Simulation of the East Asian Summer Monsoon during the ... · PDF fileSimulation of the East Asian Summer Monsoon during the Last Millennium ... from the Maunder Minimum to today

Simulation of the East Asian Summer Monsoon during the Last Millenniumwith the MPI Earth System Model

WENMIN MAN

LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, and Graduate

University of Chinese Academy of Sciences, Beijing, China

TIANJUN ZHOU

LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

JOHANN H. JUNGCLAUS

Max Planck Institute for Meteorology, Hamburg, Germany

(Manuscript received 20 August 2011, in final form 16 May 2012)

ABSTRACT

The decadal–centennial variations of East Asian summer monsoon (EASM) and the associated rainfall

change during the past millennium are simulated using the earth system model developed at the Max Planck

Institute for Meteorology. The model was driven by up-to-date reconstructions of external forcing including

the recent low-amplitude estimates of solar variations. Analysis of the simulations indicates that the EASM is

generally strong during the Medieval Warm Period (MWP; A.D. 1000–1100) and weak during the Little Ice

Age (LIA; A.D. 1600–1700). The monsoon rainband exhibits a meridional tripolar pattern during both ep-

ochs. Excessive (deficient) precipitation is found over northern China (358–428N, 1008–1208E) but deficient(excessive) precipitation is seen along the Yangtze River valley (278–348N, 1008–1208E) during the MWP

(LIA). Both similarities and disparities of the rainfall pattern between the model results herein and the proxy

data have been compared, and reconstructions from Chinese historical documents and some geological ev-

idence support the results. The changes of the EASM circulation including the subtropical westerly jet stream

in the upper troposphere and the western Pacific subtropical high (WPSH) in the middle and lower tropo-

sphere are consistent with the meridional shift of the monsoon rain belt during both epochs. The meridional

monsoon circulation changes are accompanied with anomalous southerly (northerly) winds between 208 and508N during the MWP (LIA). The land–sea thermal contrast change caused by the effective radiative forcing

leads to the MWP and LIA monsoon changes. The ‘‘warmer land–colder ocean’’ anomaly pattern during the

MWP favors a stronger monsoon, while the ‘‘colder land–warmer ocean’’ anomaly pattern during the LIA

favors a weaker monsoon.

1. Introduction

The East Asian summer monsoon (EASM) is an im-

portant component in the global climate system. Its

anomalous behavior leads to deficient or excessive pre-

cipitation and hence causes great economic and social

losses in East Asian regions [see reviews by Wang (2006)

andZhou et al. (2009a)]. TheEASMexhibits considerable

variability on a wide range of time scales. Many studies

have focused on the interannual or interdecadal variability

of the EASM (Chang et al. 2000a,b; Wang et al. 2000;Wu

et al. 2003; Yu et al. 2004; Wu et al. 2009a,b; Zhou et al.

2009b; Li et al. 2010). However, the behavior of mon-

soon variability on the decadal–centennial time scale

during the last millennium is less examined and largely

unknown.

Proxy data derived from Chinese historical documents

and speleothem records have been used to reconstruct

the past EASM variability, as well as the spatial patterns

and temporal evolutions of precipitation over eastern

China (Wang et al. 1987; Qian et al. 2003; Zheng et al.

Corresponding author address: Dr. Tianjun Zhou, LASG, In-

stitute of Atmospheric Physics, Chinese Academy of Sciences,

Beijing 100029, China.

E-mail: [email protected]

7852 JOURNAL OF CL IMATE VOLUME 25

DOI: 10.1175/JCLI-D-11-00462.1

� 2012 American Meteorological Society

2006; Zhang et al. 2008). A dataset from a 120-station

drought/flood (D/F) index (a five-grade category index)

was derived fromChinese historical documents for A.D.

1470–1979 (CMA 1981). Based on the documentary

data, Wang et al. (1981) reported that the anomaly of

summer rainfall over northern China (358–428N, 1008–1208E) was usually opposite to that over the lower-

middle Yangtze River valley (278–348N, 1008–1208E).Based on the extended dataset of the D/F index for the

period of A.D. 950–1991, it was inferred that there was

more flooding in northern China during the Medieval

Warm Period (MWP), while a similar condition was

found along the Yangtze River valley during the Little

Ice Age (LIA) (Wang et al. 1987). The flood frequency

anomalies over theYellowRiver valley (338–408N, 1058–1208E) and southern China (228–298N, 1088–1208E) in

the warm season (May–September) during A.D. 991–

1999 indicate that the flood frequency was small (1.15

decade21) over the Yellow River valley and large (2.45

decade21) over southern China during A.D. 1400–1600,

which corresponds to a weakened EASM in this period.

The flood frequency markedly increased over the Yel-

low River valley and decreased over southern China

after A.D. 1650, indicating a stronger EASM (Qian et al.

2003). Zheng et al. (2006) found that the precipitation

variation in eastern China exhibited dry/wet fluctuations

on centennial time scales. Droughts dominated in the

twelfth to fourteenth centuries, but since the middle of

the seventeenth century eastern China has been more

subject to flooding. However, Zhang et al. (2008) in-

dicated that the EASM was strong during the MWP and

generally wet over eastern China, whereas the LIA was

characterized by a period of weak EASM and drought

over all of eastern China. The Swiss record of Alpine

glaciation also captures a generally strong summer Asian

monsoon during the MWP and the prominent glacial

advances correlate with a weakening summer monsoon

during the LIA (Holzhauser et al. 2005). Since the recon-

structions have to rely on relatively sparse data sources,

there exist controversial issues in understanding the

EASM variability and the associated rainfall patterns

over eastern China.

