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Clim. Past, 5, 471–480, 2009
www.clim-past.net/5/471/2009/
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Climate
of the Past
Investigating the impact of Lake Agassiz drainage routes
on the 8.2ka cold event with a climate model
Y.-X. Li1, H. Renssen2, A. P. Wiersma3, and T. E. T¨
ornqvist1,4
1Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118-5698, USA
2Faculty of Earth and Life Sciences, VU University Amsterdam, Amsterdam, The Netherlands
3Deltares, Subsurface and Groundwater Systems, Utrecht, The Netherlands
4Tulane/Xavier Center for Bioenvironmental Research, Tulane University, New Orleans, Louisiana 70118-5698, USA
Received: 17 March 2009 – Published in Clim. Past Discuss.: 31 March 2009
Revised: 10 August 2009 – Accepted: 12 August 2009 – Published: 28 August 2009
Abstract. The 8.2ka event is the most prominent abrupt cli-
mate change in the Holocene and is often believed to result
from catastrophic drainage of proglacial lakes Agassiz and
Ojibway (LAO) that routed through the Hudson Bay and the
Labrador Sea into the North Atlantic Ocean, and perturbed
Atlantic meridional overturning circulation (MOC). One key
assumption of this triggering mechanism is that the LAO
freshwater drainage was dispersed over the Labrador Sea.
Recent data, however, show no evidence of lowered δ18O
values, indicative of low salinity, from the open Labrador Sea
around 8.2ka. Instead, negative δ18O anomalies are found
close to the east coast of North America, extending as far
south as Cape Hatteras, North Carolina, suggesting that the
freshwater drainage may have been confined to a long stretch
of continental shelf before fully mixing with North Atlantic
Ocean water. Here we conduct a sensitivity study that ex-
amines the effects of a southerly drainage route on the 8.2ka
event with the ECBilt-CLIO-VECODE model. Hosing ex-
periments of four routing scenarios, where freshwater was
introduced to the Labrador Sea in the northerly route and to
three different locations along the southerly route, were per-
formed to investigate the routing effects on model responses.
The modeling results show that a southerly drainage route is
possible but generally yields reduced climatic consequences
in comparison to those of a northerly route. This finding im-
plies that more freshwater would be required for a southerly
route than for a northerly route to produce the same climate
anomaly. The implicated large amount of LAO drainage for a
southerly routing scenario is in line with a recent geophysical
Correspondence to: Y.-X. Li
(li@tulane.edu)
modelling study of gravitational effects on sea-level change
associated with the 8.2 ka event, which suggests that the vol-
ume of drainage might be larger than previously estimated.
1 Introduction
The 8.2ka cold event is the largest abrupt climate change
over the past 10000 years documented in the Greenland ice
core records (Alley et al., 1997; Kobashi et al., 2007). This
event is characterized by a ∼160 year-long cooling (Thomas
et al., 2007) accompanied by dry and windy conditions in
Greenland (Alley and ´
Ag´
ustsd´
ottir, 2005). Proxy records
from many parts of the world, particularly the circum-North
Atlantic region (e.g., Morrill and Jacobsen, 2005; Hughes
et al., 2006; Kerschner et al., 2006; Lutz et al., 2007),
suggest that this event has been broadly felt in the North-
ern Hemisphere. This large, abrupt, and widespread cool-
ing event is often interpreted to result from the outburst of
proglacial lakes Agassiz and Ojibway (LAO) that flooded
the North Atlantic Ocean with freshwater. This would
have slowed down Atlantic meridional overturning circula-
tion (MOC), resulting in a reduction of poleward heat trans-
port and widespread cooling (Barber et al., 1999). While
far-field anomalies around 8.2ka may be compounded with
the long-term climate variability driven by solar forcing
(Rohling and P¨
alike, 2005), this causal mechanism gains
support from both emerging proxy records and climate mod-
eling data. High-resolution records from marine archives
document rapid changes in both surface and deep oceans
concomitant with the catastrophic drainage of LAO (Ellison
et al., 2006; Kleiven et al., 2008).
Published by Copernicus Publications on behalf of the European Geosciences Union.
