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

123

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

123

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

123

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

123

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

123

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

123

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

123

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

123

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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