Climate models driven by external forcing agents can

provide important insights for detecting the monsoon

variability on the decadal–centennial time scale. Studies

of these issues will improve our understanding of the

physical processes that determine the long-term mon-

soon variations. These kinds of simulations have been

done using a wide range of climate models with different

levels of complexity for the last millennium (Crowley

2000; Gerber et al. 2003; Gonzalez-Rouco et al. 2003;

Goosse et al. 2005; Ammann et al. 2007; Peng et al. 2009;

Jungclaus et al. 2010; Servonnat et al. 2010). However,

previous studies of model–data intercomparison mainly

focused on variations of surface temperatures (Stouffer

et al. 2000; Zorita et al. 2005; Wagner et al. 2005; Zhang

et al. 2011) andmajor modes of climate variation such as

ENSO (Mann et al. 2005) or the North Atlantic Oscil-

lation (Shindell et al. 2003). There has been little attempt

to interpret the causes and dynamics of the decadal–

centennial EASMvariations using themodel simulations.

Liu et al. (2011) investigated the centennial–millennial

variation of the EASM precipitation over the past 1000

years through the analysis of a millennium simulation of

the coupled ECHAM and the global Hamburg Ocean

Primitive Equation (ECHO-G) model. The model re-

sults indicate that the centennial–millennial variation of

the EASM is essentially a forced response to the external

radiative forcing. The climate response of EASM to the

external radiative forcing depends on latitude. However,

multimodel intercomparisons are still needed to inves-

tigatewhether this phenomenon ismodel dependent. The

present study aims to examine the EASM changes during

the MWP and LIA by using the millennium simulations

with a comprehensive earth system model that, in con-

trast to previous studies, offers an (albeit small) ensemble

of simulations over the last millennium. The main moti-

vation of the study is to address the following questions:

1)What are the spatial structures of theEASMduring the

MWP and LIA? 2) What are the forced responses of the

East Asian summer rainfall during the two epochs? How

about the consistency between the simulation and proxy

data? 3) What is the dominant reason for the centennial

EASM changes? The remainder of the paper is organized

as follows. Section 2 provides a description of the model

and the experimental design, as well as the details of ex-

ternal forcings used in the simulations. Section 3 presents

the results. The conclusions are given in Section 4 along

with a discussion.

2. Model and data description

a. Model description

The present study is based on the millennium exper-

iments using the Max Planck Institute for Meteorology

Earth SystemModel (MPI-ESM) (Jungclaus et al. 2010).

The model consists of the atmospheric general circula-

tionmodel ECHAM5 (Roeckner et al. 2003) and theMax

Planck Institute OceanModel (MPI-OM;Marsland et al.

2003). Modules for land vegetation [Jena Scheme for

Biosphere–Atmosphere Coupling in Hamburg (JSBACH);

Raddatz et al. 2007) and ocean biogeochemistry [Hamburg

Model of the Ocean Carbon Cycle (HAMOCC);Wetzel

et al. 2006] enable the interactive simulation of the car-

bon cycle. ECHAM5 is run at T31 resolution (;3.758)

15 NOVEMBER 2012 MAN ET AL . 7853

with 19 vertical levels, resolving the atmosphere up to

10 hPa. MPI-OM applies a conformal mapping grid

with a horizontal resolution ranging from 22 to 350 km.

The ocean model includes a Hibler-type dynamic–

thermodynamic sea icemodel with viscous plastic rheology

(Hibler 1979). Ocean and atmosphere are coupled daily

without flux corrections using the Ocean Atmosphere Sea

Ice Soil version 3 (OASIS3) coupler (Valcke et al. 2003).

b. Experimental design and forcing data

The experimental strategy is described as follows.

After a multicentury spinup phase in which the carbon

cycle was brought into equilibrium, a 3000-yr unforced

control experiment was performed under A.D. 800 or-

bital conditions and preindustrial greenhouse gas con-

centrations. Starting fromdifferent ocean initial conditions,

a five-member ensemble (E1) with the standard external

forcing spanning A.D. 800–2005 was conducted using

the earth system model.

The total solar irradiance (TSI) forcing used as the stan-

dard forcing exhibits an increase of 0.1% (;1.3 W m22)

from the Maunder Minimum to today (Viera et al. 2011),

which is in agreement with other recent evaluations, al-

though other reconstructions with higher long-term varia-

tions also exist [see the discussion in Schmidt et al. (2011)].

The volcanic forcing is calculated online in the model

using time series of aerosol optical depth and of the ef-

fective radius (Crowley et al. 2008). Anthropogenic land

cover change is considered by applying the reconstruction

of global agricultural areas and land cover (Pongratz et al.

2008). While the CO2 concentration is calculated in-

teractively within the model, the concentrations of the

other two major greenhouse gases, methane (CH4) and

nitrous oxide (N2O), are prescribed (MacFarling Meure

et al. 2006). Some other potentially important forcings

such as the orbital forcing and anthropogenic tropo-

spheric sulfate aerosols are also included in the ensemble

experiments. The orbit forcing has little effect on the

magnitude of the seasonal cycle [see Jungclaus et al.

(2010) for details].

