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We compared four simulations of the 8.2 ka event to assess climate model sensitivity and skill in responding to North Atlantic freshwater perturbations. All of the simulations used the same freshwater forcing, 2.5 Sv for one year, applied to either the Hudson Bay or Labrador Sea. This freshwater pulse induced a decadal-mean slowdown of 10–25% in the Atlantic Meridional Overturning Circulation (AMOC) of the models and caused a large-scale pattern of climate anomalies that matched proxy evidence for cooling in the Northern Hemisphere and a southward shift of the Intertropical Convergence Zone. The multi-model ensemble generated temperature anomalies that were just half as large as those from quantitative proxy reconstructions, however. Also, the duration of AMOC and climate anomalies in three of the simulations was only several decades, significantly shorter than the duration of ~150 yr in the paleoclimate record. Possible reasons for these discrepancies include incorrect representation of the early Holocene climate and ocean state in the North Atlantic and uncertainties in the freshwater forcing estimates.
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Clim. Past, 9, 955–968, 2013
www.clim-past.net/9/955/2013/
doi:10.5194/cp-9-955-2013
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Model sensitivity to North Atlantic freshwater forcing at 8.2ka
C. Morrill1,2, A. N. LeGrande3, H. Renssen4, P. Bakker4, and B. L. Otto-Bliesner5
1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
2National Oceanic and Atmospheric Administration’s National Climatic Data Center, Boulder, CO, USA
3NASA Goddard Institute for Space Studies and Center for Climate Systems Research, New York, NY, USA
4Department of Earth Sciences, VU University Amsterdam, the Netherlands
5Climate and Global Dynamics, National Center for Atmospheric Research, Boulder, CO, USA
Correspondence to: C. Morrill (carrie.morrill@colorado.edu)
Received: 31 July 2012 – Published in Clim. Past Discuss.: 21 August 2012
Revised: 12 March 2013 – Accepted: 13 March 2013 – Published: 10 April 2013
Abstract. We compared four simulations of the 8.2ka event
to assess climate model sensitivity and skill in responding to
North Atlantic freshwater perturbations. All of the simula-
tions used the same freshwater forcing, 2.5Sv for one year,
applied to either the Hudson Bay (northeastern Canada) or
Labrador Sea (between Canada’s Labrador coast and Green-
land). This freshwater pulse induced a decadal-mean slow-
down of 10–25 % in the Atlantic Meridional Overturning Cir-
culation (AMOC) of the models and caused a large-scale pat-
tern of climate anomalies that matched proxy evidence for
cooling in the Northern Hemisphere and a southward shift
of the Intertropical Convergence Zone. The multi-model en-
semble generated temperature anomalies that were just half
as large as those from quantitative proxy reconstructions,
however. Also, the duration of AMOC and climate anoma-
lies in three of the simulations was only several decades, sig-
nificantly shorter than the duration of 150yr in the paleo-
climate record. Possible reasons for these discrepancies in-
clude incorrect representation of the early Holocene climate
and ocean state in the North Atlantic and uncertainties in the
freshwater forcing estimates.
1 Introduction
The Atlantic Meridional Overturning Circulation (AMOC)
plays a key role in the climate system, particularly through
its control on heat transport and storage of carbon in the
deep ocean. Changes in the AMOC can have far-reaching
effects on the El Ni˜
no–Southern Oscillation (Timmermann
et al., 2005), Atlantic hurricane development (Zhang and
Delworth, 2006), tropical rainfall (Vellinga and Wood, 2002),
and marine ecosystems (Schmittner, 2005). Model simula-
tions of the 21st century with prescribed greenhouse gas
concentrations increasing according to the Intergovernmen-
tal Panel on Climate Change (IPCC) scenario SRESA1B
uniformly show a reduction in the strength of the AMOC
(Schmittner et al., 2005). This multi-model ensemble yields
a mean decrease of 25% by 2100, but there is a large range
in the individual model results that indicates substantial un-
certainties in the AMOC response to climate change.
Several previous model intercomparison projects were un-
dertaken to improve understanding of the large spread in
modeled AMOC. Schmittner et al. (2005) considered the
skill of nine coupled climate models in matching observa-
tions of modern hydrography. They found that the models
were more successful at reproducing temperature patterns
than either salinity patterns or pycnocline depth. Stouffer
et al. (2006) examined the response of both Earth system
models of intermediate complexity (EMICs) and coupled
atmosphere–ocean general circulation models (AOGCMs) to
North Atlantic freshwater forcings of 0.1 and 1.0Sv (Sver-
drup=106m3s1) for 100yr. While there were some ro-
bust patterns among the models, important disagreements
existed in model sensitivity and in reversibility following
AMOC shutdown. Since these were idealized experiments,
no comparison to observations was possible. Otto-Bliesner
et al. (2007) compared AMOC in four Last Glacial Maxi-
mum simulations from the second phase of the Paleoclimate
Modelling Intercomparison Project (PMIP2). These models
gave very different glacial circulations and a comparison to
Published by Copernicus Publications on behalf of the European Geosciences Union.
956 C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka
Table 1. Participating models.
Model Atmospheric model Oceanic model Citations
CCSM3 CAM3: T42 (2.8×2.8), 26
levels POP: 1× ∼ 1;0.3× ∼ 0.3in
North Atlantic, 40 levels,
volume-conserving
Collins et al. (2006)
Otto-Bliesner et al. (2006)
Wagner et al. (2013)
GISS ModelE-R ModelE: M20 (4×5), 20 levels Russell: 4 ×5, 13 levels,
mass-conserving Schmidt et al. (2006)
Russell et al. (2000, 1995)
LeGrande et al. (2006)
LeGrande and Schmidt (2008)
LOVECLIM1.2 ECBilt2: T21 (5.625×5.625),
3 levels CLIO3: 3 ×3, 20 levels,
mass-conserving Goosse et al. (2010)
paleoclimate proxy evidence indicated serious mismatches
for several of the simulations.
For the third phase of PMIP, the 8.2 ka event has been
targeted for a new model intercomparison. Of past abrupt
changes in the AMOC, the 8.2ka event provides a par-
ticularly useful case study because its duration (150yr;
Thomas et al., 2007) and forcing are constrained by the
proxy record, making an achievable target for climate model
simulations (Schmidt and LeGrande, 2005). There are still
some uncertainties regarding the hypothesized forcing of
the event, including the volume of drainage from proglacial
Lake Agassiz-Ojibway (hereafter Lake Agassiz; Barber et
al., 1999) into the Hudson Bay (northeastern Canada) and the
possibility of multiple meltwater pulses from both the lake
and the collapsing Laurentide Ice Sheet (Teller et al., 2002;
Gregoire et al., 2012). Model sensitivity to some of these
uncertainties has been explored elsewhere (Renssen et al.,
2001; Wiersma et al., 2006; LeGrande and Schmidt, 2008;
Clarke et al., 2009; Wiersma and Jongma, 2010; Wagner et
al., 2013). The target of this intercomparison is to use a me-
dian value for the forcing of the 8.2ka event and compare
model sensitivity to North Atlantic surface buoyancy anoma-
lies that have precise dating and a duration short enough to
make simulations with state-of-the-art coupled climate mod-
els feasible (Schmidt and LeGrande, 2005; Thomas et al.,
2007; Kobashi et al., 2007).
2 Models and experiments
We compare 8.2ka experiments completed with three mod-
els: the Community Climate System Model version 3
(CCSM), the Goddard Institute for Space Studies (GISS)
ModelE-R and LOVECLIM version 1.2. CCSM and
ModelE-R are atmosphere–ocean general circulation models
(AOGCMs) coupled without flux adjustments. LOVECLIM
is an Earth system model of intermediate complexity with
its most significant simplifications applied to the atmosphere
component (Table 1). These simplifications include clouds
that are prescribed and vertical profiles of temperature and
specific humidity that are limited by three atmospheric lev-
els. LOVECLIM also employs a freshwater flux correction
between the atmosphere and ocean subcomponents that re-
moves excess precipitation from the Arctic and Atlantic and
adds it to the North Pacific (Goosse et al., 2010).
