004 to explain a probable cause behind the
abnormal summer conditions in Japan. Readers are referred to the following websites:
2
(http://www.japantimes.co.jp/cgi-bin/getarticle.pl5?nn20040724f3.htm,
http://www.kyoto-np.co.jp/kp/special/ecology/eco/eco63.html,
http://www.tbs.co.jp/morita/qa_nanmon/faq_040811-1.html).
In this article, we describe salient features of this new phenomenon by depicting
the associated ocean-atmosphere conditions.
2. Datasets used
The Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST)
dataset [Rayner et al., 2003] from January 1979 to April 2005 is used in this study. Also
used are Climate Prediction Center Merged Analysis of Precipitation (CMAP) data [Xie
and Arkin, 1996] and NCEP/NCAR reanalysis products [Kalnay et al., 1996] for the
period after 1978. Merged sea surface height (SSH) data from Aviso, France, and satellite
derived winds from Quick Scat (after 2000) are also used..
3. Features of El Niño Modoki
The El Niño Modoki seen in boreal summer of 2004 is characterized by a tripolar
pattern in SSTA (Figure 2a). The central anomalous SST warming was caused from April
by a series of intraseasonal downwelling Kelvin waves in response to anomalous
westerlies over the western tropical Pacific. In response to anomalous westerlies in the
western Pacific, the thermocline became anomalously shallow there, causing colder than
normal SST in that region. Due to the excited Kelvin waves, the SSH in the central
tropical Pacific was anomalously raised (Figure 2b) along with subsurface warming.
However, as the intraseasonal westerly wind bursts were not as strong as during strong El
3
Niño years such as 1997 (figure not shown), the maximum in the Kelvin wave-induced
downwelling was rarely seen in the eastern tropical Pacific in most of the period despite
some weak and intermittent eastward propagation beyond the central tropical Pacific. The
central warming was further aided by the presence of anomalous tropical easterlies over
the tropical eastern Pacific. The same wind anomalies enhanced upwelling in the eastern
equatorial Pacific; this may be a major difference of the El Niño Modoki from a
conventional El Niño. Further, maximum temperature anomalies up to 1.5~2.5ºC along
the equator in summer and fall of 2004 are also observed at thermocline depth in the
central tropical Pacific. It is interesting to note that the warmest SSTA is located between
155ºW and 160ºE until February 2005. This occurs despite the propagation of positive
SSTA to the east between September and December also along with some spreading of
the anomalous warm waters to the west during the boreal winter (Figure 2c); also peculiar
is that the SSTA did not amplify during the eastward propagation. By the end of January
2005, anomalously cooler SSTA is again seen in both the eastern and western equatorial
Pacific flanking the warmer central equatorial Pacific SSTA. All this is different from the
post-1977 El Niños that are characterized by propagation of the warmest SSTA from the
central tropical Pacific to the coast of Peru.
The interesting patterns of SSTA (Figure 2a) in the tropical Pacific are associated
with unusual patterns of wind and rainfall anomalies during boreal summer of 2004. The
anomalous summer SST cooling in the east and west that flanks the anomalously warm
central tropical Pacific are overlaid with anomalous low level easterlies and westerlies
(Figure 2b). This is related to an above normal rainfall in the central equatorial Pacific
flanked on both sides by the negative rainfall anomalies (Figure 1). The SSTA gives rise
4
to a pair of Walker cells (Figure 3) with a common ascending limb overlying the warmer
central Pacific. This double-cell pattern shows a marked difference from the single-cell
Walker circulation in the tropical Pacific during a typical El Niño case. The surface
easterlies and westerlies associated with the Walker cells then reinforce the tripolar SSTA
through ocean dynamics. This is evidenced from the persistence of SSTA and subsurface
temperature anomalies as discussed in the previous paragraph. The El Niño Modoki
phenomenon appears to involve ocean-atmosphere coupled dynamics, as evidenced by
the interaction among SSTA, subsurface temperature, wind and rainfall anomalies.
The atmospheric condition associated with the descending branch of the western
Walker cell (Figure 3) located over the western Pacific caused the drought over the
Maritime Continent (Figure 1) and its influence extended northwest till Bangladesh,
south India and Sri Lanka. The deficit rainfall in the western Pacific region also extended
southward to southeastern Australia in the Southern Hemisphere. In contrast, positive
rainfall anomalies were observed in the Philippines, Thailand, Myanmar and northern
India near the anomalous updraft region of the northern Hadley cell. Further north,
countries including southern part of Japan suffered drought during this summer owing to
the Pacific-Japan teleconnection pattern [see Nitta, 1987]. We note that this impact in
East Asia is opposite to that of the conventional El Niño [cf. Ropelewski et al., 1987;
Diaz et al., 2001; Saji and Yamagata 2003].
