LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences

1
Institute on The Asian Monsoon System
Prediction of Change and Variability, Jan 2-12
2008, University of Hawaii, USA
The East Asian summer monsoon system Climatology
and interannual variability
Tianjun ZHOU E-mail zhoutj_at_lasg.iap.ac.cn

• LASG, Institute of Atmospheric Physics, Chinese
  Academy of Sciences

2
Outline

• The East Asian summer monsoon circulation system
• Diurnal cycle of monsoon rainfall
• Water vapor transport and rainfall anomalies of
  China
• Impact of ENSO on the climate of China varies
  with the stage of ENSO
• The relation between the Indian summer monsoon
  and East Asian monsoon

3
Division of Asian-Pacific Monsoon
(Wang and Lin,2002)
4
A schematic depiction of the summer monsoon
system in East Asia (Tao and Chen, 1987)
(Tao and Chen ,1987)
5
East Asian summer monsoon components
(Figure from Yihui Ding)
6
(Figure from Ronghui Huang)
7
Advance of the summer monsoon over the Asian
monsoon region
Figures indicate the pentad and month, for
example, 5.4 means the fourth pentad of May (Tao
and Zhang, 2004)
8
Retreat of the E. Asian summer monsoon
Retreat of the summer monsoon, 9.5 means 5th
September ( Chen et al., 1991 )
9
Migration of the summer monsoon rain belt
10
Seasonal march of East Asian summer monsoon a
stepwise northward and northeastward advance

• During the period from early May to mid May,
  southern China experiences a pre-monsoon rainy
  season.
• The monsoon rain then extends abruptly from the
  Indochina Peninsula the South China Sea from
  the Philippines to the Yangtze River valley in
  early to mid June,
• and finally penetrates northern China (34º41ºN)
  in mid July.
• The rainy season in northern China generally
  lasts for one month and ends in the early or
  middle part of August.
• From the end of August to early September, the
  monsoon rain belt rapidly moves back to southern
  China.

11
Outline

• The East Asian summer monsoon circulation system
• Diurnal cycle of monsoon rainfall
• Water vapor transport and rainfall anomalies of
  China
• Impact of ENSO on the climate of China varies
  with the stage of ENSO
• The relation between the Indian summer monsoon
  and East Asian monsoon

12
The phase and amplitude (normalized by the daily
mean) of the 1991-2004 mean diurnal cycle of
summer (JuneAugust) hourly precipitation
Yu, Zhou et al. 2007a GRL
13
Diurnal variations of the 1991-2004 mean summer
precipitation averaged over the five regions
Yu, Zhou et al. 2007a GRL
14
Hovmöller diagrams of mean diurnal variations in
hourly precipitation percentage relative to the
daily total rainfall amount (in unit of ). (a)
Time-longitude cross section for the 27?-29?N
zone, (b) Time-latitude cross section averaged
over the 110?-130?E.
Yu, Zhou et al. 2007a GRL
15
The percentages to the total warm season (May to
September) rainfall in rainfall frequency (black
isograms) and rainfall amount (colored) averaged
from 1-3 hours (a) and more than 6 hours (b)
duration rain events respectively
Yu, Zhou et al. 2007b GRL
16
The diurnal phase of the precipitation during May
to September of 1991-2004 according to raining
duration with more than 6 hours (a) and 1-3 hours
(b) respectively
Yu, Zhou et al. 2007b GRL
17
The normalized diurnal variations of
precipitation, averaged over the central eastern
China for durations of 1-3 hours (dot-dashed line
with triangle), 4-6 hours (dashed line with
filled square), 7-12 hours (dashed line with open
circle), and more than 12 hours (solid line with
filled circle)
Yu, Zhou et al. 2007b GRL
18
Normalized diurnal variation of precipitation at
three stations of (a) Shangqiu, (b) Wuhan, and
(c) Ganzhou. The grey dashed lines are the total
normalized diurnal variations. The filled circles
show the normalized diurnal variations resulted
from 1-3 hour duration rainfall events. The
filled squares present the normalized diurnal
variations in longer duration rain events, 9-12
hour duration in (a) and 13-18 hour duration in
(b) and (c)
Yu, Zhou et al. 2007b GRL
19
Diurnal cycle of precipitation amount, frequency,
and intensity
Zhou et al. (2007), J. Climate, revised
20
Mean diurnal cycle of JJA precipitation amount
(normalized by the daily mean) averaged over the
five selected regions
Zhou et al. (2007), J. Climate, revised
21
Mean diurnal cycle of JJA precipitation frequency
(normalized by the daily mean) averaged over the
five selected regions
Zhou et al. (2007), J. Climate, revised
22
Diurnal cycle of summer monsoon rainfall in China