Of relevance to this study, the ocean models of ModelE-R
and LOVECLIM are mass-conserving, in which the addition
of freshwater causes a rise in the free surface of the ocean and
reduces salinity purely through dilution. The ocean model
component of CCSM uses the rigid-lid approximation, which
does not permit vertical motion at the top of the ocean and
parameterizes the addition of freshwater as a salt extrac-
tion while keeping the volume of the ocean constant. Yin
et al. (2009) discuss the differences between these two ap-
proaches and compare results from two versions of the GFDL
CM2.1 model using each formulation. For a large freshwater
forcing that is similar in magnitude to that used in 8.2ka ex-
periments, the rigid-lid version exaggerates the forcing and
there are significant regional biases in sea surface salinity
(SSS). Despite this, the AMOC behaves similarly in the two
versions and many fundamental aspects of the two simula-
tions are qualitatively similar.
Boundary conditions specified for the control simulations
are listed in Table 2. Early Holocene orbital forcing increased
the seasonality of insolation in the Northern Hemisphere
and decreased seasonality in the Southern Hemisphere rel-
ative to the present (Berger, 1978). Greenhouse gas con-
centrations for the early Holocene were nearly identical to
those for the recent pre-industrial period (Fl¨
uckiger et al.,
2002; Monnin et al., 2004). Two of the control simulations,
CCSMall and LOVECLIM, incorporated the surface albedo
and elevation effects of the remnant of the Laurentide Ice
Sheet that was present near Hudson Bay at 8.5ka, as re-
constructed by Peltier (2004). These same control simula-
tions also included a small (0.05Sv) background flux of
Laurentide meltwater (Licciardi et al., 1999). In CCSMall,
this freshwater flux was added to the modeled St. Lawrence
River (Canadian/US Great Lakes Basin) at its outflow, and
was spread as a virtual salinity flux along the coast near
the river’s mouth. In LOVECLIM, the freshwater was added
Clim. Past, 9, 955–968, 2013 www.clim-past.net/9/955/2013/
C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka 957
Table 2. Boundary conditions for control simulations.
Simulation Orbital parameters Greenhouse gas concentrations Ice sheet Background meltwater flux
CCSMog 8.5ka CO2=260 ppm
CH4=660ppb
N2O=260ppb
none none
CCSMall 8.5ka CO2=260 ppm
CH4=660ppb
N2O=260ppb
ICE-5G 0.05 Sv added to St. Lawrence River
ModelE-R 1880 AD CO2=285ppm
CH4=791ppb
N2O=275ppb
none none
LOVECLIM 8.5ka CO2=260ppm
CH4=660ppb
N2O=260ppb
ICE-5G 0.05 Sv added to Hudson Strait
as a volume to the upper layer of the ocean at the Hud-
son Strait. Since the ocean model in LOVECLIM has a free
surface, this effectively means that the surface height was
raised. The temperature of the added freshwater in LOVE-
CLIM was assigned the same temperature as the water in the
ocean cell to which it was added. Both of these control simu-
lations with background meltwater flux were integrated until
reaching a quasi-equilibrium, in which SSS of the North At-
lantic had stabilized. Global mean ocean salinity decreases
slowly throughout these control simulations due to the back-
ground meltwater flux, a trend that parallels observed fresh-
ening since the Last Glacial Maximum (Adkins et al., 2002).
A second CCSM control simulation (CCSMog; OG=orbital
and greenhouse gas only) without a Laurentide Ice Sheet and
background meltwater flux is included in this study for a
more direct comparison to ModelE-R results.
For the 8.2ka event experiments, a meltwater pulse
(MWP) of 2.5Sv for 1yr was added to each of the control
simulations to represent the previously-mentioned drainage
of Lake Agassiz. This freshwater volume was the best esti-
mate for the drainage event based on flood hydrograph sim-
ulations (Clarke et al., 2004). Following the one-year per-
turbation, the MWP ceased and the climate was allowed to
recover. In the models with a free-surface ocean, the MWP
was added as a volume to a limited number of grid cells.
In ModelE-R, freshwater was added to the approximately
20 grid boxes in the Hudson Bay and was assigned a tem-
perature of 0C. In LOVECLIM, freshwater was added to
the upper layers of the ocean at the Hudson Strait and was
assigned the same temperature as the water in the ocean cell
to which it was added. The virtual salinity flux in CCSM re-
quired a larger area for the MWP (50–65N, 35–70W).
The control simulation for ModelE-R displayed a number
of transient, quasi-stable states with either strong or weak
AMOC (LeGrande et al., 2006; LeGrande and Schmidt,
2008). For this study, we use an experiment begun from a
period of weak AMOC. The weak case was chosen because
it exhibited the longest response to the 2.5Sv ×1yr forcing,
and because it may more closely emulate the early Holocene
than periods with strong AMOC since it lacks deep convec-
tion in the Labrador Sea between Canada’s Labrador coast
and Greenland (see LeGrande and Schmidt, 2008 for fur-
ther detail). Since the weak case exhibits some high ampli-
tude decadal variability, we examined “decadal” results for
this model (i.e., the 10-yr mean of the MWP experiment less
the 30-yr mean of the closest control years) in order to more
clearly show the transient response to the MWP.
We calculated the Student’s ttest for the differences be-
tween control and MWP experiments for the two models with
annual output (i.e., LOVECLIM and CCSM). Since annual
output is no longer available from the ModelE-R experiments
and resources do not exist to re-run these simulations, we are
unable to make statements about the statistical significance
of the model’s response to the MWP, about the skill of the
model on the annual time-scale and about the relative am-
plitude of decadal and annual variability. The model simula-
tions in the present study are an “ensemble of opportunity”,
meaning that most were completed before this intercompari-
son was planned. One limitation of not coordinating experi-
ments is that perfect comparisons are sometimes impossible
to make. However, the main conclusions of this paper would
not change were annual data available from all simulations
and ensembles of opportunity are important for informing
future coordinated intercomparisons.
3 Response to freshwater forcing
3.1 AMOC
AMOC intensity is defined here as the maximum of the
Atlantic overturning streamfunction excluding the surface
(<500m) wind-driven overturning circulation. Mean values
for the control simulations range from 16 to 20Sv (Fig. 1),
and interannual variability is small in the three simulations
www.clim-past.net/9/955/2013/ Clim. Past, 9, 955–968, 2013
958 C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka
Fig. 1. The Atlantic meridional overturning streamfunctions of the control simulations (see Table 2), in Sv (1 Sv =106m3s1). Plotted
values are 200yr means except for CCSMog, which is a 150 yr mean. Values in parentheses following the model names are long-term means
for the maximum of the streamfunction below 500m water depth.
with available annual output (standard deviations: LOVE-
CLIM=0.7, CCSMog =1.1, CCSMall =0.9Sv). AMOC
intensity is lower by severalSv in the simulations with a
background meltwater flux. AMOC has a similar structure
in all the control simulations. The northward flow of warm,
salty water occurs in the upper 1000m, while the southward
return flow of North Atlantic Deep Water occurs between
1000–3000m. The anticlockwise cell in the deep ocean,
associated with Antarctic Bottom Water formation, has a
strength of about 4Sv in all control simulations.
The values of AMOC intensity in the control simula-
tions are generally similar to the strength of the modern-day
AMOC (Meehl et al., 2007). Proxy evidence suggests that
the strength of the AMOC during the early Holocene was
probably not that different from today (Bianchi and McCave,
1999; Hall et al., 2004; Oppo et al., 2003; McManus et
al., 2004; Praetorius et al., 2008). There is some proxy ev-
idence for lack of convection and deep water formation in
the Labrador Sea during the early Holocene, however (e.g.,
Hillaire-Marcel et al., 2001; Solignac et al., 2004; Fagel
et al., 2004). To reconcile a vigorous AMOC with lack of
Labrador Sea convection, some other convection area, per-
haps in the Irminger Basin, might have been stronger in the
early Holocene to offset the weaker Labrador Sea convection
(Hall et al., 2010).
