Download Have the Tropical Pacific Ocean-Atmosphere Interactions

Global Environmental Change in the Ocean and on Land, Eds., M. Shiyomi et al., pp. 265–278.
© by TERRAPUB, 2004.
Have the Tropical Pacific Ocean-Atmosphere Interactions
Behaved as a Driver of Centennial- to Orbital-Scale
Climate Changes?
Masanobu Y AMAMOTO
Graduate School of Environmental Earth Science, Hokkaido University,
Kita-10, Nishi-5, Kita-ku, Sapporo 060-0810, Japan
Abstract. We review recent studies of the long-term El Niño-Southern
Oscillation (ENSO)-like variability and its relationship to global climate
changes on centennial to orbital time scales. A synthesis of the published
studies and data for the long-term ENSO-like variability demonstrated that an
El Niño-like pattern of the Pacific has been linked to global warming and pCO2
increase on millennial to orbital time scales, which suggests that the tropical
Pacific plays a major role in global climate changes.
Keywords: tropical Pacific, El Niño/Southern Oscillation (ENSO), long-term
ENSO, climate change, deglaciation, greenhouse gas
1. INTRODUCTION
How have the Earth’s orbital variations driven glacial-interglacial climate changes?
How have the repeated sub-orbital changes occurred? Recently, a new hypothesis
has been proposed that the tropical Pacific ocean-atmosphere interactions exert
a strong influence on global climate changes on millennial and orbital time scales
(Cane, 1998; Clement et al., 1999). This hypothesis needs to be tested by
understanding whether the tropical interactions are a driver or a passive player in
global climate changes. Here we review recent paleoceanographic studies of the
behavior of El Niño-Southern Oscillation (ENSO)-like variability on millennial
to orbital time scales (Fig. 1). We discuss the role of the long-term ENSO-like
variability in climate changes on different time scales.
El Niño-Southern Oscillation (ENSO) is a quasi-regular phenomenon of
tropical ocean-atmosphere interactions centered in the equatorial Pacific on
interannual time scales ranging mostly from 3 to 6 years. The ENSO influences
tropical and extratropical climates through atmospheric teleconnections. On
decadal time scales, the dominant climate phenomenon in the Pacific is an ENSOlike spatial distribution of surface temperature and atmospheric circulation
(Zhang et al., 1997). Climatological studies of instrumental SST anomaly data
suggest that the decadal ENSO-like SST variation in the Pacific is both of tropical
Pacific and extratopical origins (e.g., Dettinger et al., 2001). This decadal climate
process is a recent research topic in climatology.
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GISP2
ODP893,1014
MD012421
T29
M41 MD982181
M38
M63
MD982162 Huon
Palmyra
ODP806
W84
TR163-19
ODP1002
R13
Pallcacocha
Galapagos Junin
Titicaca
V21-30
Byrd
Fig. 1. Annual mean sea surface temperature (NOAA, 1998) and the locations of the sites in this
review.
On timescales longer than decadal, the behavior of the tropical oceanatmosphere interactions and its role in global climate changes are much less well
known. Very little is known about long-term changes in the frequency, amplitude
and mean state of ENSO-variability. Since the global set of instrumental SST data
is limited to the last century, the SST records derive mostly from proxy data
obtained from marine and lacustrine sediments, corals, ice cores, trees, etc.
Marine cores are widely used for paleoclimatic reconstruction, but in most cases
they are useful only for reconstructing the mean state of ENSO. Varved sediments,
corals, ice cores and trees are useful for reconstructing the frequency and
amplitude of ENSO in distinct time intervals, but their records rarely cover a
sufficient time range to discuss the changes on time scales longer than multidecadal. Compiling paleoclimatic data obtained by different proxies is therefore
of benefit to understanding the long-term change of ENSO behavior.
2. ORBITAL-SCALE VARIABILITY
The long-term variations in Earth’s orbital motion occur at cycles ranging
from about 20,000 to 400,000 years in length. They cause cyclic variations in the
amount of solar radiation received at the top of the atmosphere according to
latitude and season. Changes in solar heating driven by changes in Earth’s orbit
are the major cause of cyclic climate changes over time scales of tens to hundreds
of thousands of years (Ruddiman, 2001). After the Milankovitch hypothesis was
ENSO-like Variability in Centennial- to Orbital-Scale Climate Change
267
Fig. 2. Long-term ENSO index (∆SST; Yamamoto et al., in preparation), calculated NINO3 index
(Clement et al., 1999) and the coral ENSO variance of δ 18O (Tudhope et al., 2001) during the last
glacial-interglacial cycle.
proposed (Milankovitch, 1941), changes in the conditions of ice and ocean in the
northern high latitudes have been believed to amplify the Earth’s orbital signal to
periodic global climate changes. Recently, a new hypothesis has been proposed
that the ocean-atmosphere interactions in the tropical Pacific are an amplifier of
the orbital signals (Cane, 1998). This hypothesis is based on the results of a model
calculation that suggested the role of ENSO-like variability in driving orbitalscale global climate changes (Clement et al., 1999).