Effective radiative forcings (Fig. 1) are calculated

offline with the ECHAM5 isolated radiative transfer

code following the Wetherald and Manabe (1998) ap-

proach for calculating radiative feedbacks. The anom-

alous total radiative forcing, which represents the sum of

the solar forcing and the radiative effects of volcanic

aerosols, land cover change, and the CO2 concentration,

follows a high value during the MWP and a low value

during the LIA in the simulations (Fig. 1).

c. Data

The data used for validation of the model performance

under the present-day climate include the following:

1) The precipitation dataset compiled by Climate Pre-

diction Center (CPC) Merged Analysis of Precipita-

tion (CMAP) for the period of 1979–2005 on a 2.58 32.58 grid (Xie and Arkin 1995); and

2) The National Centers for Environmental Prediction

(NCEP) reanalysis 2 data (NCEP2) for 1979–2005 on

a 2.58 3 2.58 grid (Kanamitsu et al. 2002).

3. Results

We first present the summer [June–August (JJA)]

rainfall distribution and seasonal cycle of precipitation

on an observational basis for the model validation. Then

we focus on the large-scale monsoonal circulation and

precipitation changes during the MWP and LIA. Fi-

nally, we try to attribute the underlying causes of the

EASM variations from the perspective of land–sea

thermal contrast. In the following discussions, all the

anomalies are calculated relative to the climate mean of

the whole millennium.

FIG. 1. Effective radiative forcing anomalies at the top of the

atmosphere (W m22) associated with variations in solar irradiance

(black), volcanic aerosols (blue), CO2 emissions (orange), and land

cover changes (green). Anomalies from solar irradiance and CO2

variations are calculated relative to their preindustrial control

mean (1367 W m22 and 280 ppm, respectively). Solar forcing has

taken into account of Earth’s cross-sectional surface area (divided

by 4) and the contribution from the planetary albedo (simply use

a constant of 0.7).


a. Validation of the model performance

Faithful analysis of the EASM response to external

forcing should be based on rigorous verification of the

model performance (Liu et al. 2011). Comparisons

have demonstrated that the simulated Northern Hemi-

sphere temperature evolution and regional temperature

anomalies in China agree well with the reconstructions

(Jungclaus et al. 2010; Zhang et al. 2011).We focus on the

mean state of summer rainfall as well as the seasonal

march of the monsoon rain belt to evaluate the perfor-

mance of the model in monsoon simulation in this study.

Variations of the EASM are usually described using sum-

mer rainfall and low-level (850 hPa) winds (Zhou and

Yu 2005; Yu and Zhou 2007; Chen et al. 2010). Figures 2a

and 2b compare the simulated summer rainfall and 850-

hPa winds for the average of 1979–2005 with the observa-

tions. The observed precipitation data are derived from

CMAP, while the circulation data are from NCEP2. The

summer 850-hPa winds feature strong southwesterlies

from the Indian monsoon and southeasterlies from the

western Pacific, as well as the cross-equator flow around

1058–1208E (Fig. 2a). The observed summer precipitation

generally decreases northwestward from the Southeast

Asian marginal seas toward the arid central continental

Asia, and there is a monsoon rainband extending from

the East Asian marginal continents to Japan (Fig. 2a).

The model captures the main features of the observed

summer precipitation realistically except that it under-

estimates themonsoon rainband extending from eastern

China to Japan (Fig. 2b). The monsoon wind penetrates

into northern China in the model, resulting in a north-

ward bias of the monsoon rainband. An artificial rainfall

center located to the eastern periphery of the Tibetan

Plateau is also evident in the simulation, as in many at-

mospheric general circulation models (AGCMs) (Yu

et al. 2000; Zhou and Li 2002; Chen et al. 2010). The

pattern correlation coefficient of the summer precip-

itation between the observation and model simulation is

0.81, and the root-mean-square difference between the

observation and the simulation is 2.24 mm day21.

East Asian precipitation exhibits a robust seasonal

cycle associated with monsoon development (Zhou et al.

2009a). The seasonal cycles of extratropical (368–508N,

FIG. 2. (a),(b) Mean state of JJA precipitation (colored shading; mm day21) and 850-hPa winds (vectors; m s21)

and (c),(d) seasonal cycle of precipitation (mm day21). (a) CMAP (precipitation) and NCEP2 (850-hPa winds) data,

(b)model simulation, (c) seasonal cycle of regional (368–508N, 1008–1208E) average of precipitation, and (d) seasonalcycle of regional (218–358N, 1008–1208E) average of precipitation. All figures are for the average of 1979–2005.


1008–1208E) and subtropical (218–358N, 1008–1208E)precipitation are shown in Figs. 2c and 2d. In extratropical

East Asia (EA; Fig. 2c), the seasonal cycle is well simu-

lated with a peak rainy month in July and a minimum in

January, but the simulated spring rainfall is stronger than

that in the observation. In subtropical East Asia (Fig. 2d),

the simulation resembles the observation in the June peak

but the strength of precipitation prior to June is over-

estimated. Overall, the model performs well in the sim-

ulation of both the seasonal cycle and precipitation

amounts in different latitudinal regions of East Asia.

b. Response of EASM circulation and precipitationduring the MWP and LIA

Since the focus of the study is the dynamic structure and

physical processes of EASM variations on the decadal–

centennial time scale, we examine the features during the

MWP (A.D. 1000–1100) and LIA (A.D. 1600–1700),

which represent two typical climatic epochs of the last

millennium in China.We focus on the ensemblemean of

five realizations in the following analysis.

Anomalies of JJA mean 850-hPa winds and pre-

cipitation during the MWP and LIA are shown in Fig. 3.

