Post on 17-Mar-2023
SST and ice sheet impacts on the MIS–13 climate
Helene Muri • Andre Berger • Qiuzhen Yin •
Aurore Voldoire • David Salas Y. Melia •
Suchithra Sundaram
Received: 13 June 2011 / Accepted: 3 October 2011 / Published online: 25 October 2011
� Springer-Verlag 2011
Abstract As a first qualitative assessment tool,
LOVECLIM has been used to investigate the interactions
between insolation, ice sheets and the East Asian Monsoon
at the Marine Isotopic Stage 13 (MIS–13) in work by Yin
et al. (Clim Past 4:79–90, 2008, Clim Past 5:229–243,
2009). The results are in need of validation with a more
sophisticated model, which is done in this work with the
ARPEGE atmospheric general circulation model. As in the
Earth system Model of Intermediate Complexity,
LOVECLIM, ARPEGE shows that the northern hemi-
spheric high insolation in summer leads to strong MIS–13
monsoon precipitation. Data from the Chinese Loess Pla-
teau indicate that MIS–13 was locally a warm and humid
period (Guo et al. in Clim Past 5:21–31, 2009; Yin and
Guo in Chin Sci Bull 51(2):213–220, 2006). This is con-
firmed by these General Circulation Model (GCM) results,
where the MIS–13 climate is found to be hotter and more
humid both in the presence and absence of any added ice
sheets. LOVECLIM found that the combined effects of the
ice sheets and their accompanying SSTs contribute to more
precipitation in eastern China, whilst in ARPEGE the
impact is significant in northeastern China. Nonetheless the
results of ARPEGE confirm the counter-intuitive results of
LOVECLIM where ice sheets contribute to enhance mon-
soon precipitation. This happens through a topography
induced wave propagating through Eurasia with an
ascending branch over northeastern China. A feature which
is also seen in LOVECLIM. The SST forcing in ARPEGE
results in a strong zonal temperature gradient between the
North Atlantic and east Eurasia, which in turn triggers an
atmospheric gravity wave. This wave induces a blocking
Okhotskian high, preventing the northwards penetration of
the Meiyu monsoon front. The synergism between the ice
sheets and SST is found through the factor separation
method, yielding an increase in the Meiyu precipitation,
though a reduction of the Changma precipitation. The
synergism between the ice sheets and SST play a non-
negligible role and should be taken into consideration in
GCM studies. Preliminary fully coupled AOGCM results
presented here further substantiate the finding of stronger
MIS–13 monsoons and a reinforcement from ice sheets.
This work increases our understanding of the signals found
in the paleo-observations and the dynamics of the complex
East Asian Summer Monsoon.
Keywords MIS–13 � Monsoon � GCM � insolation �Ice sheets � SST
1 Introduction
This paper investigates the climate of Marine Isotopic
Stage 13 (MIS–13) using a state-of-the-art General Circu-
lation Model (GCM), with particular focus on the East
Asian Summer Monsoon (EASM). The MIS–13 intergla-
cial occurred approximately 500,000 years ago (500 ka). It
is important to enhance our understanding of the potential
range of behaviour of interglacials, as we are currently in
one now and large amounts of resources are being spent on
predicting the future of it. We are therefore seeking
H. Muri (&) � A. Berger � Q. Yin � S. Sundaram
Georges Lemaıtre Centre for Earth and Climate Research
(TECLIM), Earth and Life Institute (ELI),
Universite catholique de Louvain, 2 Chemin du Cyclotron,
1348 Louvain la Neuve, Belgium
e-mail: helene.muri@uclouvain.be
A. Voldoire � D. S. Y. Melia
CNRM-GAME Meteo-France/CNRS, 42, Avenue Coriolis,
31057 Toulouse Cedex, France
123
Clim Dyn (2012) 39:1739–1761
DOI 10.1007/s00382-011-1216-9
information of past interglacials through the GCMs and the
geological records. MIS–13 is of particular interest, as it
appears to have experienced strongly enhanced monsoon
systems at the same time as the marine oxygen isotope and
Antarctic ice core records indicate that it was relatively
cool compared to other interglacials (Yin and Guo 2008).
The EASM has a complex space and time structure,
stretching from the sub-tropics into the mid-latitudes. It
impacts a large part of the world’s population and current
research suggests its strengthening in the future. Hence the
importance of this work where we try to improve our
understanding of its behavioural regime by exploring the
reasons for strong EASMs in the past.
Various geological evidence from China indicate that
MIS–13 was a relatively warm and humid interglacial
(Kukla et al. 1990; Guo et al. 2009; Chen et al. 1999; Yin
and Guo 2006). Though the d18O records from benthic
foraminifera, which depends on the temperature and d18O
of its ambient waters, suggest that the interglacials between
the mid-Pleistocene transition and the Mid-Bruhnes Event,
i.e. *900–430 kyBP, were cooler than the ones after the
Mid-Bruhnes Event (Liesiecki and Raymo 2005; Imbrie
et al. 1984; Shackleton 2000), as discussed in Yin and
Berger (2011). The EPICA ice core from Antarctica show a
similar trend, as seen in EPICA (2004). According to
Jouzel et al. (2007), the MIS–13 peak temperatures were of
*1–1.5�C colder than for the last Millennium. Spahni
et al. (2005) even characterised MIS–13 as an intermediate
warm period rather than an interglacial and also speculated
that the NH ice sheets could have had more of a southerly
extent than at other interglacial.
Holmes et al. (2010) extracted the local temperature
record from ostracod data, indicating that the MIS–13
temperatures of Boxgrove (English south coast) might have
been similar to today. The seasonality, however, was lar-
ger. The annual mean and winter temperatures could have
been similar to today or a lot colder. Furthermore, deep sea
core data from the mid-latitudinal North Atlantic indicate
that the winter temperatures were of around 13–14�C
(Raymo et al. 1990), compared to *15.5�C presently
(Locarnini et al. 2006). Lea et al. (2000) use magnesium/
calcium data from foraminifera in sediment cores from the
Pacific to extract information on sea surface temperatures
of the past. The SSTs from 159�E on the eqautor were of
around 26�C, i.e. more than 2�C cooler than the modern
times. Benthic d18O isotopes in marine sediment records
from the North Atlantic, just west of the British coast, give
indications of the MIS–13 SSTs (McManus et al. 1999).
The data reveal summer SSTs of around 12�C, which is
2–3�C less than the World Ocean Atlas climatology
(Locarnini et al. 2006).
Wang et al. (2004) found indications of reorganisations
of the carbon reservoirs of the oceans from the d13Cmax.
Positive precipitation anomalies have been seen in data
from the Amazon Basin (Harris et al. 1997). Furthermore,
strong MIS–13 African precipitation rates have been found
from anomalous Sa (sapropel) in the Mediterranean Sea
(Rossignol-Strick et al. 1998).
Further evidence of unusually strong African–Asian
monsoons has been found by e.g. Rossignol-Strick et al.
(1998), Guo et al. (1998), Yin et al. (2008), Caley et al.
2011). MIS–13 Arabian Sea high productivity conditions
has been found by Ziegler et al. (2010). This can be
interpreted as a symptom of strong monsoon circulation,
though Ziegler et al. (2010) argues that it could on the
other hand indicate changes to the ocean state, primarily a
strengthening of the Atlantic overturning circulation.
However, one does not necessarily exclude the other, as
both the monsoons and the meridional overturning circu-
lation could be stronger at the same time.
The paleo-signal from MIS–13 not as clear as for other
periods, being hampered by the fact that the Antarctic ice
cores do not agree with the marine sediments that are used
for helping to date the ice stacks. As the d18O deep sea
records, the deuterium and greenhouse gas records from
Antarctica all indicate smaller variations in the climate and
ice volume between glacial and interglacial stages before
MIS–11, one might assume it was a far-reaching and
dominant feature. Though as records from Asia and Europe
show signs of pre-MIS–11 conditions being relatively
warm compared to the more recent interglacials, and
indeed with strong monsoons across Africa and China (Yin
and Guo 2008). One question arises; how can MIS–13
supposedly have experienced such wet monsoons when
globally the ice volume was larger, keeping in mind the
large-scale cooling potential of ice? Considering the myr-
iad of mixed-message paleo-observations, a MIS–13 GCM
time-slice experiment is much needed.
