CHARACTERISTICS OF WINTERTIME DAILY AND EXTREME MINIMUM TEMPERATURE OVER SOUTH KOREA

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 24: (24) Published online in Wiley InterScience ( DOI: 1.12/joc.99 CHARACTERISTICS OF WINTERTIME DAILY AND EXTREME MINIMUM TEMPERATURE OVER SOUTH KOREA SANG-BOOM RYOO, a, * WON-TAE KWON a and JONG-GHAP JHUN b a Climate Research Laboratory, Meteorological Research Institute/KMA, Seoul, Korea b School of Earth and Environment Sciences, Seoul National University, Seoul, Korea Received 28 March 23 Revised 3 October 23 Accepted 3 October 23 ABSTRACT In South Korea, consecutive positive temperature anomalies have been observed since the mid-198s. The objective of this study is to assess the recent trends in, and variability of, daily minimum temperature over South Korea with particular emphasis on its extremes. Temporal characteristics of wintertime daily and extreme minimum temperaturerelated variables were analysed on a seasonal basis for the period of to 2 1. The results show continually fewer days with extreme low minimum temperature since the mid-198s. However, no significant change in the 1 day temperature difference was observed during the same period, indicating little change in the frequency of cold surges. Also, during the period analysed, there is a significant positive trend in the seasonal mean temperature, a negative trend in the frequency of the extreme cold days, and no significant trend in the seasonal occurrence of cold surges. Northern Hemisphere geopotential height fields before and after , i.e. the start of successive positive anomalies in the winter surface air temperature over South Korea, showed a substantial decrease throughout the troposphere over the polar region. In the upper levels the overall pattern becomes more wavelike, with eddies embedded between meanders. The differences in the lower troposphere are remarkably similar to the Arctic oscillation, although the centre in the North Atlantic is shifted toward western Europe and differences in the North Pacific are relatively weaker than those in the polar region. The recent positive phase of the Arctic oscillation may contribute to these abrupt changes in wintertime daily minimum temperatures over South Korea. El Niño southern oscillation phenomena appear to contribute to the interannual variation of cold surge days in South Korea. Years with no cold surges were experienced during La Niña episodes. On the other hand, all years with more than four cases of cold surges were during El Niño episodes. Copyright 24 Royal Meteorological Society. KEY WORDS: South Korea; wintertime extreme temperature; climate change; cold surge; Arctic oscillation 1. INTRODUCTION Many studies of global or regional climate changes have focused on those with long-term average, such as annual or seasonal, temperature. It is clear from the observed record that there has been an increase in the global mean temperature of about.6 C over the last century (Houghton et al., 21), and that warming in daily minimum is stronger than that for maximum temperatures. Given these identifiable changes, it is expected that there would also be changes in extreme temperature events, such as the frequency of days with extremely low or extremely high temperatures. Houghton et al. (21) showed schematically the effect on extreme temperatures when the mean temperature or variance only increase, and when both the mean and variance increase for a normal distribution. In particular, there is little doubt that, recently, societal infrastructures have become more vulnerable to extreme weather conditions, such as flooding, tropical cyclones, and extreme heat or cold waves (Kunkel * Correspondence to: Sang-Boom Ryoo, Climate Research Laboratory, Meteorological Research Institute/KMA, Seoul, Korea; sbryoo@metri.re.kr Copyright 24 Royal Meteorological Society

2 146 S.-B. RYOO, W.-T. KWON AND J.-G. JHUN et al., 1999). Therefore, understanding changes in regional climate variability and the frequency of extreme weather, in relation to recent climatic change, is very important. Changes in the frequency of extreme weather are often the aspects of climate change to which ecosystem and societal responses are most sensitive, and are more difficult to assess than the mean (Houghton et al., 21). Change in extreme weather is thus one of the main themes of current climatological research, and many reports and papers involved with measuring and assessing global or regional changes in extremes during the last century have recently been published (e.g. Easterling et al., 2; Bonsal et al., 21; Walsh et al., 21). Changes in the frequency of wintertime extreme temperatures have been one of the key variables for investigation. These changes have a significant effect on heating degree days (Bonsal et al., 21). The majority of the studies have revealed significant decreases in days with extremely low daily