The MWP (LIA) is characterized by the strengthening

(weakening) of the 850-hPa southwesterly winds, in-

dicating a generally stronger (weaker) EASM. This re-

sult is consistent with the reconstruction from a stalagmite

record in the Wanxiang Cave, China, which indicates

a strong (weak) EASM during the MWP (LIA) (Zhang

et al. 2008).

The anomalies of monsoon rainfall exhibit a meridio-

nal tripolar pattern during both epochs. Excessive (de-

ficient) precipitation is evident over northern China

(358–428N, 1008–1208E) whereas deficient (excessive)

precipitation is seen along the Yangtze River valley

(278–348N, 1008–1208E) during the MWP (LIA). The

rainfall patterns are consistent with the reconstructions

from Chinese historical documents (Wang et al. 1987;

Qian et al. 2003), which suggest that changes of rainfall

along the Yangtze River valley are generally out of phase

from those over northern China, but different from the

reconstruction of the Wanxiang record (Zhang et al.

2008), with wet (dry) conditions over all of eastern China

during the MWP (LIA). The reason for this discrepancy

deserves further investigations. Some geological evidence

also supports more precipitation over northern China

during theMWP (Ren and Zhang 1996;Wu and Lu 2005;

Cao et al. 2004). For the rainfall along the Yangtze River

valley, the reconstructed 3000-yr precipitation curve from

the Longgan Lake (298509–308059N, 1158559–1168209E)shows that a drier climate locally occurred in most of the

MWP (Tong et al. 1997). Our result is also in accordance

with the modern definitions of the EASM in that when

southerly monsoon penetrates deeply into northern

China, extratropical EA has plentiful rainfall, but the

subtropical rainfall (known as mei-yu in China, Baiyu in

Japan, and Changma in Korea) tends to be suppressed

(Ding 1992). Note this definition is different to that

from the stalagmite proxy data (i.e., a strong EASM

often means an abundant mei-yu; Wang et al. 2005).

Our model result agrees with the observed structure on

interannual time scale. We also calculate the ensemble

spread of precipitation during the two epochs, and define

the signal-to-noise ratio as the absolute value of the en-

semble mean value dividing the ensemble spread (Zhou

and Yu 2006). The signal-to-noise ratio is greater than

1.0 over most parts of the EA region (not shown), in-

dicating that the magnitude of the forced signal is larger

than the model spread, which support the robustness of

the responses in our simulation.

FIG. 3. Anomalies of JJA mean precipitation (colored shading;

mm day21) and 850-hPa winds (vectors; m s21) for (a) the MWP

and (b) the LIA. The anomalies are calculated relative to the

millennial mean value. The regions shaded by black dots denote

areas with precipitation that are statistically significant at the 5%

level by using a Student’s t test.


c. The horizontal circulation of EASM during theMWP and LIA

The water vapor transport is crucial to monsoon

rainfall. Since the vertically integrated water vapor

transport is dominated by the lower troposphere (Zhou

and Yu 2005), an analysis of water vapor transport at

850 hPa is reasonable. There are three main branches

of climatological 850-hPa water vapor transport to EA

(Fig. 4a): a strong transport by the southwesterlies from

the Indian monsoon, a moderate transport by the South-

east Asian monsoon from the western Pacific, and a weak

transport of cross-equator flow straddling 1058–1508E.Anomalies of JJA mean 850-hPa water vapor transport

during the MWP and LIA are shown in Figs. 4b and 4c.

The anomalous water vapor transport by both the south-

westerlies from the Indian monsoon and the southeast-

erlies from the western Pacific are positive during the

MWP and, in addition to the anomalous southerly winds

over EA, the northward transport of tropical water vapor

to northern China has enhanced, but the water vapor

convergence over the Yangtze River valley has decreased

(Fig. 4b). This leads to excessive rainfall in northernChina

and deficient rainfall in central China along the Yangtze

River valley. The northward moisture transport has re-

duced during the LIA and there is more water vapor

convergence over the Yangtze River valley (Fig. 4c), re-

sulting in excessive rainfall in central China but deficient

rainfall in northern China.

To quantify the water vapor transports, estimations of

the atmospheric water budget across the four bound-

aries of northern China and the Yangtze River valley

have been calculated according to Li et al. (2009). During

the MWP, the 850-hPa net influx in northern China was

6.25 3 106 kg s21, which was much larger than that over

the Yangtze River valley (2.123 106 kg s21), resulting in

excessive rainfall in northern China but deficient rainfall

in central China along the Yangtze River valley. During

the LIA, the 850-hPa net influx in northern China (1.213106 kg s21) was much smaller than that over the Yangtze

River valley (6.05 3 106 kg s21). This leads to deficient

rainfall in northern China and excessive rainfall along the

Yangtze River valley.

The water vapor transport is closely linked to the mon-

soon circulation change. Previous studies have found that

the EASM is greatly controlled by the western Pacific

subtropical high (WPSH) in the middle and lower tro-

posphere and the subtropical westerly jet stream in the

upper troposphere (Tao and Chen 1987). The position,

shape, and strength of the WPSH dominate the large-

scale quasi-stationary frontal and associated rainband in

EA (Tao and Chen 1987; Ding 1994; Zhou and Yu 2005).

TheWPSH is conventionallymeasured by the geopotential

height at 500 hPa (Tao and Chen 1987; Ding 1994; Zhou

et al. 2009c). In Fig. 5, we present the changes of geo-

potential height of 500 hPa for both the MWP and LIA.