472 Y.-X. Li et al.: Routing effects on the 8.2ka climate event
Early modeling work with a coupled ocean-atmosphere
general circulation model has demonstrated that massive
flux of surface freshwater to the North Atlantic Ocean can
trigger rapid changes of the ocean-atmospheric system (Man-
abe and Stouffer, 1995). For the 8.2ka event, Renssen
et al. (2001, 2002) first investigated model responses to
freshwater forcing during early Holocene climate conditions
with a global coupled atmosphere-sea ice-ocean model and
showed that a pulse of freshwater perturbation can lead to
a weakening of ocean circulation and an associated cooling
anomaly consistent with proxy data around 8.2ka. Bauer
et al. (2004) simulated freshwater drainage scenarios with
a coupled atmosphere-ocean-biosphere model and showed
that a baseline freshwater flux is essential in reproducing cli-
mate anomalies around 8.2ka. LeGrande et al. (2006) and
LeGrande and Schmidt (2008) incorporated water isotope
tracers in their model simulations and showed that modeled
tracer responses are consistent with isotope-based proxy data
of the 8.2ka event. Wiersma et al. (2006) examined var-
ious aspects of freshwater perturbations with an upgraded,
global coupled atmosphere-ocean-sea ice model, and found
that freshwater volume is a decisive factor in governing the
characteristics of modeled anomalies. Comparison of model
responses with proxy records (Wiersma and Renssen, 2006)
supports the hypothesis that the 8.2 ka event was triggered by
an outburst of LAO.
In the previous modeling work, freshwater perturbations
were introduced in either the Labrador Sea (e.g., Renssen et
al., 2001, 2002; Wiersma et al., 2006) or Hudson Bay (e.g.,
LeGrande et al., 2006; LeGrande and Schmidt, 2008). In
these studies, it is either implicitly assumed in an experi-
ment design (perturbations introduced in the Labrador Sea)
or intrinsically constructed in the models (perturbations in-
troduced in the Hudson Bay) that LAO drainage was routed
through the Hudson Strait, and then spread over the Labrador
Sea before entering the North Atlantic Ocean. However,
foraminiferal δ18O data from marine sediment cores from
the Labrador Sea do not show the expected depleted δ18O
values, indicative of low salinity, that appear to occur only
on the Labrador shelf, south of the Newfoundland margin,
and as far south as Cape Hatteras, North Carolina, between 8
and 9ka (Keigwin et al., 2005) (Fig. 1). In addition, detrital
carbonate layers, representing the drainage event, do not oc-
cur across the Labrador Sea, but are mainly distributed along
the Labrador shelf (Hillaire-Marcel et al., 2007). These data
suggest that the final LAO drainage may not have spread over
the Labrador Sea as often assumed, but perhaps occurred as
a buoyant current flowing southeast along the coast reaching
as far south as Cape Hatteras before fully mixing with North
Atlantic Ocean water. The possible effects of a southerly
routing of the LAO drainage on the 8.2ka event, as implied
by oxygen-isotope data, has not yet been examined.
The objective of this study is to investigate the routing ef-
fects of LAO drainage on ocean circulation and the resulting
climate changes around 8.2 ka by introducing freshwater per-
40N
50N
60N
80W 70W 60W 50W 40W
40N
50N
60N
80W 70W 60W 50W 40W
Labrador Sea
Hudson Strait
Greenland
WGC
LC
R1
R2
R3
R4
1.03 Sv
2.06 Sv
3.09 Sv
Flux:
R0
Fig. 1. Map showing schematic circulation of surface currents to-
gether with the location of high-resolution records of Holocene cli-
mate change in the northern North Atlantic Ocean (after Keigwin
et al., 2005). Dots represent locations where low δ18O between
about 8 and 9ka was documented. Open circles represent sites
where no δ18O anomalies are observed. Sites are from Keigwin
et al. (2005) and Hillaire-Marcel et al. (1994). The northern bound-
ary of the present-day Gulf Stream (hatched area, its average posi-
tion) is shown in red and is determined based on satellite observa-
tions (Lee, 1994). The model simulates a Gulf Stream at this posi-
tion under present-day boundary conditions. In the early Holocene,
the northern boundary of the modeled Gulf Stream is located fur-
ther north at around 50◦N. Major cold currents are shown in dark
blue. WGC = West Greenland Current, LC= Labrador Current. The
possible route of the freshwater drainage during the 8.2ka event
is shown in light blue. Green areas with R1, R2, R3, R4 indicate
the approximate locations where freshwater was introduced for the
four different routing experiments. For each route, three freshwater
flux scenarios were used. Light green area with R0 indicates the
freshwater release site of Wiersma et al. (2006). Both R0 and R1
release freshwater directly into the Labrador Sea and are considered
northerly routes; R2, R3, R4 are regarded as southerly routes.
turbations to both the Labrador Sea and three locations along
the southerly drainage route. Investigating the routing effects
of LAO drainage around 8.2ka could contribute to improved
understanding of the cause of the 8.2ka climate event.
Clim. Past, 5, 471–480, 2009 www.clim-past.net/5/471/2009/
Y.-X. Li et al.: Routing effects on the 8.2ka climate event 473
2 Methods
2.1 The ECBilt-CLIO-VECODE model
We use the intermediate complexity Earth system model
ECBilt-CLIO-VECODE (version 3) to investigate the rout-
ing effects of LAO drainage on Atlantic MOC and concomi-
tant climatic responses. ECBilt-CLIO-VECODE is a three-
dimensional coupled atmosphere-ocean-vegetation model.