The variability of the Asian Monsoon system is influ-
enced by a number of factors, including the Tropical
Biennial Oscillation, the Intraseasonal Oscillation, the
Quasi-Biennial Oscillation, SST anomalies in the Indo-
Pacific regions, as well as their interactions with ENSO
(Lau et al. 2000; Meehl and Arblaster 1998; Shen and
Kimoto 1999; Meehl 1997; Ju and Slingo 1995). The
monsoon itself can impact the global climate through heat
and momentum fluxes, which could affect e.g. the ENSO
variability (Kirtman and Shukla 2000). Indo-Pacific con-
vection impacts has been found in North Atlantic decadal
climate signals (Hoerling et al. 2001), indicating the far
reaching teleconnections resulting from the monsoons.
Nitta (1987) and Huang and Sun (1992) found that strong
JJA Philippines and South China Sea convection may
trigger a Rossby wave train stretching from the North
Pacfic to the North American continent. Lau et al. (2000,
2002) confirm the teleconnections from East Asian JJA
1740 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
123
precipitation anomalies with North American’s, with SST
signals carried through in the extratropical Pacific Ocean.
The EASM is furthermore influenced by the ITCZ
(Intertropical Convergence Zone), West Pacific Subtropical
High (WPSH), the thermal properties of the West Pacific
Warm Pool i.e. the ascending branch of the Walker cir-
culation, and the thermal and dynamic effects of the
Tibetan Plateau. The East Asian jet stream is jointly con-
trolled by extratropical dynamics and tropical heating. The
monsoon is additionally impacted by land surface pro-
cesses over Eurasia (Lau et al. 2000).
The Meiyu/ Baiu and Changma fronts are responsible
for large parts of the EASM rainfall. The Meiyu front de-
velopes from a transitionary high pressure cell to the north
and West Pacific Subtropical High to the south (Chen
1983). The mid-June abrupt northwards shift of the West
Pacific Subtropical High, from say 18–25�N, causes the
Meiyu front to also shift northwards to the Yangtze River
and Huaihe Valley. This peak of the monsoon season
receives as much as 250 mm/month in precipitation. The
Meiyu front moves northwards from April to August,
before migrating southwards again. The location of the
maximum peak in precipitation closely follows the West
Pacific Subtropical High (Ding 1992; Huang and Sun
1992). When the lower level moisture convergence is
located beneath the jet entrance region at high level, it
facilitates for the development of the deep convection.
The shift in the West Pacific Subtropical High has been
found to be linked to the strength of the thermal forcing
from the convection near the Philippines (Cao et al. 2002).
When the thermal forcing reaches a threshold, the rather
sudden transition from a winter to summer circulation
regime is permitted to happen over East Asia. The thermal
forcing threshold is reached due to strong wave—mean
flow interaction an wave–wave interaction among the
waves responding to the thermal forcing. Huang (1994)
argued that the Philippine convection might be influenced
by the Madden–Julian Oscillation.
The Changma Front is characterised by zonal winds at
200–300 hPa, with few jets moving through the area. This
fascilitates for a strong uplift potential. The trough runs
perpendicular to the Korean Peninsula, and stretches from
the west Pacific, through Japan, Korea to the foothills of
the Tibetan Plateau (Oh et al. 2007). High pressure south–
southeast of Japan provides moisture and heat fluxes to
feed the front. In its northern position the front takes on a
barotropic nature.
Yin et al. (2008, 2009) used the LOVECLIM model to
investigate the MIS–13 climate response to insolation,
greenhouse gas (GHG) and ice sheet forcings. Interesting
features explaining the reinforcement of the EASM were
discovered, including the important role played by insola-
tion and a topographically induced wave train issued from
the Eurasian ice sheet. The intermediate complexity model,
LOVECLIM, contains an atmospheric component which is
relatively simple. It has a T21 resolution and three vertical
layers and it is based on the quasi–geostrophic potential
vorticity equation. Its ocean is a comprehensive general
circulation model with 20 layers and 3 9 3� grid box
resolution. In Yin et al. (2008, 2009), the atmosphere,
ocean and vegetation components are interactively cou-
pled. Due to the reduced complexity of the LOVECLIM
atmosphere, some results of Yin et al. (2008, 2009) are
needed to be confirmed with more complex tools. The
ARPEGE AGCM has therefore been used in this study as a
first attempt to evaluate how the monsoons could have been
stronger during an interglacial with a relatively high ice
load in a higher resolution model, with a more complex
physical representation than in LOVECLIM. The model
was forced with astronomically induced insolation, GHG
and sea surface temperature (SST) changes, in addition to
land ice perturbations in order to assess their relative
importance in explaining the climatic conditions seen
during the MIS–13.
The model used, experiment design and boundary con-
ditions are described in Sects. 2 and 3. The simulated MIS–
13 climate is contrasted to the control experiment in Sect. 4
The pure impacts of SST and ice sheets and their interac-
tions are evaluated in Sect. 5 The initial results from fully
coupled AOGCM experiments are presented in Sect. 6 The
conclusions are finally presented in Sect. 7.
2 The ARPEGE model
The ARPEGE (Action de Recherche Petite Echelle Grande
Echelle or Research Project on Small and Large Scales)
climate model is based on the primitive equations and is
developed by Meteo-France in collaboration with EC-
MWF. It is based on the Numerical Weather Prediction
version, but has some physical modifications (Meteo-
France 2003). The model uses hybrid r—pressure co-
ordinates, a Gregorian calendar and the solar constant takes
a values of 1370 W m-2. The GCM has a spectral trian-
gular 42 horizontal resolution transformed to a Gaussian
grid. The longitude 9 latitude grid box resolution is of
128 9 64 and there are 31 vertical levels in the atmo-
sphere, out of which 4 are in the stratosphere. Each time
step is of 30 minutes for the model when run at this res-
olution. The gravity wave drag scheme in the model
accounts for the lift and mountain blocking effects, as
described in Lott (1998) and Lott and Miller (1997). The
soil thermodynamics scheme, ISBA, uses four vertical
levels with a heat diffusion scheme, without any relaxation
towards observations or a prescribed temperature. The soil-
vegetation scheme is described in Noihlan and Planton
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1741
123
(1989) and furthered in Mahfouf et al. (1995). The model
classifies four surface types; high-, low-vegetation, sea and
ice. ARPEGE has been used for a number of monsoon
studies, including e.g. Ceron et al. (2001), Douville et al.
(2001), Garric et al. (2002), Ashrit et al. (2003), Douville
(2006), Caminade and Terray (2010).
The pre-industrial control run has been compared to the
NCEP/NCAR reanalysis data (Kalnay et al. 1996) and the
ARPEGE model has been deemed adequately close to
observations (not shown here) and therefore suitable for the
purposes of this study. The NCEP/NCAR reanalysis model
uses a fixed CO2 concentration of 330 ppm, which is 50 ppm
higher than in the ARPEGE control simulation. This dif-
ference corresponds to a raditive forcing of *0.88 W m-2,
or a global mean temperature difference of *1�C. The
comparison with the reanalysis data shows that the control
model simulates the major trends of the surface air flow
correctly by placing the pressure cells in the right regions
with the appropriate magnitudes. Temperature-wise there is
an Antarctic cold bias lasting throughout the year. It has been
suggested that a higher vertical resolution might be needed
to resolve the rather stable polar winter boundary layer in the
ARPEGE model (Byrkjedal et al. 2008). With regards to
precipitation rate, the main JJA differences lie within a
tropical belt between *100 and 160�E. Here the precipita-
tion is shifted over the sea from the Asian monsoon conti-
nent. The bias has a magnitude of*8 mm/d, suggesting that
the ARPEGE AGCM might be sensitive to SSTs.
Comparing the ARPEGE and LOVECLIM pre-indus-
trial runs (keeping in mind that ARPEGE is runs as AGCM
and LOVECLIM containd a full OGCM) shows some
differences in the mean climates. The LOVECLIM surface
air temperatures are lower across the Arctic throughout the
seasons, except during JJA. ARPEGE is much colder over
Antarctica, where the LOVECLIM temperatures are closer
to the observations. During the NH monsoon season, North
India and westwards to Saudi-Arabia is 3–6�C colder in
LOVECLIM, whilst the east Eurasian seaboard and
northeastern China and Mongolia is 2–6�C warmer. Some
of these differences can be attributed to the coarser reso-
lution of LOVELIM, especially for the high altitude
regions. Compared to ARPEGE, LOVECLIM also shows
wetter East African Monsoon, Middle East, and drier
conditions over China and the Western tropical Pacific.
