Characteristics and evolution of atmospheric circulation patterns during Meiyu over the Jianghuai...

Asia-Pacific J. Atmos. Sci., 48(2), 145-152, 2012

DOI:10.1007/s13143-012-0014-6

Characteristics and Evolution of Atmospheric Circulation Patterns during Meiyu

over the Jianghuai Valley

Danni Liu1, Jinhai He

1, Yonghong Yao

2, and Li Qi

1

1Jiangsu Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China2School of the Atmospheric Science of Nanjing University, Nanjing, China

(Manuscript received 19 April 2011; revised 25 November 2011; accepted 18 December 2011)© The Korean Meteorological Society and Springer 2012

Abstract: This study focuses on the evolution of large-scale

circulations before and after the beginning of the Meiyu and analyzes

the formations of the typical vertical circulation pattern associated

with Meiyu and its relationship with the Meiyu rainband. Results

show that the typical vertical circulation pattern during the Meiyu

season is characterized by the "two-leg"-type pattern in the vorticity

field, a typhoon eye-like structure of the equivalent potential tempera-

ture field, as well as the sharp gradient of the equivalent potential

temperature zone, i.e., the Meiyu front are typically presented in the

Meiyu season. Tracking its evolution process, we find that the typical

vertical circulation pattern is built at late March and early April

along with the rainband locating at the area south to the Yangtze

River. This typical pattern and the rainband both advance northward

affecting Jianghuai valley since the beginning of Meiyu. Moreover,

the typical vertical circulation pattern derived from Meiyu season has

been formed in April and corresponds to the reverse of the land-sea

thermal contrast between the Eastern Asia and western Pacific Ocean,

demonstrating the close relationship of the movement between the

rainband and the march of the East Asian subtropical summer

monsoon.

Key words: Meiyu, circulation pattern, East Asian subtropical summer

monsoon

1. Introduction

Meiyu is a long-lasting rainy season characterized by a quasi

stationary rainband over the middle and lower reach of the

Yangtze River from mid-June to early or mid-July in eastern

China (Zhu et al., 2000). The rainy season, Meiyu, named as

Changma in Korea and Baiu in Japan, is one of the most prom-

inent seasonal climatological phenomena. Approximately, the

Meiyu season starts on the 17th of June and ends on the 8th of

July, lasting about 22 days. As a major rainfall season in east

China, Meiyu is on the spotlight because of its relationship

with East Asia monsoon (Tao et al., 1987; Matsumoto, 1988,

1992; Lee et al., 1992; Kang et al., 1999; Zhao et al., 2007).

The breakup of the Meiyu season is closely related to the

periodical northward migration of East Asia summer monsoon.

The East Asia subtropical summer monsoon and the South

China Sea (SCS) summer monsoon are two branches of the East

Asia summer monsoon. The East Asian subtropical summer

monsoon and its associated rainband were built at the end of

March and the beginning of April. The onset of the subtropical

summer monsoon is characterized by the reverse of the me-

ridional wind, whereas the onset of the SCS is representative of

the reverse of the zonal wind. Controversy exists on whether

the shift of the Meiyu rainband is related to the East Asian

subtropical monsoon or SCS summer monsoon. Chen et al.

(2004) indicated that the subtropical rain season, Meiyu, is

different from the tropical rain season but they occur in

sequence from spring to summer. The rainband associated with

the subtropical summer monsoon develops in early April to the

south of Yangtze River, and it shifts northward with the

migration of the subtropical High after the onset of the SCS

summer monsoon. Chen et al. (2008) and Zhao et al. (2008)

and He et al. (2008) found similar conclusions and showed

that the shift of the rainband is associated with the seasonal

northward march of the East Asian subtropical monsoon. On

the other hand, some studies have shown that the large-scale

rainband affecting the eastern China is formed after the onset

of the SCS summer monsoon, and the rainband advanced

northward in sequence of time (e.g., Liu et al., 1997; Lian et

al., 2007) as a result of the seasonal migration of East Asia

summer monsoon. The main purpose of this study is to further

investigate whether the advance of the subtropical summer

monsoon or the march of the tropic summer monsoon leads to

the onset of Meiyu season.

Ding et al. (2007) showed that the wind fields at both 850

and 200 hPa displayed the similar seasonal process with that of

Meiyu rainband. It will be helpful by analyzing the formation

processes of both the rainband and the typical circulation pat-

tern around the Meiyu season to examine the relationship bet-

ween the Meiyu rainband and the East Asia summer monsoon.

