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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L21815, doi:10.1029/2006GL027449, 2006

Correction published 20 January 2007

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ArticleImpact of ENSO on landfall characteristics of tropical cyclones over
the western North Pacific during the summer monsoon season
H. Fudeyasu,1 S. Iizuka,1 and T. Matsuura1
Received 4 July 2006; revised 29 September 2006; accepted 18 October 2006; published 15 November 2006.

[1] We investigate the impact of ENSO on the landfall
characteristics of tropical cyclones (TC) over the Western
North Pacific (WNP) during the summer monsoon season.
During the early monsoon period (late May– late July), an
increase in the numbers of TCs that make landfall in the
Korean Peninsula or Japan is associated with an increasein the Niño-3.4 SST anomalies. During the peak monsoon
period (late July – mid September), the number of TCs thatmake landfall in the Indochinese Peninsula is greater in El
Niño years than in La Niña years. Furthermore, in El Niñoyears, the TCs that make landfall in the Korean Peninsula
or Japan tend to have longer lifespan and greater
intensities. These changes can be primarily attributed tothe change in the mean TC formation location as a resultof ENSO. Citation: Fudeyasu, H., S. Iizuka, and T. Matsuura(2006), Impact of ENSO on landfall characteristics of tropical
cyclones over the western North Pacific during the summer
monsoon season, Geophys. Res. Lett., 33, L21815, doi:10.1029/
2006GL027449.

1. Introduction[2] The interannual variability in tropical cyclones (TCs)over the Western North Pacific (WNP) and its relationshipwith the El Niño/Southern Oscillation (ENSO) has been
examined by several studies [e.g., Chan, 1985, 2000;
Lander, 1994]. These studies have shown that a longitudinal shift in the mean TC formation location occurs in
ENSO years. In El Niño (La Niña) years, the TCs tend toform in more eastward (westward) locations than the
climatological mean TC formation location. During thefall from September through to November, in strong El
Niño years, this longitudinal shift in the mean TC formation
location is associated with TCs that tend to curve more
northeastward, while in strong La Niña years, this longitudinal shift is associated with more TCs that tend to takea westward course [Wang and Chan, 2002]. Furthermore,
Wu et al. [2004] have shown that the mean number of TCs
that make landfall in southern China and the Indochinese
Peninsula during the fall in El Niño years is significantly lessthan normal.
[3] On the other hand, Chen et al. [1998] have shownthat the frequency of TC formation during the summer from
June through to August in La Niña (El Niño) years
increased (decreased) in the northern part of the WNP.
However, Wu et al. [2004] have reported that there is no
significant difference in the mean number of TCs that make1

Japan Agency for Marine-Earth Science and Technology, Yokosuka,
Japan.

landfall in East and Southeast Asia during the summer
between El Niño and La Niña years. Thus, ENSO does
not affect the frequency of TCs that make landfall in Eastand Southeast Asia during summer, although the mean TC
formation location in the WNP is dependent on the ENSO.
[4] In general, the spatial and temporal variation in TC
formation is primarily influenced by a large-scale circulation[e.g., Gray, 1998; Ritchie and Holland, 1999]. In the WNP
during summer, the large-scale circulation pattern changes
abruptly in accompaniment with the seasonal change in the
Asia monsoon [e.g., Matsumoto, 1992]. Previous studies[e.g., Ueda et al., 1995] have shown that the seasonalchange in the location of the monsoon trough related theAsia monsoon over the WNP strongly affects the formationand tracks of the TCs in the WNP. This seasonal change inthe monsoon trough does not depend on monthly changes.
Consequently, using calendar months in this study would beinappropriate for describing the seasonal variability in TC
activity in the WNP.
[5] During the summer monsoon season from late May
through to mid-September, which is also the most active
season for TC formation in the WNP, many TCs approachand make landfall in East and Southeast Asia where they
can cause severe damage. An understanding of the interannual variability in the landfall location of TCs during thesummer monsoon season may therefore be useful for
reducing the risk of damage and for improving water
resource management. In this study, we examine thedifference between El Niño and La Niña years in termsof the TC landfall characteristics during the summer
monsoon season.

