geology 2015 montade g36745.1

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GEOLOGY | Volume 43 |Number 8 | www.gsapubs.org 735

Teleconnection between the Intertropical Convergence Zone and

southern westerly winds throughout the last deglaciation

Vincent Montade1,2*, Masa Kageyama3, Nathalie Combourieu-Nebout4, Marie-Pierre Ledru1, Elisabeth Michel3,

Giuseppe Siani5, Catherine Kissel3

1Institut des Sciences de lEvolution de Montpellier, UM/CNRS/IRD/EPHE, Avenue Eugne Bataillon, 34095 Montpellier, Cedex, France2Ecole Pratique des Hautes Etudes, 4-14 rue Ferrus, 75014 Paris, France3Laboratoire des Sciences du Climat et de lEnvironnement/IPSL, CEA/CNRS/UVSQ, UMR 8212, Orme des Merisiers, 91191 Gif-sur-

Yvette, France4Histoire Naturelle de lHomme Prhistorique, UMR7194, Dpartement de Prhistoire du MNHN, 1 rue Ren Panhard, 75013 Paris, France5Geoscience Paris-Sud, UMR8148, Dpartement des Science de la Terre Universit Paris-Sud, 91405 Orsay, France

ABSTRACT

Comparison of environmental changes between northeastern Brazil and western Patago-

nia during the last deglaciation reveals concomitant trends in moisture from the Intertropi-

cal Convergence Zone (ITCZ) and southern westerly winds (SWW). The data confirm an

atmospheric teleconnection between the ITCZ and SWW, associated with Atlantic Meridional

Overturning Circulation (AMOC) variations. When the AMOC decreases, both the ITCZ

and the SWW shift southward; they shift northward when the AMOC increases. Climate

simulations in which the AMOC is made to vary agree with this general pattern. Additional

experiments performed with an atmosphere-only model show that the tropical Atlantic is a

key area in promoting relationships between the AMOC, ITCZ, and SWW. Our data showthat this mechanism, which transfers climate changes between low and middle latitudes to

high latitudes in the Southern Hemisphere, acted throughout the abrupt climatic events of the

last deglaciation.

INTRODUCTION

The last glacial-interglacial transition (LGIT,

ca. 2110 ka) was punctuated by abrupt climatic

events, characterized by opposite trends in tem-

perature changes between hemispheres. This

phenomenon, termed bipolar see-saw (Crow-

ley, 1992), has been attributed to changes in the

Atlantic Meridional Overturning Circulation

(AMOC). However, the atmospheric circulation

could also play a role in setting up the North

AtlanticSouthern Hemisphere teleconnection

(Lamy et al., 2007): a connection between the

Intertropical Convergence Zone (ITCZ) and

the southern westerly winds (SWW) has been

proposed as a rapid mechanism acting on the

Southern Hemisphere extratropical climate for

the beginning of the LGIT (Anderson et al.,

2009). Climate models have also shown that a

southward shift of the ITCZ related to a North

Atlantic cooling can strengthen the SWW via

the weakening of the Hadley circulation and

the Southern Hemisphere subtropical jet stream

(Lee et al., 2011). However, little is knownabout the potential role of such a teleconnec-

tion in the climatic changes recorded in South

America and, more generally, in the Southern

Hemisphere.

Here we focus on two regions of South

America, the northern part of northeastern Bra-

zil (NNEB) and western Patagonia (WP), where

the ITCZ and the SWW are the main precipi-

tation drivers, respectively. In NNEB, close to

the coast, most precipitation falls from February

to May, when the ITCZ is in its southernmost

position. In WP, the SWW generates strong pre-

cipitation, increasing southward along the coast

(Garreaud et al., 2013). The SWW spread north-

ward to 30S in austral winter, but remain south

of ~47S during the austral summer. The SWW

intensity and associated precipitation reach their

maxima at these latter latitudes. The NNEB and

WP thus offer the possibility to compare and

study atmospheric changes between the ITCZ

and SWW.

