tele-connections between east asian monsoon and the high

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


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


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


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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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.

tele-connections between east asian monsoon and the high

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