This pattern is consistent throughout the year. Overall,
however, LOVECLIM is indeed a suitable first qualitative
assessment tool.
3 The experiments
The main objective of our experiments is to quantify the
individual contributions of SST (including sea ice
concentrations) and ice sheets to the climate of MIS–13,
which is done through the factor separation method of
Stein and Alpert (1993). This is used to evaluate the pure
contributions from various forcings on the climate in the
GCM. This factor separation method is a systematic pro-
cedure to assess the impacts of multiple factors in sensi-
tivity studies. It enables the quantification of contributing
factors as well as their interaction, or synergism, in non-
linear systems, e.g. the climate system. The relevant cli-
matic forcings in this study are insolation, GHG, SST and
ice sheets.
The insolation forcing results from changing the astro-
nomical configuration of the Earth in the model to repre-
sent that of 506 ka BP (Table 2; Fig. 1). These parameters
are the eccentricity of the Earth’s orbit around the Sun, the
obliquity (or axial tilt) of the Earth, in addition to the
longitude of the perihelion. These are calculated after
Berger (1978). The 506 ka perihelion, i.e. when the Earth
is at the closest approach to the Sun, occured during the
NH summer as opposed to the present day situation when
the perihelion occurs close to the winter solstice. In Fig. 1,
the time axis is in calendar days, as opposed to the tradi-
tional one where the time axis is in true longitude of the
Sun (see Fig. S2 in Yin and Berger (2010)). The differ-
ences are due to the length of the seasons, which are
influencing the insolation deviations from present day. At
506 ka BP, the summer solstice occurred on 16.7 June, the
autumn equinox on 11.7 September and the winter solstice
on 15.7 December, it means 5.1, 11.7 and 6.5 days earlier
than today respectively. This leads to e.g. the anomaly on
October 15 (calendar day) of 83.6 W m-2 (this Fig. 1),
whereas it is -11.1 W m-2 for a true longitude of 210�,
the so-called mid-month value for October in terms of the
true longitude of the Sun (Berger 1978).
The MIS–13 GHG concentrations are the same as in Yin
et al. (2008) (Table 2). With regards to the ice sheets, both
the albedo and topography of the land has been changed in
the model where the additional ice sheets have been added
(see Fig. 2). The procedure for the ice sheet reconstruction
is explained in detail in Yin et al. (2008). The exact loca-
tion and size of the MIS–13 ice sheets are uncertain,
though the d18O from the deep sea cores indicate the
presence of more NH ice at the time (Yin et al. 2008). The
Last Glacial Maximum reconstructions of ice (Peltier 2004;
Hughes et al. 1981; Clark and Mix 2002; Lambeck et al.
2002; Bintanja et al. 2005), in addition to the d18O records
(Imbrie et al. 1984), were used as a guide with regards to
the size of the MIS–13 ice sheets. The ice sheet locations
were decided based on information of the ice sheet initia-
tion at the last glacial inception (Clark et al. 1993; Bintanja
et al. 2002; Peltier 2004). Yin et al. (2009) evaluated the
importance of the size of the MIS–13 ice sheets for the
monsoon strengthening. It is shown that the precipitation
1742 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
123
rate over Africa and India is reduced with increasing ice
sheet size, due to the ensuing southwards shift in the
Intertropical Convergence Zone. As for northeastern Asia,
the monsoon is enhanced through the ‘‘wave train’’
mechanism detected in Yin et al. (2008) independent of ice
sheet size.
The monthly mean SST and sea ice concentrations of
three LOVECLIM simulations from Yin et al. (2008) are
involved in the ARPEGE experiments. The three
LOVECLIM simulations are the pre-industrial (LPI),
506 ka with present day ice sheets (NO ICE) and a 506 ka
model run with additional north American and Eurasian ice
sheets (WITH ICE). In order to reduce the influence of the
LOVECLIM mean climate (the structural error) on the
ARPEGE simulations, the SST anomalies (NO ICE—LPI,
and WITH ICE—LPI) are used (see further explanation
below). As there are two factors to be analysed, SST and
ice sheets, four experiments are required according to the
factor separation method. These are NN, NS, IN2 and IS2,
in addition to the PI which is the simulation of the Pre-
Industrial climate with ARPEGE, as follows:
PI: The pre-industrial (PI) control model with 1860
SSTs, insolation and GHG.
NN: 506 ka orbital configuration, GHG, no additional
ice sheets, with the SST of the NO ICE on which the
LOVECLIM difference between PI and LPI is added.
NS: 506 ka orbital configuration and GHG. No addi-
tional ice sheets. SST of the WITH ICE on which the
differences between PI and LPI is added.
IN2: 506 ka orbital configuration, GHG, SSTs as in NN,
but additional North American and Eurasian ice sheets
are included.
IS2: 506 ka orbital configuration, GHG, SSTs as in NS
and ice sheets like in IN2. Whereupon, according to the
factor separation principle:
IS2�NN ¼ ðIN2�NNÞ þ ðNS�NNÞ þ ðIS2�NS�IN2
þ NNÞ:ð1Þ
i.e. the difference between the IS2 and NN models is the
combined effects of both the ice sheets and SST. IN2–NN
is the pure contribution from the ice sheets, NS–NN is the
pure contribution from SSTs and sea ice and the last term is
the synergism between the two factors.
The MIS–13 model runs are also summarised in Table 1
to illustrate the use of ice and SST forcings.
It is worth mentioning that this factor separation analysis
is actually separating the individual impacts of the ice
sheets and of their induced SST change which, due to the
coupling between the atmosphere and the ocean in
LOVECLIM, are difficult to be distinguished in Yin et al.
(2008).
4 MIS–13 compared to pre-industrial
4.1 IS2–PI: the MIS–13 climate
The combined effects of the 506 ka ice, GHG, SST and
insolational forcing as compared to the pre-industrial cli-
mate can be seen in Fig. 3. The IS2 experiment is the most
complete 506 ka simulations with ARPEGE, accounting
for multiple feedbacks. The combined effects of these
Table 1 A summary of the
ARPEGE MIS–13 experiments
and the ice and SST forcings
S corresponds to MIS–13 SST
forcing, I to ice and the number
2 indicates that the ice forcing
includes both the albedo and the
topographic changes
Exp. name Astr GHG SST and sea ice NA?EA ice
No ice Ice Topography Albedo
0 PI PI PI 1860
1 NN 506 506 H
2 IS2 506 506 H H H
3 IN2 506 506 H H H
4 NS 506 506 H
Fig. 1 The latitudinal insolation [W m-2] differences between
506 ka and the present over one year in the ARPEGE model. Time
through the year is given in calendar days
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1743
123
boundary conditions result in a global annual mean tem-
perature of 13.26�C, i.e. 0.52�C lower than in the pre-
industrial control run (Table 3). This is of comparable
magnitude to the LOVECLIM WITH ICE simulation of
Yin et al. (2008), with its -0.64�C anomaly (WITH ICE–
LPI). IS2 is the coldest out of the ARPEGE 506 ka
experiments.
Looking at the JJA surface temperatures from the IS2
experiment compared to the PI (Fig. 3a), there is a
warming of all continents, whilst there is a cooling of
North African and Indian monsoon region. There is a
substantial cooling above the North American and Eurasian
ice sheets. All these features are also seen in the
LOVECLIM results (Fig. 3d), but there the cooling over
the monsoon region appears only at the surface level as
opposed to at 2 m.
There is a substantial increase in JJA precipitation
(Fig. 3c) across North Africa, Middle East, India, China
and the east coast along the Eurasian continent (the mon-
soon belt), with up to 10 mm/day in places. There is also a
northwards shift in the precipitation along the northwest
Pacific. Subtropical SH and western North Atlantic is also
drier in this 506 ka experiment. There is an increase in
precipitation above the added land ice, though a drying
upwind and downwind in Siberia.
The JJA pressure changes in the IS2 experiment com-
pared to the PI shows a lowering of the pressure across
Europe and Asia north of the sub-tropics. Over the oceans,
most of the SH and the Arctic the MSLP (Mean Sea Level
Pressure) is higher. The Eurasian thermal low is deepened
with as much as 7 mb, whilst the northwest Pacific high is
equally heightened. This 14 mb strengthening of the land–
sea pressure gradient forces a stronger onshore moisture
flux.