METHODS

We use the following paleoclimatic records

from the NNEB and WP (Fig. 1): lacustrine core

MA97-1 (coastal area of NNEB, 3S; Ledru etal., 2006), and oceanic core MD07-3088 (off-

shore central WP, 46S; Montade et al., 2013).

Vegetation changes reflect precipitation changes

directly influenced by the ITCZ in the NNEB

and the SWW in WP. The well-constrained age

models of both records provide a reliable age

control for investigating multimillennial trends

of ITCZ and SWW changes (see the GSA Data

Repository1). We further analyze ITCZ and

SWW variations and their links with changes

*E-mail: [email protected]

GEOLOGY, August 2015; v. 43; no. 8; p. 735738 | Data Repository item 2015252 |doi:10.1130/G36745.1 | Published online XX Month 2015

2015 Geological Society of America. For permission to copy, contact [email protected].

LGMYD ACR HS1

MD07-3088

MA97-1

10 11 12 13 14 15 16 17 18 19 20Age (ka)

-1

-0.5

00.5

1

F.

13CP-B

()

4

6

8

10

12

14G.14

CB-Page(102

yr)

B

.Moraceae(%)

0102030

A.

Arborealpollen(%)

20406080

E.

Astelia(%)

0

10

2025

30

35

40

D.Palynologicalrichness

8

10

12

14

C.

SST(C)

1GSA Data Repository item 2015252, supplementary chronologic data and figures of climatic simulations,is available online at www.geosociety.org/pubs/ft2015.htm, or on request from [email protected] orDocuments Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

Figure 1. Comparison of paleo-climatic records between core

MA97-1 (coastal area of northernnortheast Brazil, 3S) and MD07-3088 (offshore central-western Pa-tagonia, 46S). YDYounger Dryas;ACRAntarctic Cold Reversal;HS1Heinrich Stadial 1; LGMLast Glacial Maximum. A, B: Ar-boreal pollen percentages indicat-ing the development of rainforestand Moraceae pollen percentagescharacteristic of rainforest undervery humid conditions (Ledru et al.,2006). C: Sea-surface temperature(SST; Siani et al., 2013). D,E: Paly-nological richness and Asteliapol-len percentages characteristic of

Magellanic moorland (Montade etal., 2013). F,G: Difference betweenthe carbon isotope compositionof benthic and planktonic foramin-ifera (13C P-B = 13C GlobigerinabulloidesCibicidoides wueller-strfi, and 14C B-P age = 14C agebenthic foraminifera-age monospe-cific planktonic foraminifera) showing the variability in vertical mixing in the Southern Oceanrelated to the upwelling activity (Siani et al., 2013). The smoothed curves use a three-pointaverage. The empty circles represent radiocarbon datings.

as doi:10.1130/G36745.1Geology, published online on 10 July 2015


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736 www.gsapubs.org | Volume 43 |Number 8 | GEOLOGY

in AMOC with numerical experiments per-

formed with the Institut Pierre Simon Laplace

climate model version 4 (IPSL-CM4), i.e., an

atmosphere-ocean general circulation model

(AOGCM) and with its atmospheric compo-

nent, the Laboratoire de Mtorologie Dy-

namique (LMDZ) atmospheric general circula-

tion model (AGCM) (Marti et al., 2010). The

resolution of the atmosphere model is 3.75 in

longitude, 2.5 in latitude, and 19 vertical levels

in both the AOGCM and AGCM. The AOGCM

runs use the Paleoclimate Modeling Intercom-

parison Project Phase II protocol for the Last

Glacial Maximum (LGM) (Braconnot et al.,

2007). The reference experiment, characterized

by an active AMOC, is labeled LGM AMOC

on (Fig. 2A). From this experiment, we start

the +0.1 Sv experiment, in which an addi-

tional freshwater flux of 0.1 Sv is added at the

surface of the North Atlantic north of 40N and

of the Arctic Ocean during 420 yr (Kageyama

et al., 2009). This flux is enough to make the

AMOC collapse after ~250 yr. At the end of the

+0.1 Sv experiment, we stop the freshwater flux(LGM AMOC off) and the AMOC does not

recover. From the LGM AMOC off run, we start

four experiments in which we impose evapora-

tive fluxes over the North Atlantic (50 to 70N).