temperatures but no significant increases at the opposite end of the distribution, i.e. days with extremely warm temperature. Among them, DeGaetano (1996) and Bonsal et al. (21) respectively showed a significant decrease in the number of days with extreme low temperature during winter in the northeastern USA and over Canada. Furthermore, Plummer et al. (1999) demonstrated a good relationship between the decreasing number of days below freezing and warming of the daily minimum temperatures in both Australia and New Zealand. As mentioned above, there have been several observational analyses involving the variability and tendencies of extreme temperature events over various regions of the globe. However, over the East Asian region, including China, Japan, and Korea, there have been relatively few results regarding extreme temperature trend and variability. Zhai et al. (1999) showed that the number of days with extreme low temperature has been decreasing in northern China. Yan et al. (22) investigated comprehensively the changes in extreme temperature over Europe and China. They showed a reduction in cold extremes and the warming trends in the lowest temperature over China during the last few decades. They related these changes to recent aspects of large-scale circulation fields: milder Siberian-high airmasses in winter, a weakening of the Meiyu phenomenon and/or greater dominance of the western Pacific subtropical high in summer. According to a recent study by Gong and Ho (24), the frequency of extremely low (below mean minus two standard deviations) daily temperature anomalies in the northeastern part of East Asia (north of 4 N and east of 11 E) is decreasing, which is statistically significant at the 9% confidence level. This trend is expressed in the rightward shift of the probability density function of the daily temperature anomalies. The sharp drop in wintertime temperature in East Asia is accompanied by widespread outbreaks of cold continental air, called cold surges or cold waves (Boyle and Chen, 1987). A cold surge is one of the most important characteristic weather phenomena of the East Asian winter monsoon on synoptic time scales (Lau and Li, 1984; Zhang et al., 1997). Although the Siberian high appears as a quasi-permanent feature in the Eurasian continent during the winter monsoon season, migratory anticyclonic cells often break away from the Siberian high and move eastward or southeastward, causing sudden outbreaks of cold air in the affected regions (Chang and Lau, 198). Therefore, any variation in the Siberian high s intensity or its breaking up exercise a strong influence on the weather, particularly the temperature, over the East Asian region. However, cold surges are neither a southeastward or eastward expansion nor a temporal variability of the Siberian high but are a separate dynamical entity (Zhang et al., 1997). And cold surges are a phenomenon in the shallow layer below 7 hpa (Ding, 1994). According to a recent study by Walsh et al. (21), neither the North American nor the European areas had seen an apparent trend in the frequency of extreme cold outbreaks, despite the recent trend toward higher minimum temperatures. Easterling et al. (2), however, summarized in their review of recent research on extreme climate events how the occurrences of cold waves in China are starting to decrease. Zhai et al. (1999) also showed that nation-wide cold wave activity has weakened in China. Since the mid-198s, consecutive positive anomalies of winter surface-air temperature have been observed in South Korea. Warm winters have been more frequent since 198, but cold winters have not appeared since the mid-198s (Ryoo et al., 22). The main objective of this study is to examine the characteristics of the variability in temperature over South Korea on a daily time scale beyond the annual/seasonal scales. Particular emphasis is given to investigating changes and trends in the extreme temperature events since the mid-198s. The data used here are described in Section 2, and results for the variability and tendencies of wintertime daily temperatures, days with extreme temperatures, 1 day minimum temperature differences, and cold surges Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

