Onset of the East Asia Monsoon in 1991 Simulated by an AGCM
K. Ninomiya, S. Emori, A. Hasegawa, T. Nishimura and S. Matsumura
Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology
Showa-machi Kanazawa-ku Yokohama 236-0001 Japan. E-mail:nmiya@jamstec.go.jp
Abstract
CCSR/NIES/FRCGC joint modeling group made a 20-year simulation for 1979-1998 by an AGCM
(primitive equation spectrum model T106L56; the triangular spectral truncation at wave-number 106
with 56 sigma levels) with the annually and seasonally varying observed sea surface temperature. This
study presents the features of the onset and seasonal evolution of the East Asia summer monsoon
simulated for 1991 in comparison with features found by observational studies for various years. TheAGCM can simulate the onset of the East Asia summer monsoon, which precedes the onset of the Indiansummer monsoon by ~1 month. The onset of the East Asia summer monsoon occurs in association withthe change in large-scale circulation over the continent heated by the sensible heat supply from thesurface.

Keywords: Onset of the East Asia summer monsoon. An AGCM simulation of the East Asia summermonsoon. Formation of the Meiyu and Baiu frontal zone.

1. Introduction

2. Model and experiment

Although the onset and the seasonal change

The model used in the present study is aof the East Asia monsoon are closely related withprimitive equation spectral model, which has

the evolution of the Indian summer monsoon,

56 sigma-levels and triangular spectralmany observational studies showed that the onset

truncation at wave-number 106 (T106L56). Itof the East Asia monsoon precedes the onset ofisthe Indian summer monsoon about one month. ItCCSR/NIES T42L20 that is developed by theis also known that the Meiyu and Baiu frontal

Center for Climate System Research and theprecipitation zone begins to shift northward

Nationalaround the early July while the Indian monsoon

Sciences (Numaguchi et al., 1997). Althoughwesterlies are sustained at the almost samethe physical processes of T106L56 are almost

latitude. Since the AGCM (atmospheric generalsame to these of T42L20, some tuning iscirculation model) simulation on the East Asia

needed when the resolution is increased tosummer monsoon has not been studied yet insimulate reasonable large-scale circulation

detail, it will be informative to compare the onsetsystems. Special tuning is not done for theof the East Asia summer monsoon simulated insimulation of the East Asia monsoon.

ahigh-resolution

Instituteversion

forof

theEnvironmental

the present AGCM with the onset described in

The 20-year integration for 1979-1998 withobservational studies. This simulation study will

the annually and seasonally varying observed sea

further our understanding on the East Asia

surfacesummer monsoon.

CCSR/NIES/FRCGC joint modeling group. The

temperatureisperformedbysimulated data of the basic atmospheric and landsurface variables recorded at 1-day interval, and

4. Features of the East Asia summer

additional variables averaged for each month aremonsoon simulated in the AGCM

used for the present study.

The onset and the seasonal changes of theEast Asia monsoon simulated in the AGCM are

3. Observed features of the East Asia

consistent with the observed features mentioned

summer monsoon

above. Figure 1 and Fig. 2 show the 10-day

The onset and the seasonal change of theaveraged precipitation and 850-hPa wind velocity

East Asia monsoon are closely related with thesimulated for 21-30 April and 21-30 June 1991,

evolutionofrespectively.

However,many observational

theIndiansummermonsoon.
(DingThe simulated East Asia summer rain season(1994), Kato (1985), Kato and Kodama (1992),

begins ~1 month before the onset of the IndianMatsumoto (1997), Ninomiya and Kobayashi

monsoon. The 850-hPa circulations in the onset(1998)) showed complicated onset process of thephase are characterized large-scale cyclonic flows

EastoverAsiamonsoon.

studies

Althoughtherearethecontinent.

During

21-30

June,

significant differences among individual years,

precipitations over the East Asia increase in

the following processes are found commonly inassociation with the increase of the Indian

many years;

monsoon westerly.

