Long-Term Trend of Summer Rainfall at Selected Stations in the Republic of Korea

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1 Long-Term Trend of Summer Rainfall at Selected Stations in the Republic of Korea Il-Kon Kim Professor, Department of Region Information Rafique Ahmed Professor, Geography and Earth Science Silla University Busan (Pusan) Republic of Korea (South Korea) University of Wisconsin-La Crosse La Crosse, Wisconsin USA ABSTRACT The Republic of Korea (or South Korea) is located in the extreme northeastern region of the East Asian Monsoon climatic zone. Summer (June September) is the rainy season in the country, which accounts for more than 60 percent of the annual precipitation. Using 50 years of summer (June September) rainfall data ( ) at 23 stations, this paper presents the geographic distribution of summer rainfall and its coefficient of variation in South Korea; and the long-term trend of summer rainfall at four selected stations. Summer rainfall in South Korea varies from less than 700 mm in the northeast to mm or more in the south and southwest. The coefficient of variation of summer rainfall ranges from less than 25 percent in the northeastern part of the country to 35 percent or more in the northwest, west and south. The long-term trend of summer rainfall at the four selected stations shows year-to-year fluctuations with negligibly small positive trend. KEY WORDS: South Korea, summer rainfall, rainfall variability, long-term trend. INTRODUCTION The Republic of Korea (or South Korea), located in the southern half of the Korean Peninsula, belongs to the mid-latitude (Lee, 2000) or warm temperate climatic region (Dege, 1999). However, being located at the boundary between the Asian landmass and the western Pacific Ocean, which is characterized by seasonal reversal of winds, South Korea also belongs to the East Asian Monsoon climate (Barry and Chorley, 1997; Oliver and Hidore, 2002). According to Dege (1999), on the basis of the Köppen s classification of climates, the northern one-third of South Korea has a cold mid-latitude climate with dry winters (Dw). The remainder of the country belongs to the mild and moist mid-latitude climate with dry winters (Cw). There are four climatic seasons in South Korea winter, spring, summer and autumn. The winter (November March) and summer (June September) are the two most dominant seasons. The 44

2 winter is very cold and dry, while the summer is warm and moist. As a result, the summer is the growing season over most of the country. Annual rainfall in South Korea ranges from less than 1000 mm in the east-central region to 1200 mm along the west coast, 1500 mm in the northeast, and 1700 mm in Cheju Island (Lee, 2000). Of the total annual rainfall, more than 60 percent comes during the summer season. Using 50 years of summer rainfall data ( ), this paper analyzes geographic distribution of summer rainfall and its coefficient of variation; also longterm trend of summer rainfall at four selected stations in South Korea is evaluated. The stations were selected in such a way that they represent the diverse physio-climatic regions of the country. LOCATION AND PHYSIOGRAPHY South Korea extends from 33 N to 38.5 N latitude, and from E to E longitude (Fig. 1). It is bordered by mountainous North Korea in the north, and by seas on three other sides the East Sea (Sea of Japan) to the east and south, and the Yellow Sea to the west. Several factors influence the climate of South Korea: (a) seasonal reversal of wind direction between summer and winter; (b) adjacent water bodies, especially the western Pacific Ocean; and (c) the peninsula s topography. Nearly 70 percent of South Korea consists of mountains and hills and dissected narrow valleys (Fig. 1a), which were carved out by streams (Jo, 2000; Kwon, 2000; Dege, 1999). The Taebaek Range, which runs in north-south direction in the eastern part of the country, forms the backbone of the mountain systems of South Korea. Several other mountains branch out from the Taebaek Range toward southwest, namely, from north to south, the Kwangju Range, Charyoung Range, and the Sobaek Range with its branch the Noryong Range. The eleva- (a) FIGURE 1. (a) Physical setting of South Korea (modified from Dege, 1999), and (b) location of the stations used in this study. (b) 45