The atmospheric component ECBilt is a spectral quasi-
geostrophic model that contains three vertical levels and has
a T21 (∼5.6◦×5.6◦) horizontal resolution (Opsteegh et al.,
1998). The ocean component CLIO is a primitive-equation,
free-surface ocean general circulation model coupled with a
thermodynamic-dynamic sea-ice model. CLIO consists of 20
vertically unevenly spaced levels and has a 3◦×3◦horizon-
tal resolution (Goosse and Fichefet, 1999). The terrestrial
vegetation component VECODE takes into account evolu-
tion of vegetation cover that comprises trees, grasses, and
desert (Brovkin et al., 2002).
The utility of this model has been demonstrated in several
previous studies, showing that it can reproduce major char-
acteristics of modern climate reasonably well under present-
day forcing conditions (Goosse et al., 2001; Renssen et al.,
2002). Also, this model has been frequently employed to
investigate Holocene climate evolution (e.g., Renssen et al.,
2005; Goosse et al., 2005) and abrupt climate changes such
as the 8.2ka event (e.g., Renssen et al., 2001, 2002). In ad-
dition, the upgraded version 3 of this model can produce
deep water formation in not only the Greenland-Iceland-
Norwegian (GIN) Sea, but also the Labrador Sea under
modern climate conditions (Wiersma et al., 2006). This
version of the model has been used to simulate freshwa-
ter forcing to ocean circulation and climate change around
8.2ka (Wiersma et al., 2006). A detailed description of the
model can be found at http://www.knmi.nl/onderzk/CKO/
ecbilt.html.
Although the ocean component, CLIO, of this model
does not have a horizontal resolution sufficient to charac-
terize eddies which are important in mixing of freshwater
and ocean water, a recent study demonstrates that responses
of the coarse-resolution ECBilt-CLIO-VECODE model are
largely similar to those of eddy-permitting resolution mod-
els (Spence et al., 2008). The coarse resolution of the
model also prevents it from describing in detail the specu-
lated southerly drainage, eastward and then southward along
the North American coast. This means that the model cannot
track freshwater drainage following a southerly route. How-
ever, a southerly drainage could be represented by introduc-
ing freshwater at a number of locations sequentially along
a southerly drainage route. Therefore, despite these limi-
tations, the ECBilt-CLIO-VECODE model can provide im-
portant insights into the impacts of routing effects on ocean
circulation and climate change.
2.2 Experimental setup and design
Wiersma et al. (2006) examined various freshwater perturba-
tion scenarios using version 3 of the ECBilt-CLIO-VECODE
model for the 8.2ka climate event. In this previous study,
freshwater was introduced over 5 years at a fixed location
R0 (Fig. 1). The present study takes Wiersma et al.’s (2006)
work a step further by investigating freshwater routing ef-
fects on ocean circulation and climate changes around 8.2 ka.
Therefore, the experimental setup is the same as that of
Wiersma et al. (2006) with greenhouse gas concentrations
(Raynaud et al., 2000) and orbital parameters (Berger and
Loutre, 1991) tuned to represent the conditions at 8.5 ka, and
a baseline flow of 0.172Sv introduced to the Labrador Sea
to account for the background Laurentide Ice Sheet melt-
ing (Teller et al., 2002). The model with these bound-
ary conditions was run for 850 years until it reaches quasi-
equilibrium in the deepest ocean layer, which is defined by
dT/dt<0.0002◦C/100yr. Since a baseline flow is included,
oceans are continuously freshening. Such an experimen-
tal design emulates the boundary conditions in the early
Holocene when the Laurentide Ice Sheet was rapidly melt-
ing and sea level was rising. Therefore, the means of the
quasi-equilibrium state prior to freshwater perturbation are
used as the background conditions to which model responses
are compared and thus represent the “control simulation” in
this study.
To examine LAO routing effects, we perturbed the early
Holocene climate system by introducing freshwater over a 5-
year period to the Labrador Sea, which represents a northerly
routing scenario (R1), and to three other locations along the
southerly drainage route near Grand Banks (R2), Georges
Bank (R3), and Cape Hatteras (R4) (Fig. 1). At each loca-
tion, freshwater is released to one grid cell at that site. After
the 5-year freshwater perturbation, the model run continues
for 500 years or more with a baseline flow at the exact same
location and rate as for the pre-perturbation state. Such an ex-
perimental design that holds both initial and boundary con-
ditions constant permits the evaluation of the sensitivity of
the climate system to changes in the location of freshwater
perturbations. The model responses to these four freshwa-
ter perturbation scenarios are therefore compared to examine
the routing effects of LAO drainage on the early Holocene
climate.
