Tele-connections between East Asian Monsoon and the High ...
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第 四 紀 研 究(The Quaternary Research) 37 (3) p. 211-219 July 1998
Tele-connections between East Asian Monsoon and the High-latitude Climate:
A Comparison between the LISP 2 Ice Core Record and the High Resolution
Marine Records from the Japan and the South China Seas
Luejiang Wang* and Tadamichi Oba*
Based on the high resolution marine sediment records from the Japan and the South China Seas, a comparison to the GISP 2 ice core record suggests a climatic tele-connections between the low-to-mid latitude East Asian Monsoon climate and that over the high latitude Greenland during the last glacial period. Episodic warm periods of Dansgaard-Oeschger events are correlated to the periods when increased monsoon precipitation caused excess of rainfall in South and East China, hence the decrease in sea surface salinity in the South China Sea, and to the development of the dark laminated sediment layers due to the reduced vertical ventilation by a fresh water lid in the Japan Sea. The possible link in this tele-connection is believed to be a counterbalance between the westerly and the southwest-to-southeast summer monsoon wind. Whenever the high latitude polar Greenland was warmed up, the westerly would have reduced in its strength and/or extension. Consequently, the monsoon circulation culminated in the East Asia due to the increased land-sea pressure contrast during summer, when the low pressure cell over mid-high latitude land areas was intensified due to the high-latitude warming.
Key Words: Tele-connection, GISP 2 ice core, Japan Sea, South China Sea, East
Asian monsoon
I. Introduction
Monsoon circulation is a thermodynamic
system in the atmosphere induced by the sea-
sonal change of heating contrast between Asian landmass and surrounding ocean areas
(Ramage, 1971; Chernia, 1980). Unlike the In-dian monsoon system, the East Asian monsoon
was controlled by the low pressure cells both over Tibet and over Siberia in summer, and
hence, summer monsoon wind directions range
from southwest in the low latitude South China
Sea to southeast in the mid latitude Japan Sea. The strong seasonality of wind directions, tem-
perature, and precipitation forms the basis of a
process that involves an extensive transport of moisture from low to high latitudes, from sea to land. Similar to the global salinity conveyer belt
in the ocean, the monsoon system in the atmos-
phere represents one of the basic elements of the global circulation. Monsoon summer rains form the main source of moisture in East Asia.
A number of studies unraveled the long-term
history of monsoonal moisture as recorded in
the loess profiles of North China (Kukla et al., 1988; Banerjee, 1995; Porter and An, 1995). Dry
phases with enhanced monsoonal dust dis-charge during winter were mainly linked to
glacial/cold stages, short-term Heinrich events, and the Younger Dryas that is also found in the
Received December 22, 1997. Accepted May, 29, 1998. Read in the Symposium of the Japan Association
for Quaternary Research 1997. * Graduate School of Environmental Earth Sciences , Hokkaido University. Nishi 5, Kita 10, Kita-ku,
Sapporo, 060-0810.
212 Luejiang Wang and Tadamichi Oba July 1998
marine sediment records near the Japan Sea Islands (Chinsei et al., 1987; Kallel et al., 1988)
and in the Sulu Sea (Kudrass et al., 1991),
whereas interglacial led to wet climate and soil
formation in China (Kukla et al., 1988) . It is well known that the global climate was
quite unstable during the last glacial cycle. One of such climatic oscillations is recognized as the
episodic warming up of the high latitude regionwhich is recorded in the Greenland ice core δ18O
signals (Dansgaard et al., 1993; Grootes et al.,
1993). Another example is the Heinrich events,
the sudden collapse of ice sheet and discharge of ice-bergs into the north Atlantic (e. g. Bond et
al., 1992) . The climatic change associated with
these regional events appeared to have been felt worldwide as evidenced by the millennium
scale of wet and dry climatic cycles in Florida
(Grimm et al., 1993) , loess deposition in central
China (Porter and An, 1995), dark and light layers in the Japan Sea sediments (Tada et al.,
1995; Nakajima et al., 1996; Tada, 1997), oce-
anic circulation changes in the Santa Barbara Basin, northeast Pacific (Behl and Kennett,
1996), foraminiferal isotopic records in a South
Atlantic deep sea core (Charles et al., 1996), and upwelling changes in the Benguela Current,
southwest Africa (Little et at.,1997).
In this paper, we would like to present the high-resolution marine records from the East
Asian monsoon regions, sediment cores from
the Japan and the South China Seas (Fig. 1), to
investigate into the possible tele-connection of the climatic events in the last glacial period, by
a comparison to the GISP 2 ice core record
(Grootes et al., 1993).
