Ki-Seon Choi

A Spatiotemporal Analysis of Historical Droughts in Korea

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)rainyseasonfollows.Summerdroughtstendtobelong(.200days)becausethedryseason (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.

Possible connection between summer tropical cyclone frequency and spring Arctic Oscillation over East Asia

This study shows that the frequency of summer tropical cyclones (TCs) in the areas of Japan, Korea, and Taiwan (JKT), which are located in the middle latitudes of East Asia, has a positive correlation with the Arctic Oscillation (AO) occurring during the preceding spring, while summer TC frequency in the Philippines (PH), located in the low latitudes, has a negative correlation with the AO of the preceding spring. During a positive AO phase, when the anomalous anticyclone forms over the mid-latitudes of East Asia, other anomalous cyclones develop not only in the high latitudes but also in the low latitudes from the preceding spring to the summer months. With this change, while southeasterlies in the JKT area derived from the mid-latitude anticyclone plays a role in steering TCs toward this area, northwesterlies strengthened in the PH area by the low-latitude cyclone plays a role in preventing TC movement toward this area. In addition, because of this pressure systems developed during this AO phase, TCs occur, move, and recurve in further northeastern part of the western North Pacific than they do during a negative AO phase.

Effects of El Niño Modoki on winter precipitation in Korea

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.

Possible relationship between North Korean total rainfall and Arctic Oscillation in May

In this study, a negative correlation between the rainfall over North Korea in May and the Arctic Oscillation (AO) in the same month is analyzed. The reasons for rainfall declines in the positive AO phase are as follows: (a) the strengthening of anomalous anticyclone in the Maritime Province of Siberia, (b) the weakening of the subtropical western North Pacific high, and (c) the stabilization across all the atmospheric layers in North Korea. Anomalous anticyclone strengthened in the Maritime Province of Siberia plays a critical role in placing the North Korean region under the influence of anomalous northeasterlies. In addition, the development of the anomalous northeasterlies results in the supply of a large amount of cold and dry air into North Korea. This consequently stabilizes the atmosphere in North Korea. Moreover, the reinforcement of anomalous cold sea surface temperature in the mid-latitude region of East Asia is found to be another reason for the atmospheric stabilization.

A Study on the Characteristics of Tropical Cyclone Passage Frequency over the Western North Pacific using Empirical Orthogonal Function

A pattern of tropical cyclone (TC) movement in the western North Pacific area was studied using the empirical orthogonal function (EOF) and the best track data from 1951 to 2007. The independent variable used in this study was defined as the frequency of tropical cyclone passage in 5 by 5 degree grid. The , and modes were the east-west, north-south and diagonal variation patterns. Based on the time series of each component, the signs of first and second mode changed in 1997 and 1991, respectively, which seems to be related to the fact that the passage frequency was higher in the South China Sea for 20 years before 1990s, and recent 20 years in the East Asian area. When the eigen vectors were negative values in the first and second modes and TC moves into the western North Pacific, TC was formed mainly at the east side relatively compared to the case of the positive eigen vectors. The first mode seems to relate to the pressure pattern at the south of Lake Baikal, the second mode the variation pattern around , and the third mode the pressure pattern around Japan. The first mode was also closely related to the ENSO and negatively related to the -3.4 index in the correlation analysis with SST anomalies.

Cluster analysis of tropical cyclone tracks around Korea and its climatological properties

After the fuzzy clustering method (FCM) that analyzes the tracks of the tropical cyclones (TCs) struck the Korean peninsula (hereafter, K-TC) for a 60-year period (1951–2010), it is found that both frequency and intensity of K-TC have been increased in recent years. In the order of the cluster number, both K-TC track pattern and its full-track pattern tended to shift southward. That is, while the passage frequency of TC in mainland China and the Manchurian regions decreased, it instead over the sea. Due to this decrease in the topographic effect on TC before reaching Korea, TC intensity around Korea became stronger. The vertical wind shear well reflected a TC intensity around Korea, which became weaker in mid-latitudes of East Asia. On the other hand, the peak month of K-TC frequency lags in the order of the cluster number. The two clusters that most TCs pass through the Korean Peninsula showed a stronger intensity and higher frequency before the 1970s. Meanwhile, another two clusters that most TCs pass through the Straits of Korea or the western region of the Japanese Islands showed those characteristics from the 1980s onward. Consequently, the changes in TC track, recurvature, frequency, and intensity around Korea were related to the southward shift of the western North Pacific high in the order of the cluster number.