3 WINTERTIME TEMPERATURE SOUTH KOREA 147 over South Korea are shown in Section 3. On a seasonal basis, the number of days with extremely low daily minimum temperature is used to assess changes in the distribution of daily temperatures over South Korea. Changes in the occurrence of abrupt temperature drops within 48 h, defined here as cold surges, are also investigated to identify the presence of any warming effect on short-term local temperature change. Section 4 discusses some explanations of recent changes in wintertime temperatures over South Korea. A summary and concluding remarks are given finally in Section. 2. DATA The main data sources used in this paper are of two different kinds. One is daily minimum temperature data derived from the climate data management system of the Korea Meteorological Administration (KMA), the other is daily surface and upper-air fields that are produced by the National Center for Environment Prediction National Center for Atmospheric Research (NCEP NCAR) reanalysis project with a resolution of To obtain a time series of regional daily minimum temperature in South Korea we simply arithmetically averaged 14 stations daily minimum temperatures over 43 winters (1 December to 28 February) from to 2 1. South Korea is very small in area, so temperatures between each of the stations correlate very strongly, with correlation coefficients above.9. This simple averaged time series was to be a good representation of the Korean regional daily minimum temperature. For simplicity, we eliminated the data for 29 February in leap years. Figure 1 and Table I show the geographical information for the 14 stations selected for this study. Stations were classified as metropolitan, urban, industrial complex, and rural, according to their characteristics with respect to population and national function. We first subdivided them into two groups: industrial complex and non-industrial station. Pohang, Ulsan, and Yeosu are three of the major industrial complexes and were classified as industrial complex. Ryoo et al. (1993) showed that, in South Korea, the warming effect due to industrialization is stronger than that due to urbanization. Ryoo and Moon (199), using the accumulated intervention (AI) model, estimated the warming effect due to industrialization at Pohang, Korea, which evolved from a small-scale fishing port into a large-scale industrial complex region after the Pohang Iron and Steel Company (POSCO) was formed in 1973, at about.4 C/1 years. Then, the non-industrial stations were subdivided into urban and rural stations with the criterion set at a population density of 1 persons per kilometre squared. Finally, we made a special classification of stations with a total population of more than 1 and population density of more than 2 km 2 as metropolitan. Most of the stations selected were classified as either urban and/or industrial stations. These urbanized and/or industrialized stations introduce a bias into local and regional climates. As we made no attempt to remove urban biases in regional daily minimum temperature time series, the findings in this study might result partly from the warming effects due to the urbanization and/or industrialization. Table II gives a summary of the statistics for regional daily minimum temperatures. Skewness and kurtosis are often referred to as shape parameters useful in identifying appropriate probability distributions of variables (Storch and Zwiers, 1999). Skewness γ 1 and kurtosis γ 2 coefficients were calculated by a moments-based measure and are summarized in Table II. These measures are neither robust nor resistant (Wilks, 199), but very convenient to calculate. The values of γ 1 andγ 2 3 suggest that the regional daily minimum temperature in South Korea follows approximately a Gaussian distribution with µ = 1.8 and σ = 3.7, where µ and σ are the mean and standard deviation respectively. We constructed the time series of 1 day and 2 day minimum temperature differences based on regional daily temperature and give a summary of the statistics in Table II. The criteria for the identification of cold surges vary depending on the region of interest (Boyle and Chen, 1987; Ding, 1994; Zhang et al., 1997), and some were purposely defined for the convenience of operational weather forecasting. The criterion for cold surges used in this study is slightly different from the operational one at the KMA. The KMA uses a 1 day temperature fall of 1 C as the cold surge criterion on an observational station basis, not as in this study on a regional basis. A 1 day temperature fall of 1 C istoo severe to determine what constitutes a cold surge in any more than one case at each station during one winter Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

4 148 S.-B. RYOO, W.-T. KWON AND J.-G. JHUN MO CHINA KOREA INDIA PACIFIC OCEAN Figure 1. Geographical locations of 14 observational stations in South Korea (inset) and of East Asia countries. The numbers in the inset are seasonal mean frequencies of cold surges at each station. Here, we used a 2 day temperature fall of 1 C asacoldsurge criterion for stations season. Thus, we used a 2 day temperature fall of 1 C as the cold surge criterion for the stations. Means for the seasonal occurrence of the cold surges for each station are shown in the inset to Figure 1 with differentsized circles and a number. The size of the circle is proportional to the seasonal mean cold surge frequency. The occurrence of regional-based cold surges over South Korea was finally defined as days with a 2 day temperature fall larger than 7. C. If days