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Choi K.-S.,Policy Research Laboratory National Institute of Meteorological Research | Kim B.-J.,Policy Research Laboratory National Institute of Meteorological Research | Cha Y.-M.,National Typhoon Center Korea Meteorological Administration | Kim H.-D.,Keimyung University
Theoretical and Applied Climatology | Year: 2015

This study analyzed the effects of thermal differences between land and the sea on tropical cyclone (TC) activity. To this end, northern China in which thermal ridges appear in summer in the continent east of Asia was defined as “Land” and an area of the sea where temperatures are low in the tropical and subtropical western North Pacific was defined as “Sea” to analyze the time series of thermal differences between the land and the sea over the last 62 years. Change-point analysis was applied to these time series. According to the results, a significant climate regime shift existed in 1978. That is, positive values were distributed from 1951 to 1978 (5178) and negative values were distributed from 1979 to 2012 (7912). Thereafter, average differences between the 5178 period during which positive values were apparent and the 7912 period during which negative values were apparent was analyzed. With regard to TC genesis, TCs during the 7912 period showed a tendency of being mainly formed in the northwestern quadrant of the tropical and subtropical western North Pacific, and those during the 5178 period showed a tendency of being mainly formed in the southeastern quadrant. With regard to TC movements, whereas TCs during the 7912 period showed a pattern of moving west from the Philippines toward the Indochina peninsular and southern China, those during the 5178 period showed a pattern of moving north from the far southeastern sea of the Philippines to pass the East China Sea and go toward Korea and Japan. Therefore, the TCs during the 7912 period showed a tendency of being formed and moving in regions further west than those of the TCs during the 5178 period. With regard to TC intensities during the two periods, TCs during the 5178 period were more intense. Large-scale environments that affected these changes in TC activity between the two periods were analyzed. During the 7912 period, since temperature in the continent were lower than those in the sea, anomalous anticyclones were reinforced in the continent East of Asia and anomalous cyclones were reinforced in the sea. Consequently, anomalous northerlies were reinforced in a region ranging from the middle-latitude region in East Asia to the Indochina peninsular so that TCs could not move to the middle-latitude region in East Asia but moved west toward the Indochina peninsular and southern China during this period. Eventually, during the 7912 period, temperatures in the land were higher than those in the sea to show the west high east low anomalous pressure system patterns so that western North Pacific subtropical highs could not develop and thus the steering flows of anomalous southerlies that would move TCs to the middle-latitude region in East Asia were weak. These results were identified from 600 hPa relative humidity, 200–850 hPa vertical wind shears, and differences in sea surface temperatures between the two periods. © 2014, Springer-Verlag Wien. Source


Choi K.-S.,National Typhoon Center Korea Meteorological Administration | Wang B.,University of Hawaii at Manoa | Kim D.-W.,Pukyong National University
Climate Dynamics | Year: 2012

This study defines the Changma onset using the available water resources index (AWRI) for 25 years (1985-2009) and verifies the validity of this definition. The three conditions for defining the Changma onset are established as follows: (i) The first day exceeding the June AWRI (threshold) averaged over the 25-year period. (ii) The continuation of the value over the threshold for at least 1 week after the onset. (iii) After the continuation of more than 1 week, the non-continuation of the value under the threshold for at least 1 week. The 25-year average Changma onset date is 24 June with a standard deviation of 9 days. The defined Changma onset is verified through the analysis on the relationship with the Antarctic oscillation (AAO). AAO in June shows a high correlation with not only the Changma onset but also the June precipitation (AWRI) in Korea. These three variables are influenced by Mascarene and Australian (positive AAO pattern) highs from in the preceding March. When these two pressure systems develop, the cold cross-equatorial flow in the direction from the region around Australia to the equator is intensified, which in turn, forces a western North Pacific high (WNPH) to develop northward; this eventually drives the rain belt north. As a result, the Changma begins early in the positive AAO phase, and the June precipitation increases in Korea. In addition, a WNPH that develops more northward increases the landfalling frequency of tropical cyclones in Korea, which plays an important role in increasing the June precipitation. © 2010 Springer-Verlag. Source


Cha Y.,National Typhoon Center Korea Meteorological Administration | Kim H.-D.,Keimyung University | Kang S.-D.,Green Simulation Co.
Theoretical and Applied Climatology | Year: 2015