Clement et al. (1999) conducted a model experiment to test the sensitivity of
the tropical Pacific to changes in solar forcing associated with changes in the
Earth’s orbital parameters. A coupled ocean-atmospheric Zebiak-Cane ENSO
model (Zebiak and Cane, 1987) was run for the past 150,000 years and forced with
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Milankovitch changes in the solar insolation. The calculation demonstrated that
the amplitude and frequency of El Niño or La Niña varied in response to a
precessional cycle (calculated NINO3 in Fig. 2). This finding suggests that the
tropical Pacific can drive globally synchronous climate change on an orbital time
scale rather than being a passive player.
When this modeling result was published in 1999, little supporting evidence
appeared in paleoceanographic records. Subsequently, Beaufort et al. (2001)
generated late Pleistocene sediment records of nanoplankton-based primary
productivity along the equator in the Indian Ocean (T29 and M63 in Fig. 1) and
the Pacific Ocean (M41, M38, W84 and R31 in Fig. 1). The study showed that
variations in equatorial productivity have been caused primarily by both glacialinterglacial variability and precession-controlled changes in the east-west
thermocline slope of the Indo-Pacific. The precession-controlled variations in
productivity were linked to processes similar to the ENSO phenomenon, and they
preceded changes in the δ18O of benthic foraminifera, which indicates that they
were not the results of ice sheet fluctuations.
Yamamoto et al. (in preparation) demonstrated a more complicated feature
of orbital-scale ENSO-like variability in the last glacial-interglacial cycle. They
generated the records of the alkenone sea surface temperature (SST) at the
western and eastern margins of the mid-latitude North Pacific (MD012421 and
ODP1014 in Fig. 1) during the last 145,000 years. The study showed that the antiphase variation between the SSTs of the northeast and northwest Pacific margins
reflected an orbital-controlled ENSO-like variability (Fig. 2). The difference
between alkenone SSTs of the northeast and northwest Pacific margins (∆SST)
was obtained by subtracting the paleo-SST of MD01-2421 from that of ODP 1014
(Fig. 1) and can be used as an indication of the long-term ENSO-like variability.
The variation of ∆SST was large and pronounced at 23,000-year cycles during 0–
60,000 years ago (MIS-1 to MIS-3) and 120,000–145,000 years ago (MIS-5e to
MIS-6). This variation agreed well with the long-term ENSO behavior predicted
by the Zebiak-Cane ENSO model (Clement et al., 1999), as regards both the
timing and frequency (Fig. 2). In contrast, the variation was relatively small and
did not show a precessional cycle during 60,000–120,000 years ago (MIS-4 to
MIS-5d), which disagreed with the model prediction (Fig. 2).
Another approach to reconstructing orbital-scale changes in inter-annual
ENSO variability was conducted by examining fossil corals. Tudhope et al.
(2001) used fossil massive Porites corals of eight different ages from Huon,
Papua New Guinea (Fig. 1), to investigate annual variability in ENSO during the
last glacial-interglacial cycle. Coral skeletal δ18O value is a good indication of
ENSO variability: Isotopically heavy skeleton oxygen results from the dry and
cool conditions during El Niño phase in this studied area. The study showed that
ENSO has existed for the past 130,000 years, operating even during glacial
periods of substantially reduced regional and global temperature. The ENSO
variance was expressed as a standard deviation of 2.5- to 7-year bandpass-filtered
δ18O time series. This ENSO variance changed greatly (Fig. 2), and the samples
taken from deglaciation and interglacial intervals show a high ENSO variance
ENSO-like Variability in Centennial- to Orbital-Scale Climate Change
269
comparable to modern corals. This phenomenon was attributed to the combined
effects of ENSO dampening during cool periods and by precessional forcing.
The above evidence supports the prediction of Clement et al. (1999) that the
long-term ENSO-like variability has been regulated by a precessional forcing.