These experiments, described by Marzin et al.

(2013), are labeled 0.1 Sv and 0.5 Sv ac-

cording to the value of the perturbation. Three

0.5 Sv experiments were performed, starting

from initial conditions, 50 yr apart. To define the

appropriate geographical domain corresponding

to central WP, we accounted for the fact that

this region, characterized by a large seasonality

of precipitation, is shifted north by ~6 in our

model (Rojas et al., 2009).

RESULTS

Atmospheric Changes

From ca. 17.5 to 15 ka, during Heinrich

Stadial 1 (HS1), the development of tropical

rainforest in NNEB indicates a significant re-

duction of the dry season (Fig. 1A), consistent

with speleothem records in the easternmost

part of this region (Cruz et al., 2009). These re-

sults are interpreted as a southward shift of the

ITCZ relative to its LGM position (Escobar et

al., 2012). Simultaneously in central WP, paly-nological richness increase shows the coloniza-

tion of vegetation, consistent with a sea-surface

temperature (SST) increase recorded in core

MD07-3088 (Figs. 1C and 1D). The benthic mi-

nus planktic foraminifera 14C age (Fig. 1G) and

13C (Fig. 1F) from the same core decrease. This

parallel decrease can only be explained by an

increase in the vertical mixing in the Southern

Ocean, i.e., an increase in the Southern Ocean

upwelling (Siani et al., 2013). Regional veg-

etation change suggesting a rapid southward

shift of the SWW south of central WP (Mon-

tade et al., 2013) explains the increased South-

ern Ocean upwelling, which is also supported

by opal productivity changes recorded in the

Southern Ocean (Anderson et al., 2009) and by

simulations (Toggweiler et al., 2006).

After HS1, although the rainforest remains

developed in NNEB, its composition reveals

changes in precipitation regimes. The Moraceae

percentages, characteristic of a rainforest devel-

oping under very humid conditions, decrease

ca. 15.5 ka and remain low from 15 to 13 ka

(Fig. 1B). This suggests a decrease of precipi-

tation consistent with the speleothem records

from NNEB (Cruz et al., 2009), while precipi-

tation increases in the Northern Hemisphere

neotropics (Peterson et al., 2000; Escobar et al.,

2012). These results mark a northward shift of

the ITCZ. In central WP, Astelia percentagesincrease abruptly at 14.5 ka and remain high

until 13 ka (Fig. 1E). Asteliais characteristic of

the humid Magellanic moorland vegetation that

develops today along the coast in southern WP,

corresponding to the core of the SWW belt, de-

fined by strong SWW intensity and precipitation

(Heusser, 1995). The development of Astelia

thus indicates a strengthening of SWW during

the Antarctic Cold Reversal (ACR) in central

WP. Simultaneously, the increase of 14C age and

13C (Figs. 1F and 1G) suggests a decrease of

wind-driven upwelling in the Southern Ocean,

as shown by Anderson et al. (2009). While

SWW intensity seems to increase in central WP,

intensity decreases over the Southern Ocean,

suggesting a northward shift of the SWW dur-

ing the ACR, also described by Garca et al.

(2012) and Moreno et al. (2012).

From 12.8 ka, during the Younger Dryas

(YD), a humidity increase is shown by a second

Moraceae increase in NNEB (Fig. 1B) while

dry conditions are observed in Cariaco Basin

record (10N), suggesting a southward shift of

the ITCZ (Haug et al., 2001). However, other

records in NNEB indicate either a slight pre-

cipitation increase (Arz et al., 1998; Wang et al.,

2004) or no particular trend (Cruz et al., 2009),

suggesting either less intense changes or a more

complex pattern than for the HS1. After the

ACR, the decrease of Asteliapercentages pointsto a reduction of SWW in the central WP (Fig.

