progress of the seasonal evolution of east asian summer ...asian summer monsoon, the delay of the...

192 ACTA METEOROLOGICA SINICA VOL.19

Progress of the Seasonal Evolution of East Asian Summer

Circulation during July-August and Its Linkages with the

Subseasonal Processes over the Western North Pacific∗

LIAO Qinghai1(��

), TAO Shiyan1( �� ), and LIN Yonghui2( �� )1Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029

2Chinese Academy of Meteorological Sciences, Beijing 100081

(Received July 8, 2004; revised November 10, 2004)

ABSTRACT

Based on the NCAR/NCEP monthly and pentad reanalysis dataset of 1961-2003, the progress of seasonalevolution of the summer atmospheric circulation in the East Asia in July to August, including the advancedand delayed cases, and their relationships with the subseasonal processes over the western North Pacific areanalyzed and compared with that of climatology. The results show that the progress of seasonal cycle isadvanced about a month ahead of the climatological time when the convection during 20-29 July is active inthe region of the subtropical West Pacific (15◦-25◦N, 150◦-165◦E), while it is delayed about one month whenweaker convections appear in the same region. Instead, the relative active convection for the latter occursin Pentad 46 (14-18 August). It is proved that the convective activities in the early July in the equatorialcentral and east Pacific, and then the convective anomalies in the subtropical western North Pacific canexcite the formation of the acceleration and delay of the seasonal circulation evolution in the East Asia inthe late summer. The preceding subseasonal processes over the western North Pacific, including the time-laginteractions among the active convection in the late June and early July, the Northwest Pacific anticyclone,the underlying sea surface temperature and low-level winds anomalies, and their relationships with theanomalous seasonal evolution of the summer atmospheric circulation in the East Asia in late July are alsoinvestigated. However, further study, especially the numerical experiments, is needed on the mechanism ofthe anomaly summer seasonal cycle in the East Asia and the Northwest Pacific.

Key words: seasonal cycle, convection, subseasonal anomalies

1. Introduction

The Asian summer monsoons are basically a re-

sponse of the atmosphere to the differential heating

between the land of the Asian Continent and the ad-

jacent oceans (Chen et al., 1992; Ding, 1994; Ye et al.,

1996). The atmospheric response, however, may be

quite complicated due to the complex topography in

the Asia, land-sea interaction, convection in the West

Pacific, including their interactions, especially in East

Asia. While the evolution of the East Asian summer

monsoon is largely phase-locked with the seasonal cy-

cle, a failure of the climatological seasonal cycle, which

leads to the hydrological disasters such as floods and

severe droughts, is not unusual. Tao and Xu (1962)

pointed out in the early 1960s that the seasonal vari-

ation of the atmospheric circulation in the flood years

in Jianghuai River Region would be delayed for one

month or so. Park and Schubert (1997) also indicated

that the seasonal cycle is advanced one month or so in

the year of severe droughts in the east of China, Japan

and the south of Korea in 1994. Although the above

results proved such facts that the unusual seasonal cy-

cle of the Asian summer monsoon is very important

for the hydrological disasters in the East Asian coun-

tries, the forcing mechanisms responsible for such an

extreme failure of the monsoon are yet to be fully un-

derstood. The further studies of this question have

significant practical meaning.

In addition, the air-sea interaction and convective

activity (Nitta et al., 1986; Nitta, 1987, 1990; Huang

and Li, 1987; Huang and Sun, 1992) also contribute to

the complexity of the seasonal cycle in the East Asian

monsoon region and the Northwest Pacific (Ueda et

∗This research is partly supported by the National Natural Science Foundation of China for Outstanding Young Scientists underGrant No. 40125014 and funded by the National Natural Science Foundation of China under Grant No. 40105010.

NO.2 LIAO Qinghai, TAO Shiyan and LIN Yonghui 193

al., 1995; Ueda and Yasunali, 1996; Ose, 1998; Lu,

2000, 2001a, b). Based on TBB data from the Meteo-

rological Research Institute of Japan, He et al. (1996)

studied the features of seasonal transition of Asian-

Australian monsoons and Asian summer monsoon es-

tablishment, indicating that the transition begins as

early as April, followed by abrupt changes in May-

June; the Asian summer monsoon is fully established

in June. The winter convective center in Sumatra

moves steadily northwestward across the “land bridge”

of the maritime continent and the Indo-China Penin-

sula as time goes from winter to summer, thus giving

rise to the change in large scale circulations that is re-

sponsible for the summer monsoon establishment over

the Southeast Asia and India; the South China Sea

to the western Pacific summer monsoon onset bears

a close relation to the active convection in the Indo-

China Peninsula and the steady eastward retreat of the

western Pacific subtropical high. The abrupt north-

ward shift of active convective region from 10◦N to

25◦N around 150◦E during 25-29 July seen in the sea-

sonal cycle of the summer monsoon over the western

North Pacific (convection jump) and other associated

phenomena preceding this convection jump are inves-

tigated in detail by Ueda et al. (1995, 1996) for the pe-

riod of 1980-1994. They noted that the tongue-shaped

warm sea surface temperatures (SST) areas (warmer

than 29 ◦C) are observed in early July around 20◦N,

150◦-160◦E, preceding the typical convection jump.

