The characteristics and the causes of extreme rainfall events inAugust over Korea
Eun-Jeong Cha1, Masahide Kimoto1, Eun-Jeong Lee2, and Jong-Ghap Jhun2
1. Center for Climate System Research, University of Tokyo, 4-6-1 Komaba,
Meguro-ku, Tokyo 153-8904, Japan, e-mail: cha@ccsr.u-tokyo.ac.jp
2. School of Earth and Environmental Sciences, Seoul National University,Seoul 151-742, Korea

ABSTRACT
We investigate the characteristics of precipitation and its extremes exceeding 80
mm day-1 in August over Korea in terms of temporal and spatial variations. In addition,
the possible causes of extreme rainfall events in seven years that have the greatest
number of heavy rainfall events are suggested. The largest monthly mean rainfall andthe greatest frequency of extreme rainfall events appear in August over Korea from1973 to 2002. The notable increasing trend in both monthly rainfall and the frequency ofextreme events is found in August. This increasing trend is mainly attributed by theseven heaviest years, 1987, 1993, 1995, 1998, 1999, 2000, and 2002.
The effects of typhoons and the large-scale circulation as the possible causes ofthe extreme rainfall in August are suggested. Among the seven heaviest years, in 1995,1999, 2000, and 2002, the effect of typhoon is important to the occurrence of heavy
rainfall events in August. Though the total number of occurrences of typhoons is under
the average from the mid-1990s, the effects of typhoons on the August rainfall over
Korea are enhanced. The spatial distribution of the frequency of the heavy rainfall
events in those years shows that the events are concentrated over the western coast of
Korea due to the track of typhoons. In addition, the spatial scale of the events is larger
than that of the other heaviest years, 1987, 1993, and 1998.
The large-scale circulation has a more significant role in the extreme events in
1987, 1993, and 1998 rather than typhoons. From the analysis of large-scale circulation,
it is found that the heavy rainfall events in August are related to the circulation of boththe midlatitudes and the tropics: the Eurasian (EU) pattern and the western North
Pacific subtropical high (WNPSH), respectively.
Keyword: extreme rainfall event, August, Korea, typhoons, Eurasian pattern, western
North Pacific subtropical high1. Introduction

becausechanges

inthevariability,In Korea the bulk of the annual

intensity and frequency could have aprecipitation occurs during the Northern

great influence on human resources andHemisphere (NH) summer (June, Julysocioeconomic activities (e.g., Bernard,and August, hereafter JJA) when the1993, Mearns et al., 1997, NDPCH,

stationary fronts are the most active and2003). However, the mechanisms of themove toward East Asia with time. The

extreme rainfall are not clear, and it is notstationary fronts are called Changma incertain whether the trends are due to theKorea and regarded as a regional

multi-decadal fluctuations, rather the partphenomenon related to the East Asianof a longer-term trend, or regionalmonsoonepisodic events.

system.

They

also

have

So

different names Meiyu in China and Baiu

farseveralstudiesabout

in Japan. Summer rainfall is mainly

extreme rainfall have been done. Over

contributed to by the stationary fronts thatthe last 50 years, the area affected by theoccupy more than half of the annual

upper 10% of the heaviest precipitationprecipitation in these regions. Therefore,has significantly increased, but there hassincebeen

thewaterresources

stronglyaslightdecreaseinannual

depend on the summer rainfall, the time

precipitation over some areas such asof the onset, evolution, and termination ofChina (Zhai et al., 1999a). Zhai et al.

summer rainfall over the East Asian(1999b) show a significant increase in thecountries has been one of the main

precipitation over the middle and lower

interests of the forecast community.

reaches of the Yangtze River and west

unusualChina during the latter part of the 20th

precipitation behavior is more frequentcentury, while also detecting a declining

these days and there are long-term

trend in precipitation over northern China.

changes

TheIt

isreported

inthatfrequency,intensity

offrequency

ofappearanceofextreme rainfall events in the world

extremely heavy daily precipitation clearly(Folland et al., 2001, and referencesshows a long-term increase in Japan fortherein). In particular, we have recently

theexperienced extreme rainfall events andYamamoto, 1993). An examination of thesevere flooding during the month ofrelationships

August when the Changma was alreadySouthernterminated, in the climatological aspect

extreme rainfall in Korea has beenfromtheperformed (Cha et al., 1999), the extreme

rainfallrainfall events are tending to occur

events have received increasing attention

frequently during the El Niño/La Niña

thebeginningmid-1990sof2000.

through

Extremelast

100years
between

Oscillation(Iwashima
theandEl

Niño(ENSO)

andyears, although statistically insignificant.