1E), while wind-driven upwelling in the South-

ern Ocean increased, as shown by 14C ages and

13C records (Figs. 1F and 1G). These changes

support a southward shift of the SWW, consis-

tent with Pesce and Moreno (2014), and opal

productivity increase recorded in the Southern

Ocean during the YD (Anderson et al., 2009).

Climate Simulations

Our freshwater flux experiments have been

designed to obtain large AMOC changes (Fig.

A

C D

B

-5

10

15

20

25

AMOCexporta

t30S(Sv

)

-40

-38

-36

-34

-32

latmax

_u

200over

the

Chileanco

as

t

4 5Precipitation NNEB

3.2 3.4 3.6 3.8 4Precipitation central WP

3210

(a) LGM AMOC on(b) +0.1 Sv(c) LGM AMOC off(d) -0.1 Sv(e) -0.5 Sv

3.2

3.4

3.6

3.8

Prec

ipita

tioncen

tra

lWP

0 2 4 5Precipitation NNEB

4

1 30 200 400 600 800 1000 1200 years

0

5

-40

-38

-36

-34

-32

latmax

_u

200over

the

Chileanco

as

t

Figure 2. Results of the Institut Pierre Simon Laplace climate model version 4 (Marti et al.,2010) run for different freshwater perturbations in the North Atlantic. All climate experimentswere run with boundary conditions for the Last Glacial Maximum (LGM) (Braconnot et al.,

2007). A: Time evolution of the export of water due to the Atlantic Meridional OverturningCirculation (AMOC) at 30S (in Sv; 1 Sv = 106m3/s) in the reference experiment (a) and the dif-ferent freshwater flux experiments (be). BD: 10 yr running averages of the annual meansof precipitation and winds illustrating the link between precipitation changes over north-northeastern Brazil (NNEB) and central western Patagonia (WP). Each dot represents a 10 yraverage over a region along the different simulations. B: Precipitation over NNEB (015S;5535W) versus central WP (3542S; 9070W). C: Latitude of the maximum zonal wind at200 hPa over the Chilean coast (latmax_u200; the zonal average is computed from 90W to70W and the maximum of this zonal average is sought from 30S to 60S) versus precipita-tion over NNEB. D: latmax_u200 versus precipitation over central WP.



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GEOLOGY | Volume 43 |Number 8 | www.gsapubs.org 737

2A). These AMOC changes are associated with

a cooling in the North Atlantic, at both extra-

tropical and tropical latitudes, and warming in

the southern Atlantic (see the Data Repository;

Kageyama et al. 2009). Significant changes

also appear in the atmospheric circulation in

the tropics and in the high southern latitudes.

When the AMOC and Atlantic Ocean northward

heat transports decrease, the atmosphere com-

pensates by expanding its northern Hadley cell

southward (Kageyama et al., 2009). The ITCZ

therefore shifts southward, which explains the

precipitation variations over the NNEB (Fig.

2B). This shift of the tropical atmospheric cir-

culation features is also found in the southern

tropical jet stream (Figs. 2C and 2D; see the

Data Repository). In our model, over the east-

ern South Pacific Ocean along South America,

in annual mean and for glacial boundary con-

ditions, there is only a single and broad west-

erly wind belt, which includes the subtropical

jet stream and, near the surface and at middle

latitudes, the SWW. These two features of the

southern atmospheric circulation are thereforestrongly connected. Because the upper level

jet stream is also connected to the Hadley cell

through angular momentum conservation, it is

a useful diagnostic for analyzing tropical-ex-

tratropical circulation interactions. The latitude

of the maximum 200 hPa zonal wind is anticor-

related with NNEB precipitation (Fig. 2C) and

correlated with WP precipitation (Fig. 2D). Our

ensemble of coupled atmosphere-ocean simu-

lations therefore shows a plausible mechanism

connecting AMOC variations to precipitation

over NNEB and WP.

