SEASONAL MARCH OF ASIAN SUMMER MONSOON
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INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 20: 1371–1386 (2000)
SEASONAL MARCH OF ASIAN SUMMER MONSOON
WEIHONG QIANa,* and DONG-KYOU LEEb
a
Department of Geophysics, Peking Uni6ersity, Beijing 100871, China
b
School of Earth and En6ironmental Sciences, Seoul National Uni6ersity, Seoul 151, South Korea
Recei6ed 7 September 1999
Re6ised 7 December 1999
Accepted 7 December 1999
ABSTRACT
This paper investigates the seasonal march of the summer monsoon through several different sub-divisions of the
Asian monsoon region. Five data sets, the Climate Prediction Center (CPC) Merged Analysis of Precipitation
(CMAP), the upper-tropospheric water vapour band Brightness Temperature (BT), the Outgoing Longwave Radia-
tion (OLR), the lower tropospheric wind and Korean daily rainfall for 1980 – 1995, are used.
Analysis shows that the largest area of deep convection in the global atmosphere is located over the tropical Indian
Ocean–equatorial western Pacific; its centre being positioned 110°E along the equator in boreal winter and then
moves northwestward to 80°E at 5°N in late July. In late April, the area of deep convection first extends northward
into the Indo-China Peninsula (ICP). In mid-May it abruptly covers over the central South China Sea (SCS) region.
After the onset of SCS monsoon, deep convection starts to develop northward along the eastern coast of China, East
Tibet Plateau (ETP) and Indian sub-continent, simultaneously. Based on the analysis of BT and lower-tropospheric
circulation, Asian summer monsoon can be divided into six inter-linked sub-regional monsoons. They are Indo-China
Peninsula monsoon, SCS monsoon, South Asian (Indian) monsoon, ETP monsoon, East Asian (south China, lower
Yangtze River and Japan) monsoon and Northeast Asian (north China and Korea) monsoon. In seasonal course,
their onset periods take place in late April–mid-May, mid-May, mid-May – late July, mid-May – early June, mid-
May–late June and late June–mid-July, respectively. Copyright © 2000 Royal Meteorological Society.
KEY WORDS: Asia; brightness temperature; composite analysis; precipitation; summer monsoon; wind
1. INTRODUCTION
From boreal winter to summer, the atmospheric circulation systems experience obvious seasonal transi-
tion in the Asian monsoon area, in which a set of abrupt changes occur in atmospheric systems, marking
the end of the winter monsoon and the beginning of the summer monsoon. Onset time and evolution
processes of summer monsoon show significant difference in various monsoon regions due to land–ocean
heating contrasts. In spite of the long history of research on the Asian monsoon, our knowledge about
the monsoon is limited in the understanding of the South Asian and East Asian components for the
months June–August (Lau and Li, 1984; Krishnamurti, 1985; Tao and Chen, 1987; Lau et al., 1988; and
others).
Recent studies using observation data showed that the summer rainfall exhibits very different
anomalous patterns in southern, central and northern China (Wang et al., 1998). From satellite proxy
data, it has been identified that the South China Sea (SCS) monsoon is linked to the deep convection over
the Indo-China Peninsula (ICP) (Ding et al., 1996; Lau and Yang, 1997) and the coastal rainfall of
southern China (Luo, 1999). It has also been understood that the Baiu rainfall regime in Japan is
associated with the onset of Meiyu rainfall in East China (lower Yangtze River valley) and that the
Changma rainfall in Korea is connected to the rainfall of northern China (Lau and Li, 1984; Krishna-
murti, 1985). Another regional monsoon, prevailing along the east edge of the Tibet Plateau from the Bay
of Bengal extending northward to the upper Yangtze River and upper Yellow River valleys, was referred
* Correspondence to: Department of Geophysics, Peking University, Beijing 100871, China; e-mail: qianwh@pku.edu.cn
Copyright © 2000 Royal Meteorological Society1372 W. QIAN AND D.-K. LEE
to as the Plateau monsoon by Chinese meteorologists (Xu and Gao, 1962; Tang et al., 1979; Tang, 1989).
