GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 2, 1096, doi:10.1029/2002GL015513, 2003
Southern Ocean sea-ice control of the glacial North Atlantic
thermohaline circulation
Sang-Ik Shin1 and Zhengyu Liu
Center for Climatic Research, University of Wisconsin-Madison, Madison, Wisconsin, USA
Bette L. Otto-Bliesner
National Center for Atmospheric Research, Boulder, Colorado, USA
John E. Kutzbach and Stephen J. Vavrus
Center for Climatic Research, University of Wisconsin-Madison, Madison, Wisconsin, USA
Received 19 May 2002; accepted 4 September 2002; published 31 January 2003.
[1] A coupled model simulates a shallower and weaker
North Atlantic Deep Water circulation at the Last Glacial
Maximum, compared to the modern, with an enhanced
intrusion of Antarctic Bottom Water into the North Atlantic.
These circulation changes are caused by the enhanced
Antarctic Bottom Water formation, which is triggered by the
enhanced equatorward sea-ice transport, ultimately by
increased westerlies, in the Southern Ocean at the Last
INDEX TERMS: 3309 Meteorology and
Glacial Maximum.
Atmospheric Dynamics: Climatology (1620); 3344 Meteorology
and Atmospheric Dynamics: Paleoclimatology; 3337 Meteorology
and Atmospheric Dynamics: Numerical modeling and data
assimilation; 4207 Oceanography: General: Arctic and Antarctic
oceanography. Citation: Shin, S.-I., Z. Liu, B. L. Otto-Bliesner,
J. E. Kutzbach, and S. J. Vavrus, Southern Ocean sea-ice control
of the glacial North Atlantic thermohaline circulation, Geophys.
Res. Lett., 30(2), 1096, doi:10.1029/2002GL015513, 2003.
1. Background
[2] Oceanic thermohaline circulation (THC) plays an
important role in global climate. At present, the THC is
characterized by a strong North Atlantic Deep Water
(NADW) and a moderate Antarctic Bottom Water (AABW).
At the Last Glacial Maximum (LGM, 21,000 years ago),
paleoclimate records suggest a shallower and weaker
NADW circulation and an enhanced AABW intrusion into
the North Atlantic [e.g., Duplessy et al., 1988]. However,
understanding the cause of these glacial THC changes has
remained a great challenge.
[3] The changes in freshwater flux to the North Atlantic
have long been suggested as a pacemaker of the oceanic
THC changes [Bryan, 1986; Stocker et al., 1992; Manabe
and Stouffer, 1997]. However, recent studies suggest that
the oceanic THC is more sensitive to the freshwater flux to
the Southern Ocean than to the North Atlantic [Seidov et al.,
2001]. In addition, the sea-ice changes in the Southern
Ocean play a significant role in modulating the oceanic
THC changes [Goosse and Fichefet, 1999].
1
Now at CIRES/NOAA Climate Diagnostics Center, Boulder, CO,
USA.
Copyright 2003 by the American Geophysical Union.
0094-8276/03/2002GL015513$05.00
[4] Previous studies on the cause of the LGM THC
changes have been focused on the role played by the
freshwater flux to the North Atlantic; either by the increased
freshwater flux [Seidov et al., 1996; Weaver et al., 1998;
Fieg and Gerdes, 2001] or by the enhanced role of freshwater flux relative to the thermal flux because of the
temperature dependence of the thermal expansion coefficient [Prange et al., 1997]. However, the estimate of
increased LGM freshwater flux is supported by sparse data
indicating reduced surface salinity [e.g., Duplessy et al.,
1991] and is poorly constrained. Meanwhile, recent studies
suggest that Southern Hemisphere climate changes lead
those of the Northern Hemisphere [e.g., Blunier and Brook,
2001]. Broecker [1998] proposed the ‘‘bipolar seesaw
hypothesis’’, which opens a possible mechanism of the
Southern Ocean control of the NADW circulation. Broecker
[2000] also suggested the enhanced AABW formation as a
cause of the onset of Little Ice Age (16th to 19th century).