Wave motion is detected in the omega field at the
600 mb level across *60�N, though the magnitudes are
naturally larger surrounding the North American ice sheet,
since it is twice the size of the Eurasian ice sheet. The
geopotential height is higher above the ice and lower up
and downstream of it. This is associated with the vortex
stretching in the regions with enhanced monsoons and
stronger updrafts. The wave might act to transport kinetic
energy to the monsoon region.
Figure 3a, c shows that the most important features
simulated by ARPEGE are in good agreement with those
simulated by LOVECLIM (Fig. 3b, d). This includes a
significant JJA warming over land, cooling over the ice
sheets and a substantial precipitation increase over the
northern monsoon regions. The two experiments also pro-
duce similar precipitation changes around the two addi-
tional ice sheets. E.g. both experiments exhibit a wet
anomaly over the Eurasian ice sheet and dry anomalies to
its east and west side. The main precipitation differences
between ARPEGE and LOVECLIM occurr in two regions;
The North American Monsoon, the South Atlantic Con-
vergence Zone and the sub-tropical North Atlantic is drier
in ARPEGE, whereas northeast Asia is drier in
LOVECLIM.
The strong MIS–13 continental heating of China
(90–120�E) during summer is seen in Fig. 4. The
Fig. 2 The topography [m] of
the added ice sheets in the MIS–
13 experiments (after Yin et al.
(2008)). The Eurasian ice sheet
is about half the size of the
North American one, with
volumes of 3.63 and 7.38 km3,
respectively
Table 2 The astronomical
parameters and GHG
concentrations for the MIS–13
and PI simulations
Obliquity (�) Eccentricity Precession (�) CH4 (ppb) CO2 (ppm) N2O (ppb)
PI 23.446 0.016724 102.04 760 280 270
M13 23.377 0.034046 274.05 510 240 280
1744 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
123
anomalous warmth starts in April/ May and lasts
throughout August, with a magnitude of 2–7�C from
*30–75�N. The accompanied MIS–13 precipitation
anomalies (Fig. 4b) exhibit a northwards shift and
enhancement in the sub-tropics–mid-latitudes. The north-
wards jump of the Meiyu Front is seen in the May–June
transition. The enhanced monsoon conditions over China
lasts until the end of August. The Meiyu precipitation is
increased by 3–5 mm/day in June and July at 30�N. This
confirms the observations of wetter than PI conditions in
China during MIS–13 (Guo et al. 1998).
It is apparent from Figs. 1, 3, and 4 that the combined
insolation, SST and ice sheet forcing at 506 ka can explain
the observations of both a cold Antarctic and a warm,
humid NH monsoon region. South of *30�S sees a year
round negative temperature anomaly in ARPEGE. Though
the peak insolation forcing of as much as 70 W m-2 con-
tribute a south polar warming of 3–5�C in September and
October. This warm anomaly is related to the different
lengths of the SH summer season at 506 ka relative to the
pre-industrial. SH spring and SH summer starting 11.7 and
6.5 days earlier at 506 ka respectively. The Southern
Ocean 10 m zonal wind is accelerated in the IS2 experi-
ment with a maximum of 2.1 m/s in the annual mean in the
Drake Passage. The stronger winds, seen in both ARPEGE
and LOVECLIM, amplify the upwelling of Circumpolar
Deep Water contributing to cool the overlying atmosphere,
whilst the Antarctic Circumpolar Current is accelerated. It
is shifted northwards due to more sea ice around Antarc-
tica, and strengthened in the higher layers in LOVECLIM.
Fig. 3 The JJA 2 m temperature (a) and precipitation (c) differences
between the ARPEGE IS2 and PI experiments. The LOVECLIM JJA
2 m temperature (b) and precipitation (d) [mm/day] differences
between the 506 ka run with ice and the PI. The contours indicate
regions with a level of confidence higher than 95%
Table 3 The ARPEGE global mean temperature differences [�C];
annual, January and July values
Exp Annual January July
IS2–PI -0.52 -1.43 0.73
NN–PI -0.21 -1.02 0.90
NS–NN -0.16 -0.16 -0.14
IN2–NN -0.13 -0.23 -0.04
IS2–NN -0.32 -0.42 -0.17
IS2–NS–IN2 ? NS -0.03 -0.03 0.01
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1745
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The modern Changma front is experimented to reside at
*38�N with maximum zonal winds at 200 hPa (Fig. 5),
which corresponds well to the NCEP/NCAR reanalysis.
For the IS2 experiment, the Changma front has strength-
ened by as much as 15 m/s and migrated northwards by
around 15�N (Fig. 5b). The MIS–13 atmospheric condi-
tions are both hotter, with a maximum at the upper trop-
osphere of 8�C, and more humid, by as much as 5 g kg-1,
confirming the paleo-observations of a hotter and more
humid EASM (Guo et al. 2009). The Changma front results
from the interaction between subtropical and mid-latitudi-
nal air masses (e.g. Ninomiya 1998; Ueda et al. 1995). In
the MIS–13 ice experiments, the mid-latitudinal air has
been cooled and dehydrated by the ice before it merges
with the subtropical air, which is even warmer and moister
in the ice experiment. This stronger moisture and thermal
contrast between the colliding air masses increases the
convective available potential energy (CAPE), fascilitating
for a strong storm potency for the MIS–13 monsoon.
4.2 NN–PI: insolation and GHG effects on the MIS–13
climate
The ice sheets in the NN and PI experiments are the
Greenland and Antarctic constellation of present day only.
These two experiments differ only through their greenhouse
gas concentrations and redistribution of seasonal and lati-
tudinal insolation. The [CO2] was 40 ppm lower at MIS–13
than the pre-industrial resulting in a radiative forcing of
-0.79 W m-2, whilst the CH4 forcing is of a mere -0.068
W m-2. The insolation forcing at MIS–13 is much larger,
on the other hand (Fig. 1). The climatic differences between
the two periods are hence mainly resulting from the inso-
lation differences. The largest difference in their astro-
nomical parameters are the longitude of perihelion, with
NH summer at perihelion at MIS–13 and NH summer at
about aphelion at pre-industrial. This leads to much higher
insolation received by the Earth during boreal summer and
much less during boreal winter at MIS–13.
As seen in previous simulations with LOVECLIM (Yin
et al. 2008), the prominent role of the latitudinal and sea-
sonal distribution of insolation is reflected in the surface
temperatures in ARPEGE. There is an insolational forcing
of 45–50 W m-2 in the early summer northwards of 10�N,
acting to heat the landmasses in particular. There is a
strong continental warming during JJA (Fig. 6a), though a
cooling in the Afro-Indian monsoon regions of up to 3�C in
the annual mean. This cooling could be a signal of
enhanced monsoonal activity resulting in a reduction of
short wave radiation reaching the surface and an increase in
the evaporative cooling. The summer (JJA) component
does indeed display a pronounced increase in the precipi-
tation rate [mm/day] in the NH monsoon regions, except
North America. There is also a northwards shift in the
proximity of the West Pacific warm pool. The same pattern
is also seen in the moisture budget (not shown), but the
signal is somewhat reduced, due to the increase in the
evaporation. The insolation forcing on the climate evi-
dently leads to such a strong heating of the summer NH
continents, increasing the land–sea thermal contrast rein-
forcing the monsoonal precipitation across Africa, Arabia
and Asia.
Fig. 4 The monthly difference in zonal mean temperature [�C] and
precipitation [mm/day] between 90–120�E between the IS2 and PI
experiments, showing a long and strong monsoon over China. This
corresponds to a region of strong continental heating from April/ May
to August
1746 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
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During DJF, the insolational and GHG forcings con-
tribute to both cool the continents, and with a 95% level of
confidence southwards of about NH mid-latitudes. The DJF
monsoons of Australasia and South America are signifi-
cantly drier (Fig. 6d). The insolational forcing is only
-5 W m-2 north of 60�N (where it is winter with an
absolute value at 506 ka of *50 W m-2), though it
reaches as much as -50 W m-2 south of *65�S (where it
is summer with an absolute value of 410 W m-2).