It is clear that the ridge of the geopotential isolines shift

northward (southward) during the MWP (LIA) com-

pared with the climate mean position (Figs. 5a,b). The

northward (southward) shift of the WPSH ridge during

the MWP (LIA) leads to stronger (weaker) southerlies

penetrating into northern China, resulting in deficient

(excessive) precipitation along the Yangtze River Val-

ley but excessive (deficient) precipitation over northern

China.

At the upper levels (200 hPa), the subtropical west-

erly jet is an important part of the Tibetan high, the

position and strength of which is closely related to the

EASM rainfall (Zhang et al. 2006). The most out-

standing features of the climatological EASM at 200 hPa

FIG. 4. JJAmean 850-hPawater vapor transport (kg m21 s21) for

(a) the millennial mean, (b) the MWP, and (c) the LIA. The

anomalies of the MWP and LIA are calculated relative to the

millennial mean value. The gray shading denotes regions with

meridional components that are statistically significant at the 5%

level by using a Student’s t test.


are the westerly jet stream centered along 408N and the

tropical easterly jet to the south of 258N (Fig. 6a). Anom-

alies of JJA mean 200-hPa zonal winds during the MWP

and LIA are shown in Figs. 6b and 6c. The subtropical

westerly jet exhibits apparent meridional shifts during the

two epochs. There is a significantly weakened (inten-

sified) westerly south to the jet axis and an intensified

(weakened) westerly north to the jet axis during the

MWP (LIA). This corresponds to the northward (south-

ward) shift of the monsoon rain belt and is generally ac-

companied by excessive (deficient) rainfall over northern

China but deficient (excessive) rainfall along the Yangtze

River valley (Lau et al. 1988; Liang and Wang 1998; Li

et al. 2004; Zhou and Yu 2005).

d. Meridional monsoon circulation changes duringthe MWP and LIA

The Hadley cell is a thermally driven meridional cir-

culation, which is characterized by a rising motion in the

tropics and a descending motion in the subtropics. The

normal Hadley cell in the East Asian monsoon region is

replaced by a meridional circulation of the opposite

sense, which is often referred to as the monsoonal me-

ridional cell (Chen et al. 1964; Ye and Yang 1979) and

has been used as observational metric in model evalu-

ations (Zhou and Li 2002; Chen et al. 2010). To examine

the meridional structure of the EASM during the MWP

and LIA, Fig. 7 shows a meridional–vertical cross sec-

tion of JJA mean winds along 1058–1228E. The clima-

tological map exhibits a strong upward motion over

Southeast Asia (;58–308N) and a weak ascent north of

about 408N(Fig. 7a). The anomalousmeridionalmonsoon

FIG. 5. Spatial distributions of JJA mean geopotential height at

500 hPa for (a) the MWP and (b) the LIA (both shown in color).

The results of the climatological millennial mean are shown in

long-dashed black. Contour interval is 10 gpm.

FIG. 6. JJA mean 200-hPa zonal wind (m s21) for (a) the mil-

lennial mean, (b) the MWP, and (c) the LIA. The anomalies of the

MWPandLIA are calculated relative to themillennial mean value.

The shading denotes regions that are statistically significant at the

5% level by using a Student’s t test.


circulation during the MWP shows upward motion over

the extratropical EA and downward motion over the

subtropical EA. This corresponds to anomalous low-level

southerlies between 208 and 508N (Fig. 7b). Anomalous

descending and ascending motion respectively dominate

the northern land and southern ocean areas during the

LIA, with anomalous northerly winds between 208 and508N (Fig. 7c). The meridional circulation changes in the

monsoon region are consistent with an enhanced (weak-

ened) EASM during the MWP (LIA).

e. The driving mechanisms of EASM change:Land–sea thermal contrast

The occurrence of the EASM variability is a conse-

quence of the atmospheric response to the diabatic

heating between the ocean and the land (Li and Yanai

1996). The spatial structure of the monsoon response can

be understood in terms of the effects of land–sea thermal

contrast. Studies of the monsoon variability from this

perspective can help us understand the physical and dy-

namical processes that determine the long-termmonsoon

variations.

The land–sea thermal contrast is attributable to ex-

ternal forcings. We term the effective radiative forcing

from the external drivers as the sum of the effective solar

irradiance, the radiative effects of volcanic aerosols, the

land-cover changes, and greenhouse-gas forcing. All of

the individual effective radiative forcings are calcu-

lated offline with the ECHAM5 isolated radiative

transfer code. The effective solar irradiance has been

multiplied by planetary albedo (we simply use a con-

stant of 0.7) and divided by 4. The effective radiative

forcing anomaly between the MWP and LIA differ by

0.23 W m22 in the simulation. The calculation of the in-

dividual component differs by 0.03 W m22 for the solar

forcing and 0.19 W m22 for the volcanic forcing over the

100-yr periods for the MWP and LIA. The result in-

dicates that volcanic forcing has a major contribution for

the total effective radiative forcing difference between

the MWP and LIA. The anomaly of the individual ef-

fective radiative forcing is20.01 W m22 for solar forcing

and 20.26 W m22 for volcanic forcing over the 100-yr

periods for the LIA compared with the long-term mean.

The calculation of the mean radiative forcing for the

FIG. 7. Latitude–height cross section of JJA mean meridional

circulation averaged over 1058–1228E (units of the vertical and

meridional velocity are 21024 hPa s21 and m s21, respectively)

for (a) the millennial mean, (b) the MWP, and (c) the LIA. The

anomalies of the MWP and LIA are calculated relative to the

millennial mean value. The gray shading denotes regions with

vertical components that are statistically significant at the 5% level

by using a Student’s t test.


