GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L10809, doi:10.1029/2005GL025380, 2006

Seasonality of westward-propagating disturbances over Southeast andsouth Asia originated from typhoons
H. Fudeyasu,1 S. Iizuka,1 and T. Matsuura1
Received 2 December 2005; revised 14 March 2006; accepted 20 April 2006; published 23 May 2006.

[1] We investigate the seasonal changes in the westwardpropagating disturbances (WDs) in Southeast and South
Asia originated from typhoons over the western North
Pacific. WDs tend to move over the northern part of the
Indochina Peninsula and South Asia in summer, while overthe southern part of the Indochina Peninsula and the Bay of
Bengal in fall. Although the number of landfall on the
Indochina Peninsula is the largest in fall, the active season
for WD with a more west-oriented track is identified in latesummer. The seasonality is related to large-scaleatmospheric conditions. The easterly region throughout
the latitude band 15° – 25°N at 700 hPa during the active
phases of intraseasonal variation in monsoon trough andthe large-scale trough in lower troposphere over the
Bay of Bengal are favorable conditions for taking themore west-oriented track of WD in late summer. Such
conditions are found neither earlier summer nor fall.

[3] The previous studies [e.g., Krishnamurti et al., 1977;
Saha et al., 1981] suggested that the WDs across the
Indochina Peninsula were closely related to the somemonsoon depressions over the Bay of Bengal. Statisticalanalysis by Chen and Weng [1999] indicated that about 60%
of monsoon depressions in summer from June to August
was related to the WDs that could be traced to typhoons in
the WNP-SCS.
[4] The attention of previous studies relating to the WDs
was primarily paid to the summer season, and did not focus
on the WDs in fall. The frequency of typhoon that makelandfall on the Indochina Peninsula, however, is greater infall rather than in summer (Figure 2). In the present study,
we investigate the seasonal changes in WDs originated fromtyphoons statistically, and how the seasonal changes are
related to those in typhoons that make landfall on the
Indochina Peninsula.

Citation: Fudeyasu, H., S. Iizuka, and T. Matsuura (2006),
Seasonality of westward-propagating disturbances over Southeastand south Asia originated from typhoons, Geophys. Res. Lett., 33,
L10809, doi:10.1029/2005GL025380.

2. Data and Definition

1. Introduction[2] Some westward-propagating residual disturbances(WDs) over Southeast and South Asia were originatedfrom the typhoons that formed in the western North Pacificand South China Sea (WNP-SCS) and made landfall on
the Indochina Peninsula. Abundant rains accompanying
typhoon-related WDs have caused several flooding resulting in heavy losses of life and widespread damages overSoutheast and South Asia, Typhoons NIKI and USAGI
being examples. The WD originated from Typhoon NIKI
that made landfall on the Indochina Peninsula in midAugust 1996, and then passed through the Indochina
Peninsula and South Asia (Figure 1b). Dartmouth
Flood Observatory reported the extreme flood events in
Bangladesh during the passage of the WD over South Asia
(http://www.dartmouth.edu/floods/index.html). In Southeast Asia, the abundant rain accompanied by the WD
originated from Typhoon USAGI also caused the extreme
flood events, devastated Northern Thailand and Central
Vietnam in early August 2001. It is important to understandthe frequency of WDs, because their associated rainfall
strongly affects human activities in Southeast and South
Asia.

