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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: vincent.montade@gmail.com
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 editing@geosociety.org.
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 editing@geosociety.org 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.
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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
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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
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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
-1
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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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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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as doi:10.1130/G36745.1Geology, published online on 10 July 2015
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7/24/2019 Geology 2015 Montade G36745.1
5/5
Geology
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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