is fully developed, then moves northward

However, long-term variations in extremeand southward over the whole country inrainfall

associatedJuly. On the other hand, the large

temporal and spatial distribution patterns

amounts of rainfall in August are caused

have not been studied in detail overby typhoons (including the direct andKorea. The objective of this study is toindirect

investigate

instability in the boundary of the Northevents

andthethecharacteristics

ofprecipitation and its extremes in August

impacts),

cyclones,fronts,

Pacific High, and regional factors.

over Korea in terms of temporal andIn order to examine the long-term

spatial variations and to suggest thevariability of the rainfall and the frequency

possible

rainfallof extreme rainfall events in summer overevents in the seven years that have theKorea, the time series of the rainfall

greatest number of heavy rainfall events.

anomaly and the frequency of rainfall

causesofextreme

events in August are obtained for the2. The characteristics of the extreme

period from 1973 to 2002. An extreme

rainfall events

rainfall event at a different weather

An investigation is carried out toassesstheclimatologyofmonthly

station in the same day is counted
individually. It is not found that anyofdominate change results either in June or

For theJuly. The notable increasing trend in both

period from 1973 to 2002, JJA rainfall

the monthly rainfall and the frequency ofoccupies more than half of annual total

extreme rainfall events is found in August.

precipitation in most regions over Korea

The enhanced summer mean rainfall inand there appeared the largest monthly

recent years is mainly contributed to by

precipitation

andthefrequency

extreme rainfall events in Korea.

-1mean rainfall in August (269 mm month )the increased precipitation in August.

and the second maximum in July (262

The increasing trend in August ismm/month). The 30-years climatologicalmainly attributed to the most extreme

results show that there are more frequent

rainfall events on the historical record in

extreme rainfall events exceeding 80 mmrecent summers (1987, 1993, 1995, 1998,-1in August than in July, although

1999, 2000, and 2002, hereafter thenearly the same monthly rainfall occurs inseven heaviest years). In the sevenboth July and August. The rainfall in July

heaviest years, the Changma was normal

is mainly attributed to the stationary front

or not fully developed with a relative lack

(Changmain Korea), which is theof rainfall and a delay in its onset andregional component of the East Asiantermination, whereas the heavy rainfall

summer monsoon and this monsoon front

was reported at most of the stations overdayrainfall

The maximum number of typhoons thatcaused heavy human resource andinfluence the heavy rainfall over Korea

propertywidespreadalso appears in August, thus the effect offlooding. Since the recent characteristics

typhoons is important in the precipitationof the summer rainy season rainfall,in August. There is an evidence for theespecially for August, show a significant

linkage between the extreme rainfall indifference

climatological

the six heaviest years except for 1998

features, we focus on the extreme rainfall

and typhoons. No typhoon appeared in

episodes in August of the seven heaviest

August of 1998 over Korea.

years.

that more than half the extreme rainfall

Korea.

The

record-breaking

lossesthroughfromtheWe note

events in August seem to be strongly
3. The causes of the extreme rainfall

affected by major typhoons, especially inevents

1995, 1999, 2000, and 2002.

To

investigate

thecausesoftyphoons

generatedinA series ofthewesternoin

Pacific centered at 20 N and contributed

summertime is difficult, because thehighly to the outbreak of heavy rainfall inmultiple

1999.

extremecyclones,

Korea

rainfallfactors

suchfronts,

eventsasinstability

typhoons,
inIn August 2000, two typhoons

theoccurred consecutively and contributed toboundary of the North Pacific High andthe excessive rainfall. From the analysis

regional factors are involved. Thoughof large-scale circulation, it is found thatmany efforts to reveal the causes of thethe heavy rainfall events are related toextreme rainfall events have been made,the circulation of both the midlatitudes

only a few studies have described how

andtyphoons impact on the extreme rainfalls.

According to Wang et al. (2000), theTherefore, we try to find out the effect of

establishment of the WNP anticyclone is

typhoons on the summer heavy rainfall inconnected to the El Nino event and theKorea, especially in August and we

high-pressure system persists throughhighlight some large-scale features thatthe positive feedback between the SST

distinguish a normal August, especially inand the wind anomalies over the tropical

1987, 1993, and 1998 among the sevenwestern Pacific. Therefore, the extreme

heaviest years, in which the effect ofevents

typhoons is relatively weak compared tofrequently, if the above two conditions

the other heaviest years.

associated with the midlatitudes and theWe haveexamined the number of occurrences oftyphoonsand

theirimpactontheprecipitation over the Korean peninsula.

theintropics:

Augusttropics are satisfied.

EUcanandWNPSH.

occur

more