Wiersma et al. (2006) showed that the volume of fresh-
water introduced is a decisive factor affecting model re-
sponses. We therefore examine the persistency of model
responses related to routing effects, if any, by introducing
varying amounts of freshwater. Specifically, we designed
three sets of experiments with an amount of freshwater in-
troduced of 1.6×1014 m3(0.45 m sea-level equivalent, SLE),
3.2×1014 m3(0.90mSLE), and 4.8×1014 m3(1.35mSLE),
over a 5-year period, corresponding to a freshwater flux
of 1.03Sv, 2.06Sv, and 3.09 Sv, respectively. For a given
amount of freshwater introduced, simulations of the four
www.clim-past.net/5/471/2009/ Clim. Past, 5, 471–480, 2009
474 Y.-X. Li et al.: Routing effects on the 8.2ka climate event
R1
R3
A
B
C
D
80N
60
40
20
EQ
80W 60 40 20 0 20 40E
00.6
1.2
-0.6
-1.2
-2.0
R1
80N
60
40
20
EQ
80W 60 40 20 0 20 40E
400 500 600
300
200
100
80N
60
40
20
EQ
80W 60 40 20 0 20 40E
00.6
1.2
-0.6
-1.2
-2.0
R3
80W 60 40 20 0 20 40E
400 500 600
300
200
100
80N
60
40
20
EQ
Fig. 2. February mean salinity anomalies (psu) (A, B) and convec-
tion depth (m) (C, D) over 20 years following the 2.06Sv freshwater
perturbation for R1 and R3. The salinity anomaly is defined as the
20 year mean salinity minus the salinity in the control simulation.
different routes (R1, R2, R3, and R4) of freshwater pertur-
bation were performed. The amounts of freshwater used in
this study are similar to those of single-, double-, and triple-
volume freshwater used in Wiersma et al. (2006).
3 Results
3.1 Oceanic responses
The three southerly routes R2, R3, and R4 yield, by and
large, similar model responses, and thus the results of R3
are henceforth presented as representative model responses
of a southerly routing scenario. The northerly R1 route
produces stronger anomalies than the southerly R3 route as
evidenced by its more pronounced decrease in sea surface
salinity (Fig. 2a, b) and the shutdown of convection in the
Labrador Sea following the freshwater perturbation (Fig. 2c,
d). The distinct oceanic responses to the R1 and R3 routes
are also characterized by marked decreases in the strength
of MOC in the North Atlantic Ocean (Fig. 3a), the GIN Sea
(Fig. 3b), and meridional heat transport in the ocean at 30◦S
(Fig. 3c). The anomalies in these parameters become more
pronounced as the freshwater flux increases from 1.03Sv,
through 2.06Sv, to 3.09Sv (Fig. 3). For instance, the At-
lantic MOC strength of R1 route decreases by ∼5Sv, 8Sv,
and 10Sv for the 1.03Sv, 2.06Sv, and 3.09 Sv freshwater
perturbation, respectively (Fig. 3a).
Fig. 3. Time series of maximum overturning stream function (Sv)
in the North Atlantic (A) and the GIN Sea (B), and meridional heat
transport in the ocean at 30◦S(C). Left, middle, and right columns
are results for 0.45m SLE, 0.90m SLE, and 1.35m SLE freshwater
perturbation, respectively. All time series are smoothed by 25-year
running means. Arrows mark the time when the freshwater pertur-
bation is introduced.
Overall, the duration of these anomalies, defined as the
departure from the mean minus one standard deviation of
the 200-year pre-perturbation values, also appears to increase
with the increasing volume of freshwater introduced, though
not as pronounced as the trend shown by the amplitudes
of these anomalies among these three sets of experiments.
For example, the MOC anomalies of R1 in the GIN Sea
last for about 150 years, 200 years, and 250 years for the
0.45mSLE, 0.90mSLE, and 1.35mSLE freshwater pertur-
bations, respectively (Fig. 3b). Within all of the three sets of
freshwater perturbation experiments, the northerly route R1
produces the larger anomalies than the southerly R3 route
(Fig. 3). Overall, durations of the anomalies produced by the
R1 route tend to be longer than those produced by the R3
route. Since the oceanic responses to different routes in each
of the three sets of freshwater perturbation experiments show
overall similar patterns of variation, we chose the 0.90m SLE
perturbation experiments as a representative set for further
detailed examination of routing effects in the rest of this pa-
per, unless stated otherwise.