Features of atmospheric circulations in the midst of the Meiyu

season have been extensively investigated in the past (e.g.,

Chang et al., 1998; Ding et al., 2007; Yang et al., 2010). Few

studies have investigated atmospheric circulations associated

with the rainband before the onset of Meiyu, specially, on the

difference between the vertical distribution of the vorticity

field. We will compare the seasonal variations of the circulation

Corresponding Author: Jinhai He, Jiangsu Key Laboratory of Me-teorological Disaster of Ministry of Education, Nanjing Universityof Information Science and Technology, 219 NinLiu Road,Nanjing 210044, China.E-mail: [email protected]

146 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

before and after the onset of the Meiyu, and detect the typical

circulation pattern of Meiyu qualitatively. To explore the rela-

tionship between the East Asia summer monsoon and the

seasonal advancement of the rainband in the east of China, vari-

ations of the large scale fields such as the vortex field and

vorticity field as well as temperature and humidity fields before

and after the beginning of Meiyu will be examined in details.

Both typical pattern in atmospheric circulations during the

Meiyu season and the formation date of the typical circulation

pattern will be presented in the study as well. Eventually the

relationship between the onset of the East Asia monsoon and the

seasonal process of the rainband will be discussed in this paper.

2. Data and methodology

The datasets used in this study include daily atmospheric

fields in the NCEP/NCAR reanalysis, and daily precipitation

data at 753 stations in China and Meiyu parameters in the

middle-lower reach of the Yangtze River determined from the

National Meteorological Information Center of China from 1954

to 2001. According to Meiyu parameters, the Meiyu season

starts on 18th, June.

The five provinces, Zhejiang, Hubei, Anhui, Jiangsu and

Shanghai, are selected to represent the region of the middle-

lower reach of Yangtze River from 28oN to 35oN, and 110oE to

122oE, i.e., Jianghuai valley. According to dates of Meiyu

onset provided by these provinces, the mean onset date of

Meiyu has been estimated. Due to diversity in the definition of

Meiyu onset among the provinces, the Meiyu onset dates could

be remarkably different. For example, the onset date of Meiyu

in 1987 provided by two neighboring Jiangsus and Anhui

provinces is on the17th of June and on the 30th of June respect-

ively. If the differences of the Meiyu onset date between five

provinces are smaller than 5 days for a certain year, this year

will be selected as a candidate for use. Otherwise, the mean

Meiyu onset date over will be unreliable and this year is not

selected for use. For example, the year of 1987 is not qualified

for use. Furthermore, the mean Meiyu onset dates of the above

selected years are compared with that from the National Mete-

orological Information Center of China. A year with the differ-

ence between the two onset dates of Meiyu smaller (larger)

than 5 days is considered as a consistent (inconsistent) year.

Table 1 lists all consistent and inconsistent years. For these

consistent years, the mean atmospheric circulation is obtained

by averaging circulations in the 10 days after the onset of

Meiyu. It is found that the mean atmospheric circulation within

10 days after the onset of Meiyu is similar to that during the

entire Meiyu season. Therefore, the mean Meiyu onset date can

be considered as a base point for detecting the evolution of large

scale Meiyu circulation. The formation of typical circulation

patterns during the Meiyu season and their evolution processes

will be investigated based on pentad mean data.

3. Characteristics of precipitation, vorticity and diver-gence before and after the onset of Meiyu

The formation of the large-scale rainband in Jianghuai valley

is considered as the beginning of the Meiyu season. Figure 1

shows that the maximum precipitation region, i.e., the main

rainband, is located at 21oN before the onset of Meiyu. The

main rainband moves northward to the Jianghuai basin after

Meiyu, about at 30oN with the secondary maximum precipi-

tation center at HuaNan in China.

The large-scale vertical motion can be forced by the conver-

gence, which transports the moisture to the precipitation area.

Figures 2a-b show that, the strong convergent area is exactly

consistent with the maximum precipitation area. Both the maxi-

mum precipitation and the strong convergent area are located to

south of the Yangtze River before the Meiyu, and then march

simultaneously over the middle-lower reach of the Yangtze

River valley after Meiyu. To the east of 115oE, the heavy

rainfall region matches the strong convergent region well. To

the west of 115oE, there is no strong convergent area corres-

ponding to the heavy rainfall area occurring there. However,

the divergence of the meridional wind component, with the

magnitude of −4 × 10−6, is consistent with this patch of heavy

rainfall area (Fig. 2d). This might suggest that the evolution of

the main rainband depends on the divergence of the meridional

wind component.