when the above criterion was satisfied occurred consecutively, then days other than the first were not regarded as cold surge days. Also, a day when the 1 day maximum temperature fall of the previous day and that particular day is less than 1. C was not defined as a cold day, taking into consideration the nocturnal radiation cooling effect on the abrupt temperature drop. These restrictions resulted in a reduction in the total number of cold surges from 88 to 8 cases during the whole period of the winter seasons from to 2 1. Among those days that were classed as cold surges in Figure 2, only in one case (8 December 1967) did the 2 day temperature fall not exceed the criterion of 1 C at all of the 14 stations. Thus, we consider that it will not create much of a problem in this study to define the days plotted in Figure 2 as regional-scale cold surge days over South Korea. Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

5 WINTERTIME TEMPERATURE SOUTH KOREA 149 Table I. Geographical characteristics and population statistics for stations based on the year 2 estimates of population by the Korea National Statistical Office Station Longitude (E) Latitude (N) Height (m) Geographical location Population density (persons/km 2 ) Total population (persons) Classification Mokpo Coast Urban Jeju Island Urban Incheon Coast Metropolitan Gwangju Inland Metropolitan Seoul Inland Metropolitan Yeosu Coast Industrial complex Chupungnyeong Inland Rural Daegu Inland Metropolitan Gangneung Coast Rural Pusan Coast Metropolitan Ulsan Coast Industrial complex Pohang Coast Industrial complex Ulleungdo Island Rural Table II. Summary of statistics of the regional daily minimum temperatures in South Korea and h temperature differences for various time periods Mean ( C) Median ( C) Range ( C) Standard deviation ( C) Skewness Kurtosis to 2 1 Temperature day difference day difference to 2 1 Temperature day difference to Temperature day difference The list of El Niño and La Niña episodes used here was provided by the NCEP Climate Prediction Center (CPC) ( The classification of each event was performed using the reanalysed sea-surface temperatures (SSTs) at a key region in the tropical Pacific (1 W 18, S N). 3. RESULTS 3.1. Intraseasonal variability of the daily winter temperatures in South Korea As seen in Figure 2, it is evident that the intraseasonal variability within the regional daily minimum temperature time series is very complex and the daily temperatures fluctuated from 13.4 C to 1.2 C, i.e. with a range of 23.6 C. This magnitude is comparable to the difference (2.7 C) between the monthly mean temperatures of the coldest month (January in South Korea) and the warmest (August). And there were abrupt temperature changes (dropping or rising) of more than C in a day. Days that experienced such Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

6 1 S.-B. RYOO, W.-T. KWON AND J.-G. JHUN Temperature, C 1 196/ / / / / / / / / / / / / / / / /1997 2/21 Figure 2. The time series of regional daily minimum temperature (line) averaged with 14 stations daily minimum temperatures during 43 winter seasons (1 December to 28 February of the following year), and its 1 day temperature difference (vertical bar). Circles indicate cold surge days in South Korea Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

7 WINTERTIME TEMPERATURE SOUTH KOREA 11 abrupt temperature changes were more than % of the total number of days analysed in this study. These intraseasonal fluctuations in the winter temperature are too large for the higher frequency variation to be smoothed out by arithmetically averaging the observations of the same calendar days for a climatological base period, e.g. for 3 years. Strong fluctuations result in a considerable saw-toothed variation in the normal curve (Gong and Ho, 23). And the pattern of intraseasonal fluctuation is also different for each winter. Figure 3 shows a Tukey box-and-whisker plot for each calendar day s temperature distribution. A box-plot series can describe intraseasonal variability of the regional daily temperatures at a glance. The box in the middle of the diagram is bounded by the 7th and 2th percentiles, and thus locates the central % of the data. Whiskers above and below the box indicate the 9th and 1th percentiles. The median (line in the box) and outlying points (circles) are also plotted. As can be seen in Figure 3, the realizations of the regional minimum temperature for the same calendar days during the 43 winters analysed here are spread widely, and almost 9 days (1 December to 28 February) experienced a range greater than 1 C. This result suggests a very weak seasonality and a weak central tendency in wintertime regional daily temperature over the Korean peninsula. It is very difficult to obtain an appropriate smooth curve for a seasonally normal course. To obtain the reference values for seasonal temperature variation, many national