The annual evolution in surface air temperature differ-
ences between the NN and pre-industrial experiments
zonally averaged between 90–120�E, illustrated in Fig. 7
highlights the strong heating of the Chinese landmasses of
up to 5�C between May to August due to the insolation.
There is also a stronger 506 ka precipitation rate over
China from May lasting through to September (Fig 7b).
and an amplification of the Meiyu front. The front is *6�further north in June, due to the shift in the West Pacfic
Subtropical High. NN–PI displays a strong continental
heating during JJA. This acts to reinforce the Eurasian
thermal low. At the same time, the mean sea level pressure
over the Pacific is heightened leading to a increased land–
sea pressure gradient accompanied by a strong onshore
advection of moisture. The insolation forcing contributes to
shift the Changma front northwards by about 15�N, the
zonal wind however is weakened (Fig. 8). The troposphere
is warmer throughout 30–60�N and lower layers are up to
4 g kg-1 more moist.
5 Pure contributions of SST, ice sheets and synergism
5.1 Pure contribution of SST
The MIS–13 climatic response to the SST forcing can be
evaluated by contrasting the NS and NN experiments. The
main differences between the SST forcing of these two
experiments (given by Yin et al. 2008) are a positive SST
anomaly of *2–3�C persisting throughout the year in the
North Pacific and a negative anomaly of 2–5�C in the
Fig. 5 The specific humidity [g kg-1] (purple dotted isopleths), air
temperature [K] (black isotherms) and zonal wind speed [m/s]
(coloured shading) of a the PI, b the IS2 experiment and c their
differences. The data is from July at 127�E, i.e. the location and
timing of the present day Changma front (Korea)
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1747
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North Atlantic. A negative anomaly is also seen across
most of the Southern Ocean of 0.5–2�C. With regards to
the sea ice concentrations, NS has more sea ice, except the
Sea of Okhotsk and the Bering Straight, which shows a
slight reduction.
The JJA SST anomaly between the 506 ka LOVECLIM
experiment with added land ice and the 506 ka with no
added ice is shown in Fig. 9, as this does not feature in
Sundaram et al. (2011b).
The NS–NN SST differences result in a cooling of
surface air temperatures at 2 m across Antarctica and the
North Atlantic and a positive anomaly in the North Pacific
(Fig. 10a). This corresponds to the regions of negative and
positive SST anomalies respectively. There is a cooling of
the surface temperatures in the Barent’s Sea region due to
the SST and sea ice impacts. The pattern of positive and
negative SST anomalies in the NH basins of the Pacific and
Atlantic respectively persists throughout the year. The
magnitude and area of changes are larger during DJF,
however, which is also the season with the largest SST
forcing. This enhancement of ice sheet induced SST is due
to the North American ice sheet impacting the wind stress
along North Atlantic, and in turn the wind driven ocean
circulation.
There is a slight reduction in DJF precipitation across
the equatorial ocean and Central America (Fig. 10d). There
is a strengthening of the DJF SH monsoon precipitation in
Indonesia and Micronesia. In regions with lower SSTs, due
to the ice sheets, the reduction in evaporation is inhibiting
the rainfall leading to the observed pattern. In the West
Pacific, the SSTs are warmer in the south and colder in the
north, contributing to the southwards shift in the annual
mean precipitation. The JJA precipitation changes are at a
much smaller scale with the only significant changes near
the Pacific Warm Pool region (Fig. 10c). This region has
somewhat lower surface air temperatures, which are
seemingly inhibiting the precipitation.
The SST and the sea ice forcing in the ARPEGE model
lead to a warming over Eurasia and the Sea of Okhotsk. It
is seen that the zonal gradient in surface air temperatures is
Fig. 6 The JJA (a, c) and DJF (b, d) temperature (a, b) and precipitation (c, d) differences between the ARPEGE NN and PI experiments
1748 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
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strengthened during the summer, by the North Atlantic
negative anomaly and a Eurasian positive anomaly. The
early summer zonal temperature gradient, with the cool
SSTs over the North Atlantic and the rapid warming of the
Eurasian land masses, acts to trigger a stationary Rossby
wave feature. It is propagating from the North Atlantic,
across Siberia and initiating a deepening of the Eurasian
thermal low and anomalously high pressure over the Sea of
Okhotsk. The 500 mb JJA geopotential height differences
between NS and NN reveals this high (Fig. 11a). This
pressure cell has an affect on the northwards penetration on
the monsoon front, acting to prevent the northwards
migration through the season. Hence a JJA southwards
shift in the precipitation is seen and an annual mean drying.
The SST forcing leads to a further strengthening of 6 m/s
of the jet stream across Korea in July (Fig. 12). Though the
jet is stronger, the precipitation anomaly of 1 mm/day
associated with it is not statistical significant. With regards to
the SH, the SST and sea ice forcing leads to cooling south of
the mid-latitudes, contributing to partly explain the cold
temperature feature seen in the EPICA ice core at MIS–13
(Jouzel et al. 2007). However, it must be stressed that these
SST and sea ice induced only by ice sheets are quite small
compared to those induced by insolation (Yin et al. 2009).
The influence of this North Atlantic temperature anom-
aly can be compared to the results by Sundaram et al.
(2011b), which shows a correlation between the positive
winter North Atlantic Oscillation (NAO) and the EASM
resulting from the Eurasian and North American ice sheets.
Fig. 7 The monthly difference in zonal mean temperature and precipitation between 90 and 120�E from the NN and PI experiments
Fig. 8 July specific humidity, temperature and zonal wind speed
anomalies at 127�E Same as Fig. 5, only for NN–PI
Fig. 9 The JJA SST difference [�C] between the NS and NN
experiments, as of the LOVECLIM 506 ka WITH ICE and NO ICE
experiments
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1749
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The LOVECLIM model with ice experiences a positive
NAO-like feature in NH winter, with a cold SST anomaly in
most of the North Atlantic, though a warm bias in the far
North Atlantic (Greenland-, Iceland-, and Norwegian Seas),
also corresponding to an area of less sea ice. According to
Sundaram et al. (2011b), this NAO pattern caused a delayed
Fig. 11 The JJA (a) and DJF (b) 500 mb geopotential height [m] differences between the NS and NN experiments, over East Eurasia and the
North Atlantic respectively
Fig. 10 The JJA (a, c) and DJF (b, d) temperature (a, b) and precipitation (c, d) differences between the NS and NN experiments
1750 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
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effect on the EASM. JJA saw a warm SST bias over Eurasia
and the Sea of Okhotsk. This matches to areas of blocking
high pressure cells. This perturbation to the circulation
serves to strengthen the Meiyu monsoonal front.
The SST and sea ice anomaly influences from this
experiment are seen in the ARPEGE results. The 500 mb
geopotential height DJF anomaly shows a deep negative
anomaly in the northern North Atlantic and a positive
anomaly south of this (Fig. 11b).
5.2 Pure contribution of ice sheets (IN2–NN)
To examine the pure contribution from changing both the
topography and the ice albedo as of the additional North
American and Eurasian ice sheets, IN2 is compared to the
NN experiment (Fig. 13). In the JJA 2 m air temperatures
(Fig. 13a), there is a cooling of the air masses above the
ice, with regions of warm anomalies downstream, sug-
gesting a wave-like phenomenon. The warm anomalies can
be attributed to anomalous southwesterly flow. These
warming and cooling anomalies induced by the ice sheets
are also simulated by the LOVECLIM model (Yin et al.
2009), suggesting that they are not model dependent.
There is a summer drying west of both ice sheets, and
there is a wave pattern east of the Eurasian ice sheet. This
is less pronounced when accounting for the evaporation,
however. There is a JJA precipitation increase in north-
eastern China. During DJF, there is a substantial cooling at
NH high latitudes of as much as -10�C. East of the Eur-
asian ice sheet there is also a drying associated with this
cooling.
The differences in geopotential height of the 500 mb
pressure level, omega at 600 mb and potential vorticity at
850 mb are the largest over North America, due to the size
and height of the ice sheet. An anti-cyclone is also estab-
lished directly above the North American ice sheet. The
JJA geopotential height difference shows a large positive
anomaly resulting from including the ice sheet topography
(Fig. 14). The vertical velocity in mPa s-1 units, i.e. in
pressure coordinates where positive values signify sinking,
shows a stronger upward motion of air masses where the
air is forced over the North American ice sheet and
increased sinking motion leewards of the ice at the 600 mb
level. There is less of a signature associated with the
Eurasian ice at this height, due to the much smaller size.