(1) Pre-summer rainfalls over Thailand begin in

Figure 3 and Fig. 4 show the 10-day averaged

middle-late April.

surface temperature and the vertical stability(2) The Meiyu-Baiu frontal precipitation zone

(thedifferenceofequivalentpotential

begins to extend from the south China to thetemperature between 850 and 500 hPa) for 21-30

Japan Islands in late April-early May.

April and 21-30 June 1991, respectively. The(3) Meridional thermal gradient over China

decrease of the meridional thermal gradient overdecrease rapidly in May in association with

China is owing to the rapid warming due to large

the rapid warming in the middle-northernsensible heat flux of ~75 W m−2 over the dry

latitudes due to the sensible heat supply fromregions in the middle-northern latitudes

the surface.

is not shown). In contrast to the decrease of the(figure(4) The vertical stability over China decrease

thermal gradient over China, fairly large thermal

rapidly in May.

gradient is sustained around the Baiu frontal(5) Precipitations over the Indochina Peninsula,zone nearby the Japan Islands. The vertical

Bay of Bengal and the South China Sea

stability over the China continent also decreases

increase in middle-late May.

rapidly.

(6) Precipitations over East Asia are intensified inThe

Indianmonsoonwesterlies

indicate

association with the increase of the Indian

maximum intensity during June and July. In thismonsoon westerly in middle June.

phase, the precipitations over the whole monsoon(7) The Meiyu and Baiu frontal precipitation zone

regions

attainmaximumintensity.

Strongbegins to shift northward early July, while

anticyclonic circulation at ~200 hPa with theIndian monsoon westerlies are sustained alonglargest heat source at ~500 hPa appears over~15N̊.

~27N̊ and 90E̊ (figure is not shown). The
Meiyu-Baiu frontal precipitation zone in themodel indicates the maximum extension andmaximum precipitation in June. The Meiyu andBaiu frontal precipitation zone in the model
begins to shift northward around the early July,
whiletheIndianmonsoonwesterlies

aresustained at the almost same latitude (figure isnot presented).

5. Conclusion
The AGCM can simulate the onset of the East
Asia summer monsoon, which precedes the onsetof the Indian summer monsoon by ~1 month. The
onset of the East Asia summer monsoon occurs inassociation

withthechange

inlarge-scale

circulation over the continent warmed by thesensible heat supply from the surface.

Fig.1: The 10-day averaged precipitation and
850-hPa wind velocity simulated for 21-30

References

April, 1991.

Ding, Y.-H., 1991: Monsoon over China, Kluwer
Academic Pub, 419 pp.
Kato, K., 1985: On the abrupt change in the structureof the Baiu front over the China Continent in late
May of 1979. JMSJ, 63, 20-36.
Kato, K. and Y. Kodama, 1992: Formation of thequasi-stationary Baiu front to the south of theJapan Island in early May of 1979. JMSJ, 70,631-647.
Matsumoto, J., 1997: Seasonal transition of summer
rainy season over Indochina and adjacent monsoon
region. Adv in Atom. Sci. 14, 231-245.
Ninomiya, K. and C. Kobayashi, 1998: Precipitationand moisture balance of the Asian summer
monsoon in 1991. Part 1: Precipitation and major
circulation systems. JMSJ, 76, 855-877
Numaguchi, A., M. Takahashi, T. Nakajima and A.
CCSR/NIESFig. 2: The 10-day averaged precipitation andatmospheric general circulation model. CGER

850-hPa wind velocity simulated for 21-30 June,

Supper Computer Monograph Report No.3, 1-48.

1991.

Sumi,

Center1997:

for

Description

GlobalofEnvironmentalResearch,

National Institute for Environmental Studies.

Fig. 3: The 10-day averaged surface temperatureand the vertical stability between 850 and 500
hPa for 21-30 June,1991.

Fig. 4: The 10-day averaged surface temperatureand the vertical stability between 850 and 500
hPa for 21-30 Juln, 1991