This warming of SST is closely related to the appear-

ance of a weak wind region (weakening of easterlies)

around 25◦N, 140◦-160◦E in late June. These weak

easterlies are likely to be associated with the propa-

gation of Rossby wave induced by the occurrence of

active convection near the Philippine Islands in the

middle to late June. Ose (1998) investigated with

a set of idealized numerical experiments on whether

the seasonal change of the prescribed diabatic heating

reproduces the seasonal change of the Asian summer

monsoon circulation from July to August. Seasonal

change from mid-summer (July) to late summer (Au-

gust) is characterized by enhanced convective activity

in the extended area of the subtropical western Pacific.

The change of the heat source over the western Pacific

solely explains the major characteristics of the clima-

tological seasonal change from July to August not only

over the Pacific but also over the Indian Ocean. The

expansion of the Tibetan High at upper-level and the

Pacific High at low-level over Japan is also simulated

only by the seasonal change of the western Pacific heat

sources. On the interannual scale, Lu (2000, 2001a,

b) studied the linkages between the atmospheric cir-

culations and SST related to the convection over the

Western Pacific Warm Pool (WPWP, 10◦-20◦N, 110◦-

160◦E). The intensity of outgoing longwave radiation

(OLR) over WPWP is well correlated with the pre-

ceding (the previous winter and spring) SSTs over the

West Pacific region (10◦-20◦N, 130◦-170◦E) and the

equatorial central and East Pacific (5◦S-5◦N, 180◦-

90◦W), but not on the same term, and vice versa for

the SST over the west of the Philippine Islands.

The datasets used in this study are:

(1) the reanalysis monthly and pentad (5-day

averaged, 73 pentads per year. Each year has

365 days with no leap day) datasets of 1961-2003

from NCEP/NCAR (National Centers for Environ-

ment Prediction/National Center for Atmospheric Re-

search) (Kalnay et al., 1996);

(2) OLR data of 1979-2003 from the NOAA;

(3) weekly SST, 1982-2003 (Reynolds and Smith,

1995; Smith and Reynolds, 1998).

An index of the acceleration and delay of the sea-

sonal evolution in the East Asia is defined, and the

circulation characteristics in the late July over the

East Asia and the Northwest Pacific are further in-

vestigated on the subseasonal and interannual scales.

The indexes used here and the corresponding circula-

tion characteristics are described in Section 2. The

variability of both OLR and weekly SST and their

linkages with the interannual variability of the index

are documented by the composite and lag-correlation

analyses in Section 3. Section 4 examines the possible

associations between the SST, convections and the cir-

culations in the low-level atmosphere. The conclusions

are summarized in Section 5.


2. The atmospheric circulations associated

with the progress of the seasonal cycle of

the summer circulations in the East Asia

To investigate the acceleration and delay in the

climatological seasonal evolution in late summer, the

climatological features of atmospheric circulation pat-

terns of the eddy height at 200 hPa are analyzed. The

results show that in July, the South Asian high center

in the southeast of the Tibetan Plateau in June con-

tinues to move northwestward, and a new west high

center develops over the Iran-Afghanistan region. The

whole South Asian high shrinks and becomes stronger

in July. The trough in the East Asia weakens and fur-

ther westward migrates, then a low center develops in

the south of the Baikal Lake. The mid-ocean upper

level trough also becomes much stronger and enlarged

in July. In August, the circulation patterns continue

to change. The west center of the high in July weak-

ens, and the corresponding monsoon rainfall (Meiyu)