II. Core settings and climatic proxy
data
For the mid latitude Japan Sea, a piston coreKH-79-3, C-3 (37°03.5'N, 134°42.6'E, water
depth 935m, length 936cm) was recovered from
the flat top of the Oki Ridge in the southern part
of the Japan Sea (Fig. 1). The core consists of alternating dark and light layers as illustrated
in Figure 2, based on the original core descrip-
tion. Based on 8 AMS 14C datings and 4 well
defined tephra layers, the upper 850cm of the
Fig. 1 Site locations of the GISP 2 ice core (on Greenland) and marine sediment cores KH-79-3, C-3 (in the Japan Sea) and 17940 (in the South China Sea)
Shaded part indicate the region affected by monsoon precipitation. Thick arrow lines indicate the trajectory of the westerly in summer (dashed line) and in winter (solid line). Thin curved arrow lines indicate the wind direction of East Asian Monsoon in summer. Ovoid circle with "L" indicate the low pressure cells on land in summer. Gray dashed line indicates the boundary between regions affected by monsoon precipitation and by the Atlantic-European west-wind drift precipitation.
1998年7月 Tele-connections between East Asian Monsoon and the High-latitude Climate 213
core revealed a sediment sequence of the last
88,000±2,000years.
The dark layers consist of a dusky brownish
olive clay which is thinly laminated on a scale of 1-2 mm, and yield abundant well preserved
planktonic f oraminif era but only a limited num-ber of species of benthic f oraminifera. In con-trast, the light layers consist of brownish to pale
olive gray homogeneously bioturbated clay
containing abundant siliceous microf ossils and
a diversified fauna of benthic f oraminif era. Based on the faunal and isotopic signals of the
dark layers, Oba et al. (1980) suggested that these layers were deposited when the upper
water column was weakly stratified due to
influxes of low salinity surface waters, which is corroborated by the previous and afterwards
studies that the dark layers were deposited
under anoxic bottom waters during glacial
periods (Ichikura and Ujiie, 1976; Oba et al., 1984, 1991; Tada et al., 1992, 1995; Ishiwatari et
al., 1994; Nakajima et al.,1996). For the low latitude South China Sea, a grav-
ity core and a giant spade box core 17940-2 and
17940-1 (20°07'N, 117°23'E; 1,727m water depth;
13.30 and 0.45m long, respectively) was ob-
tained on SONNE cruise 95 (Sarnthein et al.,
1994) from the South China continental margin, 400 km southeast of Hong Kong (Fig. 1). This
location lies close to a prominent freshwater
plume in front of the Pearl River mouth (Japan Hydrographic Association, 1978) which has the second largest water discharge of China's major
rivers (Zhang et al., 1994). Parasound sub-
bottom profiles show that the hemipelagic sedi-
ments at core location 17940 are undisturbed
(Sarnthein et al., 1994).Aδ18O record of the planktonic foraminifera
Globigerinoides ruber s. s. (white) was obtained
to document past variations in the plume of the sea-surface salinity over the last 40, 000 years
(Fig. 2). In addition to the global glacial-inter-
glacial ice effect (Fairbanks and Matthew,1978),the δ18O variations should largely reflect
the changes in freshwater discharge of the
Pearl River and hence, in the summer mon-
soonal rainfall over South China. As is shown in
Figure 2, this record has been dated by 40 AMS '4C datings of monospecif is planktonic fo-
raminif eral samples (either G. ruber or G.
sacculifer) which formed the basis of the
chronostratigraphy for the composite core of 17940, in which 0cm in 17940-2 equals 2.5cm
in 17940-1 (Wang et al., 1998).
III. Comparison, results, and discussion
In Figure 2, we plotted the marine sediment
record from the Japan and the South China
Seas together with the GISP 2 ice δ18O record
(Grootes et al., 1993) . The ice core record show clearly the Dansgaard-Oeschger (D-O) events 1 -21 during the past 85,000 years (Dansgaardet al., 1993). The planktonic δ18O records of
core 17940 shows clearly the numerous light δ18
0 values in the marine oxygen isotope stage 2 and 3, which can be correlated to each of the 1st
to 10th D-O events during the past 40,000 years.
This is justified by the comparison per analogy
of the monsoonal precipitation records at core 17940 and the air temperature changes in Green-
land to the δ18O records in stage 2 and 3, as
discussed below.