2. Data and Definition[6] The best track data compiled by the Joint Typhoon
Warning Center (JTWC) was used to identify the TC tracks.
It should be noted that the results using the data acquired
from the Regional Specialize Meteorological Center-Tokyo
Typhoon Center of the Japan Meteorological Agency were
essentially the same as the results obtained using the JTWC
data. The JTWC data includes locations and maximum windspeeds of TCs for successive six-hour intervals. In thisstudy, the TC formation location was defined as the location
where a TC first reached tropical storm intensity (maximum
sustained wind speeds of greater than 17 m s 1), while theend location was defined as the location where the TC had
for the last time tropical storm intensity. Thus, the TCs thatwere analyzed were both tropical storms and typhoons.
The landmasses that encompass the WNP are divided into
Area 1, which includes the Korean Peninsula and Japan;
Area 2, which includes China; and Area 3, which includes
the Indochinese Peninsula (Figure 1). This is the same

Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL027449$05.00

L21815

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L21815FUDEYASU ET AL.: ENSO-RELATED TC CHARACTERISTICS

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Table 1. Mean Numbers of TCs That Form in the WNP During
the Early and Peak Monsoon Periods in El Niño Years, La NiñaYears, Neutral Years, and Climatological Mean
El Niño

La Niña

5.8

5.0

11.5

10.2

Neutral

Mean

4.5

4.9

10.6

10.7

Early Monsoon
Peak Monsoon

Figure 1. Delineation of the three study areas. Area 1: theKorean Peninsula and Japan; area 2: China; area 3: theIndochinese Peninsula.

definition as that employed by Wu et al. [2004], except that thePhilippines have been excluded from the present study.
[7] Similar to Wu et al. [2004], we analyzed the TCs thatformed in the 40-years period between 1961 and 2000.
ENSO years were defined as in the work by Wu et al. [2004]
using the mean sea surface temperature (SST) anomaly in
the Niño-3.4 region (5°S – 5°N, 120° – 170°E) during theJune –November TC season in the WNP. This meant there
were nine El Niño years (1963, 1965, 1969, 1972, 1982,
1987, 1991, 1994, 1997), and six La Niña years (1964,
1971, 1973, 1975, 1988, 1999).
[8] We defined the analysis period, taking into consideration the spatial and temporal changes in the Asia monsoon
over the WNP. The lower-level easterlies are dominant in
the tropics of the WNP until late May. The westerly region
then begins to appear in the tropics in mid June, which isconsistent with the onset of the Asian monsoon in Southeast Asia [e.g., Matsumoto, 1992]. In late July, thewesterly region shifts abruptly northward with the eastward extension of monsoon trough over the WNP [e.g.,
Nakazawa, 1992]. From mid September, a gradual retreatof the westerly region begins, together with a westward
penetration of the trade easterlies in the lower latitude. For
these reasons, the periods from late May through to late
July (145– 204 Julian days) is defined as the early monsoon
period, while the period from late July through to mid
September (205 – 265 Julian days) is defined as the peakmonsoon period. It should be noted that the climatologicalmean number of TCs is 4.9 in the early monsoon period and10.7 in the peak monsoon period (Table 1). Furthermore,
there is no significant difference in the mean number ofTCs between ENSO and neutral years.