Relationship between Korean Peninsula Landfalling Tropical Cyclones and Interannual Climate Variabilities

The relationship between two interannual climate variabilities and the frequency of tropical cyclone (TC) that landed over the Korean Peninsula (KP) has investigated for the period of 1951-2004. In the analysis of the relationship between KP-landfall TC frequency and the ENSO phase, most TCs of C-14 (TCs that do not pass through mainland China before landing the KP) and C-23 (TCs that pass through mainland China before landing the KP) tended to more land in the warm phase than normal and cold phases. However, TC intensity at landfall was stronger in the cold and normal phases. In the analysis of the relationship between KP-landfall TC frequency and Arctic Oscillation (AO) phase, the TCs of C-14 tended to more land in the positive (POS) phase of AO and the negative (NEG) phase of AO for C-23. It was found that AO index was negatively correlated with the Nio-3.4 index. And then the TCs of C-14 landed more frequently over the KP in the AO POS - Nio-3.4 NEG phases and in the AO NEG - Nio-3.4 POS phases for the TCs of C-23.

Decadal Change of Frequency in Korea Landfalling Tropical Cyclone Activity

Through a statistical change-point analysis, this study found that Korea landfalling tropical cyclone (TC) frequency has increased rapidly since 1981. This increase is due to the following phenomenon. When anomalous cyclone is developed in the East Asian continent, anomalous anticyclone is reinforced in the western Pacific, which is related to the eastward shift of western North Pacific high, and thus anomalous southerly is formed to Korea from low-latitudes. This anomalous southerly plays an important role as steering flow in moving TCs toward Korea. To examine the cause of the development of anomalous cyclone in the East Asian continent, this study analyzed the water equivalent of accumulated snow depth during the preceding spring (March to May). As a result, less snow depth is observed in most regions of the East Asian continent than before 1981. Therefore, anomalous cyclone in the East Asian continent in summer can be reinforced by the land heating from the preceding spring and then the steering flow of anomalous southerly that moves TCs toward Korea can be also developed to Korea from low-latitudes in summer.

Prediction of Tropical Cyclone Intensity and Track Over the Western North Pacific using the Artificial Neural Network Method

A statistical prediction model for the typhoon intensity and track in the Northwestern Pacific area was developed based on the artificial neural network scheme. Specifically, this model is focused on the 5-day prediction after tropical cyclone genesis, and used the CLIPPER parameters (genesis location, intensity, and date), dynamic parameters (vertical wind shear between 200 and 850hPa, upper-level divergence, and lower-level relative vorticity), and thermal parameters (upper-level equivalent potential temperature, ENSO, 200-hPa air temperature, mid-level relative humidity). Based on the characteristics of predictors, a total of seven artificial neural network models were developed. The best one was the case that combined the CLIPPER parameters and thermal parameters. This case showed higher predictability during the summer season than the winter season, and the forecast error also depended on the location: The intensity error rate increases when the genesis location moves to Southeastern area and the track error increases when it moves to Northwestern area. Comparing the predictability with the multiple linear regression model, the artificial neural network model showed better performance.

A Simple Introduction of Extratropical Transition of Tropical Cyclone (TC) and a Case Study on the Latest Three TCs: Shanshan (0613), Yaki (0614), and Soulik (0618)

In this article, the extratropical transition (ET) of tropical cyclone (TC) was investigated based on the case study covering the latest three TCs (Shanshaa Yaki, and Soulik) associated with ET evolution (onset and completion) using the objective 37 diagnostics of Evans and Hart (2003) and Hart (2003). At 500-hPa level, on an onset of ET, all three TCs entered the baroclinic zone. In a vertical cross-section analysis, three TCs before and at an onset of ET kept warm and humid throughout all levels around the TC center. However, these TCs after ET onset became relatively cold and dry over the western part of TC as the typical characteristics of ET concept model. Although our case study was not sufficient, it is concluded that the diagnostics of the ET onset and atmospheric structure change associated with Evans and Hart (2003) and Hart (2003) will be useful in ET operational forecast.