• Summer precipitation over contiguous China has
  large diurnal variations with considerable
  regional features.
• Over southern inland China and northeastern China
  summer precipitation peaks in the late afternoon,
  while over most of the Tibetan Plateau and its
  east periphery it peaks around midnight.
• The diurnal phase changes eastward along the
  Yangtze River Valley, with a midnight maximum in
  the upper valley, an early morning peak in the
  middle valley and a late afternoon maximum in the
  lower valley.
• Summer precipitation over the region between the
  Yangtze and Yellow Rivers has two diurnal peaks
  one in the early morning and another in the late
  afternoon.

23
Diurnal cycle of summer monsoon rainfall in China
The longer duration rainfall events (i.e. rain
events lasting for more than 6 hours) are robust
and contribute significantly to the total
rainfall (over 60) in the key region (105-120?E,
26-36?N). The longer duration rainfall events
prefer the maximum hourly rainfall around early
morning, against the shorter duration rainfall
events (1-3 hours duration) with peak rainfall in
late afternoon. The find reveals the existence
of the two peaks in the mean diurnal
precipitation variation over central eastern
China. Two rainfall peaks in mean diurnal
variation should be mostly resulted from two
kinds of distinguishable rainfall events, the
longer duration rainfall events with maximum
around early morning and the shorter duration
rainfall events with maximum at late afternoon
respectively, rather than two peaks rainfall
always in each rainfall event.

24
Diurnal cycle of summer monsoon rainfall in China
The longer duration rainfall events (i.e. rain
events lasting for more than 6 hours) are robust
and contribute significantly to the total
rainfall (over 60) in the key region (105-120?E,
26-36?N). The longer duration rainfall events
prefer the maximum hourly rainfall around early
morning, against the shorter duration rainfall
events (1-3 hours duration) with peak rainfall in
late afternoon. The find reveals the existence
of the two peaks in the mean diurnal
precipitation variation over central eastern
China. Two rainfall peaks in mean diurnal
variation should be mostly resulted from two
kinds of distinguishable rainfall events, the
longer duration rainfall events with maximum
around early morning and the shorter duration
rainfall events with maximum at late afternoon
respectively, rather than two peaks rainfall
always in each rainfall event.

25
Diurnal cycle of summer monsoon rainfall in China
The diurnal phases of frequency and intensity are
similar to that of rainfall amount in most
regions except for middle Yangtze River valley.
Both frequency and intensity contribute to the
diurnal variation of rainfall amount over most
eastern China, as evidenced by the relative
similarity in the composite diurnal curves for
amount, frequency, and intensity. The satellite
products overestimate frequency but underestimate
intensity of rainfall. The contribution of
frequency to the diurnal cycle of rainfall amount
is generally overestimated in both satellite
products. Although the nocturnal peak over
eastern periphery of the Tibetan Plateau, the
late afternoon peak in southern China and
northeastern China are captured in both satellite
products, the quality of satellite products in
the regions between the Yangtze and Yellow
Rivers, where a robust semi-diurnal harmonic
dominates, is poor.
26
Outline

• The East Asian summer monsoon circulation system
• Diurnal cycle of monsoon rainfall
• Water vapor transport and rainfall anomalies of
  China
• Impact of ENSO on the climate of China varies
  with the stage of ENSO
• The relation between the Indian summer monsoon
  and East Asian monsoon