Xu and Gao (1962) indicated that the monsoon over the Plateau is different from that in the eastern part
of China because a heat low is usually located over the Plateau in boreal summer while a cold high in
winter. Recently, a case study showed that the earliest summer monsoon onset occurs not in the SCS
region but in the land of the ICP and the eastern coast of the Bay of Bengal (Wu and Zhang, 1998). In
this study, the domain of Asian summer monsoon is divided into several sub-regions as shown in Figure
1. The region of ICP is located in the south of Mainland China, including Myanmar, Thailand and other
counties. The SCS is between ICP and The Philippines, from the equator to the south coast of China.
South Asia (SA) includes India and its adjacent oceans. Eastern Tibet Plateau (ETP) is confined between
90 and 105°E over the upper Yangtze River valley and the upper Yellow River valley. East Asia (EA)
ranges from south China, to mid – low Yangtze River valley, to East China Sea and, finally, to Japan.
Northeast Asia (NEA) covers north China and the Korean Peninsula. According to the SCS monsoon
experiment (SCSMEX) project and the investigations of recent years, the onset of SCS summer monsoon
was found in mid-May or the fourth pentad of May (Lau and Yang, 1997; Xie et al., 1998; Ding et al.,
1999; Jin, 1999; Li and Qu, 1999; Luo, 1999). However, the seasonal march of summer monsoon in
different sub-regions and their relationship related to the SCS summer monsoon still remain unknown.
In this paper, emphasis will be placed on the seasonal march of Asian summer monsoon for these
sub-regions. The datasets used here include the satellite measured High-resolution Infrared Radiation
Sounders (HIRS) and Outgoing Longwave Radiation (OLR), the lower tropospheric winds and the
rainfall measurements from the US Climate Prediction Center (CPC) (Xie and Arkin, 1997) and the
Korea Meteorological Administration.
In Section 2, data sources are described. Basic features of the deep convection and rainfall are
illustrated in Section 3. The seasonal march of the summer monsoon before and after the SCS summer
monsoon onset is depicted in Sections 4 and 5, respectively. A schematic description of the progression of
the monsoon onset across the Asian monsoon region between April and July is provided in Section 6.
Figure 1. Map of Asia and adjacent oceans, identifying geographical features of area cited in the paper
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)ASIAN SUMMER MONSOON 1373
2. SOURCES OF DATA
In the present study, the pentad data of upper tropospheric water vapour (UTWV) contents inferred from
HIRS water vapour band Brightness Temperatures (BT), low-tropospheric winds, CPC Merged Analysis
of Precipitation (CMAP), OLR and Korean daily rainfall are used.
2.1. HIRS12 data and OLR data
To remedy the lack of precipitation data over the sea and plateau, various satellite-observed datasets
have been used over the past decades to diagnose the climatological characteristics of monsoon.
Originally, Murakami (1980) and others analysed the summer-mean pattern by using OLR data. Lau and
Yang (1997) used NOAA satellite-derived Advanced Very High-Resolution Radiometer data from the
NASA Data Assimilation Office reanalyses for 1985–1993, to study the climatology and interannual
variability of SCS monsoon. Xie et al. (1998) used the OLR data for the period 1979–1995 to define the
onset of SCS summer monsoon. Previous studies show that satellite data with various measures can reveal
some basic features of rainfall and convection over only low-latitude regions. Some investigators indicated
that the OLR data does not clearly show the East Asian rainbelts (Murakami, 1980; Lau et al., 1988).
In recent years, satellite observations have been applied to the study of climatology. For example, the
blackbody temperature (Tbb) and the UTWV band BT have been used to study the monsoon (Qian et al.,
1998a; Jin, 1999). For the water vapour band BT, three channels are sensitive to variations in tropospheric
water vapour. HIRS channel 12 (or HIRS12) is designed to measure the amount of water vapour in the
upper troposphere. The lower HIRS12 value represents a higher water vapour content in the upper
troposphere, while the higher HIRS12 value typifies the persistent subsidence region in the subtropics,
with warm temperature, but relatively little moisture. In the free-cloud area, HIRS12 retrievals tend to
provide representation at around 300 hPa for the wet tropical atmosphere. Wu et al. (1993) and Bates et
al. (1996) described the climatological meaning of HIRS12.