The location and strength of convection areas in the North
Atlantic varies significantly among the control simulations
(Fig. 2). Convection occurs primarily in the Nordic Seas in
one of the models (LOVECLIM), primarily in the Irminger
Sea in another (ModelE-R), and in both the Nordic Seas and
just east of the Labrador Sea in the third model (CCSM).
Notably, the background meltwater flux of 0.05Sv does not
shut down convection just east of the Labrador Sea in the
CCSMall control simulation (Fig. 2), as that flux is routed to
the south of the Labrador Sea by ocean surface currents.
Following the 2.5Sv MWP for one year, AMOC intensity
decreases in all simulations (Fig. 3). The maximum decadal-
mean decline in LOVECLIM and CCSM is about 10%,
Clim. Past, 9, 955–968, 2013 www.clim-past.net/9/955/2013/
C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka 959
Fig. 2. Control values of February mixed layer depth, in meters. Plotted values are 100yr means.
Fig. 3. Time series of AMOC intensity anomalies following the
MWP, expressed as a fraction of the long-term control mean. The
MWP of 2.5 Sv for one year was added at Model year 1. AMOC in-
tensity is defined as the maximum value of the overturning stream-
function below 500m water depth (excludes shallow wind-driven
overturning). Heavy lines are decadal averages. Vertical lines on the
right show the 2-sigma range of interannual variability in the con-
trol simulations, and are not shown for ModelE-R since only 30yr
control averages are available.
while for ModelE-R it is about 25%. The decline in AMOC
intensity in LOVECLIM and CCSM is relatively short-lived,
on the order of several decades, and generally within the
range of natural variability of AMOC in their control sim-
ulations. Similarly, mixed-layer depths shoal significantly
following the MWP, but this weakening of convection also
lasts several decades or less (not shown). The response in
ModelE-R is more pronounced and longer-lived, extending
on the order of 100–120yr. Proxy records do not provide
a quantitative estimate of AMOC weakening at 8.2ka, but
do suggest a duration of 100–200yr (Ellison et al., 2006;
Kleiven et al., 2008).
3.2 Ocean salinity and temperature
Significant freshening of the North Atlantic occurs follow-
ing the MWP in all simulations (Fig. 4). The largest anoma-
lies are generally along the coast of Labrador and are up to
1psu when averaged over the first fifty years following the
MWP. Areas of positive SSS anomalies at the mouth of the
St. Lawrence River in CCSMall are caused by cessation of the
0.05Sv background meltwater flux once Lake Agassiz has
drained. Globally, negative anomalies greater than 0.2 psu are
confined to the North Atlantic and Arctic oceans (not shown).
Patterns of SSS anomalies suggest that freshwater trav-
els eastward from the Labrador Sea into the North Atlantic
in all simulations. For most of the simulations, SSS de-
creases in both the Greenland-Iceland-Norwegian Seas and
in the subtropical gyre. This pathway is different from that
inferred by Keigwin et al. (2005), who used δ18O of plank-
tic foraminifera to suggest salinity was decreased near Cape
Hatteras around 8.2 ka. Also, it has been argued that freshwa-
ter released from Hudson Strait would not reach the Nordic
Seas, instead being trapped along the North American coast
(e.g., Wunsch, 2010) or circulating in the subtropical gyre
www.clim-past.net/9/955/2013/ Clim. Past, 9, 955–968, 2013
960 C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka
Fig. 4. Anomalies of annual-mean sea surface salinity in the first fifty years following the MWP relative to the control simulation, in practical
salinity units. Stippling shows statistical significance at the 95% level according to a Student’s ttest. Statistical tests were not performed for
ModelE-R since only decadal averages were available.
Fig. 5. Anomalies of annual-mean sea surface temperature in the first fifty years following the MWP relative to the control simulation, in
degrees Celsius. Stippling shows statistical significance at the 95% level according to a Student’s ttest. Statistical tests were not performed
for ModelE-R since only decadal averages were available.
Clim. Past, 9, 955–968, 2013 www.clim-past.net/9/955/2013/
C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka 961
(Condron and Winsor, 2011). However, several proxy records
from the Irminger and Labrador Seas combine δ18O and
Mg/Ca of planktic foraminera to infer decreases in δ18O of
seawater at 8.2 ka of up to 1 ‰ (Came et al., 2007; Thornalley
et al., 2009; Ellison et al., 2006; Winsor et al., 2012; Hoffman
et al., 2012), which would be equivalent to a freshening of
0.7psu assuming the Laurentide Ice Sheet meltwater was
about 25‰ (Hillaire-Marcel et al., 2007). Also, the loca-
tion of detrital carbonate layers deposited around 8.2ka in-
dicate greater freshwater transport in the outer branch of the
Labrador Current, which typically mixes with the North At-
lantic Current and travels to the Nordic Seas (Lewis et al.,
2012). The model simulations presented here, as well as oth-
ers published by Born and Levermann (2010) and Spence et
al. (2008), tend to support some amount of freshwater trans-
port into the Nordic Seas.
Likewise, sea surface cooling is concentrated in the North
Atlantic in all simulations (Fig. 5). Mean anomalies across
the North Atlantic for the first fifty years following the MWP
are on the order of 1C, though they exceed 2C locally in
the CCSM and ModelE-R experiments. Maximum anomalies
in the LOVECLIM simulation are on the order of 0.5C
and are located in the far North Atlantic. ModelE-R shows
cooling on the order of several tenths of a degree Celsius
across most of the Southern Hemisphere. The other simula-
tions show little significant change south of 30N with the
exception of CCSMall, which has some significant warming
in the south Atlantic.
3.3 Barotropic streamfunction
A common model diagnostic for the ocean circulation, in-
cluding the strengths of the subtropical and subpolar gyres
in the North Atlantic, is the vertically-integrated mass trans-
port (barotropic) streamfunction. Values for this quantity are
available for three of the simulations (Fig. 6). In these three
simulations, transports in both the subtropical gyre and the
subpolar gyre weaken for a few decades following freshwa-
ter forcing. This result is consistent with the concept that re-
duction of deep convection in the core of the subpolar gyre,
as occurs briefly in these simulations in the Labrador and/or
Irminger Seas, weakens this circulation (e.g., H¨
akkinen and
Rhines, 2004). The barotropic streamfunction is not available
as standard output for the fourth simulation (LOVECLIM)
and is not easily calculated offline. Neither deep convection
(Sect. 3.1) nor upper-ocean velocities (top 100m, not shown)
in this model show a large or long-term change forced by the
MWP, though, suggesting that the response of the gyres is at
least qualitatively similar to the other models.
On the other hand, Born and Levermann (2010) found
a prolonged strengthening of the subpolar gyre circulation
in a simulation of the 8.2ka event with the CLIMBER-3α
model. In this model, a reduction of deepwater formation in
the Nordic Seas intensified the subpolar gyre and triggered
internal feedbacks to increase and maintain deep convection
Fig. 6. Control values for barotropic streamfunction (contour lines)
and streamfunction anomalies in the first fifty years following the
MWP relative to the control simulation, in Sv (colored contours).
The contour interval for the control values is 10Sv. Dashed lines
show negative streamfunction values, or a cyclonic circulation. Pos-
itive (negative) anomalies for a cyclonic (anticyclonic) circulation
indicate weakening of the transport. Stippling shows statistical sig-
nificance for anomalies at the 95% level according to a Student’s
ttest. Statistical tests were not performed for ModelE-R since only
decadal averages were available.
in the Labrador Sea. If true, this could explain the onset of
deepwater formation in the Labrador Sea around the time of
the 8.2ka event (e.g., Hillaire-Marcel et al., 2001). The dif-
ferent response in CLIMBER-3αmight be explained by the
fact that the freshwater perturbation had less of a direct im-
pact on the Labrador Sea convection region and instead had
greater advection to the Nordic Seas (Born and Levermann,
2010).
www.clim-past.net/9/955/2013/ Clim. Past, 9, 955–968, 2013
962 C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka
Fig. 7. Control values (contour lines) and anomalies of annual-mean sea ice area in the first fifty years following the MWP relative to the
control simulation (colored contours), in percent. The contour lines show values of 5%, 25 %, 50 % and 75%. Stippling shows statistical
significance at the 95% level according to a Student’s ttest. Statistical tests were not performed for ModelE-R since only decadal averages
were available.
