Two climate factors in May that affect Korean rainfall in September
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Two climate factors in May that affect Korean rainfall in September CHOI Ki-Seon1 , MOON Il-Ju2∗ 1 2
National Typhoon Center, Korea Meteorological Administration, Seogwipo 699-942, Republic of Korea College of Ocean Science/Ocean and Environment Research Institute, Jeju National University, Jeju 690-756, Republic of Korea
Received 3 January 2012; accepted 20 June 2012 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2013
Abstract This study revealed a high positive correlation between rainfall in Korea during September and the trade wind (TW)/Arctic Oscillation (AO) index in May that combines two climate factors, low-level TWs and the AO. This correlation was identified on the basis of the difference in the 850 hPa streamline analysis between the positive and negative phases selected using the combined TW/AO index. In May, the spatial pattern of the anomalous pressure systems is similar to that in the positive AO phase. These anomalous pressure systems continue in June to August (JJA) and September, but the overall spatial distribution shifts a little to the south. Particularly in September, a huge anomalous anticyclone centered over the southeast seas of Japan strengthens in most of the western north Pacific region and supplies a large volume of warm and humid air to the region near Korea. This characteristic is confirmed by the facts that during the positive TW/AO phase, the subtropical western north Pacific high (SWNPH) is more developed to the north and that the continuous positioning of the upper troposphere jet over Korea from May to September strengthens the anomalous upward flow, bringing warm and humid air to all layers. These factors contribute to increasing September rainfall in Korea during the positive TW/AO phase. Because the SWNPH develops more to the north in the positive phase, tropical cyclones tend to make landfall in Korea frequently, which also plays a positive role in increasing September rainfall in Korea.The above features are also reflected by the differences in average rainfall between the six years that had the highest May Niño 3.4 indices (El Niño phase) and the six years that had the lowest May Niño 3.4 indices (La Niña phase). Key words: anomalous pressure, Arctic Oscillation, Korea, low-level trade wind, rainfall, tropical cyclone Citation: Choi Ki-Seon, Moon Il-Jug. 2013. Two climate factors in May that affect Korean rainfall in September. Acta Oceanologica Sinica, 32(1): 32–47, doi: 10.1007/s13131-013-0265-9
1 Introduction Summer rainfall in Korea accounts for 40%–60% of the annual rainfall, and because heavy rain often falls locally in a short time, it is a main cause of natural disasters such as floods and landslides. Since the Korean winter is dry, insufficient summer rainfall may lead to severe drought lasting from the following autumn to the spring of the next year. Therefore, the summer rainfall has been a primary research subject; many scientists have tried to understand the dynamics of the summer rains and to predict them. Chung and Yoon (2000)
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