DISCUSSION

The comparison of paleoclimatic records sug-

gests simultaneous shifts of the ITCZ and SWW,

similar to the model results. To summarize these

changes and compare with the AMOC, we cal-

culate a normalized index (Fig. 3). A rainfall in-

crease in NNEB indicates a southward shift of the

ITCZ, while a rainfall increase in the central WP

indicates a northward shift of the SWW, and con-

versely, this index is based on the ratio between

the most sensitive moisture indicators of the

ITCZ changes in core MA97-1 (Moraceae) and

of the SWW changes in core MD07-3088 (As-

telia). An index increase represents a southwardshift of atmospheric structures that occurs when

the AMOC is reduced during the HS1 and the

YD (McManus et al., 2004) and when the climate

cools over Greenland (Rasmussen et al., 2006)

and warms over Antarctica (Lemieux-Dudon et

al., 2010). However, during HS1, although the

index mainly reflects ITCZ changes because

Asteliado not yet show significant changes, the

regional comparison above supports a southward

shift of the SWW. An opposite scenario occurs

during the ACR with a northward shift of atmo-

spheric structures. Our results show that the at-

mospheric teleconnection between the ITCZ and

SWW appears to be closely related to AMOC

variations throughout the LGIT.

Such observed changes closely correspond to

the model results, in which the meridional shifts

of the ITCZ and SWW are related to the imposed

AMOC variations. Lee et al. (2011), whose re-

sults were further interpreted by Chiang et al.

(2014), proposed that ITCZ shifts modulate the

strength of the subtropical jet stream, which in

turns acts to strengthen the eddy-driven mid-lat-

itude jet stream. Such a split jet is not visible in

our simulations and our model shows a shift of

the main atmospheric circulation features rather

than a modulation of their strength. The differ-

ences in experimental designs between the stud-

ies of Lee et al. (2011) and Chiang et al. (2014)

and our study make it difficult to further ascribe

the reasons for these qualitatively different re-

sponses to AMOC changes. Common setups

would be needed to improve our understanding

of the model differences.

Using paleorecords or AOGCM results only,

it remains impossible to determine whether theconcomitant climate changes in NNEB and cen-

tral WP are the sole result of the atmospheric

teleconnection, or of other features related to

AMOC changes, i.e., SST changes at southern

extratropical latitudes. We therefore ran ad-

ditional experiments with the LMDZ AGCM

(Marzin et al., 2013) to test the importance of

the tropics in transmitting a signal from the

North Atlantic to WP. In the first experiment,

we prescribe the global SST and sea-ice cover

from 50 yr of the LGM AMOC on run, and the

same other boundary conditions and forcings as

for the coupled model. In a second run, we im-

posed the global SST and sea-ice cover from the

last 50 yr of the +0.1 Sv coupled run (Fig. 4A).

From these two simulations, we checked that

the atmospheric response to these SSTs and

sea-ice cover forcings are consistent with those

obtained in the corresponding coupled simula-

tions, which is the case (not shown). In a third

simulation, we imposed the SSTs and sea ice

from the LGM AMOC on everywhere but in

the tropical Atlantic, over which we imposed

the SSTs from the end of the +0.1 Sv simulation

(Fig. 4B). We thus tested the impact of the tropi-

cal Atlantic SST changes related to an AMOC

collapse. This response consists of a dipole,

with cooling over the northern tropical Atlantic

and warming over the southern tropical Atlan-

tic (see the Data Repository). Most of the South

10 11 12 13 14 15 16 17 18 19 20Age (ka)

C.

Index

2

1

0

-1

-2

North

South

-44

-42-40-38-36-34

A.

18O()

LGMYD ACR HS1

-440-430-420-410-400-390

D.D()