ends in the Middle and Lower Reaches of the Yangtze

River and Huaihe River. The most pronounced change

from July to August, however, is over the Northwest

Pacific region, centered over the Japan Islands, where

the upper-level trough is replaced by a ridge (a local

high center) in August. The low center in the south

of the Baikal Lake continues to develop into a long

low band covering the regions across the Atlantic, the

Mediterranean Sea, and the south of Lake Baikal (Park

and Schubert, 1997; Tao and Chen, 1987). Figure 1

can rather illustrate the seasonal evolution of the mid-

dle latitude eddy height or stationary waves. While

the seasonal evolution of the upper-level anticyclone

around the Tibetan Plateau (also called the monsoon

high) is a dominant feature in the eastern Hemisphere

during the summer, the cross sections at 40◦N in Fig.1

emphasize the evolution of stationary waves over the

East Asian sector. This is at the northern periphery

of the Tibetan anticyclone. The most pronounced sea-

sonal evolution in August is the rapid development of

Fig.1. Longitude-pressure sections of the eddy height climatology at 40◦N for (a) June, (b) July, and (c)August. Contour interval is 20 gpm.


the anticyclone near 150◦E, which is somewhat sepa-

rated from the South Asian anticyclone. The devel-

opment of this upper-level high centered around the

Japan Islands and Sea substantially changes the mid-

dle latitude circulation in the East Asia including the

vertical structure, which shows a strong dipole over

the West Pacific around 150◦E in August.

In order to describe the interannual variability of

the fast or slow seasonal evolution in late summer in

East Asia, a simple index is defined as the averaged

200 hPa eddy height in July over the key region ( 30◦-

50◦N, 120◦-160◦E). If the circulation patterns of July

in a year are similar to those of the climatology in Au-

gust, then the summertime seasonal cycle in this year

is accelerated. On the other hand, if the circulation

patterns of July in a year are similar to those of the

climatology in June, the summertime seasonal cycle

in this year is considered to be delayed. In the East

Asian summer monsoon, the delay of the seasonal cy-

cle means that the summer monsoon and rainfall per-

sist in the Huaihe River and the Middle and Lower

Reaches of the Yangtze River. The accelerated sea-

sonal cycle means the earlier ending of the Meiyu, and

also the earlier ending of the East Asian summer mon-

soon in the same regions.

Figure 2a indicates the normalized time series of

200 hPa eddy height in July averaged over the key re-

gion during 1961-2003. Years with values greater than

1.0 are selected as the positive anomaly years: 1961,

1962, 1963, 1967, 1972, 1973, 1977, 1978, and 1994,

while years with values less than -1.0 are selected

as the negative anomaly years: 1979, 1982, 1983, 1986,

Fig.2. (a) Time series of the normalized 200 hPa eddy height in July averaged over the key region (30◦-50◦N, 120◦-160◦E) during 1961-2003, and (b) the correlation coefficients between the above series and theeddy height differences which are the eddy height in July minus the climatological eddy height in August at40◦N. Light and heavy shadings indicate that their correlation coefficients exceed 95% and 99% confidencelevels, respectively.


Fig.3. The composite charts of the monthly eddy geopotential height for the positive (a-c) and negativecases (d-f) at 40◦N in June, July, and August (JJA).

1991, 1992, 1993, and 2003. Figure 2b shows the

correlation coefficients between the above series and

the eddy height differences which are the eddy height

in July minus the climatological eddy height in Au-

gust at 40◦N. Light and heavy shadings indicate that

their correlation coefficients exceed 95% and 99% con-

fidence levels, respectively. The positive eddy height

anomalies in the key region in July are associated with

the positive eddy height anomalies in the 120◦-160◦E,

especially in the upper troposphere, while negative

height anomalies in the middle-low levels of the tropo-

sphere in the area of 60◦-120◦E, and vice versa. The

above results are confirmed by the composite anal-

ysis in the next context. Figure 3 is the composite

charts of the monthly geopotential height for the posi-

tive and negative cases at 40◦N in June, July, and Au-

gust (JJA). A strong anticyclone develops rapidly for

the positive anomaly case in July, which is the same as

the climatology in August. The patterns in the nega-

tive case in July are similar to the climatology in June.

No anticyclone develops at 150◦E, and the mid-ocean

trough is weaker than that of the climatology. The

seasonal variational differences between the positive

and negative cases in the East Asia show a large zonal

asymmetric component, which means there exists pro-

nounced zonal asymmetric forcing. We will discuss it

in the later context.