However, the long-term ENSO-like variability has also been influenced by other
orbital forcing, such as changes in eccentricity and obliquity. This was suggested
by the obliquity and eccentricity cycles in ∆SST changes in mid-latitude North
Pacific (Yamamoto et al., in preparation) and the glacial damping of ENSO
variance in Huon corals (Tudhope et al., 2001). These observations suggest that
the ENSO-like variability has been linked to climatic forcing in the high latitudes
(Schmittner and Clement, 2002; Philander and Fedorov, 2003).
3. BEHAVIOR OF ENSO DURING THE LAST DEGLACIATION
The glacial periods ended with deglaciations. The deglaciation process
involves global warming, the increase of greenhouse gases such as carbon
dioxide and methane, and sea level rise due to the inflow of melt-water from the
continental ice sheets. The last deglaciation occurred in two steps with a rhythm
of fast-slow-fast. The steps are centered at ~15,000 and ~13,000 years ago
(Fairbanks, 1989).
In a classical view, the rise in summer insolation at the northern high
latitudes triggered the melting of the Northern Hemisphere ice sheets
(Milankovitch, 1941). During the last two decades, changes in the sea-ice
coverage of the northern North Atlantic and the North Atlantic thermohaline
circulation have been thought to play a major role in driving deglaciations (e.g.,
Imbrie et al., 1992). Recently, Cane (1998) proposed a new hypothesis that an El
Niño-like atmospheric pattern induced the heating of the North American Continent
which caused the melting of the Laurentide Ice Sheet. This hypothesis needs to
be tested by paleoclimatic evidence.
Heusser and Sirocko (1997) reported a large century-scale fluctuation of
pollen assemblage during the last deglaciation in ODP 893, Santa Barbara Basin,
California margin (Fig. 1). They suggested that the correlative high-frequency
variations in the northeast Pacific winter and the southern Asian summer climates
may reflect submillennial-scale ENSO-type forcing involving water vapor
transport over the Indian and Pacific Oceans.
Lea et al. (2000) reported the record of Mg/Ca ratios of planktonic foraminifera
shells in equatorial Pacific sediment cores (TR163-19 and ODP806 in Fig. 1).
They demonstrated that changes in SST coincide with changes in Antarctic air
temperature and precede changes in continental ice volume by about 3,000 years.
This result suggests that the tropical Pacific has a major role in driving glacialinterglacial climate changes.
Lacustrine cores sampled from the tropical Andes also provided evidence of
the early warming of the eastern equatorial Pacific region. Seltzer et al. (2002)
investigated magnetic susceptibility of sediment cores from Lake Junin and Lake
Titicaca (Fig. 1). They found that the deglaciation in the tropical Andes occurred
from 22,000 to 19,500 years ago, several thousand years before the Bølling-
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Fig. 3. Long-term ENSO index (∆SST; Yamamoto et al., in preparation), calculated NINO3 index
(Clement et al., 1999), δ18O of benthic foraminifera Uvigerina in MD01-2421 (Yamamoto et al., in
preparation), and the pCO 2 and δ D records from the Antarctic Vostok ice core (Petit et al., 1999)
during the last two deglaciations (Yamamoto et al., in preparation).
Allerød warming of the northern high latitudes. They suggested that the early
warming of the tropics has been transmitted both atmospherically and by ocean
circulation processes to produce the deglaciation of continental ice sheets in the
Northern Hemisphere.
Recent high-resolution paleoceanographic studies provided a more detailed
behavior of the tropical Pacific during the last deglaciation. In the eastern
equatorial Pacific, Koutavas et al. (2002) generated a SST record reconstructed
from the Mg/Ca ratios of planktonic foraminifera shells in Core V21-30 taken
from the cold-tongue region near the Galapagos Islands (Fig. 1). Cold-tongue
SST varied coherently with precession-induced changes in seasonality during the
past 30,000 years. SST increased from ~20,000 to ~12,000 years ago with highamplitude fluctuations. The warming trend is parallel to that of Antarctica
recorded in the δ 18O record of Byrd ice core. Koutavas et al. (2002) attributed this
warming to the northward displacement of the intertropical convergence zone
(ITCZ) which was related to the shift of an El Niño-like to a La Niña-like
ENSO-like Variability in Centennial- to Orbital-Scale Climate Change
271
condition.
In the western equatorial Pacific, Visser et al. (2003) analyzed the oxygen
isotopes and Mg/Ca ratios of planktonic foraminiferal shells in Core MD982162
taken from the Makassar Strait in the heart of the Indo-Pacific warm pool (Fig.
