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Choi K.-S.,National Taiwan University | Wu C.-C.,National Taiwan University | Cha E.-J.,National Typhoon Center
Journal of Geophysical Research: Atmospheres | Year: 2010

This study shows that the Pacific-Japan (PJ) teleconnection pattern has a significant influence on tropical cyclone (TC) activities over the western North Pacific (WNP) during the boreal summer (July, August, and September). During positive (negative) PJ phase, TCs form at a more northward (southward) location, recurve at a more northeastward (southwestward) location, and frequently pass over the northeast Asian (southeast Asian) region, including Korea and Japan (South China Sea and southern China). In particular, this difference in the TC track between the two phases is observed as a dipole-like pattern between the regions of Southeast and Northeast Asia. The TC characteristics during the positive PJ phase are caused by the following two stronger atmospheric circulations over the WNP: an anticyclonic circulation centered to the east of Japan and a cyclonic circulation centered to the east of Taiwan. The southeasterly between these two circulations serves as steering flow that TCs move northward toward Korea and Japan from the northeast of the Philippines. Conversely, TCs during the negative PJ phase mainly move westward toward the South China Sea and southern China by the easterly from a stronger anticyclonic circulation centered to the east of Taiwan. As a result of this feature of TC track during the negative PJ phase, TC lifetime is shorter and TC intensity is weaker. Copyright 2010 by the American Geophysical Union.

Choi K.-S.,National Typhoon Center | Wu C.-C.,National Taiwan University | Wang Y.,University of Hawaii at Manoa
Asia-Pacific Journal of Atmospheric Sciences | Year: 2011

The effect of ENSO on landfalling tropical cyclones (TCs) over the Korean Peninsula is examined. It is found that although the landfalling frequency does not show any statistically significant difference among ENSO phases, the landfalling tracks are shifted northward in response to the decrease in Niño-3.4 index. In the neutral ENSO phase, many TCs pass through mainland China before landfalling over the Korean Peninsula due to the westward expansion of the western North Pacific subtropical high. Therefore, the landfalling TC intensity over the Korean Peninsula in the neutral phase is similar to that in the La Niña phase because more than half of those TCs made landfall over mainland China. However, it is found that the preceding winter ENSO phases are not related to the landfalling TC activity over the Korean Peninsula during summer. © The Korean Meteorological Society and Springer 2011.

Kang N.-Y.,National Typhoon Center | Elsner J.B.,Florida State University
Journal of Climate | Year: 2016

Violent typhoons continue to have catastrophic impacts on economies and welfare, but how they are responding to global warming has yet to be fully understood. Here, an empirical framework is used to explain physically why observations support a tight connection between increasing ocean warmth and the increasing intensity of supertyphoons in the western North Pacific. It is shown that the energy needed for deep convection is on the rise with greater heat and moisture in the lower tropical troposphere but that this energy remains untapped when air pressure is high. Accordingly, tropical cyclone formation is becoming less common, but those that do form are likely to reach extreme intensities from the discharge of stored energy. These thermodynamic changes to the environment most significantly influence the upper portion of extreme typhoon intensities, indicating that supertyphoons are likely to be stronger at the expense of overall tropical cyclone occurrences in the western North Pacific. © 2016 American Meteorological Society.

Choi K.-S.,National Typhoon Center | Moon I.-J.,Jeju National University
Natural Hazards | Year: 2012

This study investigated the annual frequencies of tropical cyclones (TCs) that affected Korea during summer (June-September) over the last 60 years. Using a statistical change-point analysis, we found that significant regime shifts occurred in 1999 and 2005, as well as in the mid-1960s and mid-1980s. Focusing on the recent TC activities, this study analyzed the differences between the high-frequency period from 1999 to 2004 (P1) and the low-frequency period from 2005 to 2010 (P2). The analysis reveals that TCs during P2 tended to occur, move, and recurve farther to the west in the western North Pacific (WNP). This is because the WNP high expanded farther to the west during P2 compared to P1; as a result, more TCs made landfall on the west coast of the Korean peninsula (KP) during P2. In contrast, during P1, TCs tended to make landfall more frequently on the south coast of the KP. This implies that the recent TC tracks landing on the KP shifted gradually to the northwest. The analysis of streamlines at 500 hPa show that an anomalous northerly strengthened in the KP due to the formation of an anomalous anticyclone and an anomalous cyclone to the west and east of the KP, respectively. These anomalies played a role in blocking TCs from moving to the KP. At 850 hPa, the anomalous anticyclonic circulation was strengthened in most of WNP. This circulation formed an unfavorable environment for TC genesis, reducing the TC genesis frequency during P2. We verified this low convective activity in the WNP during P2 by analyzing the outgoing longwave radiation, vertical wind shear, and sea surface temperature. © 2012 Springer Science+Business Media B.V.