Five-day averaged (total 73 pentads per year) UTWV band BT dataset and OLR dataset are available.
Those datasets are the observations from NOAA polar-orbiting satellites from 1979 to 1995, with a
horizontal resolution of 2.5°. Unfortunately, HIRS12 data were not available in 1979 in high-latitude
regions for all grid points. The missing points in the HIRS12 dataset total about 5% in the tropical
regions and up to 40% in the polar regions of the total. Around cloudy points, there should be a relatively
lower BT. To obtain missing data in mid – low latitudes, the method of interpolation had been used in
previous work (Qian et al., 1998b). In this study, the HIRS12 data and OLR data covering the latitudes
between 40°S and 40°N from 1980 to 1995 were applied.
2.2. Atmospheric wind data
The low-tropospheric wind can characterize the changes of monsoon circulation, especially in the
mid–low-latitude regions. The US National Center of Environmental Prediction (NCEP) reanalysis wind
data and the US National Meteorological Center (NMC) global analysis wind data with a spatial
resolution of 2.5° were compared. They are basically similar to each other in the lower tropospheric flow
fields. In this paper, NMC and NCEP wind datasets for 1980–1995 are used to study the lower
tropospheric circulation.
2.3. Rainfall data and CMAP data
Daily surface observed rainfall data over South Korea were obtained from the Korea Meteorological
Administration for 1980 – 1995. In the early 1980s, there were more than 50 rainfall observation stations
and the number of stations has been increased to 72 in the 1990s. The network adequately covers South
Korea from south Cheju Island (34°N) to 38°N and from west coast (126°E) to the east coast (130°E).
Another rainfall dataset used is the CMAP for 1980–1995 with horizontal resolution of 2.5°. The
CMAP dataset provides the pentad precipitation by merging rain gauge, five kinds of satellite estimates
(GPI, OPI, SSM/I scattering, SSM/I emission and MSU) and numerical model predictions.
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)1374 W. QIAN AND D.-K. LEE
All the five datasets, the HIRS12, CMAP, OLR, wind and the Korean rainfall, are used in composite
analysis and then in describing the convective strength and seasonal variation of the summer monsoon in
the temporal–spatial domains over the Asian region. Multiple-year composite averages of daily data and
5-day time-means are also used to discuss the seasonal march of the Asian summer monsoon.
3. DEEP CONVECTION AND ITS VARIATIONS
There is a closed relationship between summer monsoon rainfall and deep convection. In this section,
basic features of deep convection and rainfall, particularly over the Asian region, are described.
3.1. Definition of the onset of deep con6ection
Previous findings, e.g. the studies by Lau and Yang (1997) and Xie et al. (1998), showed that the onset
date of SCS summer monsoon is around mid-May. The SCS summer monsoon onset is representative of
deep convection in the whole SCS region, so variations of BT, OLR and CMAP in the SCS region are
examined and then the criteria of deep convection is determined based on the variation of these variables.
Figure 2 shows the annual course of climatological mean CMAP, BT, OLR and NMC 850 hPa zonal
wind, averaged from 1980 to 1995 in the area of 10°–20°N, 110°–120°E. These four time series show
good similarity in phase change at the fourth pentad of May (or 28th pentad: 16–20 May). Consequently,
criteria can be determined for each variable to characterize the deep convection over the region. For
CMAP, it abruptly increases from about 4 mm/day in the 27th pentad (11–15 May) to 8 mm/day in the
28th pentad (16– 20 May), so that the criterion of deep convective precipitation may be about 5–6
mm/day. For HIRS12, the criterion of deep convection could be determined as around 244 K. The
convective criterion of OLR is about 230 W/m2. The onset date of SCS monsoon can be defined as the
time when the area-averaged 850 hPa pentad zonal wind-direction changes from easterly to westerly.