LIA further indicates that the LIA is dominated by the

volcanic forcing. When the effective radiative forcing

increases during the MWP because of the different

thermal capacity of the land and ocean, the temperature

increases over theEastAsian continent, especially in the

midlatitude, more rapidly than over the adjacent ocean,

which produces the land–sea thermal contrast during the

period.

The tropospheric mean temperature is a reasonable

indicator of thermal contrast change (Zhou and Zou

2010). The tropospheric mean (200–500-hPa average)

temperature anomalies during the MWP and LIA are

shown in Fig. 8. Warm temperature anomalies prevail

over EA with a central magnitude of 0.28C during the

MWP, while cool anomalies are seen in the tropical

western Pacific and extratropical North Pacific with a

central value of 20.258C (Fig. 8a). There exist cold

anomalies with an amplitude up to20.38C over EA and

warming anomalies with a central magnitude of 0.258Cin the tropical western Pacific and extratropical North

Pacific during the LIA (Fig. 8b). The magnitude of tem-

perature changes during the LIA is stronger than the

MWP. The position of the cooling center over EA ex-

hibits a northward displacement during the LIA. Since

the mean state of summertime tropospheric mean tem-

perature features a ‘‘warm land–cold ocean’’ condition,

the ‘‘warmer land–colder ocean’’ anomaly pattern during

the MWP favors a strong EASM circulation, while the

‘‘colder land–warmer ocean’’ anomaly pattern during the

LIA favors a weak EASM circulation. Thus the land–sea

thermal contrast change is the fundamental driver of

EASM changes.

The EASM change is caused by both zonal thermal

contrast between EA and the North Pacific and merid-

ional thermal contrast between EA and the tropical

western Pacific (Fig. 8). To have a clear picture of both

zonal and meridional land–sea thermal contrast change,

the corresponding structures at vertical cross sections are

shown in Figs. 9 and 10. The zonal land–sea thermal

contrast along 308–458Ndepicted by the height–longitude

cross section (Fig. 9) exhibits a signature of warmer land–

colder ocean during theMWP, with warmer anomalies of

0.28C over the Eurasian continent extending from 608 to1208E. The cooler anomalies with a central magnitude of

20.28C are seen over the ocean area in the middle-upper

troposphere, extending from 1208E to 1508W. Tempera-

ture anomalies of almost reversed sign are evident during

the LIA, with negative anomalies over land extending

from 758 to 1208E and positive anomalies with an am-

plitude up to 0.38C over an ocean area extending from

FIG. 8. JJA mean upper-tropospheric (500–200 hPa) tempera-

ture anomalies (8C) for (a) the MWP and (b) the LIA. The

anomalies are calculated relative to the millennial mean value. The

black stippled regions denote areas that are statistically significant

at the 5% level by using a Student’s t test.FIG. 9. Longitude–height cross section of JJA temperature av-

eraged over 308–458N (8C) for (a) the MWP and (b) the LIA. The

anomalies are calculated relative to the millennial mean value. The


at the 5% level by using a Student’s t test.


1208E to 1508W. The simulated colder land is weaker in

magnitude (;0.18C) and narrower in zonal extent, ex-

hibiting the maximum magnitude in the middle-lower

troposphere below 500 hPa. However, the warmer anom-

alies over ocean area show a deep vertical structure and

penetrate throughout the troposphere.

The height–latitude cross section measuring the me-

ridional land–sea thermal contrast also exhibits a warmer

land–colder ocean structure during the MWP (Fig. 10a).

The ‘‘warmer land’’ is evident with its maximumwarming

of 0.38C around 300–500 hPa extending from 308 to 608N.

The temperature over the ocean is also warmer during

the MWP, but the magnitude is weaker than that over

the land, so the land–sea thermal contrast still increases.

The colder land–warmer ocean structure is evident in

the troposphere during the LIA (Fig. 10b), with colder

anomalies of 20.28C over land extending from approx-

imately 308–608N and warmer anomalies of 0.28C over

ocean extending from 08 to 308N.

Following the changes of temperature gradients, anom-

alous descending and ascending motion dominate the

northern land and southern ocean areas, respectively, with

anomalous low-level northerly winds between 208 to 508N.

The strongest cooling center is around the 300-hPa level

with a magnitude of 20.28C. The cooling center results

in a southward shift of high-level subtropical westerly jet

while enhancing the anomalous low-level northerlies,

similar to what happened in the later twentieth century

associated with a weakened EASM (Yu et al. 2004; Yu

and Zhou 2007). In addition, the change of meridional

temperature gradient is also consistent with the changes

of subtropical westerly jet shown in Fig. 6, a poleward

increase (decrease) of tropospheric temperature is fol-

lowed by a weakened (intensified) westerly, based on

the principle of thermal wind balance (Zhang et al.

2006).

In summary, the spatial structure of land–sea thermal

contrast and the associated circulation changes reason-

ably explains an enhanced (weakened) EASM during

the MWP (LIA), which is further confirmed by the dif-

ferences between MWP and LIA (Fig. 11), namely that

the stronger EASM during MWP relative to LIA is

dominated by the enhanced land–sea thermal contrast.