The evolutions of the divergence field of the zonal and

meridional components of winds before and after the onset of

Meiyu (10 days before and 10 days after) are shown in Figs.

3a-b. It is found that the divergence of the zonal wind and the

convergence of the meridional wind occur at the low level at

the same time. Both the intensity of the convergence and the

divergence start to increase 5 days before the onset of Meiyu

and reach their maximum value 1 day after the onset of Meiyu.

Table 1. Consistent and inconsistent years.

31 consistent years 17 inconsistent years

1955, 1956, 1957, 1959, 1960, 1961, 1962, 1963, 1964, 1967, 1968, 1969, 1970, 1972, 1973, 1974, 1975, 1976, 1977, 1979, 1980, 1981, 1983, 1984, 1986, 1988, 1990, 1995, 1997, 2001

1954, 1958, 1965, 1966, 1971, 1978, 1982, 1987, 1989, 1991, 1992, 1993, 1994, 1996, 1998, 1999, 2000 Fig. 1. Meridional variations of 10 day mean diurnal precipitation

(mm d−1) before Meiyu (−10d) and after Meiyu (10d) from 110oE to120o

E.

31 May 2012 Danni Liu et al. 147

The magnitude of convergence of the meridional wind is larger

than that of divergence of the zonal wind, resulting in the occur-

rence of the convergence at the low level. The variations of the

divergence are comparatively complex at the high level. The

intensities of divergence and convergence start to decrease 4

days before the onset of Meiyu. The convergence of the merid-

ional wind reduces sharply 2 days before Meiyu onset, or even

turns to the divergence with its intensity larger than that of the

zonal wind. After Meiyu onset, the divergence of the meridional

wind changes from positive values to negative values process-

ively, but its divergent intensity is smaller than that of the zonal

wind. Therefore, the net divergent intensity is presented as

positive values at the high level. It is the enhancement of the

convergence of the meridional wind that leads to the net

convergent intensity at the low level. Meanwhile, the remark-

able reduction in the convergent intensity of the meridional

Fig. 2. Divergence (Solid line, unit is 10−6 s−1) at 850 hPa, precipitation (shaded area, unit is mm d−1), and wind vector (arrow) at 850 hPa before (a),and after (b) Meiyu ; divergence of the meridional wind component (solid line, unit is 10−6

s−1

) at 850 hPa, precipitation (Shaded area, unit is mm d−1

),and wind vector (arrow) at 850 hPa before (c) and after (d) Meiyu.

Fig. 3. (a) Daily variations of the 850 hPa divergence (× 10−6

s−1

) averaged over the region of 28oN to 32

oN, and 110

oE to 120

oE. D1 represents the

divergence of the zonal wind component, D2 represents the divergence of the meridional wind component, and D1 + D2 represent the totaldivergence. (b) is same as (a) except for 200 hPa.


wind and the persistent divergence of the zonal wind produce

the intensified divergence at the high level. The changes of the

meridional wind before and after the onset of Meiyu play a

leading role in the variation of the divergent field and con-

tribute significantly to the precipitation. Two separated positive

vorticity zones are always found at the low level vorticity field

before and after the onset of Meiyu. The northeastern zone is

located over the Inner Mongolia Plateau without the apparent

changes, whereas the southern zone of the positive vorticity

remains to the south of the Yangtze River and advances north-

ward to the Jianghuai Valley, which coincides with the shift of

the maximum precipitation zone (Figure not shown).

4. Vertical structures and evolution of circulation, mois-ture and temperature during the Meiyu

The characteristics and evolution of vertical circulation in

the Meiyu season are revealed by the cross section analysis

averaged from 110ºE to 120ºE during the Meiyu season (Fig.

4f), three major positive vorticity zones are located at the low-

middle level of the troposphere, centered at the South China

Sea (SCS, 10ºN to 20ºN), the Jianghuai valley (25ºN to 35ºN)

and the Inner Mongolia Plateau (north to 42ºN), respectively.