1). They found that SST increased by 3.5–4.0°C during the last two deglaciations.
This temperature increase was parallel to the global increase of atmospheric CO2
and the warming of Antarctica, but it preceded the decrease of ice volume by
2,000–3,000 years.
In the mid-latitude North Pacific, Yamamoto et al. (in preparation) showed
that a strongly El Niño-like SST pattern prevailed in the last two deglaciations.
The synchronous warming of Antarctica (Petit et al., 1999) and the tropical
Pacific (Lea et al., 2000; Koutavas et al., 2002; Visser et al., 2003) and an
increase of pCO2 (Petit et al., 1999) occurred within these strongly El Niño-like
intervals during deglaciations (Fig. 3). These findings are concordant with
Cane’s hypothesis that a long-term El Niño must have resulted in global heating
(Cane, 1998).
The studies reported above demonstrate that the warming of the tropical
Pacific coincides with the warming of Antarctica, but precedes the decrease of ice
volume during the last deglaciation. The temperature increase was not synchronous
at the eastern and western equatorial Pacific. The mid-point of deglaciation SST
increase was ~18,000 years ago at the eastern margin (Koutavas et al., 2002) and
~15,000 to ~14,000 years ago at the western margin (Stott et al., 2002; Visser et
al., 2003). This asynchronous warming suggests that the tropical Pacific was
warmed in an El Niño-like mean state during the last deglaciation.
Studies of the mid-latitude North Pacific indicate a strongly El Niño-like
pattern during the last deglaciation (Heusser and Sirocko, 1997; Yamamoto et al.,
in preparation). A new question arises why the mid-latitude North Pacific had a
stronger El Niño-like pattern than the tropical Pacific. The SST and precipitation
patterns in the mid-latitude North Pacific is influenced by the intensity and
location of the Aleutian Low that is linked to the Arctic Oscillation (Thompson
and Wallace, 2001) as well as tropical ENSO. We should consider a possible
source of extratropical forcing in ENSO-like variability in the mid-latitude North
Pacific in future discussions.
4. LAST GLACIAL MILLENNIAL-SCALE VARIABILITY
Millennial oscillations were first detected in Greenland ice cores and
referred to as “the Dansgaad-Oeschger cycle”. These abrupt climate events
appear to be paced by a 1,470-year cycle. These changes have subsequently been
detected in North Atlantic sediment cores, Antarctic ice cores, and in records
from many other regions. These changes were larger when glacial ice sheets
existed, and smaller during interglacials. The cause of the millennial oscillation
is still controversial, being attributed to either the internal behavior of the
Northern Hemisphere ice sheets, the internal interactions in climate system, or
extraterrestrial oscillation such as changes in solar radiation (Ruddiman, 2001).
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1
-35
8
12
1617
14
Greenland
GISP2
19
-40
0
WEP G. ruber
1
0
LN
2
EN
WEP G. succulifer
1
2
-35
Antarctica
Byrd
A1
ACR
A3
A2
A4
A5
-40
0
10
20
30
40
50
60
70
GISP2 age (kyr BP)
Fig. 4. Changes in the salinity effect on δ 18O (∆ δ 18O) in Core MD982181 from the western equatorial
Pacific (Stott et al., 2002) and the δ 18O of ice cores from Greenland GISP2 and Antarctic Bryd
stations (Blunier and Brook, 2001) for the past 70 ka. EN: El Niño-like period, LN: La Niña-like
period, ACR: Antarctic Cold Reversal (Jouzel et al., 1992), A1–A5: Antarctic Warm Events (Blunier
and Brook, 2001), Numbers in the top panel: Interstadial isotope stages of Greenland ice cores
(Dansgaard et al., 1993).