Freshwater perturbation through the R1 route causes ter-
mination of deepwater formation in the Labrador Sea, while
convective activity in the GIN Sea continues, though weaker
in magnitude (Fig. 4a, b, c). For the southerly routes,
Clim. Past, 5, 471–480, 2009 www.clim-past.net/5/471/2009/
Y.-X. Li et al.: Routing effects on the 8.2ka climate event 475
100 200 300 400 500 600 700 800 900
80W 60 40 20 0 20 40E
80N
60
40
20
EQ
A
(control)
80N
60
40
20
EQ 80W 60 40 20 0 20 40E
B
(R1)
80W 60 40 20 0 20 40E
80N
60
40
20
EQ
C
(R3)
60S 30S EQ 30N 60N
D
(control)
0
1000
2000
3000
4000
5000
Depth (m)
18
15
12
12
9
6
30
9
6
3
-3
-6
-6
-6
-6
-3
-3
-3
3
6
0
9
6
60S 30S EQ 30N 60N
0
1000
2000
3000
4000
5000
Depth (m)
E
(R1)
-2-2
2
64
0
02
0
4
8
10
10
8
6
6
4
2
0-2
-4
-6 -6
-4
-2
-4
-2
60S 30S EQ 30N 60N
0
1000
2000
3000
4000
5000
Depth (m)
F
(R3)
10
-2
8
8
8
6
64
4
22
000
4
2
6
6
10
12
6
-2
-2 -4
-4
-4
-6
-6
-2
-2
Fig. 4. The mean convection depth in February and stream function
for the control simulation (A, D) and for the R1 (B, E), and R3 (C,
F) simulations over 80 years following the freshwater perturbation.
The flux of freshwater introduced is 2.06Sv.
convective activities in both the Labrador Sea and the GIN
Sea remain but the GIN Sea deep convection appears to
exhibit more strength than that for the northerly R1 route
(Fig. 4b, c). In response to freshwater perturbations, the
maximum of the meridional overturning stream function de-
creases from 18Sv to 10Sv for the R1 route and from 18Sv
to 12Sv for the R3 route (Fig. 4d, e, f).
Freshwater perturbations lead to expansion of sea ice for
all four routes. The R1 route causes the largest sea-ice expan-
sion covering the entire Labrador Sea by the end of the fresh-
water perturbation, while the three southerly routing scenar-
ios only lead to a slight sea-ice expansion over the Labrador
Sea (not shown). Figure 5 shows that routes R1, R2, R3, and
R4 lead to a maximum sea-ice expansion of 12.7×, 12.2×,
12.1×, and 12.0×1012 m2, respectively. Sea-ice expansion
of the R1 route also lasts longer than for the other three routes
(Fig. 5). Also, R1 appears to trigger an immediate and rapid
sea-ice expansion, while sea-ice expansion for R2, R3, and
R4 starts about 20 or 30 years after the freshwater perturba-
tion. In addition, it appears that the further south the site is
located, the later the initiation of sea-ice expansion occurs
(Fig. 5).
Model year
200 300 400 500 600 700 800 900
Sea ice extent (x 10^12 m^2)
11.2
11.4
11.6
11.8
12.0
12.2
12.4
12.6
12.8
13.0
R1
R2
R3
R4
perturbation
Fig. 5. Changes in sea-ice extent in the Northern Hemisphere after
the freshwater perturbations for the northerly R1 and southerly R2,
R3, and R4 routing scenarios. The time series are smoothed by 25-
year running means for a 0.90m SLE freshwater perturbation over
5 years (2.06 Sv flux).
3.2 Atmospheric responses
Both northerly and southerly routes of freshwater release
lead to widespread cooling (Fig. 6a, b, c). To further eval-
uate the modeled atmospheric response to the perturbation
scenarios, we analyzed the simulated air temperature at two
locations: Greenland Summit and Ammersee, Central Eu-
rope. Both sites have well-established proxy records of the
8.2ka event (e.g., Alley et al., 1997; Kobashi et al., 2007;
von Grafenstein et al., 1998) (Fig. 6d). For the northerly R1
route, the maximum modeled temperature drop at Greenland
Summit is about 2.5◦C (Fig. 6e), which is comparable to the
reconstructed 3.3◦C of Kobashi et al. (2007), while simu-
lations for Central Europe exhibit a maximum temperature
drop of only about 0.8◦C (Fig. 6f), which is about half of
the reconstructed 1.5◦C drop for the Ammersee (von Grafen-
stein et al., 1998). Both at Greenland Summit and in Central
Europe, the R1 route leads to more pronounced temperature
anomalies than does the southerly route, which is evidenced
by its comparatively larger amplitude and longer duration
(300 to 400 years) (Fig. 6e, f). The temperature anoma-
lies produced by the southerly routes (e.g., R3) display simi-
lar patterns of variation with comparable magnitudes at both
Greenland and Ammersee sites. The temperature change in
Greenland is characterized by a brief warming, probably due
to local and temporary intensification of convective activity
(see Sect. 4.1), followed by a ∼200 year cooling at Green-
land Summit (Fig. 6e). The temperature in Central Europe
exhibits a ∼100 year weak cooling in response to the fresh-
water perturbation (Fig. 6f).