The positive vorticity zone at the SCS represents the activity of

the SCS monsoon and the height of its core (> 3 × 10−6 s−1)

Fig. 4. Meridional cross section of 5 day mean temperature in oC (dash line), specific humidity in g kg−1 (solid line) and vorticity in10−6 s−1 (shaded)averaged between 110

oE and 120

oE for the period of (a) from 11

th to 12

th pentad, (b) from 22

nd to 23

rd pentad, (c) from 26

th to 27

th pentad, (d) from

28th to 29th pentad, (e) before Meiyu, and (f) after Meiyu.


hardly reaches the middle level of the troposphere. The positive

vorticity zones at the Jianghuai valley and Inner Mongolia

Plateau penetrate the entire troposphere. These two zones are

separated by a negative vorticity zone at the low level but

merge at the high level to form an integrated part. This pattern

of the vorticity field is nicknamed as “two leg pattern”. One of

the two legs refers to the positive vorticity zone at the Jianghuai

valley, and the other at the Inner Mongolia Plateau, termed as

the south leg and the north leg respectively. The south leg

corresponds to the interface of the Meiyu front between the

upward warm air mass and the downward cold air mass. The

south leg of the positive vorticity tilts northward with height,

indicative of a baroclinic structure with the positive vorticity at

the low level and the negative vorticity at the middle and high

level of the troposphere. The baroclinic structure implies the

juxtaposition of the Meiyu front at the low level with the sub-

tropical high at the middle level, and the south Asia subtropical

high at the high level of troposphere (Zhu et al., 2000). The

north leg of the two leg pattern manifests itself as a less

baroclinic structure.

Our results in Fig. 5 are broadly consistent with features of

temperature field and moisture field in the meiyu season sum-

marized in Ding et al. (2007). Figure 5 shows that a high

specific humidity zone lays inside the Meiyu region at the low

level of the troposphere. From the meridianal cross section, the

southern leg of the “two leg pattern” is accompanied by both

temperature ridge and humidity ridge through the entire tropo-

sphere. The northern leg of the “two leg pattern” is also

Fig. 5. Meridional cross section of 5-day mean θse (dash line, unit is K) transected between 110

oE and 120

oE for the period of (a) from 11

th to 12

th

pentad; (b) from 22nd to 23rd pentad; (c) from 26th to 27th pentad; (d) from 28th to 29th; (e) before Meiyu; (f) after Meiyu .The solid line represents θse

equal to 328K. The dense θse zone with its value smaller than 12K is shaded.


collocated with a weak temperature ridge at lower troposphere.

This distribution of temperature makes the Meiyu region sitting

in a cold temperature trough at the low level. The occurrence of

the weak temperature ridge implies a warm and moist air mass

located to south of the “south leg” positive vorticity zone. This

feature can be found easily from the field of equivalent poten-

tial temperature (Fig. 5f). The sharply changing zone of θse

(shaded), represents the Meiyu frontal zone. In the Meiyu

season, a low θse zone, located to north of the Meiyu frontal

zone, indicates the existence of a dry and cold air mass. In the

meantime, a high θse zone lies to the south of the Meiyu frontal

zone, acting as a warm and moist air mass. At the flank of the

warm air mass, θse decreases with height at the low level,

indicating that the air there is moist unstable. At the middle

level, θse is homogeneous and reaches a minimum. Then θ

se

increases with height at the upper troposphere. This vertical

structure of equivalent potential temperature resembles the

structure of a typhoon eye (Ding et al., 2007), characterized by

the ridge of the temperature and the ridge of humidity through

the entire troposphere, and the high value of θse

locating to the

south of the Meiyu front. In contrast to the vertical structure of

vorticity, temperature as well as humidity after the onset of

Meiyu, the feature of a “two-leg” pattern in the vorticity field,

a typhoon-eye-like vertical structure at the side of warm and

moist air and a sharp gradient zone in the equivalent potential

temperature field also appear before the onset of Meiyu, but

with their locations situated more south at the low level of the

troposphere (Fig. 5a). And the southern leg in the two-leg

pattern, around 27oN, is quite weak in terms of positive vor-

ticity before the onset of Meiyu, accompanied by a tempera-

ture ridge and a humidity tongue. With the onset of the Meiyu,

the southern leg of the two-leg pattern is enhanced in term of

increased positive vorticity and shifts toward 30oN, with the

consistent progress of both the temperature ridge and the

humidity tongue. The characteristic contour line of θse with the

value of 328 k withdraws to the north, while the sharp gradient

zone of θse shifts from 30oN to 33oN.