3.4 Sea ice
All of the simulations have areas of significantly expanded
sea ice following freshwater forcing, particularly in the
Labrador Sea and in the Norwegian and/or Barents Sea
(Fig. 7). Generally, these changes for the first fifty years fol-
lowing the MWP are on the order of 5–10%, although they
can be as large as 20–25% in some areas. Sea ice changes in
the Southern Ocean have a heterogeneous spatial pattern and
generally are not statistically significant.
3.5 Surface air temperature
The North Atlantic region and the Arctic become signifi-
cantly colder in most simulations during the first fifty years
following the MWP, with mean annual temperatures in the
multi-model ensemble decreasing less than 0.5C over Eu-
rope and 1.0C over Greenland (Fig. 8). These results
hold for individual ensemble members, as well, for both
Europe (40N–60N, 10W–30E; anomalies are LOVE-
CLIM=0.0C, CCSMog = −0.3C, CCSMall = −0.5C,
ModelE-R= −0.6C) and Greenland (60N–80N, 60W–
20W; anomalies are LOVECLIM =0.0 C, CCSMog =
0.6C, CCSMall = −0.4C, ModelE-R= −0.8C). Tem-
perature changes are minimal in the tropics and the South-
ern Hemisphere. This spatial pattern agrees well with proxy
records, which clearly indicate colder conditions across the
Northern Hemisphere during the 8.2 ka event but suggest that
any Southern Hemisphere temperature changes were likely
regional (Fig. 8).
The magnitude of circum-North Atlantic temperature
changes inferred from proxies is somewhat larger than those
in the models. Temperature reconstructions from pollen and
δ18O in Europe consistently show anomalies of about 1.1 to
1.2C in mean annual temperature during the 8.2 ka event,
although standard errors of these reconstructions are nearly
as large as the anomalies themselves (Veski et al., 2004; von
Grafenstein et al., 1998; Sarmaja-Korjonen and Sepp¨
a, 2007;
Feurdean et al., 2008). Nitrogen isotopes from Greenland in-
dicate temperatures decreased about 2.2 C averaged over the
duration of the event, with an even larger decrease of 3.3 C
during the most extreme 60yr period (Kobashi et al., 2007).
Anomalies over the North Atlantic in the LOVECLIM and
CCSM experiments are short-lived; generally, temperature
values are outside the range of natural variability (defined
as the mean ±2 standard deviations of the control) for less
than two decades (Fig. 9). Anomalies are longer-lived in the
ModelE-R simulation, lasting on the order of 100yr. These
longer-lived anomalies are a better match to high-resolution
proxy records from Europe and Greenland, which consis-
tently show an event duration of 100 to 150yr (Morrill et
al., 2013).
Clim. Past, 9, 955–968, 2013 www.clim-past.net/9/955/2013/
C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka 963
Fig. 8. (Top) Multi-model ensemble mean anomalies of annual-
mean 2-meter air temperature in the first fifty years following
the MWP relative to the control simulations, in degrees Celsius.
Stippling shows grid cells where at least three of the simulations
agree on the sign of the temperature anomaly. (Bottom) Qualita-
tive and quantitative mean-annual temperature anomalies relative to
the early Holocene background climate, in degrees Celsius, inferred
from proxy records for the 8.2ka event, as summarized by Morrill
et al. (2013).
3.6 Precipitation
Despite the noise inherent in precipitation, a number of fea-
tures are common among the model simulations for the fifty
years following the MWP. In all cases, the most important
changes are a reduction in precipitation over the North At-
lantic and Northern Hemisphere tropics, and an increase in
precipitation over the Southern Hemisphere tropics (Fig. 10).
The tropical pattern, consistent with a southward shift of
the mean position of the Intertropical Convergence Zone, is
clearest over the Atlantic Ocean (Fig. 11). Tropical proxy
records from both speleothem δ18O measurements and in-
dicators of lake water balance support this spatial pattern
(Fig. 10).
Several quantitative estimates of drying exist from prox-
ies in high northern latitudes; these include an 8% reduc-
tion in accumulation in central Greenland ice cores and an
17% reduction in rainfall inferred from pollen north of the
Mediterranean, although again the standard errors of these
reconstructions are nearly as large as the anomalies them-
selves (Feurdean et al., 2008; Pross et al., 2009; Hammer et
Fig. 9. Time series of annual-mean surface air temperature aver-
aged over the region 50–70N, 60W–10E in the North Atlantic,
expressed as anomalies in degrees Celsius from the long-term con-
trol average. The MWP of 2.5Sv for one year was added at Model
year 1. Vertical lines on the right show the 2-sigma range of inter-
annual variability in the control simulations, and are not shown for
ModelE-R since only 30-yr control averages are available.
al., 1997; Rasmussen et al., 2007). The model simulations
generally match the magnitude of drying in central Green-
land, but typically do not match either the direction or mag-
nitude of change in southeastern Europe. Additionally, ev-
idence for wetter conditions at 8.2ka from pollen and lake
geochemical records in northern Europe is not matched by
the freshwater experiments (Fig. 10).
4 Discussion and conclusions
To summarize, the models generally do a good job in repro-
ducing large-scale patterns of temperature and precipitation
changes at 8.2 ka inferred from proxy records. These patterns
include cooling across most of the Northern Hemisphere and
a southward shift of the Intertropical Convergence Zone. The
models have less success in matching the magnitude and
duration of climate anomalies. Temperature changes in the
multi-model ensemble are about half the size of those of
quantitative proxy records from Europe and Greenland. For
all but one of the simulations, the duration of the 8.2ka cli-
mate anomalies is on the order of several decades rather than
the 150yr observed in proxy records. Also, there are dis-
crepancies between model and data for some regional-scale
anomaly patterns, including precipitation changes in Europe.
These patterns are less well-constrained by proxy evidence,
however.
The background climate state of the early Holocene, and
the location of convection areas in the North Atlantic more
specifically, might explain some of the differences we see
between models and proxy data. The ModelE-R simulation
has the best match to proxies for event duration, and it has
www.clim-past.net/9/955/2013/ Clim. Past, 9, 955–968, 2013
964 C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka
Fig. 10. (Top) Multi-model ensemble mean anomalies of annual-
mean precipitation in the first fifty years following the MWP rela-
tive to the control simulations, in % change from control. Stippling
shows grid cells where at least three of the simulations agree on the
sign of the precipitation anomaly. (Bottom) Qualitative and quanti-
tative annual-mean precipitation anomalies, in % change from early
Holocene background climate, inferred from proxy records for the
8.2 ka event, as summarized by Morrill et al. (2013).
been previously demonstrated for this model that the lack
of Labrador Sea convection is essential for this response
(LeGrande and Schmidt, 2008; LeGrande et al., 2006). Previ-
ous work with the ECBilt-CLIO model also supports this in-
terpretation; when Labrador Sea convection is weakened by
the background meltwater flux, the ocean’s ability to trans-
port freshwater anomalies away from the North Atlantic is di-
minished and the response to freshwater forcing is prolonged
(Wiersma et al., 2006). On the other hand, lack of convection
in the Labrador Sea does not lead to a long-lived climate re-
sponse in this model’s successor, LOVECLIM. While the ex-
act reasons for this have yet to be determined, the background
meltwater flux used in the LOVECLIM experiment is less
than that in the ECBilt-CLIO experiments (0.05 vs. 0.17Sv;
Wiersma et al., 2006; Li et al., 2009), and it seems that
LOVECLIM is also less sensitive to freshwater perturbations
than its predecessor. Also adding uncertainty to the impor-
tance of convection strength in the Labrador Sea, proxies in-
dicate that AMOC strength was not too different from today
during the early Holocene. In this case, some other convec-
tion area, perhaps in the Irminger Basin (between Greenland
Fig. 11. (Top) Annual-mean precipitation zonally-averaged across
the Atlantic (90W–40E) in the control simulation, in cmyr1.