[5] Coupled models have been used to simulate the LGM
THC [Hewitt et al., 2001; Kitoh et al., 2001], but these
models show enhanced LGM NADW circulation, which is
inconsistent with the paleoclimate records [e.g., Duplessy et
al., 1988]. Here, we present a coupled model that simulates
the major features of the reconstructed glacial THC
changes. Furthermore, we propose the Southern Ocean
sea-ice as the major control of the glacial THC changes.
2. Experiments
[6] We used the National Center for Atmospheric Research-Community Climate System Model (NCAR-CCSM),
whose modern control simulation shows good agreement
with the present-day climate [Otto-Bliesner and Brady,
2001]. The modern simulation is described in Shin et al.
[2002]. The LGM climate is simulated by setting the orbital
parameters [Berger, 1978], the height and the extent of icesheets and the associated 105 m reduction of sea level
[Peltier, 1994], and the concentrations of atmospheric greenhouse gases [Raynaud et al., 1993] to the LGM conditions.
The LGM simulation was started from the modern ocean
state with an accelerated deep-ocean [Bryan, 1984] and
integrated for 300 surface model years, equivalent to
15,000 abyssal model years, at which time the global ocean
had reached a quasi-equilibrium state. Although the global
ocean had reached a quasi-equilibrium state, it should be
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SHIN ET AL.: GLACIAL THERMOHALINE CIRCULATION
Figure 1. Annual mean overturning streamfunction (Sv, 1
Sv = 106 m3s 1) to the north of 30°S in the Atlantic Ocean
for (a) Modern and (b) LGM, as obtained from the climate
model simulations. (c) The oceanic mixed-layer depth (m)
in the North Atlantic (upper panel) and the Weddell Sea
(lower panel) at the LGM. Hatched regions indicate where
the modern oceanic mixed-layer depth is deeper than 200 m.
The oceanic mixed-layer depth is defined as in Large et al.
[1997].
the LGM by increased sea-ice melting, surface cooling of
8°C [Shin et al., 2002] compensates for this freshening and
makes the surface waters dense enough to form NADW at a
rate similar to the modern. Therefore, it can be concluded
that the LGM THC changes are not caused by the surface
density flux change in the North Atlantic.
[9] The Southern Ocean provides the dominant forcing
for the LGM North Atlantic THC changes. In the Southern
Ocean, the AABW formation rate is increased from about
2.5 Sv at the modern (Figure 2a) to 25 Sv at the LGM
(Figure 2b), mainly due to increased haline density flux.
About 80% of the LGM AABW formation rate can be
explained by the increased brine release in the Southern
noted that the model only simulates the first stage of the
ocean turnover, which has the timescale of about a
thousand year. This LGM simulation closely approaches
the final state of a previous LGM simulation started from a
glacial-like ocean conditions without deep-ocean acceleration; both LGM simulations compare well with paleoclimate records from the ocean and the land surface for the
LGM [Shin et al., 2002]. The results presented here are
averaged over the last 50 years of both the modern and
LGM simulations.
3. Potential Mechanism for LGM THC Changes
[7] The simulations capture the major features of reconstructed LGM THC changes in the Atlantic [e.g., Duplessy
et al., 1988], with a reduced NADW circulation, shallower
than the modern by about 1,000– 2,000 m and weaker in
strength by about 30%, and an enhanced AABW intrusion
into the North Atlantic (Figures 1a – 1b). The formation site
of the LGM NADW migrates southward from the Labrador
Sea, the region south of Iceland and the Nordic Seas at the
modern to a position mainly south of Greenland at the LGM
(Figure 1c). A significant increase of deep-water formation
in the Weddell Sea at the LGM is apparent in the expanded
area of deep oceanic mixed-layer in the Southern Ocean
(Figure 1c).