Based on the above analysis, the typical circulation pattern

during the Meiyu season is summarized as follows (1) a “two-

leg” pattern in the vorticity field, (2) a typhoon eye-like struc-

ture in the equivalent potential temperature field, and (3) a

sharp gradient zone of θse i.e., the Meiyu front. These promin-

ent features occur around the onset of the Meiyu. How do they

evolve before and after the onset of the Meiyu? The vertical

structure of pentadly mean vorticity, equivalent potential tem-

perature, and their relationships to the Meiyu rainband is

further examined next.

At the end of February (Fig. 4a), the positive vorticity zones

associated with the “two-leg” pattern hang above 850 hPa.

Negative vorticity dominates at the low level in east China,

caused by the stagnant continental high pressure system at the

low level in this region during the winter. A cold and dry air

mass is reflected in the fields of temperature and humidity in

this region, indicating the dominance of a typical winter pattern.

Meanwhile, a warmer and moist air still stays in the tropics.

Starting at the end of March, the southern leg and the northern

leg of the positive vorticity zone (i.e., “two-leg” pattern) extend

downward to the low level, with the eastward migration of the

continental cold high pressure system (Fig. 6a). The “two-leg”

pattern starts to take shape. The southern leg of the positive

vorticity zone hits the ground and moves to the Jianghuai

valley since the middle of April (Fig. 4b). The northern leg of

the positive vorticity zone also hits the ground but still stays to

the north of 42oN. This transition in the positive vorticity zone

indicates the retreat, i.e., the northward migration, of the

continental cold high pressure system in the eastern China and

the reversal of the ocean-land surface pressure contrast between

the eastern Asia and western pacific (Guo, 1983). In the

middle of May, the northward migration of the South Asian

High at the upper level leads to the poleward tilting of the

Fig. 6. (a) The migration of the continental cold high pressure systemat 925 hPa from 8

th pentad to 19

th pentad. The solid circle represents

the center of the continental cold high pressure system, and thenumber with the solid circle represents the time of the pentad, (b)Meridional variations of the mean wind field (stream line), divergence(Solid line, unit is 10−6 s−1), the ridge of the Weastern North PacificHigh (long dashed line) and the negative vorticity (shaded area) at 500hPa from 110o

E to 120oE.


south leg of the positive vorticity zone with height (Fig. 4c).

With the progression of the Subtropical High (Fig. 6b), the

southern leg of the “two-leg” positive vorticity zone starts

moving northward at the middle level. Meantime, the “two-leg”

positive vorticity zone is stagnant to the south of the Yangtze

River at the low level, resulting in the poleward tilt of the south

leg of the “two-leg” positive vorticity zone. Thus, a strong

baroclinic zone starts to take shape over the Yangtze River,

with the vertical structure of the positive vorticity zone at the

low level and the negative vorticity at the high level (Fig. 4d).

At the same time, another positive vorticity zone forms over

the SCS (South China Sea) and stretches up to the middle level

after the onset of SCS monsoon (Fig. 4d).

Also demonstrated are the temporal variations of equivalent

potential temperature field. In the end of February (Fig. 5a), the

characteristic θse contour line of 328 K, representative of warm,

moist unstable air mass, is located to the south of the Tropic of

Cancer. And a dry and cold air mass, represented by low θse,

dominates the continental area in East China. Meanwhile, a

broad and sharp horizontal θse gradient zone lies to the north of

the characteristic θse line, suggestive of a strong cold air and

thus a winter circulation pattern. At the end of April (Fig. 5b),

the characteristic θse line of 328 K moves northward to the

south of the Yangtze River, a subtropical area. The sharp gra-

dient zone of θse turns narrower and a front or frontal zone

becomes clearer. In the meantime, with the increasing of θse at

the low level, the instability layer extends to 850 hPa. The

smaller value of the equivalent potential temperature locates

around the south and north area of the dense gradient zone of

θse (shaded area in the plot) respectively, with one of the

smallest center over the SCS at the middle level. The larger

value of the equivalent potential temperature locates at the low

level and the high level to the south of the dense gradient zone.