The Eurasian ice sheet sets up an anomalous anti-cyclonic
flow bringing in northerly winds to the east of the ice sheet
across Central Eurasia. The ice sheets also act to separate
the storm tracks. An increased descent is seen along the
northwest Pacific coast line, whilst there is a band of
increased ascent south of this at the equator. The potential
vorticity differences, i.e. the spin of the air masses, shows a
compression of the air as it passes over ridge of North
American ice sheet with vortex stretching as air passes
around the ice mass. The same is seen over West Eurasia,
though at a smaller scale as one might expect due to the
smaller size of this ice sheet. The Eurasian ice sheet
advects the JJA 850 mb potential vorticity disturbances
down wind across Eurasia in a wave-like manner, inducing
a band of unstable air across eastern China, as seen in
Fig. 15.
IN2–NN shows an annual large increase in geopotential
in the North Pacific and over the North American ice.
Dwap (vertical wind in pressure co-ordinates) at 600 mb is
negative, with a magnitude of 10 mPa s-1, over northeast
China indicating a stronger updraft as a result of the ice
sheet induced atmospheric circulation changes. Cold wind
coming off the ice sheets meet strong onshore moist warm
Pacific air results in a cell of increased convection over NE
China, in the ARPEGE model. Adiabatic warming of the
air leeward of the ice sheet results in a positive surface air
temperature anomaly. The Eurasian summer thermal low
pressure centre is strengthened, at the same time as the high
pressure over the North Pacific is heightened. The pressure
gradient is therefore sharper facilitating for stronger
onshore easterlies in NE China and an intensification of the
monsoonal precipitation from the Changma front.
The ice sheet’s impact on the zonal wind of the
Changma front is a further strenghtening of the jet at
200 hPa of 6 m/s (Fig. 16), i.e. of similar magnitude as in
the influence of the SST forcing. The ice sheets lead to a
cooling and drying of the atmosphere north of the mid-
latitudes, with a -3�C peak at 400 hPa and a lower level
specific humidity reduction of 1g kg-1. The reinforcement
of the Changma front by the ice sheet results in the 2 mm/
day precipitation anomaly seen across the Manchurian
Plain (Fig. 13c) with stronger vertical winds and
Fig. 12 The July specific humidity, temperature and zonal wind
speed anomalies at 127�E, as in Fig. 5c, only for NS–NN
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1751
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convergence. The meridional temperature gradient is
increased due to the ice sheets, creating stronger jets, which
presence indicates high potential energy and flow
instability.
The ice sheets do not contribute to any significant
temperature changes in the SH, confirming that the MIS–13
SH cold bias (Jouzel et al. 2007) was likely caused by the
combined effects of the insolation, GHG, sea ice and SST,
as also concluded by Yin and Berger (2011).
5.3 Combined effect of SST and ice sheets
The difference between IS2 and NN (Fig. 17) gives the total
combined impact of ice sheets and their induced SST
changes. It is equivalent to the difference between the
506 ka LOVECLIM experiments WITH ICE and NO ICE
in Yin et al. (2008). The JJA average shows a wave like
pattern of precipitation anomalies across Eurasia associated
with the Eurasian ice sheet, with a rain shadow down stream
Fig. 13 The JJA (a, c) and DJF (b, d) temperature (a, b) and precipitation (c, d) differences between the IN2 and NN experiments
Fig. 14 The JJA geopotential
height [m] differences at
500 mb between the IN2 and
NN experiments
1752 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
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of the ice (Fig. 17). There is, however, a more complex
picture in North America, with an upstream enhancement of
the precipitation and patches of change above the ice itself.
The annual and JJA moisture budget show a drying above
the North American ice sheet due to the increased elevation
and a lessened availability of water vapour due to the colder
air. The wave pattern seen in the precipitation is enhanced
in the JJA precipitation–evaporation, due to the effect of the
ice sheets on the evaporation rate.
There is a distinctive wave like pattern seen in 2 m air
temperatures in JJA across Eurasia, with a cooling of the
air directly above the ice, a warming to the east–southeast
and the southern rim of the Tibetan Plateau. This is also
seen in the annual mean, though less distinctively, though
still confirming the results of Yin et al. (2008).
The mean sea level pressure (MSLP) [mb] is heightened
in the IS2 run compared to the NN globally in the annual
mean. IS2–NN shows a lowering of the pressure over Eur-
asia during JJA, higher pressure of the northwest Pacific,
leading to an onshore shift in the precipitation (near Japan).
The 850 mb annual mean geopotential height difference
between IS2 and NN shows an increase of 45 m above the
North American ice and a belt of 20–30 m increase across
northern Russia. The combined effects of GHG, SST and ice
forcings lead to an increased JJA precipitation rate in
northeast China, where the Changma front has been
empowered (Fig. 18). A southwards shift in the precipitation
is seen in the West Pacific, which could possibly be due to the
cooling potential of the large North American ice sheet. DJF
shows a positive NAO-like temperature anomaly over the
North Atlantic (Sundaram et al. 2011b), from the SST
forcing, though there is not warming over Eurasia, due to the
cooling influence of the ice sheet. The SST effect has
seemingly been trumped by the ice sheet impact.
The JJA shows, however, a lowering of the surface
pressure over Eurasia. With regards to the 850 mb potential
vorticity, the air is compressed above the North American
ice and stretched as it passes around the ice, hence the
increase in potential vorticity units (PVU)
[10-6 m2 s-1 K kg-1] surrounding the ice. There are
smaller changes in the potential vorticity induced by the
Eurasian ice. There is a slight compression above the ice
and 2PVU worth of divergence/ stretching to the west and
southeast of the added land ice. The 600 mb annual mean
omega (X) [mPa s-1] pattern shows again wave like
behaviour associated with the ice sheets with an increase
uphill and decrease on the down-slope of the ice continuing
into a further positive anomaly, which is similar to what
was found in LOVECLIM (Fig. 4a in Yin et al. (2008)).
5.4 Synergism
The synergism between ice sheets and sea surface tem-
peratures can be found by investigating IS2–NS–
IN2 ? NN, c.f. the method of Stein and Alpert (1993).
Synergism can be defined as the additional contribution
due to the variables when they are added together com-
pared to the sum of the individual contributions.
The synergetic effect leads to a strenghtening of the
polar jet at 65�N and subtropical jet at 35�N, whilst the
Fig. 15 The JJA potential vorticity [PVU = 10-6 m2 s-1 K kg-1] differences at 850 mb between the IN2 and NN experiments
Fig. 16 The July specific humidity, temperature and zonal wind
speed differences between IN2 and NN at 127�E (same as Fig. 5)
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1753
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subtropical July jet is weakened (Fig 20). This leads to a
slight enhancement of the Meiyu rain in Central China and
a reduction of the Changma (Fig. 20) and Baiu rain in
northeast China, Korea and Japan (Fig. 19c). The impact of
the ice sheets in the presence versus absence of the SST
forcing shows that the SSTs reduce the ice sheet impact on
the temperatures across Eurasia (Fig. 19a). The magnitude
of the ice sheet induced wave is dampened in the presence
of the SSTs.
5.5 Relative contributions of SST and ice sheets
to EASM
The JJA precipitation differences [mm/day] averaged over
eastern China (90–120�E; 23–40�N) (Fig. 21a) show that
the combined effects of the forcings result in a *1.7 mm/
day increase compared to the pre–industrial, whereupon
this is largely insolation driven, a result also found in the
LOVECLIM simulation (Yin et al. 2008). The pure con-
tribution from the SSTs result in a decrease of *0.3 mm/
day, whilst the ice sheets account for less than 0.1 mm/day.
The combined effects of the ice sheets and SSTs cause a
small increase entirely due to the synergism. This makes
for a 0.5 mm/day increase highlighting the importance of
Fig. 17 The JJA (a, c) and DJF (b, d) temperature (a, b) and precipitation (c, d) differences between the IS2 and NN experiments
Fig. 18 The July specific humidity, temperature and zonal wind
differences between IS2 and NN at 127�E
1754 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
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the mutual interactions of ice sheets and SST in the
atmosphere–ocean coupled system for explaining the
summer precipitation over eastern China.