27
P-J Oscillation (Nitta, 1987) EAP Pattern
teleconnection (Huang and Li, 1987,1988)
(Figure from Ronghui Huang)
Schematic diagram of the East Asia/Pacific
pattern teleconnection of summertime circulation
anomalies over the Northern Hemisphere.
28
H
warming state
Anomaly
H
H
cooling state
Anomaly
Schematic map of the relationships among the SST
in the tropical western Pacific (TWP), the
convective activities around the Philippines, the
western Pacific subtropical high, the onset of
the South China Sea summer monsoon (SCSM) and the
summer rainfall in China and its surrounding
regions.
(Figure from Ronghui Huang)
29
Standard deviation of JJA mean rainfall revealed
by the station data (mm/day). Dots indicate
locations of the observational stations.
Vertically integrated climate mean (1951-1999
average) JJA water vapor transport (kgm-1s-1).
Zhou and Yu 2005 JGR
30
The leading modes of JJA mean rainfall for
1951-1999. (a) EOF1, (b) EOF2. Patterns are shown
as percentage of rainfall anomaly relative to
climate mean state associated with one standard
deviation of the corresponding PC change.
Zhou and Yu 2005 JGR
31
The normalized principal component corresponding
to the leading mode of (a) EOF1, and (b) EOF2.
Zhou and Yu 2005 JGR
32
Anomalies of (a) vertically integrated water
vapor transport in kgm-1s-1, (b) 500-hPa
geo-potential height in m and (c) 200-hPa zonal
wind in ms-1 regressed upon the PC of rainfall
EOF1. The coloring in (a) indicates the magnitude
of the moisture flux vector. The climate mean
conditions of Z500 and U200 are the colored
backgrounds of (b) and (c) respectively.
Zhou and Yu 2005 JGR
33
Anomalies of (a) vertically integrated water
vapor transport in kgm-1s-1, (b) 500-hPa
geo-potential height in m and (c) 200-hPa zonal
wind in ms-1 regressed upon the PC of rainfall
EOF2. The coloring in (a) indicates the magnitude
of the moisture flux vector. The climate mean
conditions of Z500 and U200 are the colored
backgrounds of (b) and (c) respectively.
Zhou and Yu 2005 JGR
34
Water vapor transport and rainfall anomalies
The origins of water vapor supply related to
anomalous rainfall patterns are different from
those related to the normal monsoon rainfall.
Anomalous Pattern I with a heavier rainbelt
along the middle and lower reaches of the Yangtze
River valley follows from a convergence of the
tropical southwest water vapor transport with the
midlatitude northeast water vapor transport the
tropical water vapor transport comes directly
from the Bay of Bengal and the South China Sea
but originally from the Philippine Sea. The
anomalous water vapor transport is associated
with a southwestward extension of the western
Pacific subtropical high and a southward shift of
the upper East Asian jet stream.
35
Water vapor transport and rainfall anomalies
Anomalous Pattern II with a main rainbelt along
the Huaihe River valley is supported by the
convergence of the subtropical southwest water
vapor with the midlatitude water vapor transport.
The subtropical branch comes directly from the
South China Sea but originally from the East
China Sea and the adjacent subtropical Pacific to
the further east along 20-25ºN. The background
large-scale circulation change includes a
northwestward extension of the western Pacific
subtropical high and an eastward shift of the
upper jet stream. The EAP/PJ pattern stands out
in the anomalous circulations. Although the
cross-equator flows including the Somali jet
supply abundant water vapor for the normal
condition of JJA rainfall over China, the
tropical water vapor transports related to
typical anomalous rainfall anomalies originate
from the tropical western Pacific Ocean.
36
Outline

• The East Asian summer monsoon circulation system
• Diurnal cycle of monsoon rainfall
• Water vapor transport and rainfall anomalies of
  China
• Impact of ENSO on the climate of China varies
  with the stage of ENSO
• The relation between the Indian summer monsoon
  and East Asian monsoon