(Bottom) Anomalies of Atlantic annual-mean precipitation for the
first fifty years following the MWP relative to the control simula-
tion, in cm yr1.
and Iceland), might have been stronger in the early Holocene
to offset the weaker Labrador Sea convection (Hall et al.,
2010). If this was true, the strengthened convection areas
elsewhere might be able to compensate for decreased fresh-
water divergence in the Labrador Sea.
Another factor in the model-data mismatch could be the
size or the complexity of the MWP. The model simula-
tions were forced with 2.5Sv for one year, which was the
best estimate of the flood hydrograph simulations of Clarke
et al. (2004). As these authors point out, though, the total
volume of Lake Agassiz could have generated twice this
forcing and more complex multipulse patterns are possible
(Teller et al., 2002). Their flood model generates a stable
drainage channel that prohibits complete drainage, but this
result might be unlikely for an outburst flood from Lake
Agassiz. In addition, uncertainties in the reconstructed po-
sition of the ice-margin on the northern side of Lake Agas-
siz translate into a range of possible lake volumes spanning
45–200% of the best estimate (Tornqvist and Hijma, 2012).
Clim. Past, 9, 955–968, 2013 www.clim-past.net/9/955/2013/
C. Morrill et al.: Model sensitivity to North Atlantic freshwater forcing at 8.2ka 965
Reconstructions of sea level rise at 8.2ka support the idea
of a larger freshwater drainage. Using peat deposits from the
Mississippi River delta (US), Li et al. (2012) reconstructed a
total eustatic sea level rise of 0.8 to 2.2m at 8.2 ka. Another
reconstruction from the Rhine-Meuse delta (the Netherlands
and Belgium) implies a sea level rise of 3.0±1.2 m (Hi-
jma and Cohen, 2010). These are significantly larger than the
forcing of 2.5Sv for one year (0.2m sea level equivalent)
or even than the best estimate of the entire volume of Lake
Agassiz (0.4m sea level equivalent). Recent model simu-
lations suggest that the collapse of the Laurentide ice-sheet
saddle around 8.2ka provided this larger volume of fresh-
water (Gregoire et al., 2012), and that this forcing results in
a cooling event that matches many proxy records (Wiersma
and Jongma, 2010; Wagner et al., 2013).
The difference in boundary conditions between the control
simulations does not obviously account for divergent model
responses. As shown in the comparison of the two CCSM
simulations, CCSMog and CCSMall, the addition of a rem-
nant Laurentide Ice Sheet and a background meltwater flux
does not alter the model response to freshwater forcing, ei-
ther in magnitude or duration. It is worth noting, however,
that these boundary conditions were important in previous
experiments with ECBilt-CLIO for prolonging the AMOC
response to Lake Agassiz drainage (Wiersma et al., 2006).
Thus, the effects of these boundary conditions might be very
model-dependent. Differences between early Holocene and
preindustrial orbital forcing and greenhouse gas concentra-
tions are relatively minor, and are not expected to have an
important influence. This should be verified, though, with ad-
ditional model experiments.
Another explanation for the model-data discrepancies is
that the models are not sensitive enough to freshwater per-
turbations. If true, this finding would have important impli-
cations for future climate projections, particularly as models
suggest that continued melting of the Greenland Ice Sheet at
its current rate will have a significant impact on the AMOC
(Hu et al., 2009). There are few model intercomparisons to
determine whether the sensitivity of these three models to
freshwater perturbations is representative of coupled climate
models as a whole. For hosing experiments of 0.1Sv for
100 yr under modern boundary conditions, earlier versions of
the CCSM3 (CCSM2) and LOVECLIM (ECBilt-CLIO) have
AMOC and surface air temperature responses close to the
multi-model ensemble mean (Stouffer et al., 2006). For hos-
ing experiments in a Last Glacial Maximum climate, how-
ever, AMOC decreases somewhat less in the CCSM3 and
LOVECLIM compared to the multi-model ensemble mean
(Kageyama et al., 2012). Improved constraints on the size
of freshwater forcing and its location with respect to early
Holocene convection areas are necessary to rule out the pos-
sibility of inadequate model sensitivity.
Acknowledgements. We thank Lauren Gregoire and an anonymous
reviewer for their helpful comments. Funding for the CCSM
simulations was provided by grants from the U.S. National Science
Foundation, Office of Polar Programs, to CM (ARC-0713951)
and BLO-B (ARC-0713971), and supercomputer time was
provided by a grant from the National Center for Atmospheric
Research (NCAR) Computational Information Systems Laboratory
(CISL). CM and BLO-B thank Nan Rosenbloom for running the
CCSMall simulations, Ellen Ward for assistance with figures, and
Esther Brady and Amy Wagner for helpful discussions. ANL
thanks NASA GISS for institutional support. This is Past4Future
contribution no. 37. The research leading to these results has
received funding from the European Union’s Seventh Framework
programme (FP7/2007–2013) under grant agreement no 243908,
“Past4Future. Climate change – Learning from the past climate.
Edited by: M. Kageyama
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... A grande liberação de água doce oriunda das camadas de gelo continentais (ex. Groenlândia) pode causar grandes perturbações oceânicas e climáticas, como por exemplo interferir na sensibilidade da circulação termohalina, na TSM, na variação da salinidade nas altas latitudes do oceano Atlântico Norte (Manabe & Stouffer, 1997;Morrill et al., 2013), do Atlântico Sul (Weaver et al., 2003), Atlântico Tropical (Goelzer et al., 2006) e na circulação atmosférica. A influência da adição de água doce na circulação oceânica depende da intensidade e duração do fluxo extra de água doce, do local onde o mesmo é liberado e da presença de local de formação de águas profundas Zhang et al., 2016). ...
... No Pacífico Norte e oceano Ártico é vissível o predomínio de alta salinidade (tons azuis), com valores superiores a 1.6 g/kg (Figura 4A). A adição de água doce sobre o HN influencia a TSM em grande parte do globo (Bond et al., 1992;Bard et al., 2000;Stouffer et al., 2006;Morrill et al., 2013), sugerindo uma possível teleconexão (impacto em áreas remotas do globo) produzida ao adicionar água doce extra nos mares do Ártico e costa do Canadá e Groenlândia. O fluxo extra de água doce tende a propagar-se pela CCA e, posteriormente, adentra na AMOC, enfraquecendo--a. ...
... O fluxo extra de água doce tende a propagar-se pela CCA e, posteriormente, adentra na AMOC, enfraquecendo--a. Morrill et al. (2013) Por sua vez, a redução da salinidade superficial aumenta a temperatura do ponto de congelamento, favorecendo a expansão do GM no HS, entre -45ºE e 60ºE, onde a cobertura de GM chegou até 80% (Figura 5A). No modelo modificado a cobertura de GM é super-estimada sobre o HN, corroborando com os resultados obtidos por Jansson (2004). ...
... Furthermore, the proxy data are underpinned by model simulations indicating increased precipitation for the duration of this event as a result from the southward migration of the ITCZ (e.g. Broccoli et al., 2006;Morrill et al., 2013). (Bard et al., 2007), and δ 18 O records from Diva Cave, northeastern Brazil and the Net Balance Accumulation (in meters per year water equivalent), an indicator or regional precipitation estimated for the Quelccaya Ice core (Thompson et al., 2013). ...
... Although disruption of AMO has been previously suggested as a consequence of the 8.2 ka event (Hillaire-Marcel et al., 2007), no previous record of such disruption is yet available. Moreover, relevant so-called "hosing" experiments (Broccoli et al., 2006;LeGrande et al., 2006;Morrill et al., 2013) are yet to provide detailed information on the periodic ocean-atmosphere interactions (such as AMO) during anomalous paleoceanografic conditions. ...