In ARPEGE, the spatial structure of the JJA precipita-
tion (e.g. Fig. 17c for the total impact and 13c for the ice
sheets) shows northeastern China (120–130�E; 40–45�N)
(Northeastern, or Manchurian, Plain) responding signifi-
cantly to the forcings. Precipitation response to different
forcings is hence also discussed for this region (Fig. 21b).
The exact location for this domain is model specific to
ARPEGE, as it is where the model is exhibiting a response
to the forcing. LOVECLIM, on the other hand, has a centre
of precipitation increase due to ice sheets further south on
*30�N (Yin et al. 2008; Sundaram et al. 2011b). The
climatic response in the larger domain (90–120�E;
23–40�N) is to a lesser degree model dependent, as it
covers the typical large scale summer monsoon region. The
Changma front is responsible for the summer precipitation
in the northeastern region. Here the ARPEGE MIS–13
precipitation compared to pre-industrial is of 2.4 mm/day,
i.e. 0.7 mm/day more than eastern China. In this region, the
ice sheets contribute to the majority of this increase, i.e.
1.75 mm/day. The insolation and SSTs are equally
important for this regions in the MIS–13 summer rainfall
and with an *0.5 mm/day increase each. The interplay of
Fig. 19 The JJA and DJF temperature and precipitation differences from the synergisms between SST and ice sheets
Fig. 20 Same as Fig. 5, only for IS2–NS–IN2?NN i.e. the syner-
gism between SST and ice
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1755
123
ice sheets and SST (IS2–NS–IN2 ? NN) cause a drying of
0.4 mm/day, contrary to its wettening over East China.
Finally, the combined effects of ice and SSTs also lead to a
large increase (1.8 mm/day).
There are two main cyclogenesis regions for Asian lows
to develop (NAVPACMETO 1997). The southern lows are
generated by upper level trough movements over southern
and central China. These southern lows include the Tai-
wan-, Yellow Sea- and Shanghai Lows mainly affecting the
precipitation in east China and the Meiyu region. The
northern lows originate from the Icelandic Low, where-
upon the cyclones are guided by the jet stream southeast-
wards towards Mongolia and northern China in the vicinity
of Lake Baikal. The Manchurian-, South Mongolian- and
Lake Baikal Lows can be formed from the Icelandic Low.
These influence northeastern China and the Baiu–Changma
areas.
The ARPEGE experiments show that the southern lows
are mainly influenced by the insolation, whilst the northern
lows are more sensitive to the ice sheets and to a lesser
extent the SST forcing.
The large NH JJA insolation increase acts to deepen the
Asiatic summer thermal low, at the same time as the North
Pacific High is equally heightened. The thermal low
anchors the Intertropical Convergence Zone at the western
end. The driving force behind the monsoonal moisture flux
is therefore strengthened. The thermal forcing acts to shift
the ITCZ further north, including further inland China,
explaining the characteristics averaged over East China as
seen in Fig. 21a. The tropical SSTs are already very high,
so a small change to them at these latitudes will have less
of an impact on the moisture availability for the Meiyu
front.
At the northern lows, however, SSTs do have a stronger
impact. The warm SSTs of the West Pacific Warm Pool
feed the Meiyu rain, but for the Manchurian precipitation,
the subtropical and extratropical SSTs are important. The
SST forcing in the experiment experiment is of 1–2�C
higher north of 25�N in the Pacific, explaining the positive
correlation with the Manchurian precipitation. The
Changma Front is strengthened by the ice sheets increasing
the Manchurian Plain precipitation (see IN2–NN,
Fig. 21b). The Changma Front is characterised by zonal
winds at 300 hPa, with few jets moving through the area.
This fascilitates for a strong uplift potential. The trough
runs perpendicular to the Korean Peninsula, and stretches
from the west Pacific, through Japan, Korea to the foothills
of the Tibetan Plateau (Oh et al. 2007). High pressure
south–southeast of Japan provides moisture and heat fluxes
to feed the front. The mean sea level pressure south of
Japan is higher in the NS experiment compared to the NN,
explaining the strengthening of the Changma frontal pre-
cipitation as seen in 21(b). In its northern position the front
takes on a barotropic nature. Wang et al. (2000, 2001) also
identified the importance of the presence of an anti-
cyclonic anomaly in the Japan Sea/ northwest Pacfic for
stronger precipitation in the north east region. The contri-
bution of positive SST anomalies on the northeastern
monsoon precipitation was also identified, using the NCEP/
NCAR reanalysis data. These mechanisms were confirmed
in these results from the ARPEGE model.
The JJA zonal wind shear differences [m/s] at 200 hPa
between the experiments is shown in Fig. 22. The gradient
can be expressed as:
RM ¼ uð200 hPaÞ½40�50�N; 110�150�E�� uð200 hPaÞ½25�35�N; 110�150�E�: ð2Þ
after Lau et al. (2000), i.e. the zonal wind gradient between
the mid-latitude and sub-tropics, which is an expression of
the subtropical jet stream displacement. This is partly
caused by changes to the Hadley circulation locally, in turn
caused by the East Asian and Southeast Asian monsoonal
Fig. 21 The area averaged JJA
precipitation differences [mm/d]
between IS2–PI, NN–PI, NS–
NN, IN2–NN, IS2–NN and
(IS2–NS–IN2 ? NN).
Averaged over a East China:
90–120�E, 23–40�N, and
b northeastern China:
120–130�E, 40–45�N
1756 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
123
heat sources. The IS2–PI anomaly is of 8.8 m/s, i.e the jet
stream has moved much further north in the MIS–13 cli-
mate. The insolation and GHG forcings result in a RM
index of 5.4 m/s. Comparatively, the ice sheets contribute
to 3.7 m/s, similar to their contribution in the presence of
the SST forcing (IS2–NN). The NS–NN comparison results
in near–zero index, i.e. no apparent shift in the jet stream.
Which is also the result from the synergisms between the
various factors (IS2–NS–IN2 ? NN).
6 AOGCM results substantiating initial LOVECLIM
findings
The FAMOUS (Fast Met Office/U.K. Universities Simu-
lator) fully coupled AOGCM was used to follow up on the
MIS–13 EMIC results of Yin et al. (2008). FAMOUS uses
the same code as HadCM3 and is described in Smith et al.
(2008), Jones et al. (2005) as well as at http://www.
famous.ac.uk, with further OGCM description in Jones and
Moberg (2003). The atmosphere is run with a 1 h time
stepping scheme and a 5 9 7.5� horizontal resolution with
11 vertical levels. The oceanic component is the same as
for HadCM3L (Cox et al. 2001), with 20 vertical levels, a
12 h time step and 3.75 9 2.5� longitude–latitude resolu-
tion. The same sets of experiments were performed with
FAMOUS as previously done with LOVECLIM:
1. Pre-Industrial (PI) model run.
2. MIS–13 No Ice: the orbital parameters and the GHG
values are altered to those of 506 ka.
3. MIS–13 With Ice: orbital parameters and GHGs as in
Experiment 2, with additional ice sheets over North
America and Eurasia.
6.1 The MIS–13 no ice experiment
The preliminary results from the FAMOUS experiments
are summarised in this section and furthered in Muri et al.
(2011). They show the same general features as
LOVECLIM with regards to the GHG and insolation
forcing. This includes a JJA warming of the continents
accompanied by a northwards shift in the ITCZ, enhancing
the NH summer monsoons (Fig. 23). FAMOUS does not
however simulate much enhancement of the monsoon over
eastern China, as the main centre of amplification is over
northern India.
The FAMOUS MIS–13 No Ice SSTs display a larger
response to the forcing than LOVECLIM. A common
feature of the JJA SSTs is a warming of the North Pacific
Ocean along the Kuroshio and California Currents. The SH
ocean basins are all cooler at the surface in both
LOVECLIM and FAMOUS. With regards to the JJA North
Atlantic, again the amplitude of the response is larger in
FAMOUS. Both models display a MIS–13 cooling along
the storm tracks with a slighter warming south and north of
this, i.e. the models agree on the general response. The
FAMOUS JJA MSLP and geopotential height reveal that
there is a strong lowering of the pressure over land centred
over western Mongolia/north western China. This is cou-
pled to a heightening of the pressure over Bay of Bengal.