B.231P

a/230Th0.10

0.090.08

0.070.06

-60

-45

-30

-15

0

-60

-45

-30

-15

0

-120 -90 -60 -30 0

-300 -240 -180 -120 -60 0 30024018012060

A

B

5

4

31

2

-5-6 -4 -3

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1

3 2

1

-1

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1

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3

0

0

0

0

0

1

1

1

3

3

2

2

-2

-5

-6

-4-3-1

-1

Figure 3. A: North Greenland Ice Core Proj-ect (NGRIP) ice core 18O (Rasmussen et al.,2006). YDYounger Dryas; ACRAntarc-tic Cold Reversal; HS1Heinrich Stadial 1;LGMLast Glacial Maximum. B: Sedimen-tary 231Pa/230Th from marine sediment coreGGC5 near the Bermuda Rise, North Atlantic(McManus et al., 2004). C: Normalized indexsummarizing common latitudinal shifts ofboth the Intertropical Convergence Zoneand the southern westerly winds. D: Ice-coredeuterium (D) based on the age scale ofLemieux-Dudon et al. (2010). The smoothedcurves use a three-point average (for A andD) and a locally weighted polynomial regres-sion (for C).

Figure 4. Atmospheric changes, computedwith the Laboratoire de Mtorologie Dy-namique model, resulting from sea-surfacetemperature (SST)/sea-ice changes due toAtlantic Meridional Overturning Circula-tion (AMOC) variations. A: Difference in an-nual precipitation (mm/yr, colors) and 200hPa zonal wind (m s1, contours) due to theglobal SST/sea-ice changes simulated byInstitut Pierre Simon Laplace climate modelversion 4 (IPSL-CM4; Marti et al., 2010) for acollapse of the AMOC [the fields shows the(collapsed AMOC reference state) anom-aly]. The difference is computed between areference run using 50 yr of the SST/sea icefrom the reference IPSL-CM4 Last GlacialMaximum run and a second simulation us-ing the SST/sea ice from the final 50 yr ofthe +0.1 Sv simulation. B: The same fieldsbut for the difference due to SST changesrestricted to tropical Atlantic.



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738 www.gsapubs.org | Volume 43 |Number 8 | GEOLOGY

American precipitation change obtained with

global SST changes is obtained by only pre-

scribing tropical Atlantic SST changes. In both

cases, the southern subtropical jet stream moves

southward. This gives us confidence that a tele-

connection involving simultaneous movements

of the ITCZ and the southern subtropical jet

stream initially triggered by AMOC variations

and mediated through tropical Atlantic SST

changes is a major component in the explana-

tion of the concomitant precipitation changes in

NNEB and WP. From our experiments, we can-

not rule out that the tropical Pacific could also

play a role in tropical-extratropical interactions,

because in our AOGCM experiments, SST vari-

ations over this region were much smaller than

over the tropical Atlantic. Other models simu-

late SST changes over the Pacific in response to

AMOC changes (Kageyama et al., 2013), and

these could be used to compare the role of the

Pacific versus Atlantic SST changes.

CONCLUSION

These results confirm the teleconnectionbetween the ITCZ and the SWW related to the

AMOC changes during the LGIT. We further-

more show that the tropical Atlantic appears to

be a key area in promoting rapid north-south

teleconnections, from the North Atlantic to

South America. Such a teleconnection, which

seems to be crucial to transfer abrupt climatic

changes between low latitudes and middle to

high latitudes over South America, could have

played a key role on the Southern Ocean CO2-

degassing via the southward intensification of

SWW during the HS1 or the YD.

ACKNOWLEDGMENTSFinancial support was provided by the Pachiderme proj-ect from the Institut National des Sciences de lUnivers(Les Enveloppes Fluides et lEnvironnement), andMontade benefited from a postdoctoral position fundedby Fundao Cearense de Apoio ao DesenvolvimentoCientfico and Ecole Pratique des Hautes Etudes. Thisis Institut des Sciences de lEvolution de Montpellierpublication #2015-115 and Laboratoire des Sciencesdu Climat et de lEnvironnement contribution #5506.

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Manuscript received 2 March 2015Revised manuscript received 4 June 2015Manuscript accepted 4 June 2015

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doi: 10.1130/G36745.1published online 10 July 2015;Geology

Michel, Giuseppe Siani and Catherine KisselVincent Montade, Masa Kageyama, Nathalie Combourieu-Nebout, Marie-Pierre Ledru, Elisabethwesterly winds throughout the last deglaciationTeleconnection between the Intertropical Convergence Zone and southern

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