Negative rainfall anomalies over the
equatorial eastern Pacific in response to the descending branch of the eastern Walker cell
extend to Mexico, whereas surplus rain is seen further north up to 40ºN over North
America. This is again different from the characteristic El Niño influence of surplus
rainfall over the west coast of U. S. Further details of boreal summer (and winter) global
5
teleconnections associated with the tropical Pacific SSTA during 2004, along with
discussion on the relevant mechanisms that explain the teleconnection in tropics and
higher
latitudes,
are
discussed
in
Ashok
et
al.
[2006;
available
at
http://www.jamstec.go.jp/frsgc/research/d1/iod/].
By carrying out an EOF analysis of the tropical Pacific (20ºS-20ºN) SSTA from
January 1979 to February 2005, we have identified that the SSTA shown in Figure 2a
largely corresponds to the EOF2 of the tropical Pacific SST variability that explains 12%
of variance (see Figure 2b of Ashok et al., 2006). The principal component (Figure 4) of
the EOF2 (PC2 hereafter) is positive almost throughout 2004, and its amplitude exceeds
0.8 of its standard deviation during boreal summer of 2004 and 0.7 during the second half
of the following boreal winter. As demonstrated in PC2, there are several other years
such as 1986, 1990, 1991, 1992, 1994 and 2002 in which the SST conditions in the
tropical Pacific and their relevant teleconnections resemble those of 2004 from boreal
summer through winter [Ashok et al., 2006]; incidentally, we notice that the amplitudes
of the SSTA during many of these events are stronger than that of the 2004 El Niño
Modoki event. Interestingly, PC3 was also stronger than normal during 2004. Hence the
tripolar SSTA during 2004 was amplified (Figure A1 in auxiliary section) in the tropics
[see Figure 3 of Ashok et al., 2006]. The robustness of the present EOF analysis is also
verified by rotation of first 10 EOF modes, Also, the composite tropical Pacific SSTA
during El Niño Modoki years confirms the warmest SSTA being restricted to the central
tropical Pacific from boreal summer through next winter [see Figure 5 of Ashok et al.,
2006], demonstrating that these events are not a part of the ENSO evolution.
6
The weak amplitude and changing phases of NINO3 and NINO3.4 indices also
show that the anomalous conditions in 2004 are quite distinct from those of the so-called
“protracted El Niño” [Allan, 2003]. The event is also not adequately captured by the
definition of “dateline El Niño” [Larkin and Harrison, 2005a, b]. This is because the
definition is based only on the SST warming in the NINO3.4 region. Larkin and Harrison
[2005a] have mentioned correctly that there is no need of any significant warming in the
cold tongue region, but they did not distinguish the existence of even significant cooling
in the eastern and western tropical Pacific, which is associated with tripolar SSTA. We
note that the magnitude of the cooling in the eastern tropical Pacific during 2004 summer
is comparable to that of the central tropical Pacific warming (Figure 2a). Based on lead
and lag correlation analyses, Ashok et al. [2006] found that the maximum lead/lag only
explains 30% of either of the EOF1 or the EOF2 type phenomena. This further supports
that the 2004 El Niño Modoki event is not a part of ENSO evolution. Rather, it should be
classified into another coupled mode in the tropical Pacific.
The characteristics of ENSO as defined by Trans-Nino index (TNI) by Trenberth
and Stepaniak [2001] (henceforth TS01) is also examined here in the context of El Niño
Modoki. The TNI is a measure of zonal SSTA gradient between the central equatorial
Pacific and the eastern equatorial Pacific (Table 1). As reported by Trenberth et al.,
[2002], the TNI is highly correlated with the principal component of SVD2 of SSTA and
several atmospheric fields such as precipitation. Further, they find that NINO3.4 and TNI
are significantly correlated at different leads/lags. Based on this, they claim that the TNI
explains a pattern, which is a part of ENSO evolution. We notice, however, that the
lag/lead correlations between TNI and NINO3.4 not only weakened after 1976 but also
7
changed signs (see Figure 2 of TS01). This suggests that, depending on the background
state, the TNI is composed of several different modes of variability.