www.clim-past.net/5/471/2009/ Clim. Past, 5, 471–480, 2009
476 Y.-X. Li et al.: Routing effects on the 8.2ka climate event
4 Discussion
Our model responses exhibit overall similar variation pat-
terns to those of Wiersma et al. (2006) (Fig. 7). The model
experiments show that larger freshwater perturbations lead to
more pronounced MOC anomalies, characterized by larger
amplitudes and longer durations (Fig. 3a). These results cor-
roborate the findings by Wiersma et al. (2006) that the vol-
ume of freshwater introduced is the premier factor affect-
ing the MOC and corresponding climate anomalies. For a
given freshwater perturbation, a northerly route (R1) appears
to produce distinctively stronger responses than a southerly
route (R3) (Figs. 3, 6). This is also true when the R1-induced
anomaly is compared with that of R0 of Wiersma et al. (2006)
(Fig. 7). The three southerly routes often yield largely sim-
ilar anomalies in terms of amplitude and duration, yet show
a tendency that anomalies become weaker as sites of fresh-
water perturbation are located farther south. Overall, these
two features appear to be persistently present regardless of
the exact amount of freshwater introduced (Fig. 3), and are
thus considered to be robust. Therefore, these two features
are regarded as consequences caused by different routes and
are discussed below in detail.
4.1 Comparing model results for northerly vs.
southerly routes
The northerly route (R1) produces distinctively stronger and
often longer anomalies than the southerly routes in response
to freshwater perturbations (Figs. 3, 6). The distinct differ-
ence between the northerly and the southerly routes must
be related to the degree to which deep convection in the
Labrador Sea and the GIN Sea is affected. The convection
depth data show a marked difference between the northerly
route and the southerly routing scenarios (Fig. 4). The
R1 route leads to disappearance of deep convection in the
Labrador Sea and subdued deep convection in the GIN Sea.
The southerly routes show slightly stronger and more ex-
panded deepwater formation in the GIN Sea than the R1
route. In comparison to the R1 scenario, some convective
activity occurs in the Labrador Sea, though very weak, in all
southerly routes.
The strong perturbation to the ocean and atmosphere in
the R1 scenario may primarily result from the shutdown of
deepwater formation in the Labrador Sea. The mean sea
surface salinity in the Labrador Sea over 20 years following
the freshwater perturbation drops more than 2psu for the R1
route and only ∼0.5psu for the southerly R3 route (Fig. 2a,
b). The intense freshening prevents deepwater formation in
the Labrador Sea and leads to pronounced anomalies in the
R1 scenario. The southerly routes cause only minor fresh-
ening in the Labrador Sea due to dilution and thus their ef-
fective forcing to ocean circulation is much smaller. As a
result, anomalies produced by southerly routes are weaker
than those of the northerly R1 route. The difference in the
80W 60 40 20 020
40E
80N
60
40
20
EQ
A
(control)
Temperature(°C)
80N
60
40
20
EQ 80W 60 40 20 020
40E
80N
60
40
20
EQ 80W 60 40 20 020 40E
B
(R1)
C
(R3)
Model year
200 300 400 500 600 700 800 900
Temperature (C)
-32
-31
-30
-29
-28
-27
-26
R1
R3
Model year
200 300 400 500 600 700 800 900
Temperature (C)
12.2
12.4
12.6
12.8
13.0
13.2
13.4
13.6
R1
R3
Cal yr BP
78007900800081008200830084008500
d18O of NGRIP and GRIP
-37.0
-36.5
-36.0
-35.5
-35.0
-34.5
-34.0
-33.5
-33.0
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
NGRIP
GRIP
Ammersee
D
(proxy)
E
(Greenland)
F
(Ammersee)
Fig. 6. Atmospheric responses to 2.06 Sv freshwater perturbations
of the R1 and R3 routes. (A) shows the annual mean surface air
temperature distribution of the control simulation and temperature
anomalies caused by R1 and R3 routes are shown in (B) and (C),
respectively. The temperature anomalies are defined as the differ-
ence between R1 and R3 route-induced temperature response and
the temperatures of the control simulation. (D) shows the tempera-
ture proxy data from the Greenland ice cores and Lake Ammersee,
Germany (data from Rasmussen et al., 2006 and von Grafenstein et
al., 1998) to which model responses for R1 and R3 routing experi-
ments at these two locations (E, F) are compared. The time series
in E and F are smoothed by 25-year running means.
magnitude of the freshwater forcing between the northerly
and southerly routes could also explain the distinct difference
in the temperature anomalies at Greenland Summit and in
Central Europe (Fig. 6). Since the freshwater release site for
R1 is close to where deepwater formation takes place in the
Labrador Sea and the freshwater anomaly appears to spread
out quickly (Fig. 2), the R1 scenario causes more rapid, more
dramatic and prolonged expansion of sea ice in the Northern
Hemisphere than in the three southerly routes (Fig. 5). The
weaker effective forcing of the southerly routes is evidenced
by the fact that sea-ice expansion does not occur immedi-
ately following the freshwater perturbation, but instead starts
a few decades later (Fig. 5). This delay effect probably in-
dicates the time necessary for freshwater perturbations pre-
scribed in a southerly route to be transmitted to the deepwater
formation sites and to cause anomalies in model responses.