1

National Research Institute for Earth Science and Disaster Prevention,
Tsukuba, Japan.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2005GL025380$05.00[5] The best track data compiled by the Joint Typhoon
Warning Center is used to identify the typhoons that makelandfall on the Indochina Peninsula. In this study, typhoons
refer to both tropical storms and typhoons, so the analyzed
typhoons that make landfall on the Indochina Peninsula aredefined as typhoons crossing the longitude of 110°E withmaximum sustained wind speeds greater than 17 m s 1.
In order to search their subsequent movements overSouth Asia, we use relative vorticity derived from theEuropean Center for Medium range Weather Forecast
40 year re-analysis (ERA-40) data with a resolution of 2.5°.
[6] Figure 1a shows the relative vorticity of WD originated from Typhoon NIKI with respect to longitude at 850,
700, 500, and 300 hPa levels. At 110°E, the WD has alarger relative vorticity in the lower level. During thepassage of WD over the Indochina Peninsula (100°–
110°E), the relative vorticity decays rapidly at 850 hPa
level compared with those in upper levels, because a lowlevel disturbance is greatly influenced by mountainous
terrain. Since the relative vorticity at 700 hPa is the largest
than those at all levels in the west of the Indochina
Peninsula, it would be an indicator of the movements of
WD.
[7] It is noted that the higher relative vorticity at 700 hPa
associated with the WD is accompanied by the active
convection, according to the satellite data (not shown).
Therefore, we define the WD as the relative vorticity at
700 hPa more than 5.0  10 5 s 1. The disappearance
location of WD is identified when the intensity of WD
eventually reaches below the defined criterion. It is notedthat the results using other criterions of 4.0  10 5 or 6.0 
10 5 s 1 are essentially same as those using this definition.

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FUDEYASU ET AL.: SEASONALITY OF WD OVER SEA AND TYPHOON

Figure 1. The case of Typhoon NIKI in 1996: (a) relative
vorticity of WD with respect to longitude at 850 (cross),
700 (closed circle), 500 (square), and 300 (triangle) hPa
levels, and (b) the relative vorticity at 700 hPa level.
Contours interval is 2.0  10 5 s 1. Dark and light shading
indicate areas of anomalies greater than 3.0  10 5 s 1 and5.0  10 5 s 1, respectively.
[8] The analysis is confined to the period of 36 years
from 1966 to 2001 when satellites have operated and the
ERA-40 data completely covers. During the analysis period,a total of 170 typhoons are identified as typhoons that makelandfall on the Indochina Peninsula. These typhoons form
over the SCS (54%) and the east of the Philippines (46%).
To examine the seasonal changes, the numbers of thesetyphoons are counted within a 10-day period (Figure 2). It isfound that there are the seasonal changes with three peaks
near early-July (190), late-August (240), and midNovember (320). As pointed out in the previous studies[Harr and Elsberry, 1991; Chen and Weng, 1996], thecharacteristics of typhoon tracks are influenced by thelarge-scale circulation relating to the Asia monsoon. By
considering the short-term variation of typhoons that makelandfall on the Indochina Peninsula, we here define three
periods of 151 – 210 days (31 May – 29 July), 211 – 270 days
(30 July – 27 September), and 271– 330 days (28 September –26 November) as P1, P2, and P3, respectively.

L10809westward crossing over the southern part of the Indochina
Peninsula, and then finally disappear in the western part of
the Indochina Peninsula or the eastern part of the Bay of
Bengal.
[10] In order to see the frequency of WDs in each period
in a more quantitative way, the histogram of passage
number of WDs with respect to longitude is shown in
Figure 3. The number of WDs at 110°E, namely, thefrequency of typhoons that make landfall on the Indochina
Peninsula is the largest in P3 (Figure 2). The most active
season for WD with a more west-oriented track is, however,identified in P2. The disappearance rate of WD during thepassage over the Indochina Peninsula (110°– 100°E), which
is defined as the ratio of the number of disappearing WD tothat of passage, is about 40% in both P2 and P3, while thelargest rate more than 60% is in P1. Between the westernpart of the Indochina Peninsula and the central Bay of
Bengal (100°– 90°E), however, the rate in P3 increases by90%. By contrast, the rate in P2 is about 40% even over thewestern region of the Indochina Peninsula, which meansthat the WDs can move more westward in P2 than in P3.
Thus, the seasonal change in the frequency of WDs with amore west-oriented track is inconsistent with that oftyphoons that make landfall on the Indochina Peninsula.
[11] The seasonal change in tracks of WDs is not related
to the characteristics of typhoons: their intensities andgenesis locations. It is greatly influenced by large-scaleatmospheric circulation patterns in the lower-middle
troposphere, consisting of the Asia monsoon, mid-latitude
westerly, and trade easterly. Figure 4 shows the time seriesof 3-day mean zonal winds at 700 hPa averaged the area
between 90° –110°E from June through November 1996,
with the tracks of WDs in 1996. In P1 corresponding to theactive monsoon season, the westerly region appears fromsouth of 10°N up to 30°N. It extends far to the SCS from theIndian Ocean, which is favorable for disturbing the westward moving typhoons over the Indochina Peninsula. Even
after a few typhoons make landfall, the WDs move somewhat northwestward and eventually disappear around thenorthern part of the Indochina Peninsula and the southernpart of China, the WD related Typhoon FRANKIE being
one example.