We find that the principal component of EOF2 is positive for most of the time
after March 2002 (Figure 4). In contrast, the phase of NINO3 and NINO3.4 indices
change sign several times. The amplitude of the EOF1, which is basically related ENSO,
is also weak after 2003 summer. Conversely, the SSTA over the central pole of the El
Niño Modoki is consistently warm from March 2002 through February 2005(Figure 4).
This indicates that the tropical Pacific has been dominated by the EOF2 mode since 2002
spring. Thus, the El Niño Modoki in summer 2004 is neither preceded by La Niña nor
succeeded by El Niño; this situation is different from the hypothesis of TS01. In fact, out
of the three major El Niño events after 1977, only the 1982-83 event fits to the condition
stipulated by TS01.
All these indicate that the anomalous tropical Pacific condition in
2004, which largely resembles the EOF2 pattern, is not a manifestation of ENSO
evolution. More detailed analyses to be reported elsewhere [Ashok et al., 2006] also
suggest that similar events represented by the EOF2 can well be described as a distinct
class of coupled phenomenon in tropical Pacific. The El Niño Modoki events seem to
have become more prominent statistically because of the changes in background
conditions of the tropical Pacific [Ashok et al., 2006].
4. Conclusions
In boreal spring of 2004, the equatorial central Pacific SSTA was anomalously
warm with colder than normal SSTA on its both sides in the tropical eastern and western
Pacific. This tripolar SSTA configuration with the warmest anomaly in the tropical
Pacific lasted through boreal winter without extending to the Peruvian coast.
8
This
situation was much different from that of typical El Niño events. Because of this
distinction, we have described this tripolar SSTA structure in the equatorial Pacific
during 2004 as a new class of phenomenon called El Niño Modoki [see Ashok et al., 2006
for details]. This classification as another mode in the tropical Pacific is strongly
supported by the fact that this event involves unique coupled ocean-atmosphere dynamics
and has its own teleconnections over the globe. In particular, its impacts on summer
conditions in the Far East are just opposite from those of El Niño. The SSTA pattern is
captured basically by the EOF2 pattern of the tropical Pacific SST variability for the
period from Januray 1979 to February 2005. Some aspects in the spatial structure and
temporal evolution of El Niño Modoki were captured in earlier works by Meyers et al.
[1999] and Weare et al. [1976]. However, they did not deal with the variance
contributions from important El Niño Modoki events in 1994, 2002, 2004 and a major La
Niña Modoki event in 1998. We have also demonstrated that the 2004 El Niño Modoki
event is not a part of the evolution of El Niño. Since changes in characteristics of the
tropical Pacific coupled system may have vast impact on global teleconnection patterns
[Navarra et al., 1999; Meyers et al., 2006], we believe that the present topic is of
considerable importance even from a societal viewpoint. In a companion paper [Ashok et
al., 2006], we have examined general properties and teleconnection features of the El
Niño Modoki event using ocean/atmosphere data since 1979.
Acknowledgements The authors were encouraged by discussions with Prof. R. Lukas.
Useful comments from the reviewers of an earlier version are acknowledged. Data from
Aviso is acknowledged. Figures in this paper have been prepared using the
GrADs/COLA and FERRET softwares.
9
Table 1: Different SSTA indices used in this study
Index
NINO3.4
NINO3
NINO1+2
NINO4
Trans-Niño
index
Definition
The area-averaged SSTA over 5ºN-5ºS, 170ºW-120ºW.
The area-averaged SSTA over 5ºN-5ºS, 150ºW-90ºW.
The area-averaged SSTA over equator-10ºS, 90ºW-80ºW.
The area-averaged SSTA over 5ºN-5ºS, 160ºE-150ºW.
difference of normalized SSTA between NINO4 and NINO1+2
10
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Figure 1 June-August rainfall anomalies (mm.day-1) for 2004.
14
(a)
(b)
(c)
Figure 2 Abnormal conditions during JJA in 2004: (a) SSTA in ºC, (b) evolution of
SSHA in cm (shaded) and wind anomalies(m.s-1; vectors), averaged between 2ºS-2ºN,
and (c) evolution of SSTA averaged between 5ºS-5º N.
.
15
Figure 3 Anomalous Walker circulation averaged over 10ºS to 10ºN during JJA in 2004.
Figure 4 Normalized time series of the PC2of tropical Pacific SST variability (Bar) and
PC3 (yellow), NINO3 SSTA (blue), and NINO4 SSTA (red).
16