services, such as the KMA, operationally used the normal values. However, because of the weak seasonality, the weak central tendency and the abrupt day-to-day fluctuation, which are an inherent property of daily temperature variation in the countries located in the Northern Hemisphere at mid-latitudes, the normal values sometimes have no meaning in terms of representing the climatological aspect of temperature for a given calendar day. These strong fluctuations of the wintertime daily temperature in South Korea result mainly from the variability of East Asian synoptic weather systems, the configuration of the pressure system surrounding the Korean peninsula. Local nocturnal radiation cooling also influences the level of the daily minimum temperature at the observing station. A high-pressure system, the Siberian high, has a very strong cold dome near the Siberian region and covers almost the entire East Asian continent during the winter season and yields a northerly flow over a large part of Asia. The Siberian high contributes the most variance in winter daily temperatures over East Asian regions. Gong and Ho (22) suggested that about 34% of the variance in temperature anomalies over middle- to high-latitude Asia is related to the Siberian high. Geographically, furthermore, the Korean peninsula is located at the eastern margin of the East Asian continent, where the Siberian high exerts less influence than it does on the inner continent. Thus, the correlation between the temperature in South Korea and the Siberian-high intensity is not strong, but moderate (Ryoo et al., 22). 1 1 Temperature, C Days from 1 December Figure 3. Time series of box-and-whisker plot for calendar days regional daily minimum temperatures in South Korea. The upper and lower boundaries of the box indicate the 7th and 2th percentiles respectively. Whiskers above and below the box indicate the 9th and 1th percentiles respectively. The median (line in box) and outlying points (circles) are also plotted Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

8 12 S.-B. RYOO, W.-T. KWON AND J.-G. JHUN The surface-level pressure over the Korean peninsula does not correlate significantly with the Siberian high s intensity alone (see Ryoo et al. (22: Figure 2)). The irregular outbreak of cold surges from the continental interior sometimes results in an extratropical cyclogenesis off the East Asian coast. The development and movement of cyclones over the East China Sea and the northwestern Pacific also play a significant role in the change of regional daily temperature over the Korean peninsula. Abrupt temperature drops in South Korea usually appear under a synoptic situation of a Siberian high west of the Korean peninsula and an extratropical cyclone east of the peninsula. In Section 3.3 we will give detailed analyses of the 1 day temperature differences and cold surges (abrupt temperature drops) over South Korea. Traditionally, it is believed in East Asia that there are regular spells of cold days and warm days (so called three cold days and four warm days ) in winter daily temperature fluctuations. In order to find and discuss dominant peaks of period in intraseasonal variation of the daily winter temperatures in South Korea, Blackman and Tukey s power spectral analysis (WMO, 1966) was carried out for each of 43 winters, and then we composed 43 normalized spectral densities for each of periods. Figure 4 shows dominant peaks of 4.1,.3, and 7. days. These peaks support the idea of a climatologically regular spell of cold days and warm days in winter temperature fluctuations over South Korea Winter seasonal mean temperatures and days with extreme low daily temperatures Many studies on secular temperature trends have indicated that, during the winter season, there is a significant warming of the Northern Hemisphere surface air temperatures. In South Korea also, the warming of winter surface air temperatures is significant at the 99% confidence level (e.g. Ryoo et al., 1993), and substantial positive anomalies since the mid-198s have been observed in the time series of the winter seasonal mean temperature (see Ryoo et al. (22: Figure 1)). Here, we analysed changes in the frequency of extremely cold days over South Korea. An extremely cold day was defined as a day with an extreme low daily minimum temperature, below. C,basedonthe regional daily minimum temperature in Section 2. And an extremely warm day is a day with an extreme warm daily minimum temperature, over 2. C. Although a criterion of. C for an extremely cold day was objectively chosen from the value of mean minus one standard deviation, 2. C for an extremely warm day is slightly higher than that of the mean plus one standard deviation for computational convenience. Normalized Spectral Density day.3 day 7. day. Col Period (days) Figure 4. Power spectrum of wintertime regional daily minimum temperatures in South Korea. The values corresponding to each period were composed with 43 normalized spectral densities and carried out for each of 43 winters Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