The “saddle” shaped typical structure of equivalent potential

temperature has been formed (Fig. 5c). After the onset of the

SCS monsoon (Fig. 5d), the warm and moist air mass

advances northward, corresponding to the same movement of

the dense gradient zone of θse, affecting the region south to the

Yangtze River and the Jianghuai valley.

Therefore, the winter patterns in the fields of the vorticity,

temperature and humidity start to change in the end of March.

The positive vorticity zone stretches downward to the low level.

The “saddle” shaped equivalent potential temperature is pro-

duced progressively. Corresponding to the eastward shifting of

the weakening continental High (Fig. 6a), the warm air invades

toward the continent in Eastern China (Fig. 5b-f). The circula-

tion pattern of the “two-leg-shape” vorticity structure and the

“saddle” shaped equivalent potential temperature field have

been set up in late April, and the features of the circulation

pattern is consistent with that of the typical pattern during the

Meiyu season. Accordingly, the typical pattern representative

of the onset of the Meiyu has been built since April, and the

typical circulation pattern advances northward with the warm

and moist air mass to affect the region of southern China and

the Jianghuai valley. The circulation patterns presented in the

fields of the vorticity, temperature, and the humidity after the

onset of Meiyu are very similar to their corresponding typical

patterns before the onset of the Meiyu, which has been set up

since April before the onset of Meiyu. This typical pattern

shifts northward right after the onset of the SCS monsoon.

There have been a number of published studies showing that

the set up of the East Asia Summer Monsoon is closely related

to the seasonal reverse of the land-sea thermal contrast between

Eastern Asia and northwestern Pacific (Guo, 1983; Qi et al.,

2007). The vertical distribution of the circulation structure

suggests that, the typical pattern of Meiyu is formed progres-

sively during the rainy season in Southern China and its

formation coincides with the time of the reverse change of the

pressure difference from the positive value to the negative one

between Eastern Asia continent and the northwestern Pacific.

After the onset of the SCS monsoon, the typical pattern ad-

vances northward with the summer monsoon and affects the

south region to the Yangtze River and Jianghuai valley,

accompanied by the northward shift of the rainfall band in

eastern China. Therefore, the typical pattern of the “two-leg”

vorticity, the equivalent potential temperature whose vertical

structure is quite similar to that in a typhoon eye presented after

the Meiyu has been detected since the late March and the early

April. The typical pattern presented during the Meiyu season is

considered as the result of the northward shift of the circu-

lation set up to south of Jianghuai Valley before the onset of

Meiyu. The existence of the typical pattern relating to the

building of the east subtropical summer monsoon is a signal of

the seasonal transition in Eastern China.

5. Summary

In this study, variations of the large scale circulation and the

typical circulation pattern as well as its evolution before and

after the onset of Meiyu have been examined with the analysis

of the cross sections passing through the eastern China. The

typical circulation structure during the Meiyu season is charac-

terized by the “two-leg” vorticity field, and the equivalent po-

tential temperature field whose vertical structure is quite similar

to that in a typhoon eye as well as the sharp gradient zone of

θse (frontal interface or frontal zone). The vertical circulation

structure moves from south before Meiyu to north after the

onset of Meiyu without change. Tracing the formation and the

evolution of pentad mean of the circulation, the result shows

that the transition of the circulation pattern at the end of March

and the early April corresponds to the beginning of the rainy

season in south China. The circulation pattern of “two-leg”

shaped vorticity field, and a typhoon eye-like structure of the

equivalent potential temperature field as well as the sharp gra-

dient zone of θse has been formed on April. The reverse change

of the pressure difference between the East Asia continent and

the northwest Pacific occurs simultaneously. Both the typical

circulation structure and the rainfall band shift northward after

the onset of Meiyu. The spring season is the first stage of the

rainfall season associated with the subtropical monsoon. So we


conclude that the seasonal transition of the subtropical monsoon

leads to the onset of Meiyu. The appearance of the typical

circulation pattern with the rainfall band is representative of

the onset of the East Asia subtropical summer monsoon. Both

the precipitation and the circulation pattern move simultan-

eously with the seasonal process of East Asia subtropical

summer monsoon.

Acknowledgements. This study is jointly supported by National

Natural Science Foundation of China (Grant No. 41075068 and

40875044) and National Public Benefit Research Foundation of

China (GYHY200706005).

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Characteristics and evolution of atmospheric circulation patterns during Meiyu over the Jianghuai...

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