[8] We first investigate the cause of the THC changes
from the oceanic perspective. The water mass formation
rate (Sv) is diagnosed using the surface density flux
(kgm 2s 1) [Shin et al., 2002]. The density flux is divided
into thermal and haline density fluxes to evaluate the
relative contribution of the two fluxes on the THC. The
water mass formation rate of LGM NADW is about 50 Sv
and shows little change in magnitude between the modern
(Figure 2a) and the LGM (Figure 2b). The LGM NADW
forms at slightly higher surface density (1,027.6 – 1,028.1
kgm 3) than the modern (1,027 – 1,027.5 kgm 3) due to
the overall increase of surface density at the LGM [Billups
and Schrag, 2000]. As in the modern climate, the LGM
NADW formation rate is primarily controlled by the thermal density flux, even though we consider the nonlinear
temperature effect on the thermal expansion coefficient.
Although the negative haline density flux is increased at
Figure 2. Simulated water mass formation rate (Sv) as a
function of sea surface density (kgm 3) in the Southern
Ocean (South Atlantic including the Weddell Sea; dotted
black, blue and red lines indicate the water mass formation
rate by total density, thermal density and haline density
fluxes, respectively) and in the North Atlantic (solid black,
blue and red lines indicate the water mass formation rate by
total density, thermal density and haline density fluxes,
respectively) for (a) Modern and (b) LGM. In (c), the LGM
water mass formation rate by haline density flux in the
Southern Ocean (dotted red line) is further divided into the
haline density flux due to surface freshwater flux (evaporation minus precipitation, dotted black line) and due to seaice effect (solid black line). A positive (negative) water
mass formation rate indicates that the surface density fluxes
produce a denser (lighter) water mass. Whereas the dense
water mass tends to sink to the subsurface to form deepwater mass, the light water mass tends to remain on the
surface. The density ranges of the NADW and the AABW
are shaded in gray.
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SHIN ET AL.: GLACIAL THERMOHALINE CIRCULATION
Figure 4. The north-south cross section of the zonallyaveraged; (a) salinity change (LGM-Modern, psu); (b)
potential temperature change (LGM-Modern, °C); and (c)
potential density change (LGM-Modern, kgm 3) in the
Atlantic Ocean. Gray-shaded regions in (a) and (c) indicate
relative freshening and lightening at the LGM.
Figure 3. (a) The annual haline density flux change (LGM
minus modern, 10 6 kgm 2s 1) in the Southern Ocean.
Positive values (red) indicate the regions where the surface
water density is increased, while negative values (blue)
indicate the regions where the surface water density is
decreased. The solid line indicates the sea-ice margins in JJA
(June, July and August), and the dotted line indicates the seaice margin in DJF (December, January and February) in the
LGM simulation. The simulated seasonal sea-ice margin is
similar to the recent LGM sea-ice reconstruction in the
Southern Ocean [Crosta et al., 1998]. (b) The annual sea-ice
motion change (LGM-Modern, ms 1) in the Southern
Ocean.
Ocean (Figure 2c). Whereas the LGM AABW forms under
regions of year-round sea-ice cover due to brine release in
the sea-ice production zone (>60°S), the density of the polar
surface waters in the region of wintertime extension of the
sea-ice margin (50 – 60°S) is reduced significantly due to
sea-ice melting at the LGM (Figure 3a). This horizontal
asymmetry is closely related to the oceanic salt pump
[Broecker and Peng, 1987]. The growth and melting of
sea-ice results in a vertically asymmetric redistribution of
salt in the ocean. It transports salt from the surface to the
deep-ocean, while making the surface waters near the seaice margin fresher.
[10] The salt transported by enhanced AABW makes the
deep-ocean saltier than the modern by 1.75 psu in the North
Atlantic at the LGM (Figure 4). This saline bottom water
intrusion makes the deep-ocean heavier, thereby increasing
oceanic vertical stability at the LGM. Because the glacial
surface density flux in the North Atlantic has not changed
significantly (Figures 2a – 2b), deep convection is confined
to a depth shallower than the modern due to the increased
oceanic vertical stability that is caused by the dense water
intrusion from the Southern Ocean.