3. Results[9] Figure 3 shows the tracks of typhoons and WDs ineach period. During P1 and P2, typhoons make landfall on
the northern coast of the Indochina Peninsula. However,
WDs in P1 rapidly disappear in the northern part of the
Indochina Peninsula and the southern part of China, while a
lot of WDs in P2 move westward over the northern part of
the Indochina Peninsula, and even some WDs arrive at thenorthern part of the Bay of Bengal or South Asia. Furthermore, a few WDs in P2 reach the northern part of India.
During P3, typhoons make landfall on the whole eastern
coast of the Indochina Peninsula. Most of WDs move

Figure 2. Number of typhoons that made landfall on the
Indochina Peninsula during 1966– 2001 counted in 10-day
periods. Typhoons generated in the Philippine Sea arerepresented by black histograms, while the South China Sea
represented by open histograms on the top of black
histograms.

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L10809Figure 3. The typhoon tracks (dashed line), distribution of WD tracks (contour), disappearance location of WD (shading),
and histogram of passage number of WD with respect to longitude in each season. Contour interval is 10. Black, dark, andlight shading indicates areas of number greater than three, two, and one, respectively.
[12] On the other hand, during P3 corresponding to thewithdrawal season of the Asian monsoon, the easterly
region spreads between 5° – 20°N relating to the tradeeasterlies. The trade easterlies over the WNP intrude
westward over the Bay of Bengal through the Indochina
Peninsula. The condition facilitates the westward movementof WD related to typhoons that make landfall on the wholeeast coast of the Indochina Peninsula. However, the midlatitude westerly region simultaneously shifts to the lower
latitude and spreads to South Asia and the northern part of
the Indochina Peninsula, accompanied by the weakening of
the Tibetan High, and therefore WDs in P3 take the way
only in the southern part of the Indochina Peninsula and the
Bay of Bengal, the tracks of WDs related Typhoons BETH
and ERNIE being examples.
[13] It is of interesting that, only in P2, some WDs moves
more westward through the northern part of the Bay of
Bengal and South Asia. Although the climatological zonal
winds over the Indochina Peninsula averaged during theperiod shows the westerly, the zonal wind often weakens or
becomes easterly associated with the intraseasonal variationin the monsoon trough with a period of 30– 60 days [e.g.,
Nakazawa, 1992], facilitating the westward movement ofdisturbances across the Indochina Peninsula. As the intense
westerlies are dominant in the lower latitude corresponding
to the active phases of monsoon trough near 20 August and
20 September 1996, easterly region simultaneously appear
between 15°– 25°N. The easterly region extends far to India
from the WNP-SCS during the active phases, while monsoon westerly region extends eastward arriving at the east of
the Philippines in the lower latitude. These easterlies seemto act as a steering flow of typhoon and WD moving to thewest. The WDs related Typhoons NIKI, SALLY, and
WILLIE move westward arriving at South Asia just during
the active phases of intraseasonal variation in the monsoon
trough. In contrast, during the inactive phases of intraseasonal variation, the westerly regions spread around thelatitude of 20°N near 10 August 1996, which would disturb
the westward movement of WD through the path over the
Indochina Peninsula, Typhoon MARTY being examples.
The similar relations between the frequency of WDs and theintraseasonal variation of monsoon trough in P2 are foundin the other years as suggested by Chen and Weng [1999].