Choi K.-S.,National Typhoon Center | Moon I.-J.,Jeju National University
Dynamics of Atmospheres and Oceans | Year: 2012

This study analyzes the characteristics of Western North Pacific (WNP) tropical cyclone (TC) activity and large-scale environments according to the Western Pacific (WP) teleconnection pattern in summer. In the positive WP phase, an anomalous cyclone and an anomalous anticyclone develop in the low and middle latitudes of the East Asia area, respectively. As a result, southeasterlies are reinforced in the northeast area of East Asia (including Korea and Japan), which facilitates the movement of TC to this area, whereas northwesterlies are reinforced in the southwest area of East Asia (including southern China and the Indochina Peninsula) which blocks the movement of TC to that area. Due to the spatial distribution of this reinforced pressure system, TCs that develop during the positive WP phase move and turn more to the northeast of the WNP than TCs which develop during the negative WP phase.The characteristics of this TC activity during the positive WP phase are associated with the upper tropospheric jet being located farther to the northeast. TCs during the negative WP phase mainly move to the west from the Philippines toward southern China and the Indochina Peninsula. Due to the terrain effect caused by the passage of TCs in mainland China, the intensity of TCs during the negative WP phase is weaker than those during the positive WP phase. © 2012 Elsevier B.V.

Choi K.-S.,National Typhoon Center | Cha Y.-M.,National Typhoon Center
Natural Hazards | Year: 2012

Differences in atmospheric and oceanic environments which affect the tropical cyclone (TC) activity between the late twenty-first century (2071-2100, A1B) and the late twentieth century (1971-2000, 20C3M) are analyzed using multi-model ensemble from 15 general circulation models. Six factors (vertical wind shear, 700 hPa relative humidity, 850 hPa relative vorticity, outgoing longwave radiation, precipitation, and sea surface temperature) related to TC genesis predicts that more TCs in the future will occur than in the present. The result of maximum potential intensity analysis shows the frequency of occurrence and influence of stronger TCs will increase over the western North Pacific in the future. Anomalous northerly in the mid-latitudes of East Asia due to the strengthening of west-high and east-low pressure system pattern in the future plays an important role in blocking TC from moving toward mid-latitudes of East Asia. The multiple linear regression model (MLRM) developed using six predictors (independent variables) analyzed from NCEP-NCAR reanalysis data predicts that total TC genesis frequency during July to October (JASO), which predicted using data of 20C3M, will have more (2-3) TCs than in the present. © 2012 Springer Science+Business Media B.V.

Kang K.,National Typhoon Center | Kim S.,Weather Information Service Engine Institute
Ocean Engineering | Year: 2015

Wave propagation conditions and wave-tide interactions are studied in a tide-dominant bay area during a strong storm event with a speed of 15-20 ms-1 during the study period of March 3-8, 2007. The study area is Kyunggi Bay, which is a large estuarine system in the west coast of Korea, a well-known macrotidal area. To investigate wave-tide interactions, a series of numerical experiments were designed to determine significant factors that contribute to wave height deformation and periodic wave-height modulations during a strong passing storm. The findings suggest that the periodically occurring wave height modulations at onshore and near coastal areas are mainly caused by the water depth changes by tide, which means that more wave energy can propagate into the coastal area increasing the wave height. The wave propagation was changed by tide, and the peak wave period was increased during the flood and decreased during the ebb tide. Although the effect was relatively weak, the tidal current also contributed to the increase and decrease of wave height during the flood and the ebb. It is worth noting that the wind was not a factor of periodic wave-height modulations, but it played a role in increasing the wave height. © 2015 Elsevier Ltd. All rights reserved.