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A Holocene stalagmite from Botuverá Cave, southeastern Brazil was analyzed by LA-ICPMS for Mg/Ca, Sr/Ca, Ba/Ca. The observed variability in the record was demonstrated to be modulated by prior calcite precipitation, and, thus, is interpreted to reflect monsoon intensity. We find that the calcite δ18O is strongly correlated with Sr/Ca, indicating that atmospheric circulation over South America and monsoon intensity have been tightly correlated throughout most of the Holocene, both directly responding to solar precession. Comparison with other contemporaneous high-resolution hydroclimate records reveals that SAMS has shown a degree of complexity during the Holocene not previously detected, with periods where the South American Convergence Zone (SACZ) expanded to cover most of the South American sub-continent, and coincident with periods of low-SST in the north Atlantic. We also detect periods where rainfall amount in northeastern and southeastern Brazil are markedly anti-phased, suggesting a north-south migration of SACZ, which it appears to be mediated by solar irradiance. The high-resolution nature of our record allow us to examine the effect that Holocene climate anomalies had upon SAMS dynamics and hydroclimate in southeastern Brazil, in particular the 8.2ka event and the Little Ice Age. In addition to confirm the internal structure of the events, we also detect the possible consequences of the climatic anomalies upon ocean–atmosphere interactions through its effects upon SAMS.
... Furthermore, the proxy data are underpinned by model simulations indicating increased precipitation for the duration of this event as a result from the southward migration of the ITCZ (e.g. Broccoli et al., 2006;Morrill et al., 2013). (Bard et al., 2007), and δ 18 O records from Diva Cave, northeastern Brazil and the Net Balance Accumulation (in meters per year water equivalent), an indicator or regional precipitation estimated for the Quelccaya Ice core (Thompson et al., 2013). ...
... Although disruption of AMO has been previously suggested as a consequence of the 8.2 ka event (Hillaire-Marcel et al., 2007), no previous record of such disruption is yet available. Moreover, relevant so-called "hosing" experiments (Broccoli et al., 2006;LeGrande et al., 2006;Morrill et al., 2013) are yet to provide detailed information on the periodic ocean-atmosphere interactions (such as AMO) during anomalous paleoceanografic conditions. ...
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A Holocene stalagmite from Botuverá Cave, southeastern Brazil was analyzed by LA-ICPMS for Mg/Ca, Sr/Ca, Ba/Ca. The observed variability in the record was demonstrated to be modulated by prior calcite precipitation, and, thus, is interpreted to reflect monsoon intensity. We find that the calcite δ 18 O is strongly correlated with Sr/Ca, indicating that atmospheric circulation over South America and monsoon intensity have been tightly correlated throughout most of the Holocene, both directly responding to solar precession. Comparison with other contemporaneous high-resolution hydroclimate records reveals that SAMS has shown a degree of complexity during the Holocene not previously detected, with periods where the South American Convergence Zone (SACZ) expanded to cover most of the South American sub-continent, and coincident with periods of low-SST in the north Atlantic. We also detect periods where rainfall amount in northeastern and southeastern Brazil are markedly anti-phased, suggesting a north-south migration of SACZ, which it appears to be mediated by solar irradiance. The high-resolution nature of our record allow us to examine the effect that Holocene climate anomalies had upon SAMS dynamics and hydroclimate in southeastern Brazil, in particular the 8.2 ka event and the Little Ice Age. In addition to confirm the internal structure of the events, we also detect the possible consequences of the climatic anomalies upon ocean–atmosphere interactions through its effects upon SAMS.
... The Lower Mississippi Basin fluvial activity chronology also indicates increased fluvial activity during the 8.2-ka cooling event (Figure 6). Increased activity on the Lower Mississippi during this period was potentially related to hydroclimate variation associated with the 8.2-ka event (Morrill et al., 2013). This regional approach improves insight into fluvial response to large-scale hydroclimate variation in the early Holocene. ...
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In the eastern United States, existing paleo-reconstructions in fluvial environments consist primarily of site-specific investigations of climate and human impacts on riverine processes. This paper presents the first meta-analysis of fluvial reconstructions focused on regional watersheds of the eastern United States, including the Lower Mississippi, Tennessee, South Atlantic–Gulf Coast, Ohio, Mid-Atlantic, and New England regional watersheds. Chronologies of fluvial activity (i.e. alluvial deposition) and stability (i.e. landscape stability) were developed by synthesizing data from existing, published, and site-specific fluvial reconstruction studies conducted across the eastern United States. Overall, regional watersheds show variable patterns of synchronicity across watersheds and did not demonstrate cyclic behavior through the Holocene. During the last millennium, only the Lower Mississippi and Ohio regional watersheds exhibit high rates of fluvial activity active during the ‘Medieval Climate Anomaly’ (650–1050 yr BP), while nearly all other regional watersheds in the eastern United States were active during the ‘Little Ice Age’ (100–500 yr BP). These findings imply that fluvial activity may be more spatially restricted during warmer/drier climatic conditions than during cooler/wetter periods. We find an increase in fluvial activity during the era of Euro-American colonization (400 yr BP to present) in the southeastern United States but not the northeastern United States, implying a heterogeneous response of fluvial systems to human activities in the eastern United States related to climatic, cultural, and/or physiographic variability. These new insights gained from fluvial chronologies in the eastern United States demonstrate the utility of regionally synthesized paleo-records to understand large-scale climate variation effect on rivers.
... The combined effect of these two types of LIS meltwater forcing on the early-Holocene climate has been investigated with a variety of coupled models. Simulating solely the lake discharge in climate models by prescribing a meltwater input of 2.5 Sv in a single year resulted in a climate perturbation that recovers over several decades (Morrill et al., 2013b;Wagner et al., 2013). As for the background freshwater fluxes, they have been the subject of a greater number of studies assuming various hosing rates. ...
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The earliest part of the Holocene, from 11.5k to 7k (k = 1000 years before present), is a critical transition period between the relatively cold last deglaciation and the warm middle Holocene. It is marked by more pronounced seasonality and reduced greenhouse gases (GHGs) than the present state, as well as by the presence of the Laurentide Ice Sheet (LIS) and glacial meltwater perturbation. This paper performs experiments under pre-industrial and different early-Holocene regimes with AWI-ESM (Alfred Wegener Institute–Earth System Model), a state-of-the-art climate model with unstructured mesh and varying resolutions, to examine the sensitivity of the simulated Atlantic meridional overturning circulation (AMOC) to early-Holocene insolation, GHGs, topography (including properties of the ice sheet), and glacial meltwater perturbation. In the experiments with early-Holocene Earth orbital parameters and GHGs applied, the AWI-ESM simulation shows a JJA (June–July–August) warming and DJF (December–January–February) cooling over the mid and high latitudes compared with pre-industrial conditions, with amplification over the continents. The presence of the LIS leads to an additional regional cooling over the North America. We also simulate the meltwater event around 8.2k. Big discrepancies are found in the oceanic responses to different locations and magnitudes of freshwater discharge. Our experiments, which compare the effects of freshwater release evenly across the Labrador Sea to a more precise injection along the western boundary of the North Atlantic (the coastal region of LIS), show significant differences in the ocean circulation response, as the former produces a major decline of the AMOC and the latter yields no obvious effect on the strength of the thermohaline circulation. Furthermore, proglacial drainage of Lakes Agassiz and Ojibway leads to a fast spin-down of the AMOC, followed, however, by a gradual recovery. Most hosing experiments lead to a warming over the Nordic Sea and Barents Sea of varying magnitudes, because of an enhanced inflow from lower latitudes and a northward displacement of the North Atlantic deep convection. These processes exist in both of our high- and low-resolution experiments, but with some local discrepancies such as (1) the hosing-induced subpolar warming is much less pronounced in the high-resolution simulations; (2) LIS coastal melting in the high-resolution model leads to a slight decrease in the AMOC; and (3) the convection formation site in the low- and high-resolution experiments differs, in the former mainly over northeastern North Atlantic Ocean, but in the latter over a very shallow subpolar region along the northern edge of the North Atlantic Ocean. In conclusion, we find that our simulations capture spatially heterogeneous responses of the early-Holocene climate.