According to the criterion of HIRS12, the deep convection (shaded areas less than 244 K) of
multiple-year climatological mean for 1980 – 1995 is given in Figure 3. The largest area of deep convection
over the low-latitudes is located near the Asian monsoon region from the equatorial eastern Indian Ocean
to the equatorial western Pacific. Another two areas of deep convection appear in equatorial America and
equatorial central-western Africa. Areas with high values in sub-tropical regions show strong subsidence.
In Asia, the area of largest deep convection gradually or abruptly extends northward depending upon
the sub-regions, and finally leads to summer monsoon onset over the whole of Asia from late April to
July. In July, the deep convection can extend to the mid-latitude region, such as the Korean Peninsula.
Figure 2. Climatological mean time series of CMAP (mm/day), HIRS12 (K), OLR (W/m2) and 850 hPa zonal wind (UWIND, m/s)
in the SCS area of 10° – 20°N, 110° – 120°E
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)ASIAN SUMMER MONSOON 1375
Figure 3. HIRS12 distribution of multiple-year climatological mean for 1980 – 1995. The criterion of deep convection is defined as
244 K. Three deep convective (shaded) areas located in the global tropical region
To determine the representativeness of HIRS12, the daily area-averaged rainfall of South Korea is used
to calibrate the BT against the local precipitation. Figure 4 shows both temporal evolutions of the BT
with the average of two grids (35°N, 127.5°E and 37.5°N, 127.5°E) and the averaged precipitation (solid
line with circle) in South Korea. The dashed line shows that two major wet periods appear in mid-July
(39th pentad) and late July (41st pentad). Although a rain peak accompanied by a relatively low BT
occurs during 22 – 25 June, the major summer rainfall appears in 10–16 (mid-)July and late July.
According to Figure 4, the relatively higher rainfall (around or greater than 10 mm/day in July) is
associated with BT lower than 244 K. Therefore, the BT with the criterion less than 244 K in the
mid-latitude can also be used to represent the local rainfall measurement in the monsoon season.
3.2. Seasonal 6ariation of deep con6ection
Figure 5 shows the annual course of climatological mean BT with 73 pentads from January to
December along 30°, 20°, 10°N and the equator, respectively. The cross-shaded areas (less than 244 K)
indicate the wet upper troposphere or deep convection. It can be clearly seen that there are three
concentrated deep convection regions along the equator. The changes in the strength of deep convection
with time in the equatorial region between 60° and 170°E are smaller than those to the north of the
equator. The heavy solid lines indicate the seasonal variation of deep convective centre with time. The
centre gradually shifts westward from boreal spring to late June. In July, the centre reaches 80°E, just in
the meridian of the Indian sub-continent. After July, the centre begins to retreat eastward and returns to
Figure 4. The temporal variations of climatological mean BT (K) in South Korea (dashed line) with the average of two grids (35°N,
127.5°E and 37.5°N, 127.5°E) and the daily area-averaged rainfall (solid line, mm/day) of South Korea from June to July (total 61
days). Data collected between 1980 and 1995. Numbers in the figure indicate pentads related to the calendar year
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)1376 W. QIAN AND D.-K. LEE
110°E at the end of year. Along the equator, there is no change in deep convection or without dry/wet
transition in the upper troposphere. However, off the equator, transitions in deep convection obviously
exist, especially along 10° and 20°N. Deep convection starts from April at 100°E and ends in November
near the ICP, along 10°N. Also along 10°N there are four centres of deep convection in the boreal
summer. The largest centre appears in the meridian (80°E) of the Indian sub-continent. The other three
centres are related to the African monsoon at 15°E, SCS monsoon at 115°E and North American
monsoon at 90°W, respectively. Along 20°N, two centres of deep convection are to be found in Southeast
Asia and North America. Along 30°N, a small cross-shaded area at 85°E in August shows probably the
northern edge of the Indian summer monsoon but East Asian summer monsoon and North American
summer monsoon also appear in boreal summer. An important feature in Figure 5 is that the deep
convection centres gradually move westward from boreal spring to summer.