years (Figures 2a and 2b). The number of TCs that form inthe southeast of the climatological mean TC formation
location (132.7°E, 15.6°N) increases (decreases) in El Niño(La Niña) years. In El Niño years, many TCs that form inthe southeastern part of the WNP tend to move northwestward, and, thus, make landfall in Area 1. Table 2 shows
the mean numbers of TCs that make landfall in Areas 1, 2,
and 3 during the early and peak monsoon periods in
ENSO years, neutral years, and climatological mean. The
difference in the mean number of TCs that make landfall
in Areas 2 and 3 during the early monsoon period between
El Niño and La Niña years is not statistically significant.
On the other hand, the mean number of TCs that makelandfall in Area 1 is greater in El Niño years than in La Niñayears. A Student’s t-test shows that this difference isstatistically significant at the 95% level.
[10] Figure 3 shows the relationship between the numbers of TCs that made landfall in Area 1 during the earlymonsoon period for 1961 and 2000 and the Niño-3.4 SST
anomalies in July. The increases in the numbers of TC
landfalls are found to be associated with the Niño-3.4 SST
anomalies. We find a correlation coefficient of 0.51, which
does not agree with Wu et al. [2004] that consideredsummer to be from June through to August.
[11] During the peak monsoon period, the mean TC
formation location in El Niño years is different from thosein La Niña years (Figures 2c and 2d). In El Niño years, thenumber of TCs that formed south of the climatologicalmean TC formation location (138.1°E, 19.3°N) and in theSouth China Sea (SCS) increases, while the number of TCs
that formed north of the climatological mean formation
location decreases. The reverse situation occurs in La Niñayears. This south-north difference in the active region for
TC formation between El Niño and La Niña years is inagreement with a previous study [Chen et al., 1998] that

Table 2. Mean Numbers of TCs That Made Landfall in Areas 1, 2,
and 3 During the Early and Peak Monsoon Periods in El NiñoYears, La Niña Years, Neutral Years, and Climatological MeanArea

3. Result[9] Figure 2 shows the TC tracks during the early andpeak monsoon periods in El Niño and La Niña years.
During the early monsoon period, there is a difference in
the TC formation locations between El Niño and La Niña2 of 5

El Niño

La Niña

Neutral

Mean

1
2
3

1.3
1.6
0.3

Early Monsoon
0.2
1.7
0.6

0.6
1.1
0.4

0.7
1.3
0.4

1
2
3

2.0
2.2
1.7

Peak Monsoon
1.8
2.5
0.7

2.3
2.4
0.8

2.2
2.4
0.9

L21815FUDEYASU ET AL.: ENSO-RELATED TC CHARACTERISTICS

L21815Figure 2. Tracks and formation locations for TCs that form during (a) the early monsoon period in El Niño years, (b) the
early monsoon period in La Niña years, (c) the peak monsoon period in El Niño years, and (d) the peak monsoon period in
La Niña years, respectively. The track is shown using a line, while the formation location is shown with a square. El Niñoyears were defined as 1963, 1965, 1969, 1972, 1982, 1987, 1991, 1994, and 1997, while La Niña years were defined as
1964, 1971, 1973, 1975, 1988, and 1999.

considered summer to be from June through to August.
Chen et al. [1998] suggested that the increase in TC
formation in the northern part of the WNP in La Niñayears is related to the appearance of an anomalous cycloniccell situated in a summer teleconnection wave train emanating from the western tropical Pacific that progresses
along the edge of the WNP.
[12] During the peak monsoon period in El Niño years,
most of the TCs that form in the SCS and the Philippine Sea

Figure 3. Scatter diagram showing the relationshipbetween the Niño-3.4 SST anomaly in July and the numbersof TCs that make landfall in Area 1 during the earlymonsoon period for 1961 and 2000.

take a straight track towards the Indochinese Peninsula andsouthern China (Figure 2c). The mean number of TCs thatmake landfall in Area 3 during the peak monsoon period is,thus, larger in El Niño years than in La Niña years (Table 2).
This difference between El Niño and La Niña years is
statistically significant at the 95% level. Harr and Elsberry[1991] found that most of the TCs that formed in the SCS
and the Philippines Sea tended to take a straight track, thatis, they moved westward and finally made landfall in the
Indochinese Peninsula and southern China. Therefore, in El
Niño years, the increase in the number of TCs that madelandfall in Area 3 during the peak monsoon period could be
primarily attributed to an increase in the TC formations in
the SCS and the Philippine Sea, which corresponds to a

Figure 4. Occurrence rate for TCs that make landfall in
Area 1 during the peak monsoon period based on lifespan in
El Niño years, La Niña years, and neutral years.

