Climatic features of the water vapor transport around East Asia during June andSeptember* Takao YOSHIKANE1, Fujio KIMURA 2(1: The Frontier Research Center for Global Change, 2: Institute of Geosciences, University of Tsukuba)* Yokohama Institute for Earth Science 3173-25, Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001
JAPAN, e-mail: yoshikat@jamstec.go.jp
Abstract
Climatic features of water vapor transport, which is closely related to precipitation in the middle latitudes around East Asia,
are investigated using a reanalysis data in June and September. The maximum pole-ward water vapor transport is formed
along the continental coast in June, although it is formed over the western Pacific Ocean. The water vapor in June isquasi-stationary transported from low latitudes to high latitudes. On the contrary, the intermittently transportation of water
vapor is clearly confirmed in September. It is indicated that the water vapor is primary transported by the southwesterly
low-level wind due to the heat contrast between land and ocean in June. In September, it reveals that almost 92% of the total
amount of pole-ward water vapor is transported by typhoons in 1998. It is suggested that Typhoon has an important role toform the climatic atmospheric field around East Asia in September.
Keyword: Typhoont, Baiu, Akisame,Water vapor transport.

1. Introduction
Baiu and the Akisame front formed in the early summer
and early autumn. Many studies suggested that thepole-ward water vapor is very important to form therainfall zones. Asakura (1971) indicated that the amount ofwater vapor transport from the southwest to rainfall zone isquite large in early summer (Baiu season). Murakami et al.
(1962) indicated that the water vapor from the southeast torainfall zone is remarkable in early autumn (Akisameseason). However, there are few studies considered thedifference of the water vapor transport and amountbetween Baiu and Akisame season from the climatic view
point. In this study, difference in feature of the water vapor
transport is investigated between Baiu and Akisame using areanalysis data.

difference of rainfall distribution between those months(Akiyama, 1975; Matsumoto, 1988). In June, it is foundthat the rain band, which is called the Mei-yu front inChinese, is formed over the China continent, although theMei-yu front is unclear in September. The same feature ispointed out by the study of T. C. Chen et al. (2004). It canbe also found that the precipitation of Akisame front in
September is smaller than that of Baiu front in June (Fig.
1c and 1d).
(a)(b)
Region A
3x10102x10101010
ton ⋅ m −1

2. Features of water vapor transport
The water vapor transport is evaluated the verticalaccumulated value from 1000hPa to 300hPa of reanalysis
data every six hours of National Centers for Environmental
Prediction (NCEP) and National Center for AtmosphericResearch (NCAR). Figure 1 shows that the 20 year
accumulated monthly water vapor distribution of June(Fig.1a) and September (Fig.1b), and the 20 year averaged
monthly precipitation of June (Fig.1c) and September(Fig.1d) from 1980 to 1999. In the area between 25-30N
degrees, water vapor is strongly transported pole-ward
along the continental coast in June, although the watervapor is transported over the Ocean in September. Amountof the 20 year accumulated water vapor transport in June
through the Region A (30N, 120E-150E) is about 2.3 times
as in September. Accumulated water vapor transport of 20
year through the Region A in June and September areestimated about 3.38×1016 ton and 1.49×1016 ton,respectively. It is indicated that the difference of theamount of water vapor between June and September iscorresponded to that of rainfall in the mid latitudes (Fig.1cand 1d). Many meteorologists have been indicated that thedifference of the location of maximum water vapor
between June and September is also corresponded to the3x1010 ton ⋅ m(d)(c)−1AKISAME

BAIU

mm / month

Fig. 1: 20 year accumulated monthly water vapor
distribution of June (Fig.1a) and September (Fig.1b),
and the 20 year averaged monthly precipitation of June(Fig.1c) and September (Fig.1d) from 1980 to 1999.
Figure 2 shows the water vapor transport every six hourin June (Fig.2a) and September (Fig.2b) 1998 in theRegion A, as well as Typhoon tracks indicating the path ofthe minimum pressure center of Typhoon, in September
1998 (Fig.2c). The dark and semi-dark color in Fig.2a and2b indicates the equator-ward and pole-ward componentsof water vapor, respectively. In Fig.2c, Typhoon number(e.g. T9804) was defined by Japan Meteorological Agency