Figure 5. The annual course of climatological mean UTWV band BT (K) with 73 pentads of the year along 30°, 20°, 10°N, and
the equator, respectively. The cross-shaded areas indicate relatively high water vapour content or deep convection (BT less than
244 K)
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)ASIAN SUMMER MONSOON 1377
Figure 6 shows the annual course of BT along 80° and 115°E. In Figure 6(a), the transition in deep
convection appears between 5° and 30°N. Between 5°S and 5°N there is no dry/wet transition, except that
its wet centre gradually moves northward from January–February to July–August at 5°N. The summer
monsoon in the south of the Indian sub-continent (10°N) starts in mid-May and ends in November.
Along 20°N, the summer monsoon starts in June and ends in September. The northern edge of the Indian
summer monsoon is located near 30°N and its climatological mean period is only about one month
starting from late July. Starting from the summer monsoon onset in India, the strengthened subsidence
appears in the sub-tropical Southern Hemisphere with higher BT.
In Figure 5, there is a convective centre in the SCS region along 10°N. The relative distribution of BT
along 115°E is shown in Figure 6(b). This figure depicts the seasonal variation of the SCS monsoon. It
can be noted that in the SCS region between 10° and 20°N, the deep convection or summer monsoon has
an abrupt outbreak in mid-May or about the 28th pentad. Also in Figure 6(b), the retreat of the summer
monsoon starts from southern China in September and ends late in the year.
From Figures 5 and 6, it can be identified that the most intense summer monsoon appears in the Indian
sub-continent along 80°E, while the abrupt onset of the summer monsoon occurs in the central SCS.
4. EARLY SUMMER MONSOON
In previous studies, the Asian monsoon is divided into several sub-parts according to the geographical
location. Lau and Yang (1997) divided the Asian monsoon into South Asian, East Asian and Southeast
Asian sub-monsoons. The Southeast Asian sub-monsoon prevails over the land region of Indo-China,
southern China, the SCS and the Philippines. Here, we divide the Asian monsoon into the early summer
monsoon and the summer monsoon. The regions of the Indo-China Peninsula and the SCS are included
within the area of the early summer monsoon because the seasonal transition of atmospheric circulation
in the Asian area happens when the SCS summer monsoon begins. Firstly, we describe the basic
characteristics of the early summer monsoon as compared with the situation after the SCS monsoon
onset. Figure 7 shows daily mean precipitation before onset of the SCS summer monsoon and after the
onset for five-pentad mean based on the CMAP data. The time-mean data before the SCS monsoon onset
from the 23rd to the 27th pentad (22 April–15 May) show that there are two rainbelts along the
low-latitudes and the mid-latitudes. The latter rainbelt extends from southern China to the northwest
Pacific Ocean, including the south part of Japan. Two rainbelts are separated in the central SCS region.
The former is referred to as the tropical convection rainfall; the latter being referred to as the westerly
frontal rainfall (Bao, 1987) in South China, from April to early May. After the onset of the SCS summer
monsoon for the period between the 28th pentad and the 32nd pentad (16 May–10 June), the rainbelts
obviously undergo a significant change. From the low-latitude Indian Ocean through to the SCS to the
northwest Pacific Ocean, including the south part of Japan, there is no gap along the rainbelt. After the
onset of SCS monsoon, another feature emerges: the precipitation centres are located in oceanic regions,
such as the Arabian Sea, the Bay of Bengal, SCS and the equatorial western Pacific Ocean.
Now, HIRS12 and OLR are simultaneously applied to examine the precipitation, the five-pentad mean
BT and OLR. The results are shown in Figures 8 and 9. Obviously in Figure 8, the UTWV measurements
support the CMAP data in tropical and mid – low latitudes, but the centres of deep convective precipita-
tion are not well indicated by BT in detail.