3 tions of most of these mountains generally range from 800 to 1000 m, with some peaks exceeding 1500 m (e.g., Mt. Taebaek, 1773 m, Mt. Chiri, 1915 m, not shown on the map). Besides the hills, mountain ranges, and the narrow basins located between them, there are a few small alluvial plains in the country which are located in the western and southeastern part of the country (Fig. 1a). SEASONALITY AND GENERAL CLIMATIC CONDITIONS It was noted that there are two dominant seasons in South Korea. The cold and dry winter season occurring from November to March, and the warm and moist summer season extending from June to September, with two intermediate transitional seasons of spring and the autumn. During the winter season, the Siberian high pressure generates a land-to-sea wind. This is the typical winter monsoon in the country, which brings in bitterly cold and dry continental polar air (cp air mass) from Siberia to South Korea (Lee, 2000; Dege, 1999). Average temperature in January ranges from about 5 C in the north to 2 3 C in the south. Due to the land-tosea air flow in winter, precipitation is very low during this season which totals about 10 percent of the annual precipitation. During the spring season (April May), the Siberian high pressure wanes, and the weather in South Korea is influenced by the northern fringes of the extratropical cyclones. June through September is the warm and rainy season, when the high pressure over the Pacific Ocean (the Hawaiian High) generates the sea-to-land wind. This is the typical summer monsoon in South Korea. Moist air masses that move in from the western Pacific Ocean form the polar front, which is known as the Changma Front in Korea. The mechanism of rainfall during this season is provided by a series of cyclones that develop in the Yangtze River basin of China at an interval of 2 3 days, and move into South Korea along the Changma Front, causing heavy rainfall in the country. July is the rainiest month in the country, after which the rainfall generally decreases through the rest of the season (Lee, 2000). Geographic distribution of summer rainfall in South Korea reflects the effects of topography, direction of the prevailing winds and the tracks of the cyclones during the summer season. Very heavy rainfall occurs in the southern coastal zone and in the adjacent mountains, where it ranges from 800 mm to 900 mm or more. It may be noted that the tropical cyclones from the western Pacific Ocean, known as typhoons in the region, reach the southern coastal area of the country during the late summer season, at a frequency of once every two years, and may reach the central region of the country once every four years (Lee, 2000). These typhoons cause torrential rains resulting in severe floods, soil erosion and devastating landslides, especially in the southern part of the country. Summer is warm in South Korea. August is the hottest month over most of the country due to less cloud cover and rainfall than in July. Average temperatures in summer varies from C in June, to 22 C 25 C in July, C in August, and C September (Lee, 2000). The period from mid-september through October is the autumn season in South Korea. In mid-september, the high pressure begins to build up over Siberia. As a result, South Korea begins to come under the influence of a series of anticyclones that travel from China to Korea. The period from late September through October is characterized by clear skies and pleasant weather. In November, the Siberian high pressure begins to intensify, which generates land-to-sea winds of the winter season over the Korean Peninsula (Lee, 2000; Dege, 1999). DATA AND ANALYSIS Data needed for this study include 50 years of monthly summer rainfall records (June through September). The data ( ) were selected for 23 stations, which were summed to obtain the total seasonal rainfall. These stations represent the diverse physio-climatic regions of the country, and are illustrated in Fig. 1b. These data were obtained from the Korea Meteorological Administration (KMA), Seoul, South Korea. 46

4 Data analysis include the following: (a) Calculation of the mean (x ), standard deviation (r), and the coefficient of variation (CV) of the summer rainfall at 23 stations. Coefficient of variation (CV) is a measure of the variability, and is given as: CV ((r/x ) 100). The methods of obtaining these statistical parameters are given in most standard textbooks on statistical methods (e.g., Manly, 2001). (b) Determination of the long-term trend of summer rainfall at four selected stations. General physio-climatic characteristics of the four stations are described below which were used to determine longterm trends of summer rainfall: (1) Busan: Located on the Nakdong River delta, backed by the coastal hills. Rainfall is enhanced in this area by the orographic effect of the coastal hills. (2) Gangneung: Coastal location in the dry northeast, in the rain-shadow zone of the Taebaek Range. (3) Gwangju: Interior location in the humid southwest, on an alluvial plain. The mountains in the nearby downwind region produce orographic uplifting which enhances the rainfall. (4) Seoul: Located on the windward side of the coastal hills in the northwest, at the head of the Han River delta. Here too, the coastal hills enhance the rainfall due to their orographic effect. To determine the long-term trend of summer rainfall at the above four selected stations, time series of the summer rainfall were plotted, and the trend line equations were calculated for each station (i.e., y-intercept, and the slope of the trend line), along with their respective correlation coefficients (r). Time series analysis and the method of obtaining the trend-line equation and correlation coefficient are described in most standard textbooks on statistical methods (e.g., Manly, 2001). RESULTS AND DISCUSSION SUMMER RAINFALL Distribution of the average summer rainfall and its coefficient of variation in South Korea are shown in Fig. 2a and Fig. 2b, respectively. It was mentioned earlier that the geographic distribution of summer rainfall reflects the effects of topography and the direction of the prevailing wind during the summer season. In general, summer rainfall decreases from south to north, with great regional variations. Maximum rainfall occurs in the coastal south and in the adjacent mountains which is the entry point of the summer monsoon flow. Summer rainfall in this region varies from 800 mm to 900 mm (Fig. 2a). However, Jeju Island, located off the southwest coast, receives the highest amount of rainfall, mm. Very heavy rainfall on this island is caused by its location on the path of the oncoming moisture flow and by the high mountains (1000 m or more in elevation, with the highest peak in excess of 1900 m). Rainfall along the west coast varies between 700 mm and 800 mm, and increases inland toward the higher elevation of the mountains, obviously due to the orographic effect. Thus, the region of secondary maximum rainfall is the area that stretches from the Pukhan River Valley in the north to the Samjin River Valley (Fig. 1a and Fig. 2a). Rainfall in this narrow region ranges from 800 mm to 900 mm. The northeastern part of the country receives the lowest summer rainfall, generally less than 700 mm, due to the rainshadow effect of the mountain ranges. The variability (i.e., coefficient of variation) of summer rainfall in South Korea ranges from less than 25 percent in the northeast to 35 percent or more in the northwest, west and south (Fig. 2b). LONG-TERM TREND OF SUMMER RAINFALL Graphical plots of the time series of the summer rainfall for the 50-year period 47