(JMA). The solid and dot line indicates that Typhoon isexisted in the Domain B (20N-40N, 120E-150E) and inouter area of the Domain B, respectively. The terms of thecenter of typhoon existence in Domain B (Typhoon term)
are shown in the right side of Fig.2b. The typhoon has
never existed in June 1998 in the same region. The watervapor fields of June and September in 1998 are likely tothe climatic fields. The water vapor in June isquasi-stationary transported from the low latitudes to highlatitudes. On the other hands, the intermittently
transportation of water vapor is clearly confirmed in
September. The strong water vapor transport corresponds
to the Typhoon in Domain B as shown in Fig.2b. The watervapor transport is very small in the term without Typhoon(non Typhoon term). Almost 92% of total amount ofpole-ward water vapor through the Region A is estimatedto bring due to Typhoon in shown Fig.3, although theappearance term of Typhoon (Typhoon term) is only 15 day
in September (30 day). The amount of accumulatedpole-ward water vapor transport during Typhoon term andnon-Typhoon term are estimated about 4.84×1014 ton /15days and 3.58×1013 ton / 15days, respectively. It isfound that the pole-ward water vapor transport due to theTyphoon is extremely large from this analysis result. We
defined the water vapor transport due to the Typhoon when
the center of Typhoon is in the Domain B (Typhoon term)
after that.
June 1998

September 1998

(a)

(b)

2626

21

21

T9809T9807
T9808
T9806

161611116

T9805

6kg⋅ m−1 ⋅ s−1

1kg ⋅ m −1 ⋅ s −1

Equatorward

T9804

1Poleward

500

Longitude

TyphoonNumber (JMA)

(c)

T9804

T9809

T9805
T9806
T9807
T9808

Fig. 2: Time latitude section of water vapor transport
every 6 hour in June (a) and September (b) 1998 inRegion A, and Typhoon track in September 1998 (c).
The solid and dot line in Fig.2c indicates that Typhoon isexisted in the Domain B (20N-40N, 120E-150E) and inouter area of Domain B, respectively.

3. Discussion
In general, the monthly mean of the meridional watervapor transport can be divided into the products of theaverage of the mean and its deviations. The same methodof Mirakami (1962) is used to calculate the water vapor
except for the calculation region. The zonal domain in thisstudy is applied form 120E to 150E. This narrow zonal
region is reasonable to estimate the water vapor transport
in the limited area around East Asia. In this limited area, it
is speculated that the constant flow such as the monsoon
should be made a contribution to the water vapor transportof the average of the means, because the a mean variable isconstant over the period of averaging (Holton, 1992). And
the disturbances such as Typhoon should be made acontribution to the water vapor transport of the average ofthe deviations. The time averaged zonal mean water vapor
transport is calculated by the equation as follows.
(1)
v h = v h + v ′ h′where is meridional component of wind andismixing ratio. The time average and zonal means areindicated by over bars, and the fluctuating components byprimes. The first and second terms on the right indicate thewater vapor due to the product of the average of the means(120E-150E) and the eddy flux, respectively. Thosecomponents are evaluated by the vertically accumulationfrom 1000hPa to 300hPa.
Figure 4 shows the monthly meridional water vapor
transport divided into the product of the average of themeans and the deviations in June and September 1998. The
maximum of the total pole-ward water vapor is remarkablearound 30N in June. Almost all of water vapor transport isdue to the constant flow. In September, the water vapor
around 30N is much smaller than in June. Almost 75% and25% of the total water vapor transport are due to theaverage of the means and the deviations, respectively.
There are some studies to indicate that the average of themeans dominates the total water vapor transport.
Simmonds et al. (1999) investigated the interannual
variations of water vapor flux over China in summer
season (June - August) considering the mean and transient
eddy transport. They concluded that the transient eddytransport along the path of the Indian monsoon is verysmall. The average of means component of water vapor
should be very important in June. However, there are someobjections in September. The water vapor transport byTyphoon is intermittently caused with the large amount ofwater vapor as shown in Fig.2b. It means that this transport
is not caused by the constant flow but by the disturbance.
However, it is quite difference between the estimated water
vapor in Fig.3 and calculated in Fig.4. The Typhoon
contribution is almost 92% of total in Fig.3, although thewater vapor transport of the average of the deviations isalmost 25% of that in Fig.4. Some reasons are consideredas the cause of such difference. The meridional water vapor
transport due to disturbance caused by the baroclinic
instability is largely different from that due to the Typhoon
disturbance. Therefore it is suggested that the water vapor
transport due to the Typhoon could not be exactlyestimated by the deviation component of the water vapor.
kg ⋅ m −1 ⋅ s −1

Poleward Water Vapor Transpot

1200
Typhoon term

1000

Jun.1998
Sep.1998

800
600
400
2000-2000120

240

360
480time(hour)

600

Accordingly, it is indicated that the large amount ofwater vapor is transported along the southwesterly Low
Level Jet (LLJ) generated by the strong heat contrast
between the continent and ocean in June. The
quasi-stationary water vapor transport is produced by thefixed heat contrast. This mechanism has been showed by
the study of Yoshikane et al. (2001) and Yoshikane andKimura (2000) using a regional climate model. In
September, the heat contrast is getting small due to theseasonal radiation change and the Typhoon disturbance isremarkable in the Northwestern Pacific Ocean. As theresults, the large water vapor is intermittently transportedby the Typhoon.