3. Variations of OLR and SST in the equato-

rial central Pacific and subtropical west-

ern North Pacific

Ueda et al. (1995, 1996) stated that the sud-

den onset of the summer monsoon over the subtrop-

ical western North Pacific in late July is associated

with an abrupt northward shift of enhanced convec-

tion around 20◦N, 150◦E−“convective jump”. The

composite OLRA for the positive minus negative cases

is shown in Fig.4a. There are centers over the equa-

torial central Pacific, the subtropical Northwest

Pacific east of the Philippine Islands, especially east of


150◦E, and the southeast of the exit of the East Asian

Jet Streams. The time series of OLR over the east cen-

ter around 150◦E in the subtropical Northwest Pacific

are made for the positive cases, negative cases and the

climatology through Pentad 36 to 50 (Fig.4b). The

seasonal change of the climatological case reveals a re-

markable minimum in Pentad 42 (25-29 July), which

indicates an enhanced convection in this pentad. An

enhanced convection occurs in Pentad 41 (20-24 July)

for the positive cases, while in Pentad 46 (14-18 Au-

gust) a minimum of OLRA (OLR anomalies) appears

for the negative cases. A gap is pronounced for the

two kinds of cases. No big convection differences exist

between the positive cases and the climatology on the

subseasonal time scale.

The enhancement of convection in Pentads 41-

42 may be possible to associate with preceding SST

anomalies in the West Pacific. A longitude-time sec-

tion of linear correlation coefficients between the OLR

in Pentads 42-43 averaged over the same area of Fig.4b

and the weekly SST in 15◦-25◦N is shown in Fig.5. The

significant negative correlations around early June and

early to mid July clearly demonstrate that positive

SSTA is correlated to the enhanced convection (neg-

ative OLRA) in the early June over the subtropical

central and East Pacific and also an enhanced convec-

tion in the early and mid July (Week numbers 5-6)

over the region of 165◦E-170◦W. On the other hand,

a positive correlation in late July (Week numbers 8-9)

means the negative SSTA over the subtropical

Fig.4. (a) The composite OLRA for the positive minus negative cases. (b) The time series of OLR over thekey region (15◦-25◦N, 150◦-165◦E) in the subtropical Northwest Pacific for the positive cases (solid line withopen square), negative cases (solid line with dark square) and the climatology (solid line with dark circle)through Pentads 30 to 50. Pentad 30 corresponds to 26-30 May, while Pentad 50 to 3-7 August.


Fig.5. A longitude-time section of linear correlation coefficients between the OLR in Pentads 42-43 averagedover the key region and the weekly SST in 15◦-25◦N in the northern summer. Number 1 in the ordinaterepresents the first week of June, and 12 represents the last week of August. The shading indicates correlationcoefficients exceeding 95% confidence level.

Northwest Pacific matches with the active convection

in the region of 15◦-25◦N, 145◦-160◦E. That is SST

decreases after the enhanced convection. The decrease

of radiation for the existence of the convective clouds,

strong evaporation cooling and upwelling of cold sea

water due to the increase of wind speeds lead to the

above SST decreases.

Figure 6 indicates the time-lag correlations be-

tween the 200 hPa eddy height averaged over the key

region at Pentads 42-43 and the OLR averaged in Re-

gion 1 (5◦S-5◦N, 170◦-190◦E), Region 2 (15◦-25◦N,

150◦-165◦E), Region 3 (10◦-20◦N, 100◦-120◦E), and

Fig.6. The time-lag correlations between the 200 hPa eddy height averaged over the key region at Pentads42-43 and the OLR averaged in Region 1 (solid line with dark circle, 5◦S-5◦N, 170◦-190◦E), Region 2 (solidline with dark square, 15◦-25◦N, 150◦-165◦E), Region 3 (dashed line with no marker, 10◦-20◦N, 100◦-120◦E),and Region 4 (solid line with open circle, 7.5◦-17.5◦N, 115◦-125◦E). The time-lag correlations between OLRin Region 2 and Region 1 (solid line with open square) are also presented. Pentad 31 corresponds to 31 Mayto 4 June; Pentad 49 corresponds to 29 August to 2 September.


Region 4 (7.5◦-17.5◦N, 110◦-125◦E), which lie respec-

tively in the equatorial central Pacific, the subtropical

western North Pacific, the west of the Philippine Is-

lands, and the region around the Philippine Islands.

Significant (above 95% significance level) positive cor-

relation coefficients exist in Pentads 36-42 in Region 1.

Instead, negative correlation coefficients exist in Pen-

tads 38-43 in Region 2. Negative correlation between

OLR in Regions 2 and 1 demonstrates the existence of

a convective wave-like pattern during the early July to

late July and early August. The above results prove

that the convective activities in the early July in the

equatorial central and East Pacific, and then the con-

vective anomalies in the subtropical western North Pa-

cific can excite the formation of the acceleration and

delay of the seasonal circulation evolution in East Asia

in the late summer.