Since sea ice acts as an insulator of heat flux between ocean
and atmosphere, the more expanded sea-ice cover in the R1
Clim. Past, 5, 471–480, 2009 www.clim-past.net/5/471/2009/
Y.-X. Li et al.: Routing effects on the 8.2ka climate event 477
Model year
200 300 400 500 600 700 800 900
(Sv)
10
12
14
16
18
20
22
R1 (this study)
R0 (Wiersam et al., 2006)
perturbation
Fig. 7. Time series of maximum overturning stream function (Sv)
in the Atlantic for a comparable freshwater perturbation of 3.09Sv
introduced at R1 (the present study) and R0 (Wiersma et al., 2006)
(Fig. 1). The time series are smoothed by 25-year running means.
The arrow indicates the time when freshwater perturbation is intro-
duced.
scenario could cause more effective reduction of heat flux
from ocean to atmosphere. Together with the increase in
surface albedo associated with expanded sea-ice cover, this
leads to the more pronounced cooling than in the southerly
routing scenarios (Fig. 6).
The different routes probably affect the deep convection
in the GIN Sea in a similar fashion. The R1 route causes
stronger anomalies in the GIN Sea than the southerly routes
(Fig. 3b). The mean sea surface salinity over 20 years fol-
lowing the freshwater perturbation (Fig. 2a, b) shows that
the freshwater anomaly in R1 appears to be more effectively
transported to the GIN Sea via the North Atlantic drift than
southerly routes where a comparatively larger fraction of the
freshwater anomaly apparently dissipates southward (south
of 40◦N) by gyres (Fig. 2a, b). The notable decrease in the
MOC strength in the GIN Sea must also contribute to the
more pronounced reduction in heat transport for the northerly
route R1 than for a southerly routing scenario (Fig. 3c).
Atmospheric responses share the similarity with oceanic
responses in that R1 route also causes larger and longer tem-
perature anomalies than do the southerly routes (Fig. 6).
This, again, is a consequence of the comparatively more
effective impacts of the R1 route than southerly routes on
convection in the Labrador Sea and the GIN Sea. One no-
tably different feature is that atmospheric responses of the
southerly route R3 display a brief warming preceding an ex-
pected cooling anomaly at Greenland Summit (Fig. 6e). Such
a brief warming is not shown in proxy records (Fig. 6d) and
is also absent from the atmospheric responses of the Am-
mersee site (Fig. 6f), suggesting that this short-lived warm-
ing is likely a local phenomenon. This brief warming over
Greenland is probably associated with a temporary and local
intensification of convective activity near Iceland that com-
pensates for local weakened deep convection near Svalbard,
leading to local warming in the Iceland-Greenland region
(Fig. 2). This convective activity near Iceland is short-lived
due to the continued freshening of the surface ocean. The at-
mospheric response of R1 does not show such a short warm-
ing episode because freshwater forcing in R1 is most effec-
tive among all four routes, causing the GIN Sea to be rela-
tively fresh (Fig. 2a, b), preventing deep convection to take
place near Iceland. Therefore, the brief warming observed in
the atmospheric responses of the southerly routes at Green-
land Summit may not indicate important, regional changes in
climate conditions.
4.2 Comparing model results for the three
southerly routes
The three southerly routes yield similar model responses.
While the magnitude and duration of the anomalies are
largely comparable, anomalies tend to become weaker in
magnitude and shorter in duration as the freshwater pertur-
bation site moves farther south from R2 through R4 (Fig. 1).
This pattern is most evident in the sea-ice data (Fig. 5) that
show a decrease in the maximum of sea-ice extent and short-
ening in the duration of sea-ice expansion from route R2
through R4 (Fig. 5). In addition, the initiation of the sea-
ice expansion appears to occur sequentially from route R2
through R4 (Fig. 5). The observed pattern may arise from
the difference in the distance from the freshwater perturba-
tion site to the deepwater formation site in the Labrador Sea.
As the location at which freshwater is introduced is moved
farther away (southward) from the deepwater formation site
in the Labrador Sea, the effective freshwater forcing and the
corresponding impacts on deep convection in the Labrador
Sea weaken.
4.3 Implications for the amount of LAO freshwater
drainage
Our modeling results show that a southerly route of freshwa-
ter forcing can produce a climate anomaly largely similar to
that of a northerly route, but the anomaly is weaker in inten-
sity and shorter in duration than that of a northerly route.