Recently, Stott et al. (2002) found the evidence that long-term ENSO-like
variability was related to millennial oscillation during the last glacial. They
reconstructed a salinity record of Core MD982181 from the offshore of the
Mindanao Island (Fig. 1), the western equatorial Pacific, during the last 70,000
years by analyzing Mg/Ca ratios and oxygen isotopes of planktonic foraminifera
shells. The salinity effect on δ18O (∆δ18O) was used as an ENSO index, because
the salinity in this region is influenced mainly by ENSO variability. The changes
in ∆δ 18O demonstrated that El Niño-like conditions dominated in stadials (cold
periods of Greenland), while La Niña-like conditions dominated in interstadials
(Fig. 4). Stott et al. (2002) suggested the contribution of La Niña-like condition
to global warming. It is, however, notable that El Niño-like periods correlate with
ENSO-like Variability in Centennial- to Orbital-Scale Climate Change
273
gradual warming periods of Antarctica, and La Niña-like periods correlate with
gradual cooling periods of Antarctica (Fig. 4). The abrupt warming of Greenland
delayed behind the warming of Antarctica by 1,500–3,000 years (Blunier and
Brook, 2001). It is also noted that the millennial-scale change of pCO2 recorded
in Taylor Dome ice core was parallel to that of air temperature recorded in Vostok
ice core (Indermühle et al., 2000). This relationship is the same as that observed
in the orbital-scale ENSO-like variability reported by Yamamoto et al. (in
preparation). This common feature suggests that the correlation of the ENSO
mode to global climate change is constant on millennial to ten millennial time
scales.
5. HOLOCENE CENTENNIAL TO MILLENNIAL-SCALE VARIABILITY
Millennial-scale ENSO-like variability in the Holocene epoch was reported
by Moy et al. (2002). They generated a color record of a sediment core from Lake
Pallcacocha, southern Ecuador (Fig. 1), which is strongly influenced by ENSO
variability, and covers the last 12,000 years continuously. They found that
changes on a time scale of 2–8 years, which is attributed to El Niño events,
became more frequent over the Holocene until ~1200 years ago, and then declined
towards the present. This long-term trend is concordant with the prediction by
Clement et al. (1999) using the Zebiak-Cane ENSO model. The fluctuation of
ENSO activity at a time scale of ~2,000 years was also observed. This fluctuation
appears to be linked to the depositional events of ice-rafted detritus in the North
Atlantic (Bond et al., 1997). The events tend to occur during the period of low
ENSO activity immediately following a period of high ENSO activity, suggesting
some linkage between ENSO and North Atlantic oceanic dynamics.
Dumber et al. (1994) generated the last 400-year continuous δ 18O record of
coral Pavona clavus and Pavona gigantea in the Galapagos Islands. A number of
low δ18O excursions during the 17th and 18th centuries indicate that the ENSO
events are missing from the historical ENSO reconstruction reported by Quinn et
al. (1987) and the recently revised reconstruction by Ortlieb (2000). Spectral
analysis indicated several shifts in dominant oscillatory modes. ENSO variability
during 17th century is concentrated at periods of 4.6 to 7 years. In the early to mid18th century, the lower frequency of ENSO variability diminishes and the
dominant ENSO period is 4.6 years with a second peak at 3 years. During the early
to mid-19th century, ENSO variability shifts to a central period of about 3.5 years.
A longer climatic record was generated by Cobb et al. (2003) using fossil
corals from the Palmyra Island in the central tropical Pacific (Fig. 1). The record
provided 30–150 year windows of tropical Pacific climate variability within the
last 1,100 years (Fig. 5). Since the climate of the Palmyra Island is warm-humid
in El Niño years and is cool-dry in La Niña years, the δ18O variance of banded
coral is a good indication of the amplitude of ENSO variability. The record
demonstrated that the ENSO variance was weak in the 14th century, while it was
stronger in the 17th century. The record also indicated that the climates of the 10th
and 12th centuries were cool and dry, which is analogous to a modern La Niña
condition (Fig. 5). The climate of the 17th century was warm and humid, which
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200
400
600
800
1000
1200
1400
1600
1800
2000
0
-0.1
Northern Hemisphere
-0.2
-0.3
0.05
-0.4
El Niño
0.10
EN
LaNiña
EN
LN
LN
-5.6
0.15
EN
-5.4
0.20
LN
-5.2
0.25
El Niño
-5.0
-4.8
-4.6
-4.4
LaNiña
-385
-390
-395
1368
1367
1366
1365
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1364
1363
1362
MWP
200
400
600
800
1000
Little Ice Age
1200
1400
1600
1800
2000
Year (A.D.)
Fig. 5. Changes in the surface temperature anomalies of the Northern Hemisphere (Mann and Jones,
2003), the titanium concentration in ODP Site 1002, Cariaco Basin (Haug et al., 2001), the annual
mean δ 18O in a Porites coral from the Palmyra Island, central equatorial Pacific (Cobb et al., 2003),
the smoothed δ D of Dome C ice core (Lorius et al., 1979), the solar irradiance reconstructed from
10
Be concentration in PS1 ice core from the South Pole (Bard et al., 1997) and the atmospheric CO2
concentration recorded in the Law Dome ice core (Langenfelds et al., 1996). EN: El Niño-like
period, LN: La Niña-like period.
is analogous to a modern El Niño condition (Fig. 5). They could not find any
relationship between ENSO behavior and global climate changes, and concluded
that the majority of ENSO variability over the last millennium may have arisen
from dynamics internal to the ENSO system itself.