[11] From the coupled climate perspective, the Southern
Ocean control of the NADW circulation is caused by the
stronger sea-ice sensitivity to the LGM conditions in the
Southern Ocean than in the North Atlantic. The sea-ice
change at the LGM shows strong inter-hemispheric differences (Table 1). In the Southern Ocean, the increase in seaice covered area dominates over the slight sea-ice thickness
increase at the LGM. In the Northern Hemisphere, the seaice covered area is reduced at the LGM, but it was thicker,
especially in the Arctic Ocean. This difference in interhemispheric response of sea-ice to the glacial climate
forcing is related to oceanic stratification and land/sea
configuration. The Southern Ocean is characterized by deep
convective mixing, which prevents thickening of sea-ice.
Open ocean on the equatorward side further favors the
wind-driven sea-ice spread. As a result, the sea-ice covered
area is almost doubled at the LGM relative to the modern
Table 1. The Simulated Total Sea-Ice Covered Area (m2) and
Mean Sea-Ice Thickness (m) at Modern and at the LGM During
DJF (December, January and February) and JJA (June, July and
August)
Total sea-ice
covered area
(1012 m2)
Mean sea-ice
thickness (m)
Modern
LGM
Modern
LGM
Southern Ocean
DJF
JJA
4.9
15.0
11.2
29.6
1.9
1.0
2.0
1.1
North Atlantic/Arctic Ocean
DJF
JJA
12.4
7.2
8.2
6.6
1.4
2.0
4.2
5.0
The sea-ice covered area is reduced at the LGM in the Arctic Ocean
because the area of the Arctic Ocean is reduced from about 70 1012 m2 in
the modern to about 50 1012 m2 at the LGM due to the 105 m lowering of
sea level [Peltier, 1994].
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SHIN ET AL.: GLACIAL THERMOHALINE CIRCULATION
mainly due to the enhanced equatorward sea-ice drift by
increased westerlies in the Southern Ocean (Figure 3b),
while the sea-ice thickness remains little changed (Table 1).
In contrast, the Arctic Ocean surface is insulated from the
subsurface by a strong halocline. Stable stratification allows
surface cooling to increase the sea-ice thickness by about
250% at the LGM (Table 1). Thin sea-ice in the Southern
Ocean is more sensitive to climate forcing than thick sea-ice
in the North Atlantic/Arctic. This difference in inter-hemispheric sea-ice sensitivity to the glacial climate, ultimately
caused by the enhanced equatorward sea-ice transport by
increased westerlies in the Southern Ocean at the LGM,
provides a mechanism of Southern Ocean control of the
NADW circulation by a sea-ice triggered haline density flux
increase at the LGM.
4. Concluding Remarks
[12] In this study, we show that a shallower and weaker
LGM NADW circulation, compared to the modern, is
caused by the enhanced AABW formation and an accompanying increase of oceanic vertical stability in the Atlantic
Ocean, which is triggered by sea-ice change and an associated haline density flux increase in the Southern Ocean.
From the coupled climate perspective, the Southern Ocean
control of the North Atlantic THC is caused by the stronger
sea-ice sensitivity to the glacial climate forcing in the
Southern Ocean than in the North Atlantic, triggered by
the enhanced equatorward sea-ice transport by increased
westerlies in the Southern Ocean at the LGM.
[13] Acknowledgments. This work was supported by a grant from
the NSF (ATM-9905285) to the University of Wisconsin-Madison and by a
computer-time grant from the NCAR. Center for Climatic Research Contribution Number 769.
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B. L. Otto-Bliesner, National Center for Atmospheric Research, P.O. Box
3000, Boulder, CO 80307-300, USA. ([email protected])
J. E. Kutzbach, Z. Liu, S.-I. Shin, and S. J. Vavrus, Center for Climatic
Research, 1225 W. Dayton St., Madison, WI 53706-1695, USA. ( jek@
facstaff.wisc.edu; [email protected]; [email protected];
[email protected])