In the present study, the fact that strong positive correlations have existed between East Asian summer monsoons (EASMs) and western North Pacific tropical cyclone (TC) genesis frequency over the last 37 years was found. To figure out the cause of these correlations, 7 years (positive East Asian summer monsoon index (EASMI) phase) that have the highest values and 7 years (negative EASMI phase) that have the lowest values in the normalized EASM index were selected and the differences in averages between the two phases were analyzed. In the positive EASMI phase, TCs mainly occurred in the northwestern waters of the tropical and subtropical western North Pacific and showed a tendency to move from the far eastern waters of the Philippines, pass the East China Sea, and move northward toward Korea and Japan. On the 500 hPa streamline, whereas anomalous anticyclones developed in the East Asia middle-latitude region, anomalous cyclones developed in the tropical and subtropical western North Pacific. Therefore, in this phase, whereas EASMs were weakened, western North Pacific summer monsoons (WNPSMs) were strengthened so that some more TCs could occur. In addition, in the case of the East China Sea and the southern waters of Japan located between the two anomalous pressure systems, TCs could move some more toward the East Asia middle-latitude region in this phase. According to an analysis of the 850 hPa relative vorticity, negative anomalies were strengthened in the East Asia middle-latitude region while positive anomalies were strengthened in the region south to 25 N. Therefore, in the positive EASMI phase, whereas EASMs were weakened, WNPSMs were strengthened so that some more TCs could occur. According to an analysis of the 850 and 200 hPa horizontal divergence, whereas anomalous downward flows were strengthened in the East Asia middle-latitude region, anomalous upward flows were strengthened in the tropical and subtropical western North Pacific. According to an analysis of 200–850 hPa vertical wind shear and 600 hPa relative humidity, negative anomalies and positive anomalies were strengthened in the tropical and subtropical western North Pacific, respectively, to provide good atmospheric environments in which some more TCs could occur in the positive EASMI phase. According to an analysis of sea surface temperatures (SST) too, whereas cold SST anomalies were strengthened in the East Asia middle-latitude region, warm SST anomalies were strengthened in the tropical and subtropical western North Pacific to provide good marine environments in which some more TCs could occur in the positive EASMI phase. © 2015 Springer-Verlag Wien Source


Choi K.-S.,National Typhoon Center Korea Meteorological Administration | Cha Y.-M.,National Typhoon Center Korea Meteorological Administration | Kang S.-D.,National Typhoon Center Korea Meteorological Administration | Kim H.-D.,Keimyung University
Theoretical and Applied Climatology | Year: 2015

The study surveys the tropical cyclone (TC) activities in the Northwestern Pacific region during the summer monsoon season (SMS) (June–July) in the Northeast Asian region (North China, Korea, and Japan). The positive (negative) SMS rainfall years in the region show that not only the TC genesis frequency, but also the TC frequency which affects the Northeast Asian countries, are low (high) in the tropical and subtropical northwest Pacific. That is, the TCs mainly move to the southern part of China or up to the east sea far from Japan in the positive SMS rainfall years. The study analyzes the difference between the two groups in the 500 hPa streamline to investigate TC activities for the groups. The large-scale anomalous anticyclone and the anomalous cyclone were enlarged in the tropical and subtropical Northwestern Pacific and from Manchuria to the eastern Japan, respectively. Due to such anomalous pressure system patterns being strengthened during the positive SMS rainfall years, the tropical Northwestern Pacific below 20°N fortified anomalous easterlies, causing a lot of TCs during those years to move to the southern part of China along with these anomalous steering flows (anomalous easterlies). In addition, the anomalous anticyclone that is located in the tropical and subtropical Northwestern Pacific caused a low TC genesis frequency during the positive SMS rainfall years. © 2014, Springer-Verlag Wien. Source


Choi K.-S.,National Typhoon Center Korea Meteorological Administration | Cha Y.-M.,National Typhoon Center Korea Meteorological Administration | Kang S.-D.,National Typhoon Center Korea Meteorological Administration | Kim H.-D.,Keimyung University
Theoretical and Applied Climatology | Year: 2015

This study analyzed that there was a strong negative correlation between sea surface temperature (SST) in the equatorial Eastern Pacific and tropical cyclone (TC) frequency that affects Korea in summer season in 1951 to 2012. Then, 12 years that had the highest region-averaged SST values in the equatorial Eastern Pacific (warm SST years) and 12 years that had the lowest SST values (cold SST years) were selected, followed by analyzing average difference between two groups to identify any causes of possible correlation between SST in the equatorial Eastern Pacific and TC frequency. In the warm SST years, not only TC frequency that affected Korea but also TC genesis frequency was low, as well as TC intensity. A difference of 500 and 850 hPa streamlines between two groups showed that anomalous northwesterlies were strengthened in Korea due to anomalous cyclonic circulations developed in the mid-latitude region in East Asia, so that such anomalous steering flows played a role in preventing TCs from moving to Korea. Furthermore, anomalous anticyclonic circulations were strengthened in the east of Taiwan, which was related to weakening of the monsoon trough, thereby causing lowering TC genesis frequency in the warm SST years. In addition, as anomalous cyclonic circulations were strengthened in the southeast quadrant of the tropical and subtropical western North Pacific, it created high TC genesis frequency in the sea in the warm SST years. Low TC frequency, low TC genesis frequency, and low TC intensity that affected Korea in the warm SST years were clearly seen by the analysis of differences between two groups with respect to 500 and 850 hPa air temperature, 600 hPa relative humidity, precipitable water, 200–850 hPa vertical wind shear, and sea surface temperature. © 2014, Springer-Verlag Wien. Source

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