... Farther south, at Padre Cave in eastern Brazil, Cheng et al. [2009] used a stalagmite to study the 8.2 kyr event and showed that the South American monsoon intensified during this time (Figure 4), and they argued for a "two-stage" 8.2 kyr event as having occurred at Padre Cave and low-latitude sites elsewhere, such as in China and Oman. The wetter conditions at Padre Cave and the drier conditions at sites in the Caribbean and Central America are consistent with model results that indicate a reduction in precipitation over the northern tropical Atlantic, and an increase in precipitation in the southern tropical Atlantic, in response to sudden meltwater forcing in the North Atlantic Ocean [Morrill et al., 2013]. During the 8.2 kyr event, a southward displacement of the North Atlantic anticyclone would have resulted in a more southerly Intertropical Convergence Zone (ITCZ), which would have increased the intensity of the trade winds in the northern tropical Atlantic and kept it dry [Giannini et al., 2000;Lachinet et al., 2004;Wiersma and Renssen, 2006]. ...
Article
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We use sediments from a flooded sinkhole (Cenote Jennifer) in northern Cuba to provide new, well-dated, high-resolution evidence for the 8.2 kyr event. From ~7600 to 8700 cal yr BP the sinkhole contained shallow, low-salinity water, which supported a marsh dominated by cattail and grass. Peaks in Cl and Br – occurring at 8150, 8200, and 8250 cal yr BP – are attributable to increased evaporation due to regional drying associated with the 8.2 kyr event. The three peaks in these elements also closely correspond to the greyscale record from the Cariaco Basin, indicative of increased upwelling in the southern Caribbean Sea at this time, supporting the notion of a multi-stage 8.2 kyr event. Our work provides new data that help to clarify the initiation, behavior, and impacts of the 8.2 kyr event in the northern tropics.
... Furthermore, the early Holocene background climate state was not too dissimilar from the present, with two main differences: the increased (decreased) seasonality of insolation in the Northern (Southern) Hemisphere due to orbital forcing and the presence of a remnant Laurentide Ice Sheet both before and after the 8.2 ka event (Carlson et al., 2008;Renssen et al., 2009). For these reasons, the 8.2 ka event was selected for a model intercomparison for the third phase of the Paleoclimate Modelling Intercomparison Project (PMIP3; Morrill et al., 2012). ...
Article
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The Paleoclimate Modelling Intercomparison Project (PMIP3) now includes the 8.2 ka event as a test of model sensitivity to North Atlantic freshwater forcing. To provide benchmarks for intercomparison, we compiled and analyzed high-resolution records spanning this event. Two previously-described anomaly patterns that emerge are cooling around the North Atlantic and drier conditions in the Northern Hemisphere tropics. Newer to this compilation are more robustly-defined wetter conditions in the Southern Hemisphere tropics and regionally-limited warming in the Southern Hemisphere. Most anomalies around the globe lasted on the order of 100 to 150 yr. More quantitative reconstructions are now available and indicate cooling of ~ 1 °C and a ~ 20% decrease in precipitation in parts of Europe as well as spatial gradients in δ18O from the high to low latitudes. Unresolved questions remain about the seasonality of the climate response to freshwater forcing and the extent to which the bipolar seesaw operated in the early Holocene.
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There is a converging body of evidence supporting a measurable slowdown of the Atlantic Meridional Overturning Circulation (AMOC) as climate warms and Northern Hemisphere ice sheets inexorably shrink. Within this context, we assess the variability of the AMOC during the Holocene based on a marine sediment core retrieved from the deep northwest Atlantic, which sensitively recorded large‐scale deglacial transitions in deep water circulation. While there is a diffuse notion of Holocene variability in Labrador and Nordic Seas overturning, we report a largely invariable deep water circulation for the last ~11,000 years, even during the meltwater pulse associated with the 8.2‐ka event. Sensitivity tests along with high‐resolution ²³¹Pa/²³⁰Th data constrain the duration and the magnitude of possible Holocene AMOC variations. The generally constant baseline during the Holocene suggests attenuated natural variability of the large‐scale AMOC on submillennial timescales and calls for compensating effects involving the upstream components of North Atlantic Deep Water.
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The 8.2 ka event is a period of abrupt cooling of 1-3 °C across large parts of the Northern Hemisphere, which lasted for about 160 years. The original hypothesis for the cause of this event has been the outburst of the proglacial Lakes Agassiz and Ojibway. These drained into the Labrador Sea in ~0.5-5 years and slowed the Atlantic Meridional Overturning Circulation, thus cooling the North Atlantic region. However, climate models have not been able to reproduce the duration and magnitude of the cooling with this forcing without including additional centennial-length freshwater forcings, such as rerouting of continental runoff and ice sheet melt in combination with the lake release. Here, we show that instead of being caused by the lake outburst, the event could have been caused by accelerated melt from the collapsing ice saddle that linked domes over Hudson Bay in North America. We forced a General Circulation Model with time varying meltwater pulses (100-300 year) that match observed sea level change, designed to represent the Hudson Bay ice saddle collapse. A 100 year long pulse with a peak of 0.6 Sv produces a cooling in central Greenland that matches the 160 year duration and 3 °C amplitude of the event recorded in ice cores. The simulation also reproduces the cooling pattern, amplitude and duration recorded in European Lake and North Atlantic sediment records. Such abrupt acceleration in ice melt would have been caused by surface melt feedbacks and marine ice sheet instability. These new realistic forcing scenarios provide a means to reconcile longstanding mismatches between proxy data and models, allowing for a better understanding of both the sensitivity of the climate models and processes and feedbacks in motion during the disintegration of continental ice sheets.
Chapter
The paper reviews finds and analyses that give insights into social life in the Neolithic lake side settlement of Zürich-Parkhaus Opéra. Symbols and indications of ritual performances are equally discussed as health status, clothes, mobility and long distance exchange contacts. Special attention is payed to the strong evidence of separate quarters within the same settlement, that differed in terms of raw material supplies, tools and other aspects.
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Full-text available
The Paleoclimate Modelling Intercomparison Project (PMIP3) now includes the 8.2 ka event as a test of model sensitivity to North Atlantic freshwater forcing. To provide benchmarks for intercomparison, we compiled and analyzed high-resolution records spanning this event. Two previously-described anomaly patterns that emerge are cooling around the North Atlantic and drier conditions in the Northern Hemisphere tropics. Newer to this compilation are more robustly-defined wetter conditions in the Southern Hemisphere tropics and regionally-limited warming in the Southern Hemisphere. Most anomalies around the globe lasted on the order of 100 to 150 yr. More quantitative reconstructions are now available and indicate cooling of ~ 1 °C and a ~ 20% decrease in precipitation in parts of Europe as well as spatial gradients in δ18O from the high to low latitudes. Unresolved questions remain about the seasonality of the climate response to freshwater forcing and the extent to which the bipolar seesaw operated in the early Holocene.
Article
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The 8.2 ka event is the most prominent abrupt climate 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 8.2 ka. 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 examines the effects of a southerly drainage route on the 8.2 ka event with the ECBilt-CLIO-VECODE model. Hosing experiments 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 performed 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 implies 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 modelling study of gravitational effects on sea-level change associated with the 8.2 ka event, which suggests that the volume of drainage might be larger than previously estimated.
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Seven freshwater perturbation experiments were performed with a global atmosphere—sea-ice—ocean model to study the mechanism behind the 8.2 kyr BP Holocene cooling event. These experiments differed in initial state and duration of the applied freshwater pulse, while the amount of freshwater was kept constant (4.67 × 1014 m³). One of the scenarios, with freshwater added to the Labrador Sea at a rate of 0.75 Sv during 20 years, resulted in weakening of the North Atlantic thermohaline circulation during 320 years and surface cooling varying from 1 to 5°C over adjacent continents. This result is consistent with proxy data, suggesting that a meltwater-induced weakening of the thermohaline circulation caused the event. Moreover, our results indicate that the time-scale of the meltwater release and the initial state are important, as both have a strong effect on the magnitude and duration of the produced model response.