In the climatological mean map (Figure 9(a)), centres of relatively lower OLR located near or over
tropical lands or islands, appearing in ICP, Sumatera, Kalimantan and New Guinea. These distributions
are consistent with the precipitation centres appearing in the tropical region; however, from the southern
coast of China to the northwest Pacific there is no information to show the rainbelt. After the onset of
SCS monsoon, the area of lower OLR expands and its centre is positioned in the Bay of Bengal and the
ICP, which agrees with the deep convective precipitation in the tropical region. Note in Figure 9(b) that
relatively higher OLR appears in the northwest SCS and Taiwan regions, while the lower OLR occurs
over central-eastern China and Japan. Obviously, OLR does not indicate precipitation in the mid-latitude
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)1378 W. QIAN AND D.-K. LEE
Figure 6. Same as in Figure 5 but with 73 pentads along (a) 80°E and (b) 115°E, respectively
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)ASIAN SUMMER MONSOON 1379
Figure 7. Daily climatological mean precipitation (mm/day) in the periods of (a) 23 – 27 pentads and (b) 28 – 32 pentads according
to CMAP data
region either before or after the onset of the SCS summer monsoon. This result is in agreement with the
investigation by Murakami (1980) and Lau et al. (1988).
Finally, NMC wind data are needed to display climatological mean flow fields at the 850 hPa level
(Figure 10). Before the onset of the SCS monsoon (Figure 10(a)), a sub-tropical anticyclone in the
northwest Pacific controls the central SCS region and southeast ICP. Along 10°N, the southerly flows
converge only in central ICP near 100°E, while other areas including South Asia and east–northeast Asia
are controlled by northwesterly flows. From southern China to northwest Pacific near southern Japan,
there are southwesterly flows, which are the marginal flow of the sub-tropical anticyclone and consistent
with the rainbelt variations in time and in space. After the onset of the SCS monsoon (Figure 10(b)), an
anticyclone circulation, related to the sub-tropical Pacific high, has retreated eastward to the east of the
Philippine islands and the southwesterly monsoon flows become dominant over the SA and EA regions.
Also, the strengthened convergence of southwesterly, southerly and southeasterly winds extend from the
SCS to northwestern Pacific, which is favourable for the deep convection in this area. Another two
features in Figure 10 are that the ridgeline (dashed line) of the sub-tropical high has retreated from the
north of the Philippine islands to the mid-part of the Philippine islands and the westerly wind has
strengthened in the south Bay of Bengal after onset of the SCS summer monsoon.
The four figures depicted above show that HIRS12 data can be used to characterize deep convective
rainfall in both tropical and mid-latitude regions, and describe the descending domain of the sub-tropical
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)1380 W. QIAN AND D.-K. LEE
Figure 8. Same as in Figure 7 but for climatological mean UTWV band BT (K) in the periods of (a) 23 – 27 pentads and (b) 28 – 32
pentads based on the NOAA satellite measurement
high very well, while the OLR data are particularly useful for the description of deep convection in
tropics. However, it should be pointed out that the deep convection concerned with monsoon rainfall
must link directly to the largest centre of tropical deep convection. In other words, the Asian summer
monsoon should be defined as the expansion of deep convection from the largest convective region over
the tropical Indian Ocean. According to this definition, the earliest monsoon in the Asian region appears
in the Indo-China Peninsula from late April to mid-May (27 pentad). The precipitation that appears in
southern China and the northwest Pacific before the onset of SCS monsoon possesses the nature of the
westerly frontal one (Bao, 1987). As shown in Figure 8(b) there is a tendency for deep convection to
advance northward over the south Indian sub-continent, ETP along 100°E and East China Sea. It means
that since mid-May not only the SCS monsoon rapidly breaks out but also the SA monsoon, ETP
monsoon and EA monsoon start to set up from south to north in coming pentads.
5. MARCH OF SUMMER MONSOON
Around mid-May (the 28th pentad; 16 – 20 May), the Somali jet strengthens and the westerly winds
become dominant over the SA region, turning southwesterly over the ETP, SCS and EA regions (Figure
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)ASIAN SUMMER MONSOON 1381
Figure 9. Same as in Figure 7 but for climatological mean OLR (W/m2) in the periods of (a) 23 – 27 pentads and (b) 28 – 32 pentads
based on the NOAA satellite measurement
10(b)). These changes are associated with the onset of the summer monsoon in all of these regions.
According to Figure 6(a), the mean dates of the summer monsoon onset along the Indian meridian with
the BT less than 244 K start in the 28th pentad (mid-May) and end in the 41st pentad (late July).