In core 17940, the δ18O records from Bφlling/
Allerφd (B/A) to Younger Dryas (YD) events
is correspond to an increase in δ18O of O.7‰,
reflecting a decrease in monsoonal precipita-
tion and/or river runoff in the YD. In the GISP
2 ice core, δ18O record indicates warm tempera-
ture in the B/A and a cooling in the YD. Hence,
light δ18O values in core 17940 indicate the
increased monsoon precipitation and can be
correlated to the D-O warming events in oxy-
gen isotope stage 2-3. On the other hand, based on studies of the
dark layers in the Japan Sea cores, it is suggest-
ed that the formation of the dark laminated layers was under the condition of the stratified
upper water column caused by influxes of low
salinity surface waters (Oba et al., 1991; Tada, 1997) which were induced by an excess of
monsoonal precipitation over the Japan Sea
and/or by an increase in the inflow of the low
214 Luejiang Wang and Tadamichi Oba July 1998
Fig. 2 Comparison between the Greenland ice core GISP 2δ18O record and the high resolution
marine sediment records in the Japan and the South China Sea
salinity waters from the Yellow River through
the Tsushima Strait. Under this scenario, on
Figure 2, the dark layers of core KH-79-3, C-3
were also correlated to match each of the 2nd to 21st D-O events during the past 85,000 years
(Fig. 2).
In making such correlation, one has to bear in mind that the correlation has to be made by the
above outlined scenarios instead of based on
absolute chronological time scale. This is due
to the uncertainties in dating of the marine records, especially in the oxygen isotope stage 2 -3 , not only because of the somewhat larger standard errors in datings for the older sedi-ments, but also because of the changing (yet
partly unknown) 14C production rate during
1998年7月 Tele-connections between East Asian Monsoon and the High-latitude Climate 215
the stage 2-3 (Duplessy et al., 1989; Stuiver and Braziunas, 1993; Lai et al., 1996; Volker et
al., 1998; Wang et al., 1998).
However, our correlation between the high latitude climate warming events and the low-to-
mid latitude East Asian monsoon precipitation
point to a hemispherical tele-connection of the climate change during the last glacial periods. Although no modeling work has been done on
the mechanisms of this tele-connections, we
proposed the following process which may ex-
plained the observed correlation. Today, the mid-high latitude wind system is
dominated by the westerly, the main trajectory
of which centers at about 40-50°N in summer
and at about 35-45°N in winter (Fig. 1) (Zhang
and Ma, 1989). However, the westerly is re-
placed by the monsoon circulation in the East Asia region, with northwest-to-northeast winds
prevailing in winter and southeast-to-southwest winds dominate in summer. It is during the
summer months when the landward monsoon wind brings moisture from the western Pacific
onto land and hence the precipitation in East
Asia. During the D-O warming events, the high
latitude, especially the Norwegian-Greenland
Sea, was warmed up by invasion of the exten-
sions of the Gulf Stream (Sarnthein et al., 1995; Hebbeln and Wefer, 1997) , and the westerly was
weakened and moved in the culminate direction in the downwind side, opposed to the strength-
ened westerly during the cold Heinrich events,
when the North Atlantic and Norwegian-Greenland Sea were kept cold and the high
atmospheric pressure over the seas were
maintained. The relatively warm and less dust-load of the westerly wind in the central Asia
would lead to a milder surface condition than
that of the general cold-dry glacial regime. This
amelioration in the land surface (e. g. soil for-mation and better development of the land
vegetation) would lead to a decrease in albedo,
and hence, the increased heating of the bottom of the atmosphere in these regions. This would
result in an increase in the development of the
low pressure cells on land in summer, and there-
fore, leading to an increase in the land-sea
pressure contrast which, finally, resulted in a
strengthened summer monsoon wind. This sum-
mer monsoon brought more moisture onto land.
In the low latitude monsoon region, precipita-
tion increased both over South China and the
South China Sea, caused increased riverine
input into the South China Sea and reduced the
salinity of the surface water, as recorded in the
light δ18O values in core 17940 (Fig. 2). In the
mid latitude East Asia, increased southeast
monsoon wind caused increased rainfall over
East Asia (including the Japan Sea). The Yel-
low River discharge increased, and might well enter the Japan Sea, as the river mouth was
more closer to the Tsushima Strait during the
glacial low sea levels, which enhanced the low salinity surface layer and the stratification of the water column of the Japan Sea. These
resulted in the sluggish ventilation of the Japan
Sea and was recorded by the sedimentation of the dark laminated layers (Fig. 2) with less
diversified benthic faunas due to the relatively
anoxic water conditions and less bioturbation during the warm episodes of D-O events (Tada
et al., 1997) .
At the end of the warm episode of D-O events,
evidence indicates the collapse of the high lati-tude ice-sheet and the following discharge of
ice-bergs into the high latitude North Atlantic, i.e. the Heinrich events by Bond et al. (1992)
and/or Heinrich-like events by Stoner et al.