The spatial pattern of MWP–LIA tropospheric mean

temperature differences reveals warmer anomalies with

an amplitude up to 0.458Cover EAand colder anomalies

with a central magnitude of 20.58C in the tropical west-

ern Pacific and extratropical North Pacific. Following the

land–sea thermal contrast change, the southerlies pene-

trate northward into higher latitudes in the MWP than in

the LIA.

4. Summary and discussion

a. Summary

The EASM changes and the corresponding rainfall

patterns over EA during the MWP and LIA are ana-

lyzed by using the output of the MPI Earth System

Model. The association between the EASM and land–

sea thermal contrast changes is studied. Themain results

are summarized below:

1) The EASM during the MWP is stronger than that

during the LIA, as the proxy data indicated. Follow-

ing the intensified (weakened) EASM during the

MWP (LIA), the summer precipitation over eastern

China exhibits a meridional tripolar pattern. Exces-

sive (deficient) rainfall is found over northern China

but deficient (excessive) rainfall along the Yangtze

River valley during the MWP (LIA). Both similari-

ties and disparities between our model results and

the available estimates from the proxy data have been

compared: reconstructions from Chinese historical

documents and some geological evidence support our

results, but reconstructions from the Wangxiang re-

cord show disparities with the model results.

FIG. 10. Latitude–height cross section of JJA temperature av-

eraged over 1058–1228E (8C) for (a) theMWPand (b) the LIA. The

anomalies are calculated relative to themillennial mean value. The


at the 5% level by using a Student’s t test.


2) The northward water vapor transport has enhanced

(reduced) during theMWP (LIA), which leads to less

(more) moisture convergence and thus deficient

(excessive) rainfall along the Yangtze River valley

but excessive (deficient) rainfall in northern China.

Both the changes of the subtropicalwesterly jet stream

in the upper troposphere and theWPSH in themiddle

and lower troposphere are consistent with the merid-

ional shift of themonsoon rainbelt during both epochs.

A stronger (weaker) WPSH along with a northward

(southward) shift of the subtropical westerly jet stream

is evident in the warm (cold) period.

3) The meridional monsoon circulation changes show

anomalous ascending (descending) and descending

(ascending) motion respectively over the northern

land and southern ocean areas during theMWP (LIA).

An anomalous southerly (northerly) wind is seen

between 208 and 508N, which corresponds to an en-

hanced (weakened) summer monsoon.

4) The land–sea thermal contrast changes caused by the

effective radiative forcing lead to the MWP and LIA

monsoon changes. The EASM is dominated by both

the zonal thermal contrast between EA and the

North Pacific and the meridional thermal contrast

between EA and the tropical western Pacific. The

‘‘warmer land–colder ocean’’ anomaly pattern dur-

ing the MWP favors a strong EASM, while the

‘‘colder land–warmer ocean’’ anomaly pattern dur-

ing the LIA favors a weak EASM.

b. Discussion

Based on the output of MPI Earth System Model

millennial climate simulations, our analysis shows that

the EASM during the MWP is stronger than that dur-

ing the LIA. There is excessive (deficient) rainfall in

northern China but deficient (excessive) rainfall along

the Yangtze River valley during the MWP (LIA). We

compared our results with the available estimates de-

rived from the proxy data. Reconstructions from Chi-

nese historical documents and some geological evidence

support our results, but reconstructions from the Wan-

xiang record show disparities with the model results.

The reason deserves further investigation.

The model output analyzed in Liu et al. (2011) was

obtained from the millennium integrations of the cou-

pled ECHO-G model, which consists of the spectral at-

mospheric model ECHAM4 and the global ocean

circulationmodelHOPE-G. The simulationwas forced by

three external forcing factors: solar variability (Crowley

2000), greenhouse gas concentrations in the atmosphere

including CO2 (Etheridge et al. 1996) and CH4 (Blunier

et al. 1995), and an estimated radiative effect of volcanic

aerosols (Robock and Free 1996). Note that the level of

solar irradiance used to drive themodel in Liu et al. (2011)

exhibits larger change in comparison to the ‘‘state of the

art’’ estimates for solar variability applied in this study.

Both our result and the work by Liu et al. (2011) indicate

that centennial variation of the EASM is essentially a

forced response to the external radiative forcing. Besides

the similarities, however, there are also disparities be-

tween the two simulations. The results from our study

indicate that the summer precipitation over eastern China

exhibits a meridional tripolar pattern, which is charac-

terized by an out-of-phase relationship between the sub-

tropical and extratropical rainfall. This feature is consistent

with the observed structure on the interannual time scale.

The subtropical and extratropical rainfall increases si-

multaneously in Liu et al. (2011). This feature implies

that the forced mode is characterized by an in-phase

relationship between the subtropical and extratropical

rainfall. The main discrepancy in the rainfall between our

study and the work by Liu et al. (2011) is in the Yangtze

River valley whereas the extratropical (corresponding

to Northern China) response is consistent. Since the

warming in high latitudes is much larger than that over

the tropical regions during theMWP, both of themodels

FIG. 11. The differences between MWP and LIA (MWP minus

LIA) for (a) JJA mean precipitation (colored shading; mm day21)

and 850-hPa winds (vectors; m s21) and (b) JJA mean upper-tro-

pospheric (500–200 hPa) temperature (8C). The black stippled

regions denote areas that are statistically significant at the 5% level

by using a Student’s t test.


show enhanced southerly winds across the subtropical

and extratropical monsoon regions, indicating that the

responses of the circulations within the two models are

similar. However, the precipitation could differ signifi-

cantly even though the atmospheric circulations are the

same between different models. We suggest that the dis-

parities of the precipitation in the two simulations could

be model dependent; thus, multimodel intercomparisons

are needed in future studies. We should note that the

subtropical region (218–358N, 1008–1208E) in Liu et al.