The geopotential at 850 hPa reveals a ridge of higher
pressure over the Japan Sea and far eastern Eurasia. These
regions of higher pressure are acting to prevent the mon-
soon enhancement in eastern China in FAMOUS. The
differences in the meso-scale climates between the models
are due to differences in the dynamics and physical rep-
resentations, in addition to resolution.
6.2 The MIS–13 ice sheet impacts
Comparing the FAMOUS MIS–13 experiment with ice to
the one without any extra land ice, reveals a localised JJA
cooling over and around the ice sheets. The cooling effect
is the largest in DJF, where most of the NH high latitudes
(north of *60�N) are colder in the With Ice experiment.
As for the precipitation; the JJA ITCZ across the Pacific
and Indian Oceans is stronger, a feature also seen in
LOVECLIM. The precipitation disturbances from the
Eurasian ice sheet propagate in a south easterly fashion in
FAMOUS, whereas LOVECLIM is somewhat more zonal
in its behaviour. There is a 0.5–1.5 mm/day JJA reduction
on the southern flank of the Eurasian ice sheet and a
0.5–3 mm/day drying southwest of the North American
one (Fig. 24a). On the northern flank of the Eurasian ice
sheet, there is a 0.5–1 mm/day increase in precipitation,
whilst the positive anomaly is north-northeast of the North
American ice sheet. This is due to an anomalous anti-
Fig. 22 RM: The JJA area averaged u shear differences [m/s] at
200 hPa between IS2–PI, NN–PI, NS–NN, IN2–NN, IS2–NN and
(IS2–NS–IN2 ? NN). Averaged over 110–150�E and differenced
40–50�N and 25–35�N
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1757
123
cyclonic flow around the ice sheets. The ITCZ precipitation
band across the Atlantic, including the North American
Monsoon region, is reduced by 1–3 mm/day. A drying is
seen on the southeast side of the Himalayas (Bhutan side).
The precipitation is indeed stronger over southern and
north western China, confirming the work of Yin et al.
(2008), where ice sheets were found to contribute to
enhance the EASM monsoon precipitation. The Eurasian
ice sheet introduces a gravity wave disturbance (Fig. 24b),
which influences the monsoon flow in FAMOUS.
Figure 25a emphasises how the large-scale model
response over China is comparable in ARPEGE (Fig. 21a),
FAMOUS and LOVECLIM, i.e. the MIS–13 monsoon
precipitation over China is stronger in all the experiments,
even in the presence of ice sheets. It also illustrates that the
exact regional response to the forcings in northeastern
China is model dependent (Fig. 25b vs. Fig. 21b). All three
models exhibit the same wave-like feature across Eurasia,
though the specific location of the resulting precipitation
increase is different between the models. The strength and
size of the induced high pressure anomaly in the Japan
region contributes to determine the precipitation increase
and location.
7 Discussion and concluding remarks
The relative importance of several factors on the MIS–13
climate are estimated in the analysis, using linear combi-
nations of a number of simulations. The Stein-Alpert
method yields quantitative isolation of the contributions
from ice sheets and SST and is here demonstrated as a
useful tool in climate analysis. The ‘‘pure’’ effect of one
factor, is relative, however, as it is actually referring to the
Fig. 23 The JJA temperature (a) and precipitation (b) [mm/day] differences between the FAMOUS MIS–13 No Ice and the PI experiments
(a) (b)
Fig. 24 a The JJA precipitation rate differences [mm/day] and b the JJA u-component of the gravity wave stress differences [kg m-1 s-2]
between the FAMOUS MIS–13 with ice and the MIS–13 no ice experiments
1758 H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate
123
effect of one particular factor separated from the other
represented factors (Stein and Alpert 1993) in comparison
with all the others. The results indicate how a difference
map of two simulations, considering more than one forcing
factor, can be somewhat deceptive.
We have shown how it is possible to still have strong
monsoon systems in climates with larger ice volumes than
present. Though the ice sheets have a significant cooling
effect, the astronomical forcing is so large that during
May to September, the continental heating south of the
ice sheets enhances the land–sea thermal contrast. This
works to reinforce the African and Asian monsoon sys-
tems in the MIS–13 climate. The insolation forcing alone
is enough to explain the warmer and more humid con-
ditions of the MIS–13 EASM as compared to the pre-
industrial indicated in the paleo-records. The impact of
ice sheets can be sensed on remotely located climates.
The ice sheets perturb the circulation as the air is forced
around and over the ice, triggering a wave-like feature
across Eurasia. The wave leads to an onshore shift in the
Changma front precipitation near Japan. The ice sheet
induced wave seems to break down the Okhotskian High
triggered by the SST perturbation. The southwards dis-
placement of the ITCZ precipitation in West Pacific could
be due to the cold potential of the North American ice
mass.
Whilst the insolational forcing is a strong driver of the
enhanced monsoon activity seen at 506 ka, the ice sheets
and SST forcing play more subdued and complicated roles.
The NS experiment shows how the SST and sea ice
induced North Atlantic surface temperatures trigger a
Rossby wave propagating across Eurasian, deepening its
JJA thermal low and inducing a blocking Okhotskian High,
leading to a southwards shift in the monsoon.
The tropical easterly jet is more important for the pre-
cipitation in the south, whilst the subtropical jet is more
influential in the north and both jets are strengthened in the
MIS–13 experiment compared to the PI.
The intermediate complexity model LOVECLIM was
used as first qualitative assessment of the relationship
between insolation, ice sheets and monsoon activity during
MIS–13 (Yin et al. 2008, 2009). Analysis was made with
the AGCM ARPEGE in this work to test the main
LOVECLIM results. ARPEGE simulations can be consid-
ered as more detailed simulations of the atmospheric
response to the MIS–13 climate, but these simulations are
forced with LOVECLIM SSTs so they share an important
ingredient with LOVECLIM. Both models find that the
high NH summer insolation contributes to a significant
increase in precipitation over East Asia, India, and North
Africa at MIS–13. In LOVECLIM, the combined effect of
ice sheets and their induced SST contributes to increase the
summer precipitation over eastern China. In ARPEGE, the
summer precipitation over east China is also reinforced by
the combined effect of SST and ice sheets, although the
precipitation increase is more important over northeastern
China. Anyway, the ARPEGE model confirms the results
found in Yin et al. (2008), showing how ice sheets can
counter-intuitively contribute to increase monsoonal
precipitation.
The FAMOUS AOGCM experiments, briefly presented
here, underpin the findings of LOVECLIM, including the
insolation driven MIS–13 monsoon and the cross-Eurasian
atmospheric disturbance which acts to further enhance the
monsoon regionally.
Compared to the proxy records, the GCM shows that the
cold Antarctic temperatures can be explained by the inso-
lation and ice sheet induced SST forcings. One might
speculate that the ice sheets at MIS–13 could have resided
in the SH rather than the NH, due to the cooler conditions
there. Hot NH summers create more unstable circum-
stances for ice sheets with large seasonal melting.
With regards to Ziegler et al. (2010)’s hypothesis that
the anomalous Arabian Sea fluxes at MIS–13 was caused
by a stronger North Atlantic overturning circulation rather
than enhanced monsoon activity, it has been shown that the
Fig. 25 The FAMOUS (bluebars) and LOVECLIM (redbars) area averaged JJA
precipitation differences [mm/d]
between MIS–13 with ice—pre-
industrial. MIS–13 no ice—pre-
industrial and the MIS–13
experiment with ice—the MIS–
13 without any added ice sheets.
Averaged over a East China:
90–120�E, 23–40�N, and
b northeastern China:
120–130�E, 40–45�N, c.f.
Fig. 21
H. Muri et al.: SST and ice sheet impacts on the MIS–13 climate 1759
123
ARPEGE GCM simulates a much stronger monsoon across
North Africa, the Arabian Peninsula, India and much of the
Asian continent. The meridional overturning circulation is
investigated by Sundaram et al. (2011a).
Acknowledgments We thank the anonymous reviewers for their
constructive comments. The EMIS project is funded by the ERC
Advanced Grant N�227348. The NCEP Re-analysis data was pro-
vided by the NOAA/ OAR/ ESRL PSD, Boulder, Colorado, USA and
obtained from their Web site at http://www.cdc.noaa.gov. Q. Yin is
supported by the Belgian National Fund for Scientific Research
(F.R.S.-FNRS). Access to computer facilities was made easier
through sponsorship from S.A. Electrabel, Belgium.
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