Previous researches (Nitta et al., 1986; Nitta,

1987, 1990; Kurihara and Tsuyuki, 1987; Huang and

Sun, 1992) also pointed the importance of the convec-

tive activity around the Philippine Islands. However,

no significant correlations exist between the height in-

dex (Fig.2) and the OLR variation in the west of the

Philippine Islands or around the Philippine Islands

during the late June and mid July until Pentads 43-

44 and Pentad 46. The above results indicate that

the convective activities in the west of and around the

Philippine Islands in the early summer contribute lit-

tle to the formation of the acceleration and delay of

the seasonal circulation evolution in East Asia in the

late summer.

4. Low-level circulation patterns associated

with OLR and SST anomalies

In the previous section, we have shown the strong

relationship between the height change at 200 hPa over

the key region and the convections, and weekly SST

in the equatorial central Pacific and subtropical west-

ern North Pacific. The variation of SSTA in the two

regions occurs under the influence of the Subtropical

High in the Pacific. Figure 7 depicts the linear regres-

sion fields of 850 hPa geopotential height in Pentad

36, wind differences and 850 hPa height change be-

tween Pentads 36-37 and Pentads 34-35 (Fig.7a), and

850 hPa geopotential height in Pentad 42, wind differ-

ences and 850 hPa height change between Pentads 42-

43 and Pentads 40-41 (Fig.7b) against the normalized

time series of the OLR over Region 2 in the context

based on the 1-σ standard. In the late June and early

July, the Subtropical High extends westward. Posi-

tive 850 hPa height change appears in the end of west

part of the Subtropical High, while negative height

change around 160◦E in the subtropics. The low-level

winds in the above two regions match with the increase

of SST in the west part of the Subtropical High and

with the decrease of SST around 160◦E in the subtrop-

ics. The convection enhanced around the Philippine

Islands and the Rossby wave propagation contribute to

the strengthening of the west part of the Subtropical

High. About 20 days later, due to continuous impacts

from the negative height change, the decrease of low

level winds and the Rossby wave excited by the

Fig.7. Linear regression fields of 850 hPa geopotential height at Pentad 36, wind differences and 850 hPaheight change between Pentads 36-37 and Pentads 34-35 (a), and 850 hPa geopotential height in Pentad42, wind differences and 850 hPa height change between Pentads 42-43 and Pentads 40-41 (b) (light shadesindicate negative height anomalies and heavy shades positive anomalies) against the normalized time seriesof the OLR over Region 2 in the context based on the 1-σ standard.


convection in the equatorial central Pacific, the SST

in the west of the subtropics continues to increase.

Then the convection in the subtropics around 160◦E

becomes more active, and the last response is the pos-

itive height change in the upper troposphere (Gill,

1980).

5. Concluding remarks

Through the analyses of convection and weekly

sea surface temperature in the subtropical western and

central Pacific, the possible mechanism for the anoma-

lies of the seasonal evolution of the atmospheric circu-

lations in East Asia in the late July has been discussed.

The roles of the convection around 160◦E in the west of

subtropics and the convection in the equatorial central

Pacific, SST anomalies and their subseasonal evolution

in the subtropics, their interactions with the low-level

subtropical high, and the continuing impacts from the

Rossby wave exited by the convective anomalies in

the above two regions are emphasized. The Rossby

wave patterns found by Nitta (1987) and Huang and

Sun (1992) are important only in early and middle

summer. The climatological change of the convection

around 20◦N, 160◦E is similar to the typical convec-

tion jump in Ueda (1995, 1996), but the convection

activity for the accelerated and delayed seasonal cycle

of the late summer circulation evolution has quite dif-

ferent meanings. Time-lag correlation and regression

analyses indicate that in the positive cases, the western

part of the Subtropical High around 160◦E is weakened

and the west tip of the high system becomes stronger

in late June, the convection in the equatorial central

Pacific continuous to impact the low-level subtropical

circulation in the first region, both of which lead to

the increase of SST in the western North Pacific, then

the convection is enhanced. The enhanced convective

activities trigger the formation of the local high over

the Japan Islands in the upper troposphere. The local

high around the Japan Islands pushes the Meiyu or

Baiu front northward, then ends the monsoon rainfall

in the regions of the Yangtze and Huaihe Rivers. Fur-

ther investigations will be needed on the formation of

the local high over and around Japan (Kawamura et

al., 1998), the impacts of El Niño on the convection,

especially over the equatorial central Pacific and the

MJO (Madden-Julian Oscillation) propagating from

the Indian Ocean and equatorial West Pacific.

Acknowledgments. The authors will be grate-

ful to the two anonymous reviewers for many valuable

comments and suggestions, which are helpful to the

improvement of this paper.

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