Haug et al. (2001) generated a record of titanium content of a sediment core
from ODP Site 1002, the Cariaco Basin, off the Venezuelan coast (Fig. 1). Since
ENSO-like Variability in Centennial- to Orbital-Scale Climate Change
275
the titanium content of Cariaco sediments reflects the precipitation of northern
South America, thereby the content is a good indication of the latitudinal position
of the Intertropical Convergence Zone (ITCZ). The titanium record showed the
wetter Bølling-Allerød period, the drier Younger Dryas period, and a long-term
trend from wetter conditions in the early Holocene to dryer condition in the late
Holocene, with pronounced short dry periods. During El Niño events, the
exposure of cold subsurafce waters in the eastern equatorial Pacific is greatly
reduced, and the temperature structure of the region becomes more symmetric
about the equator. As a result, El Niño events are characterized by a mean
southward shift of the ITCZ in the Pacific Basin. Haug et al. (2001) thus attributed
the latitudinal shift of the ITCZ to the long-term variability of ENSO behaviors.
The titanium record indicates that an El Niño condition dominated in LIA (Fig.
5), which is concordant with the observation by Cobb et al. (2003).
Observations by Stott et al. (2002) and Yamamoto et al. (in preparation)
demonstrated that El Niño conditions correlate with the cold phase of Greenland
and the warming phase of Antarctica. This relationship also holds for the
centennial-scale ENSO-like variability during the last two millennia observed by
Haug et al. (2001) and Cobb et al. (2003). The δ D record of ice core from the
Antarctic Dome C indicates that the warming of Antarctica prevailed within El
Niño-like periods such as 8th–9th, 12th and 15th–18th centuries (Lorius et al.,
1979). However, the changes of ENSO mode show no systematic relationship
with the mean temperature change of the Northern Hemisphere (Mann and Jones,
2003). Furthermore, there is no clear correspondence between the Antarctic
temperature and the atmospheric CO2 concentration recorded in the Law Dome
ice core (Etheridge et al., 1996). This does not fully agree with the cases of the
last glacial millennial-scale variability and orbital-scale variability. It is curious
that El Niño conditions dominated in the intervals in which the solar irradiance
increased, such as 15th-18th centuries, while La Niña conditions dominated in the
intervals in which the solar radiation decreased (Fig. 5). This relationship
suggests that the change of solar radiation is an external forcing of centennialscale ENSO-like variability.
6. SUMMARY
A new perspective of long-term ENSO-like variability is useful for
understanding the cause of global climate changes. The paleoceanographic
studies reviewed above indicate that the climate changes of the tropical Pacific
preceded those of the northern high latitudes, suggesting that the tropical Pacific
plays an important role in global climate change. Some of the studies suggest that
the long-term ENSO-like variability should be involved in the climate driving
processes.
There has been a question whether El Niño conditions correlate with global
warming, warm, cooling or cool phase. This question, however, derives from the
confusion of most authors who correlate the changes in ENSO behavior with
Greenland ice core records or ice volume changes. Recent ice core studies have
demonstrated that the warming of Antarctica preceded the warming of Greenland
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by 1,500–3,000 years (e.g., Blunier and Brook, 2001), suggesting that the
Antarctic climate responded more directly to external forcing. It is therefore more
reasonable to correlate ENSO behavior with the climatic phase of Antarctica.
This new perspective indicates that El Niño-like conditions correlate with the
warming periods of Antarctica and the coldest periods of Greenland, while La
Niña-like conditions correlate with the cooling periods of Antarctica and the
warm periods of Greenland. This correspondence suggests that the shift to an El
Niño-like condition in the tropical Pacific is essential to global warming on
different time scales. There is still a question whether or not the tropical Pacific
ocean-atmosphere interactions behave as an amplifier of the external forcing of
Earth’s climate system. More detailed features of the ENSO behavior in different
climate phases or different time scales need to be an important research objective
in future climate science.
Acknowledgements—The author would like to thank Tomohisa Irino, Tadamichi Oba,
Yuichiro Tanaka and an anonymous reviewer for their comments on the manuscript.
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