(2011) covers southern China and the Yangtze River

valley, and excessive (deficient) rainfall is found over

southern China but deficient (excessive) rainfall along

the Yangtze River valley during the MWP (LIA) in our

simulation. If we calculate the regional-mean precip-

itation value for the subtropical region defined by Liu

et al. (2011), we also find that subtropical precipitation

was strong during MWP and weak during LIA.

We use the model output from the full-forcing exper-

iments in this study, so it is difficult to assess responses

from one specific forcing component. However, exami-

nation of the specific response fromone individual forcing

is quite important; for example, the land-use forcing could

be an important aspect for regional climate during the last

millennium. By using an atmospheric general circulation

model, Takata et al. (2009) suggested that the land cover/

use change between 1700 and 1850 could result in the

weakening of theAsian summermonsoon through changes

in the energy and water balance at Earth’s surface. Thus,

the effect of land-use forcing on the regional climate de-

serves further study by experiments in which just one

forcing component is applied.

A map with the change in land use between the MWP

and LIA is further provided in order to discuss possible

local reinforcement of the land–sea contrast due to land use

between the two periods (Fig. 12). Large areas of crop-

land were deduced for China during theMWP (Fig. 12a).

The spread of crops further developed during the LIA

compared with the MWP (Fig. 12b). The distribution

of pasture was quite different from that of cropland

(Figs. 12c,d). The pasture lands during both periods

were mainly located in Mongolia and Tibet, where herd-

ing was the traditional form of agriculture. Many parts of

eastern China showed little pasture area during those two

FIG. 12. Historical cropland and pasture area during the MWP and LIA: (a) cropland for the MWP, (b) cropland

for the LIA, (c) pasture for the MWP, and (d) pasture for the LIA. Units are percentage of grid cell. Values less than

1% are white.


periods. The total areas of cropland and pasture were

larger during theMWP than that during the LIA over this

region. The land-use change would favor a land–sea

thermal contrast change between the two periods.

We state the major features of symmetries between

the patterns during theMWPandLIA, and it is interesting

to see that it is not entirely symmetrical. The anomaly

of the effective radiative forcing is 20.03 W m22 for the

MWP and 20.26 W m22 for the LIA compared to the

long-term mean value, which is not symmetrical in am-

plitude between MWP and LIA. The asymmetry of the

effective radiative forcing between MWP and LIA is

consistent with the asymmetrical responses between the

two typical periods. The possible reasons for the asym-

metry in the patterns are both important and interesting.

The asymmetry between theMWPandLIA in the rainfall

pattern displays as significant negative–positive–negative

anomaly pattern extending from northwestern China to

the East Asian marginal continents. Corresponding to the

rainfall pattern, a wave train structure is evident with neg-

ative and positive anomalies by turns in the geopotential

height at 200 hPa (not shown). The wave structure of the

200-hPa geopotential height is consistent with the asym-

metry between the MWP and LIA in the rainfall pattern.

The asymmetry of the patterns and possible reasons for

the asymmetry still deserve further diagnosis in our fu-

ture study.

Additionally, our study shows evidence that the changes

of land–sea thermal contrast associated with the effec-

tive radiative forcing dominate the EASM response.

Previous studies suggest that the direct radiative effect

of solar forcing variations on the monsoon change is re-

latively weak and that dynamical responses may be more

important (Fan et al. 2009). Major climate modes during

different epochs, such as ENSO and the Pacific decadal

oscillation (PDO), may also contribute to the changes of

EASM and deserves further study. In addition, the cur-

rent diagnosis is basedon theMPImodel driven by a solar

irradiance reconstruction of weaker variability, and thus

the forcing of effective solar irradiance to EASM varia-

tion on decadal–centennial time scales may be under-

estimated. To account for uncertainty in the solar forcing,

another set of three-member ensemble simulations (E2)

with an alternative solar irradiance reconstruction of

stronger variability has been done inMPI and the results

will be used in our future diagnosis.

Furthermore, the EASM changes during the MWP

and LIA are primarily dominated by the natural vari-

ability, such as solar and volcanic forcing variability dur-

ing the last millennium. However, the future monsoon

variations could be affected by human activities, such as

anthropogenic forcings. Further, the EASM was stronger

during the MWP, whereas the monsoon has weakened

during the latter half of the twentieth century when the

warming was rapid. Further study is necessary to un-

derstand the reasons behind the EASM changes under

two similar climate backgrounds. Thus, it is not appro-

priate to suggest that the monsoon variability during the

last millennium is a possible analog for future monsoon

changes.

Acknowledgments. This work was jointly supported

by the National Natural Science Foundation of China

under Grants 40890054 and 41125017 and the Visiting

Student Program of the International Max Planck Re-

search School on Earth SystemModelling (IMPRS-ESM)

at theMax Planck Institute forMeteorology (MPI-M) in

Hamburg, Germany. The IMPRS-ESMVisiting Student

Program is funded by the ZEIT Foundation ‘‘Ebelin and

Gerd Bucerius,’’ Hamburg. We thank Drs. Chao Li and

Hongmei Li at MPI-M for the helpful comments and

discussion.

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Simulation of the East Asian Summer Monsoon during the ... · PDF fileSimulation of the East Asian Summer Monsoon during the Last Millennium ... from the Maunder Minimum to today