5 FIGURE 2. (a) Distribution of summer rainfall (mm), and (b) the coefficient of variation of summer rainfall (%) in South Korea. ( ) at four selected stations are illustrated in Fig. 3 and Fig. 4. The longterm trend of summer rainfall at a station is represented by a dashed line, with the appropriate trend equation (y...)and the correlation coefficient (r) written on the graph. These plots show year-to-year fluctuations in the summer rainfall, but the trend line at each station has a very small slope, and also has a negligibly small correlation coefficient (r). The r-values at Busan, Gwangju, Gangneung, and Seoul are , , , and , respectively. Very low slope in the trend line and a negligibly small r-value at each station suggest that the time series of the summer rainfall at these stations indicate a negligibly small positive trend. This is especially true about the rainfall trend in Seoul where the correlation coefficient (r) is only CONCLUSIONS Rainfall data for the four summer months (June September) at 23 stations in South Korea, for a 50-year period, were statistically analyzed to obtain the mean and the coefficient of variation of the summer rainfall; and to determine the longterm trend of summer rainfall at four selected stations (Busan, Gwangju, Gangneung, and Seoul) in the country. Summer rainfall in South Korea varies from less than 700 mm in the northeast to 800 mm in the northwest and west, and 900 mm or more in the south and southwest. High rainfall in the south, and along a strip that stretches from the Pukhan River Valley in the north to the Samjin River Valley in the south is caused by the additional effect of orographic uplifting provided by the mountains. The variability (i.e., coefficient of variation) of summer rainfall in South Korea ranges from less than 25 percent in the west-central and northeastern part of the country to 35 percent or more in the northwest, west and south. The time series of long-term rainfall in summer at four selected stations suggest year-to-year fluctuations, and indicate 48

6 FIGURE 3. Time series of summer rainfall (a) at Busan and (b) at Gangneung. The dashed line represents the long-term trend ( ) in the summer rainfall. The trend equation (y) and the correlation coefficient (r) are also shown in the graphical plots. 49

7 FIGURE 4. Time series of summer rainfall (a) at Gwangju, and (b) at Seoul. The dashed line represents the long-term trend ( ) in the summer rainfall. The trend equation (y) and the correlation coefficient (r) are also shown in the graphical plots. 50

8 a negligibly small positive trend, which is especially evident in the Seoul trend data. ACKNOWLEDGEMENTS The authors express their thanks to Silla University, Busan, South Korea, and to the University of Wisconsin-La Crosse, La Crosse, Wisconsin, USA, for funding to acquire precipitation data from the Korea Meteorological Administration; and to Mrs. Jang-Hee Suk of the Department of Region Information, Silla University, for her assistance in data processing. REFERENCES Barry, R. G. and Chorley, R. J Atmosphere, Weather and Climate, 7 th Edition, New York, Routledge. Dege, E Korean Peninsula: A Geographical Introduction, International Geographical Union, 49(2), pp Jo, W-R Geology and Geomorphology [of Korea]. IN: Korea: The Land and People (Eds: Kwon, H-J. and Huh, W-K.), 29 th International Geographical Congress, South Korea, Seoul, pp Kwon, H-J Introduction [to Korea]. IN: Korea: The Land and People, (eds: Kwon, H-J. and Huh, W-K.), 29 th International Geographical Congress, South Korea, Seoul, pp Lee, H-Y, Climate [of Korea]. IN: Korea: The Land and People, (eds: Kwon, H-J. and Huh, W-K.), 29 th International Geographical Congress, South Korea, Seoul, pp Manly, B. F. J., Statistics for Environmental Science and Management, Boca Raton and New York, Chapman and Hall/CRC. Oliver, J. E. and Hidore, J. J Climatology: An Atmospheric Science, 2 nd Edition, New York, MacMillan. 51