As shown in Figure 8(b), deep convection obviously starts to develop along the ETP near 100°E in
mid-May (28 pentad). Figure 11 shows the climatological mean time-latitude section of the BT along
100°E. There are three times of northward expansions (heavy lines) or jumps of deep convection in April,
May and June, respectively. The first rapid jump, reaching 15°N, happens in the end of April; the second
reaching 25°N occurs in mid-May; and the last one reaching 35°N appears in mid-June. The first one
relates to the onset of ICP summer monsoon; the second appears to be the time of the onset of SCS
monsoon and the last one is associated with the outbreak of ETP summer monsoon around the 32nd
pentad. Between 30° and 35°N along 100°E, there are two periods in June and September when the deep
convection expands northwards.
To visualize the domain of ETP summer monsoon, deep convection of the Asian region in the 33rd
pentad (10–14 June) is shown in Figure 12. The centre of major deep convection is still located in between
the equatorial Indian Ocean and equatorial western Pacific. At this time, deep convection (BT less than
244 K) along 100°E has extended northward to the upper Yellow River from the northern Bay of Bengal
through Myanmar and the upper Yangtze River. In India, deep convection has reached the central
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)1382 W. QIAN AND D.-K. LEE
Figure 10. Same as in Figure 7 but for climatological mean streamline and wind speed (m/s) in the periods of (a) 23 – 27 pentads
and (b) 28–32 pentads using the NMC data. The dashed line indicates the ridgeline of subtropical high
sub-continent. In the East China Sea and Japan, deep convection has developed, but central China
remains relatively dry. According to the criterion of BT, the Meiyu deep convection in the mid–low
Yangtze River does not start at this time.
As the summer rainfall character in the lower Yangtze River is usually different to that in the Yellow
River (Wang et al., 1998), two regions have been identified: one relates to East Asia and another to
Northeast Asia. Then, representativeness of BT for EA and NEA summer monsoons are examined.
Figure 13 shows the climatological mean of UTWV band BT for the period 1980–1995 along 125°E.
From April to early May, the shaded area between 21° and 30°N shows the sub-tropical convection
discussed in Section 4. From mid-May (SCS monsoon onset) to the beginning of June, deep convection
appears between 16° and 31°N. In early June there is a short dry period in the EA and NEA between 20°
and 38°N. The Meiyu with the criterion of 244 K in mid-latitude near 30°N starts from mid-June to July.
In mid-July, deep convection develops at the Yellow Sea (35°N) near South Korea. It is found that the
periods of the Meiyu in East China and the Changma in Korea are relatively shorter than the rainy season
in the tropical region.
Since the onset of SCS monsoon, southwesterly wind steadily increases and extends northward in
southern China and East China Sea. From the climatological mean in July along 120° and 125°E (figure
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)ASIAN SUMMER MONSOON 1383
Figure 11. The annual course of climatological mean UTWV band BT (K) from January to December (73 pentads) along 100°E.
The shaded areas indicate relatively high water vapour content or deep convection (BT less than 244 K). The heavy lines indicate
the northward expansions of deep convection
Figure 12. Climatological mean UTWV band BT (K) over the Asian region for the 33rd pentad (10 – 14 June). The shaded areas
(less than 244 K) represents wet upper-troposphere or deep convective areas
omitted), the southerly component calculated by NMC and NCEP data steadily passes 35°N after 6
July and then stagnates near 37°N up to mid-July. The feature of convergence is seen more clearly in
the 700 hPa-wind field (Figure 14) than in the 850 hPa-wind field, calculated by NCEP wind data.
During 2–6 July, the convergence line of both southwesterly and northwesterly winds is located in the
Huaihe River between lower Yangtze River and lower Yellow River but during 7–11 July the conver-
gence line migrates northward to the lower Yellow River. In 2–6 July, the southwesterly wind has
started to influence the southern edge of the Korean Peninsula. Therefore, according to Korean
precipitation or BT and lower tropospheric circulation, the onset date of NEA summer monsoon can
be determined as the period of 37 – 39 pentads (late June–mid-July) while EA monsoon as 28–36
pentads (mid-May – late June).