Comparison of atmospheric responses to the 0.90mSLE
freshwater forcing with proxy data from Greenland ice cores
and Lake Ammersee sediments indicates that durations of the
temperature anomalies of southerly routes better resemble
those of proxy-based temperature anomalies than a northerly
route (Fig. 6e, f). Therefore, our modeling results suggest
that a southerly route is feasible and can explain the pattern
of δ18O data from the Labrador Sea and its vicinity.
Since a southerly route produces a largely similar but
weaker perturbation to the climate system than does a
northerly route, this would imply that more freshwater would
www.clim-past.net/5/471/2009/ Clim. Past, 5, 471–480, 2009
478 Y.-X. Li et al.: Routing effects on the 8.2ka climate event
be required for a southerly route than a northerly routing
scenario in order to produce a climatic anomaly similar
to that of the 8.2ka event. Estimates of the amount of
freshwater released from LAO drainage range widely from
∼1.2×1014 m3to 5.0×1014 m3(or ∼0.35m to 1.5mSLE)
(De Vernal et al., 1997; Barber et al., 1999; Leverington et
al., 2002; Von Grafenstein et al., 1998; Renssen et al., 2002).
A sea-level study in the Mississippi Delta (T¨
ornqvist et al.,
2004) inferred an upper limit of 1.2±0.2 m for an abrupt sea-
level rise associated with the LAO drainage. However, recent
geophysical modeling suggests that the local sea-level rise in
the Mississippi Delta only represents about a fifth of the eu-
static sea-level rise resulting from LAO drainage, because the
perturbation of the gravitational field would lead to a non-
uniform pattern of sea-level change (Kendall et al., 2008).
Hence, a larger amount of freshwater release than inferred
by T¨
ornqvist et al. (2004) remains a possibility. Our find-
ings of a larger amount of drainage for a southerly routing
scenario would thus be in line with these geophysical model
results.
4.4 Limitation of this study
One important assumption in our modeling experiments is
that 100% of the freshwater drained from the Hudson Bay
was transported to the four respective sites of release and
that there was no mixing between freshwater and ocean wa-
ter along the way. In reality, dynamic mixing of freshwater
and ocean water could have occurred during transport. Since
the ECBilt-CLIO-VECODE model has a coarse spatial res-
olution (3◦×3◦) of the ocean component, it is not possible
to simulate the detailed dynamic mixing process of fresh-
water and ocean water. Spence et al. (2008) investigated
the sensitivity of the responses to a coarse spatial resolu-
tion model and an eddy-permitting resolution model for the
8.2ka event, and showed that responses of two models of
coarse and fine spatial resolution are not significantly differ-
ent. Therefore, the differential climatic responses we observe
in our modeling experiments, particularly the distinctly dif-
ferent responses between the northerly R1 route and the three
southerly routes, represent impacts due to different routing
scenarios. Future work should focus on improving the spa-
tial resolution of the climate model to adequately describe
the topography of continental slopes along the drainage route
and to characterize the dynamic dissipation of freshwater into
the North Atlantic Ocean. This line of research will fur-
ther refine our understanding of effective freshwater forcing
to ocean circulation for different routes, especially the three
southerly routing scenarios. Furthermore, additional detailed
paleoceanographic proxy records from the Labrador Sea and
northwestern North Atlantic Ocean are needed to provide
further constraints on the freshwater drainage pathway.
5 Summary and conclusions
We have simulated four different routing scenarios of LAO
drainage around 8.2ka, with varying amounts of freshwa-
ter perturbation to examine the routing effects on climatic
responses of freshwater perturbations to oceanic and at-
mospheric circulation. The modeling results suggest that
changes in drainage routes could result in significantly dif-
ferent climatic responses and a southerly route of LAO fresh-
water drainage as suggested by oxygen-isotope data may be
plausible. Overall, the modeling results show that a southerly
routing scenario would lead to a weaker climatic anomaly
than a northerly routing scenario.
In addition, our modeling results could provide insight into
estimating the amount of freshwater drainage associated with
the 8.2 ka event. A southerly route would require more fresh-
water drainage than a northerly route in order to produce a
similar climatic anomaly. Future research should focus on
improving the spatial resolution of the climate model, in-
corporating dynamic mixing of freshwater and ocean water
during the catastrophic freshwater drainage event, as well as
collecting detailed paleoceangraphic proxy records to further
constrain the freshwater drainage pathway.
Acknowledgements. Funding for this work was provided by
the Earth System History program of the US National Science
Foundation (OCE-0601814) and the McWilliams Fund of Tulane’s
Department of Earth and Environmental Sciences. We thank two
anonymous reviewers and the journal editor U. Mikolajewicz
for their constructive comments that helped improve the earlier
versions of the manuscript.
Edited by: U. Mikolajewicz
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