[14] Furthermore, the large-scale environment over the
Bay of Bengal in P2 seems to sustain the intensity of WD
reaching South Asia. Lower troposphere over the northernpart of the Bay of Bengal is characterized by the existenceof large-scale trough extending southward (not shown). The
large-scale trough gives rise to large positive relative
vorticity over South Asia and the northern part of Bay of
Bengal. Additionally the southwesterlies around the largescale trough over the Bay of Bengal bring the warm moist
air to South Asia from the equatorial Indian Ocean(Figure 5). Since the condensation heating associated withcumulus convection is the most significant energy source
for the tropical disturbances [Lau and Lau, 1992], the largescale environment over the Bay of Bengal in P2 causes the
slowdown of the decay of WDs in this region, resulting inthat WDs tend to have a longer lifetime and be able to reachSouth Asia. Some WDs during the passage over South Asia

Figure 4. Time series of 3-day mean zonal winds at
700 hPa averaged the area between 90°– 110°E from June
through November 1996 and the tracks of WDs (line).
Contour interval is 5 m s 1. Light and dark shading indicate
areas of greater than 0.0 m s 1 and 5.0 m s 1, respectively.
The disappearance locations depending on the longitude at
105°E (square), 100°E (triangle), and 95°E and 85°E
(circle) of WDs are indicated.

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FUDEYASU ET AL.: SEASONALITY OF WD OVER SEA AND TYPHOON

Figure 5. Vertical integrated water-vapor flux (kg m 1 s 1
arrows, scale at bottom) and its divergence from 925 to
300 hPa level in P2. The regions smaller than 3.0 
10 5 kg m 2 s 1 are shaded.
during P2 are redeveloped, as shown in the WD relating to
Typhoon NIKI (Figure 1). During P3, on the other hand,such conditions are not realized over the Bay of Bengal
because of retreat of the Asian monsoon (not shown).

4. Summary[15] We investigated the seasonal changes in the typhoonrelated residual disturbances (WD). WDs tend to move overthe northern part of the Indochina Peninsula and South Asia
during P1 (June– July) and P2 (August – September), while
over the southern part of the Indochina Peninsula and Bay
of Bengal during P3 (October – November). Although thenumber of typhoons that make landfall on the Indochina
Peninsula is the largest during P3, the most active season for
WD with a more west-oriented track is identified during P2.
[16] The seasonality of WD seems to be related to largescale atmospheric condition. During the active phases of
intraseasonal variation in the monsoon trough in P2, theeasterly region at 700 hPa level appears throughout
the latitude band 15°– 25°N extending far to India fromthe WNP-SCS. These easterlies seem to act as a steering
flow of typhoon and WD moving to the west. Furthermore,

L10809the large-scale environment in the lower troposphere overthe Bay of Bengal in P2, which is characterized by theexistence of large-scale trough, large positive relative
vorticity, and environmental inflow of warm moist air fromwarm oceans, cause the slowdown of the decay of WDs
during the passage over South Asia and the Bay of Bengal.
It facilitates the more west-oriented track of WDs over thenorthern part of the Indochina Peninsula and South Asia.
Such conditions are found neither in P1 nor P3.
[17] Since the typhoon-related WDs affect the amountof rainfall around the countries of Southeast and South Asia,
it is important to understand rainfall accompaniedwith typhoon-related WDs as well as the Asia monsoon.
Observation of rainfall with better spatial and temporal
resolutions will allow us a more quantitative understandingof this phenomenon as well as predicting water resources
over Southeast and South Asia.
[ 18 ] Acknowledgments. The authors would like to thankJ. Matsumoto, T. Satomura, and T. Yamada for helpful discussions
and suggestions. This work is partly supported by the Research Project for
Suitable Coexistence of Human, Nature, and the Earth under the Ministry of
Education, Sports, Culture, Science and Technology.

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H. Fudeyasu, S. Iizuka, and T. Matsuura, National Research Institute for
Earth Science and Disaster Prevention, 3-1 Tennoudai, Tsukuba, Ibaraki,
Japan. (fude@bosai.go.jp)

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