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The beginning of the current interglacial period, the Holocene epoch, was a critical part of the transition from glacial to interglacial climate conditions. This period, between about 12,000 and 7,000 years ago, was marked by the continued retreat of the ice sheets that had expanded through polar and temperate regions during the preceding glacial. This meltdown led to a dramatic rise in sea level, punctuated by short-lived jumps associated with catastrophic ice-sheet collapses. Tracking down which ice sheet produced specific sea-level jumps has been challenging, but two events between 8,500 and 8,200 years ago have been linked to the final drainage of glacial Lake Agassiz in north-central North America. The release of the water from this ice-dammed lake into the ocean is recorded by sea-level jumps in the Mississippi and Rhine-Meuse deltas of approximately 0.4 and 2.1 metres, respectively. These sea-level jumps can be related to an abrupt cooling in the Northern Hemisphere known as the 8.2 kyr event, and it has been suggested that the freshwater release from Lake Agassiz into the North Atlantic was sufficient to perturb the North Atlantic meridional overturning circulation. As sea-level rise on the order of decimetres to metres can now be detected with confidence and linked to climate records, it is becoming apparent that abrupt climate change during the early Holocene associated with perturbations in North Atlantic circulation required sustained freshwater release into the ocean.
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The 8.2 ka event was the last deglacial abrupt climate event. A reduction in the Atlantic meridional overturning circulation (AMOC) attributed to the drainage of glacial Lake Agassiz may have caused the event, but the freshwater signature of Lake Agassiz discharge has yet to be identified in δ18O of foraminiferal calcite records from the Labrador Sea, calling into question the connection between freshwater discharge to the North Atlantic and AMOC strength. Using Mg/Ca-paleothermometry, we demonstrate that ∼3°C of near-surface ocean cooling masked an ∼1.0‰ decrease in western Labrador Sea δ18O of seawater concurrent with Lake Agassiz drainage. Comparison with North Atlantic δ18O of seawater records shows that the freshwater discharge was transported to regions of deep-water formation where it could perturb AMOC and force the 8.2 ka event.
Article
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Fresh water hosing simulations, in which a fresh water flux is imposed in the North Atlantic to force fluctuations of the Atlantic Meridional Overturning Circulation, have been routinely performed, first to study the climatic signature of different states of this circulation, then, under present or future conditions, to investigate the potential impact of a partial melting of the Greenland ice sheet. The most compelling examples of climatic changes potentially related to AMOC abrupt variations, however, are found in high resolution palaeo-records from around the globe for the last glacial period. To study those more specifically, more and more fresh water hosing experiments have been performed under glacial conditions in the recent years. Here we compare an ensemble constituted by 11 such simulations run with 6 different climate models. All simulations follow a slightly different design but are sufficiently close in their design to be compared. All study the impact of a fresh water hosing imposed in the extra-tropical North Atlantic. Common features in the model responses to hosing are the cooling over the North Atlantic, extending along the sub-tropical gyre in the tropical North Atlantic, the southward shift of the Atlantic ITCZ and the weakening of the African and Indian monsoons. On the other hand, the expression of the bipolar see-saw, i.e. warming in the Southern Hemisphere, differs from model to model, with some restricting it to the South Atlantic and specific regions of the Southern Ocean while others simulate a wide spread Southern Ocean warming. The relationships between the features common to most models, i.e. climate changes over the North and tropical Atlantic, African and Asian monsoon regions, are further quantified. These suggest a tight correlation between the temperature and precipitation changes over the extra-tropical North Atlantic, but different pathways for the teleconnections between the AMOC/North Atlantic region and the African and Indian monsoon regions.
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A cold event at around 8200 calendar years BP and the release, at around that time, of a huge freshwater outburst from ice-dammed glacial Lake Agassiz have lent support to the idea that the flood triggered the cold event. Some suggest that the freshwater addition caused a weakening of the North Atlantic meridional overturning circulation (MOC) thereby reducing the ocean transport of heat to high northern latitudes. Although several modeling efforts lend strength to this claim, the paleoceanographic record is equivocal. The authors’ aim is to use a coupled ocean–atmosphere model to examine the possibility that the two events are causally linked but that MOC reduction was not the main agent of change. It is found that the outburst flood and associated redirection of postflood meltwater drainage to the Labrador Sea, via Hudson Strait, can freshen the North Atlantic, leading to reduced salinity and sea surface temperature, and thus to increased sea ice production at high latitudes. The results point to the possibility that the preflood outflow to the St. Lawrence was extremely turbid and sufficiently dense to become hyperpycnal, whereas the postflood outflow through Hudson Strait had a lower load of suspended sediment and was buoyant.
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Boreal summer insolation during the last interglaciation (LIG) generally warmed the subpolar to polar Northern Hemisphere more than during the early Holocene, yet regional climate variations between the two periods remain. We investigate northeast Labrador Sea subsurface temperature and hydrography across terminations (T) I and II and during the LIG to assess the impact of two different magnitudes of boreal summer insolation increase on the northeast Labrador Sea. We use Mg/Ca ratios in Neogloboquadrina pachyderma (sinistral) as a proxy of calcification temperature to document changes in subsurface tem- peratures over Eirik Drift. Our corresponding record of d18O of seawater documents changes in water mass salinity. Mg/Ca calcification temperatures peak early in the Holocene coincident with peak boreal summer insolation. In contrast, LIG temperatures are relatively constant through the interglaciation, and are no warmer than peak Holocene temperatures. During the first half of the LIG, d18O of seawater remains depleted, likely from southern Greenland Ice Sheet retreat and enhanced Arctic freshwater and sea-ice export to the Labrador Sea. The consequent stratification of the Labrador Sea and attendant suppressed convection explains delayed deep-ocean ventilation and a cooler subsurface in the northeast Labrador Sea during the LIG.
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The climatic perturbation at ca. 8.2 kyr B.P. is the strongest short-term climate anomaly within the Holocene. It is generally attributed to a meltwater-induced slowdown of the thermohaline circulation in the North Atlantic. Model simulations and available proxy data suggest that it was strongest in the high to middle latitudes around the North Atlantic. Based on new pollen data from Tenaghi Philippon, northeastern Greece, we provide evidence for a massive climate-induced turnover in terrestrial ecosystems of the Aegean region associated with the 8.2 kyr B.P. event. The reconstructed winter temperature decline of >4 °C is much stronger than suggested by model simulations and proxy data from more northern latitudes of Europe, although the latter provide a direct downstream response to a North Atlantic thermohaline circulation slowdown. We attribute this discrepancy to mesoclimatic effects; a stronger influence of the Siberian High during the 8.2 kyr B.P. event may have enhanced the katabatic air flow from the mountains bordering the study site via a larger, longer persisting snow cover. Our data demonstrate that high-amplitude temperature anomalies and increased seasonality connected to the 8.2 kyr B.P. event may also have occurred in the lower mid-latitudes, much farther south than previously thought. The magnitudes of these anomalies appear to have been strong enough to have seriously affected Neolithic settlers in the northeastern Mediterranean region.
Article
Previous model experiments of the 8.2 ka event forced by the drainage of Lake Agassiz often do not produce climate anomalies as long as those inferred from proxies. In addition to the Agassiz forcing, there is new evidence for significant amounts of freshwater entering the ocean at 8.2 ka from the disintegration of the Laurentide ice sheet (LIS). We use the Community Climate System Model version 3 (CCSM3) to test the contribution of this additional meltwater flux. Similar to previous model experiments, we find that the estimated freshwater forcing from Lake Agassiz is capable of sustaining ocean and climate anomalies for only two to three decades, much shorter than the event duration of ~150 years in proxies. Using new estimates of the LIS freshwater flux (~0.13 Sv for 100 years) from the collapse of the Hudson Bay ice dome in addition to the Agassiz drainage, the CCSM3 generates climate anomalies with a magnitude and duration that match within error those from proxies. This result is insensitive to the duration of freshwater release, a major uncertainty, if the total volume remains the same. An analysis of the modeled North Atlantic freshwater budget indicates that the Agassiz drainage is rapidly transported out of the North Atlantic while the LIS contribution generates longer-lasting freshwater anomalies that are also subject to recirculation by the subtropical gyre back into the North Atlantic. Thus, the meltwater flux originating from the LIS appears to be more important than the Agassiz drainage in generating 8.2 ka climate anomalies and is one way to reconcile some model-data discrepancies.