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)1384 W. QIAN AND D.-K. LEE
Figure 13. Same as in Figure 11 but along 125°E
6. A SCHEMA OF ASIAN MONSOON ONSET
Detailed examination of monsoon onset in a number of different sub-regions allows us to build up a
schematic view of the progression of monsoon onset across the Asian monsoon region between April and
July. The schematic representation is depicted in Figure 15.
The domain of the Asian summer monsoon includes the Indian sub-continent, the Yangtze and Yellow
River valleys, South China, Korean Peninsula, mid-south Japan, ICP, SCS, and all coastal seas from the
Arabian Sea in South Asia to the Japan Sea in Northeast Asia. In the west of the Asian summer monsoon
domain, there is the driest region ranging from North Africa to Southwest Asia. In the south of the
domain, there is the largest centre of deep convection in the atmosphere. In the equatorial Indian Ocean
between 5°S and 5°N there is no dry/wet transition in the deep convective region. Normally, tropical deep
convection over the Indian Ocean extends northward from late April to July.
Southwesterly flows (850 hPa) first reach the ICP and the coastal region of southern China from the
24th (27–31 April) to the 27th pentad (11 – 15 May). In this period, deep convection first develops from
south to north along the ICP. So, in the domain of the Asian summer monsoon, the earliest summer
monsoon breaks out from the ICP. At this time, the sub-tropical anticyclone in the northwest Pacific
Ocean still controls the SCS and the southeast edge of the ICP. The precipitation in southern China and
the northwest Pacific Ocean possesses the nature of the westerly frontal one.
Around mid-May (the 28th pentad), the Somali jet strengthens and the westerly winds become
dominant over the SA region, turning southwesterly over the ETP, SCS and EA regions, and these
changes are associated with the onset of monsoon rainfall in all of these regions. During this time, several
events occur in the SCS region. The wind direction turns from easterly to westerly and the sub-tropical
anticyclone retreats from the SCS region to the western Pacific. Under these circumstances, the deep
convection indicated by HIRS12, OLR and strong precipitation abruptly take place in the central SCS
region.
From mid-May, the summer monsoon starts to break out over South Asian and East Asian regions.
The South Asian summer monsoon gradually starts to set up from the southern edge (around 10°N) of
the Indian sub-continent in the 28th pentad (16–20 May) to 30°N in the 41st pentad (20–24 July). ETP
summer monsoon starts to break out from south to north within the period of 28–32 pentads (between
16–20 May and 6 – 10 June) near 100°E. The East Asian summer monsoon, in general, begins in the 28th
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)ASIAN SUMMER MONSOON 1385
Figure 14. Climatological mean wind fields at the 700 hPa for (a) 2 – 6 July and (b) 7 – 11 July. Depth — solid line indicates the
convergence over central eastern China. A scale of the wind (m/s) is given in the lower right corners
Figure 15. A schematic representation of the progression of monsoon onset across the Asian monsoon region between April and
July. Pentads (defined with respect to the calendar year) are indicated
Copyright © 2000 Royal Meteorological Society Int. J. Climatol. 20: 1371 – 1386 (2000)1386 W. QIAN AND D.-K. LEE
pentad (16–20 May) from southern China and the East China Sea and then gradually advances to the
mid–low Yangtze River valley in the 36th pentad (26–30 June). The Meiyu, the summer monsoon rainfall
in the mid–low Yangtze River valley, usually follows the rainy spell in southern China and Japan after
the SCS monsoon onset. Northeast Asian summer monsoon onset starts mainly from the 37th pentad
(late June) around 35°N to the 39th pentad (mid-July) in the lower Yellow River valley and Korean
Peninsula.
ACKNOWLEDGEMENTS
The authors wish to thank Dr P.P. Xie for providing CMAP data sets. They extend their thanks to two
anonymous reviewers for many useful comments. The Korea–China Exchange Program of Scientific
Exchange supported the first author’s visit to Seoul National University. This research was also supported
by the National Natural Foundation of China (Contract No. 49975023) and the Development of Regional
Climate Model/Technology for Climate Change Prediction of the Korean government. The first author
would like to thank Professors S. Wang and D. Shen for help with the English text.
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