Choi J.-W.,National Institute of Meteorological science | Cha Y.,National Typhoon Center
Dynamics of Atmospheres and Oceans | Year: 2016

This study has developed the index for diagnosis on possibility that tropical cyclones (TCs) affect Korean Peninsula. This index is closely related to the strength of the western North Pacific subtropical high (WNPSH), which is calculated as a difference in meridional wind between at the highest correlation area (around Korean Peninsula) and at the lowest correlation area (sea southeast of Japan) through a correlation analysis between TC frequency that affects Korean Peninsula and 500 hPa meridional wind. In low frequency years that selected from Korea affecting TC index, anomalous northeasterly is strengthened from Korea to the South China Sea because the center of anomalous anticyclonic circulation is located to northwest of Korean Peninsula. Thus, TCs tend to move westward from the sea east of the Philippines to the mainland China. On the other hand, in high frequency years, anomalous southwesterly serves as steering flow that more TCs move toward Korean Peninsula because the center of anomalous anticyclonic circulation is located to sea east of Japan. Consequently, this study suggests that if this index is calculated using real time 500 hPa meridional winds that forecasted by dynamic models during the movement of TCs, the possibility that TCs approach Korean Peninsula can be diagnosed in real time. © 2016 Elsevier B.V.

Kim D.-W.,National Institute of Meteorological Research | Byun H.-R.,Pukyong National University | Choi K.-S.,National Typhoon Center | Oh S.-B.,Pukyong National University
Journal of Applied Meteorology and Climatology | Year: 2011

The climatological characteristics of drought in South Korea were investigated using daily precipitation data for 1777-2008. The effective drought index was used to quantify the drought intensity. As a result, five characteristics were discovered. First, South Korea can be divided into four drought subregions (the central, southern, and east coastal regions and Jeju Island) using hierarchical cluster analysis. Second, a map for longterm drought conditions in the four subregions is created that allows identification of the spatiotemporal distribution of droughts for the 231 yr at a glance. Third, droughts in South Korea have time scales that depend on the onset season. Spring (March-May) droughts tend to be short (≤200 days) because the summer (June-September) rainy season follows. Summer droughts tend to be long (>200 days) because the dry season (October-February) follows. In the dry season, droughts tend to be sustained or become severe rather than being initiated or relieved. Fourth, 5-, 14-, 34-, and 115-yr drought cycles were identified by spectral analysis. The 5-yr cycle was dominant in all of the regions, the 14-yr cycle was observed over the southern and east coastal regions, and the 34-yr cycle was observed over the central region. Fifth, the most extreme drought occurred in 1897-1903 (return period: 233 yr) and was associated with the 115-yr drought cycle. After this drought, severe droughts (return period of >10 yr) occurred in 1927-30, 1938-40, 1942-45, 1951-52, 1967-69, and 1994-96; they were caused by the consecutive shortage of summer rainfall for two or more years. © 2011 American Meteorological Society.

Kim D.-W.,National Institute of Meteorological Research | Choi K.-S.,National Typhoon Center | Byun H.-R.,Pukyong National University
Climate Dynamics | Year: 2012

This study compares the impacts of El Niño Modoki and El Niño on precipitation over Korea during the boreal winters from 1954 to 2009. Precipitation in Korea tends to be equal to or greater than the normal level during an El Niño Modoki winter, whereas there is no significant change during an El Niño winter. Greater than normal precipitation during El Niño Modoki was also found over the lower reaches of the Yangtze River, China and much of southern Japan. The latitudes of these regions are 5-10° further north than in southern China, where precipitation increases during El Niño. The following two anomalous atmospheric circulations were found to be causes that led to different precipitation distributions over East Asia. First, an atmospheric wave train in the lower troposphere, which propagated from the central tropical Pacific (cyclonic) through the southern Philippine Sea (anticyclonic) to East Asia (cyclonic), reached the southern China and northern Philippine Sea during El Niño, whereas it reached Korea and southern Japan during El Niño Modoki. Second, an anomalous local meridional circulation, which consists of air sinking in the tropics, flowing poleward in the lower troposphere, and rising in the subtropics, developed between the southern Philippine Sea and northern Philippine Sea during El Niño. During El Niño Modoki, however, this circulation expanded further to the north and was formed between the southern Philippine Sea and regions of Korea and southern Japan. © 2011 Springer-Verlag.

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