37
Wang, Wu, Li 2003 J. Climate
S-SVD mode of 850 hPa winds and SST
anomalies (19572001)
El Nino evolution ?
38
A positive thermodynamic air-sea feedback
mechanism (Wang et al. 2000)
El Nino heating ? atmospheric Rossby wave
response ? cold SSTA in WNP ? anomalous PSAC ?
EAM
39
Two dominant modes of EAM
JJA(0)
SEOF1 SEOF2
SON(0)
D(0)JF(1)
MAM(1)
Wu, Zhou and Li 2008, J. Climate, submitted
40
Principal components and eigenvalue
eigenvalue
PCs
Wu, Zhou and Li 2008, J. Climate, submitted
41
Fractional variances accounted by the first two
leading modes
JJA(0)
SON(0)
D(0)JF(1)
MAM(1)
Wu, Zhou and Li 2008, J. Climate, submitted
42
Circulation and SST anomalies corresponding to
the first dominant mode
Wu, Zhou and Li 2008, J. Climate, submitted
43
Circulation and SST anomalies corresponding to
the second dominant mode
JJA(0)
SON(0)
D(0)JF(1)
MAM(1)
Wu, Zhou and Li 2008, J. Climate, submitted
44
?
v
?
?
1982
1983
Strong ENSO
Rainfall anomalies of JJA 1983
Xue Feng et al. 2007, Chinese Science Bulletin
45
?
v
?
?
???
2002
2003
Moderate ENSO
Rainfall anomalies of JJA 2003
Xue Feng et al. 2007, Chinese Science Bulletin
46
?

?
?
1991
1992
Moderate ENSO
Rainfall anomalies of JJA 1992
Xue Feng et al. 2007, Chinese Science Bulletin
47
?

?
?
?
?
1994
1995
Moderate ENSO
Rainfall anomalies of JJA 1995
Xue Feng et al. 2007, Chinese Science Bulletin
48
ENSO-related rainfall anomalies of China
The impact of ENSO on rainfall change over China
is phase-dependent. The first dominant mode of
SEOF analysis corresponds to the decaying phase
of ENSO, while the second mode corresponds to
turnabout of ENSO. Positive rainfall anomalies
along the Yangtze River and Huaihe River valley
mostly develops in the summer of ENSO decay
phase. This relationship varies with the
intensity of ENSO. How the ENSO anomalies can
affect the East Asian monsoon during and after
their mature phases from boreal winter to the
early summer of the following year? There is a
teleconnection between the central Pacific and
East Asia during the extreme phases of ENSO
cycles. This Pacific-East Asian teleconnection is
confined to the lower troposphere and
characterized by a westward and poleward
emanation of alternative cyclonic and
anticyclonic vorticity anomalies from the
equatorial central Pacific toward East Asia. The
anomalous anticyclone over the Philippine Sea
during mature phases of ENSO results from a
combined effects of remote ENSO forcing, local
air-sea interaction, and midlatitude-tropical
interaction.
49
Outline

• The East Asian summer monsoon circulation system
• Diurnal cycle of monsoon rainfall
• Water vapor transport and rainfall anomalies of
  China
• Impact of ENSO on the climate of China varies
  with the stage of ENSO
• The relation between the Indian summer monsoon
  and East Asian monsoon

50
Correlations of NC rainfall index with rainfall
anomalies elsewhere (1951-1998) (Wang and Huang
2007)
160 stations
NCEP
Hlume
Xie-Arkin
51
Correlations of Indian rainfall index with
rainfall anomalies elsewhere (1951-1998) (Wang
and Huang 2007)
160 stations
NCEP
Xie-Arkin
Hulme
52
30-yrs moving correlation between NC-Indian
summer rainfall
(Wang and Huang 2007)
53
Rainfall anomalies over China and the
correlations with Indian rainfall index
1827-1856
1945-1974
(Wang and Huang 2007)
54
The relation between the Indian summer monsoon
and East Asian monsoon
The summer (June to September) monsoon rainfall
variations over northern China are closely
correlated to the Indian summer rainfall. The
correlation coefficient is 0.65 for the period
from 1951-1980, significant at the 99.9 level of
confidence. However, this relation is unstable.
A recent examination using rainfall data covering
1813-2005 found that the correlation between
Indian and East Asian summer monsoon rainfall is
extremely low around 1840. Further analyses
suggest that a significant positive correlation
only exists during the period when the frequency
of La Niña is high.
55
Thank you !
E-mail zhoutj_at_lasg.iap.ac.cn http//web.lasg.ac.c
n/staff/ztj/index_e.htm