(1996). The melting of the ice-bergs chilled the surface ocean and the surrounding land, as is
the case of the oldest Dryas (Sarnthein et al.,
1995); and the westerly was strengthened
(Porter and An, 1995). These caused the cold-dry conditions on Asian landmass with en-
hanced loess deposits, increase in land surface
albedo, the reduced summer pressure low on land, which would lead to a reduced summer
monsoon due to weakened land-sea pressure
contrast. The precipitation remained low at
glacial level and was recorded as heavy valuesof δ18O in the South China Sea. The relative low
216 Luejiang Wang and Tadamichi Oba July 1998
input of fresh surface water into the Japan Sea,
hence, helped it resume its vertical ventilation and the deposits of light layers full of diversified
benthic faunas indicating an well oxygenated
bottom water condition.
IV. Conclusion
Based on the comparison between the Green-
land ice core GISP 2 δ18O record and the high
resolution marine sediment records in the Japan
and the South China Sea, a hemispherical tele-
connection is established between the high lati-
tude climate and the low-to-mid latitude East Asian monsoon climate. The intensified sum-
mer monsoon which brought increased precipi-
tation coincides with the warm episode of the Dansgaard-Oeschger events in the high latitude
region as recorded in the Greenland ice core. It
is suggested that a mechanism involving the
counterbalance between the westerly and the southwest-to-southeast summer monsoon wind
plays an important role in this climatic tele-connection. The East Asian monsoon circula-tion culminated during the warm episodes of the
Dansgaard-Oeschger events due to an increase
in land-sea pressure contrast in summer, when the low pressure cells over mid and high latitude
land areas were intensified by the high-latitude
warming.
Acknowledgement
The authors would like to express their sin-
cere thanks to all those people who contributed
to set up the marine records for this comparison work, esp. to the chief scientist Michael Sarnth-
ein of the SONNE 95 cruise in the South China
Sea during which the core 17940 was raised, to
Helmut Erlenkeuser and Pieter M. Grootes for conducting the stable isotope and AMS 14C
dating measurements at Leibniz Laboratory in
Kiel University, to the scientists on board and
the crew members of the research vessels "Hakurei -Maru" for collecting the core KH-79 -3 , C-3 from the Japan Sea. This study was supported by the Japan Society for the Pro-
motion of Science and L. Wang gratefully
acknowledge the generous funding from the
German Ministry of Research and Technology
and the Deutsche Forschungsgemeinschaf t for
his study on the SONNE 95 cruise samples.
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1998年7月 Tele-connections between East Asian Monsoon and the High-latitude Climate 219
東 ア ジ ア モ ンス ー ン と高 緯度 気候 の テ レ コネ ク シ ョン:
日本海および南シナ海の海底コアの高解像度記録 と
グリーンラン ドGIPS2氷 床 コアとの比較
王 律 江 ・大 場 忠 道
(要 旨)
日本海 お よび南 シナ海 の海底堆 積物 の高解像 度記録
と,グ リーンラン ドGIPS2氷 床 コアの記録 を比較 した
ところ,最 終氷期 にお いて低-中 緯度 の東 ア ジア モ ン
スー ンと高緯度のグ リー ンラン ドとの間 に気候上の テレ
コネ クションがあったこ とが示唆 され た。
Dansgaard-Oeschgerサ イクルにお いて急激 に温暖
化 した時代は,モ ンスー ンが強化 されて南 シナ海や東 シ
ナ海への降水量が増加 した時代 と一致す る.そ うした時
代 には,南 シナ海では海洋表 層の塩分が低下 し,日 本海
では淡水流入に よって海水 の鉛 直混合 が弱 ま り,暗 色の
細互層が堆積 してい る.こ の ようなテレコネ クションが
起 こった原因は,夏 季の南西モンスー ンあるいは南東モ
ンスー ン と偏西風 との間の相互作用 によって生 じた と考
え られる.す なわち,グ リー ンラン ドのよ うな高緯度 が
暖め られる と,偏 西風の まっす ぐ進 もうとす る力が弱 ま
り,東アジアではモンスー ン循環が南北方向に蛇行す る.
その結果,夏 季 に陸 上の中-高 緯度が暖め られて,陸 上
の低 気圧 帯 と海 との間 の気圧傾度 が増加 し,南 西 モ ン
スー ンや南東 モンスー ンが強め られ るか らである.
*北 海道大学大学院地球環境科学研究科 〒060-0810札 幌市北区北10条 西5丁 目.
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