720(a)

Fig. 3: Time variation of water vapor through the Region
A (30N, 120E-150E) in September 1998. The slashes
part of bottom of graph shows the water vapor transport
when the Typhoon exists in Domain B (20N-40N,120E-150E).
kg ⋅ m −1 ⋅ s −1

kg ⋅ m −1 ⋅ s −1

250

250

(b)
0.75x1010
0.5x1010
0.25x1010
ton ⋅ m −1(c)(d)
0.75x1010

Water vapor transport (meridional)

0.5x1010

200

200

150

150

100

100

50

50

00-50

0EQ

3060-50090
North EQ

0.25x1010

TotalMeanEddy

ton ⋅ m −1

1.5x1010 ton ⋅ m−1

Fig. 5: 20 year accumulated water vapor transport of non
Typhoon term and Typhoon term in June (a) (b) and
September (c) (d). The shadow shows the pole-wardwater vapor transport.
3060

90
NorthFig. 4: Latitudinal distribution of meridional water vapor
transport in limited area (120E-150E) in June and
September 1998. The bold, solid, dash, and dash dot
dash lines are the monthly water vapor transport, theproduct of average of the means, the eddy flux.
Figure 5 shows the 20 year accumulated water vapor
transport of Typhoon existence case and the no existencecase in June and September. The shadow shows thepole-ward water vapor transport. In June, the term ofTyphoon existence is almost 13% of the total term. There
are little amount of water vapor transport due to theTyphoon and most of water vapor transport is formed by
the constant flow such as the summer Asian monsoon. InSeptember, the term of Typhoon existence is almost 36%of the total term. It is clearly found that the strong
pole-ward water vapor transport is formed by typhoon
around Region A. Table 1 indicates that the difference of
the water vapor transport between Typhoon term and nonTyphoon term in June and September. In September, thewater vapor transport due to the Typhoon is almost 54% ofthe total, although the term of Typhoon existence is almost
only 36% of the total. The water vapor transport due to theTyphoon can never be ignored from the view of climate.

JuneSeptember

20-year averaged Typhoon term 20-year accumulated poleward water vapor transport (ton ⋅ m −1 )Total
Typhoon TermRatio
Term par month (day) Ratio
2.8
9.50%
4.50E+06
5.90E+05
13%
10.8 36.30%
1.97E+06
1.08E+06 54.80%

Table 1: Amount of the 20-year accumulated pole-wardwater vapor transport and 20-year averaged Typhoon
term in June and September.

4. Conclusion
Climatic features of water vapor transport, which is
closely related to precipitation in the middle latitudes
around East Asia, are investigated using a reanalysis data
in June and September. The maximum pole-ward watervapor transport is formed along the continental coast inJune, although it is formed over the western Pacific Ocean.
The water vapor in June is quasi-stationary transported
from southwestward to north. On the contrary, theintermittently transportation of water vapor is clearlyconfirmed in September. It is indicated that the water vapor
is primary transported by the southwesterly low-level wind
due to the heat contrast between land and ocean in June. In
September 1998, it reveals that almost 92% of the total
amount of pole-ward water vapor is transported byTyphoons, although the appearance term of Typhoon isabout only 15 day in September 1998. However, the watervapor transport of the average of the deviations is almost
25%, although the typhoon contribution is almost 92% oftotal. It is suggested that the water vapor transport due tothe Typhoon could not be exactly estimated by thedeviation component of the water vapor, because themeridional water vapor transport due to disturbance causedby the baroclinic instability is largely different from that
due to the Typhoon disturbance.
From the result of climatic water vapor transport from1980 to 1999, the term of Typhoon existence in June isalmost 13% of the total term. There are few amount ofwater vapor transport due to the Typhoon and most ofwater vapor transport is formed by the constant flow suchas the summer Asian monsoon. The term of Typhoon
existence in September is almost 36% of the total term. Itis clearly found that the strong pole-ward water vapor
transport is formed by typhoon around Region A. It is
indicated that the large amount of water vapor is constantly
transported along the southwesterly Low Level Jet (LLJ)
generated by the strong heat contrast between the continentand ocean in June. In September, the large water vapor isintermittently transported by the Typhoon due to the weakheat contrast and the intensified Typhoon disturbance overthe Northwestern Pacific Ocean.

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