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L21815FUDEYASU ET AL.: ENSO-RELATED TC CHARACTERISTICS

Figure 5. Same as Figure 4 except the occurrence rate ofTCs making landfall in Area 1 based on the Saffir-Simpson
scale. TD: tropical depression; TS: tropical storms; C1– C5:
Category 1 – 5 typhoons.

southward shift in the mean TC formation location. Itshould be noted that the correlation between the numbersof TCs that made landfall in Area 3 and a Niño-3.4 SST
anomaly during the years surveyed was not observed todiffer markedly due to the large variability in the number ofTCs that formed in the Philippine Sea during the neutral
years.
[13] On the other hand, during the peak monsoon period,
more than 20% of all TCs that form over the WNP movenorthward and make landfall in Area 1. Thus, as has beenshown previously (Table 2), no significant difference isapparent in the mean numbers of TCs that make landfall in
Area 1 between El Niño and La Niña years, which isconsistent with the previous studies [Wu et al., 2004;
Saunders et al., 2000]. However, the landfall characteristics
of TCs in Area 1 differ between El Niño and La Niña years.
Figure 4 shows the percentage of total TCs with a given
lifespan that make landfall in Area 1 during the peakmonsoon period. The climatological mean lifespan of TC
that make landfall in Area 1 is 6.3 days. The occurrence rateof TCs with a lifespan of between 7 and 12 days in El
Niño years is more than that in neutral or La Niña years,
while the occurrence rate of TCs with a lifespan of
between 1 and 6 days in El Niño years is smaller. The
difference in the occurrence rate of TCs that have a longer
lifespan between El Niño and La Niña years is statisticallysignificant at the 95% level. Since lifespan and TC
formation location are related [Wang and Chan, 2002],
an increase in the number of TCs that form in thesouthern part of the WNP in El Niño years causes anincrease in the occurrence rate of TCs that have a longer
lifespan.
[14] For TCs that make landfall in Area 1 during the peakmonsoon period, the occurrence rate of intense TCs, which
are defined as TCs that can be classified as being category
3 – 5 on the Saffir-Simpson scale [Simpson, 1974], is greaterin El Niño years than in La Niña years (Figure 5). Thisdifference in the occurrence rate of intense TC between El
Niño and La Niña years is statistically significant at the
90% level. Chia and Ropelewski [2002] have statisticallystudied the relationship between the TC intensity and theformation locations during the periods from July throughto October. They demonstrated that intense TCs (category
3 – 5 on the Saffir-Simpson scale) were more likely todevelop from TCs that formed in the southeastern part of
the WNP than from those that formed in the northwesternpart of the WNP. Thus, TC intensity is related to their

L21815formation locations. During the peak monsoon period in El
Niño years, TCs that form in the southeastern part of the
WNP and make landfall in Area 1 tend to have a longer
lifespan and develop into more intense TCs. The south-northdifference in the TC formation location during the peakmonsoon period between El Niño and La Niña years does
not affect the number of TCs that make landfall in Area 1.
Instead, the characteristics of TCs are different between El
Niño and La Niña years. However, there is no difference inthe characteristics of TCs that make landfall in Area 2
between El Niño and La Niña years.

4. Summary[15] We investigated the impact of ENSO on the characteristics of TCs that made landfall during the earlymonsoon period (late May –late July) and the peak monsoon period (late July – mid September). It was found thatan increase in the numbers of TCs that made landfall in
Area 1 during the early monsoon period was associatedwith an increase in the Niño-3.4 SST anomalies. This
increase during El Niño years could be primarily attributed
to an increase in TCs that formed in the southeastern part of
the WNP.
[16] During the peak monsoon period, the number of TCs
that made landfall in Area 3 was greater in El Niño years
than in La Niña years. This was caused by an increase in
the formation of TCs in the SCS and the Philippine Sea,
which corresponded to a southward shift in the mean TC
formation location. Furthermore, in El Niño years, thoseTCs that made landfall in Area 1 were found to have longer
lifespan and greater intensities. This increase could be
attributed to an increase in the number of TCs that formedin the southeastern part of the WNP.
[17] The findings of this study suggest that ENSO not
only affect the characteristics of TCs during the fall, but also
have an effect on the landfall characteristics of TCs during
the summer monsoon season.
[18] Acknowledgments. The authors would like to thank S. Suzuki
and K. Ichiyanagi for their 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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