9 WINTERTIME TEMPERATURE SOUTH KOREA 13 The number of days with an extreme low daily temperature was counted for each of 43 winters and plotted in Figure together with the values of seasonally averaged daily minimum temperatures. A substantial reduction in the frequency of extremely cold days was found in South Korea since the mid 198s. This decrease coincides with warming in the seasonal mean temperatures with a correlation coefficient of.89. The strong negative correlation means that the level of seasonal mean temperature in South Korea is dependent on the frequency of extremely cold days occurring during the season. The change in the distributional characteristics of daily minimum temperatures was further examined by comparing two 1 year periods, viz to and to 2 1. The latter is a period of substantial positive temperature anomalies and small number of extremely cold days in South Korea, and the former is a relatively normal period. Figure 6 and Table II respectively show the relative frequency polygons and the summarized statistics of regional daily minimum temperatures for the two periods. A significant difference is found in the shapes of the two polygons in Figure 6. The occurrence of days with extreme low temperatures (lower than. C) during the recent 1 year period (123 days) was less than in the preceding 1 year period (276 days), but the range between the highest and the lowest values did not change significantly. Houghton et al. (21) referred briefly to how this leads to new record high temperatures: a Number of Cold Surges Number of Days Less Than -. C S. Korea # of Days Temperature 196/ / / /1991 2/21 E - L L+ L - E - E- E E- E E E- E - L - L - L - L L + E + E L + LL L - E - E- L - E + E + E E + Seasonally Averaged Minimum Temperature, C Figure. Time series of the winter seasonal mean temperatures, the number of days less than. C (upper panel) and the number of cold surges in South Korea. Here, the winter season is from 1 December to 28 February. The characters in the lower panel indicate the El Niño and La Niña episodes from October to December Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

10 14 S.-B. RYOO, W.-T. KWON AND J.-G. JHUN /87-/1 71/72-8/86 Relative Frequency. % Temperature, C Figure 6. Relative frequency distributions of wintertime regional daily minimum temperatures over South Korea during the period from to 2 1 (full line), and the period from to (dotted line with circles) change from 9.6 C to 1. C in the maximum value of the regional daily minimum temperature over South Korea. However, the difference between the number of days with an extremely warm minimum temperature (over 2. C herein) is not as strong as for the extremely cold days: 26 days for the recent period and 17 days for the preceding period. The above results, apparently fewer days with extremely low temperatures and slightly more days with high temperatures, imply a change in the distributional characteristics of daily temperatures in South Korea. There is a significant increase in the mean of daily minimum temperature from 2.3 to 1. C since the mid-198s over South Korea. The polygon for the recent period resembles a normal distribution more than does that for the previous one (compare the skewness and kurtosis coefficients in Table I). And the statistics in Table II also provide evidence of changes in the distributional characteristics of daily minimum temperature. Consequently, changes in the daily minimum temperature in South Korea led to more moderate temperatures in the range of 2 to+2 C for the recent period than for the previous one The 1 day temperature differences and cold surges This section examines the distributional characteristics of wintertime 1 day minimum temperature differences over South Korea by comparing the two 1 year periods defined in Section 3.2, and serves to complement the analysis of daily minimum temperatures. On a regional and daily basis, the 1 day temperature differences over South Korea are displayed using vertical bars in Figure 2. Two relative frequency polygons for these differences during the period to 2 1 and the period to are shown in Figure 7. No significant difference is found in the main bodies of the two polygons. A small change is observable outside the main body of the polygon, in the tail areas: the record temperature drop since the mid-198s becomes lower than in the previous period, but the record for the temperature rising is the opposite, i.e. higher. The robustness of the 1 day temperature differences can be confirmed from the similarity of the two periods statistics, which are also summarized in Table II. The mean, median, range, and standard deviation of the 1 day temperature differences are exactly the same for the two periods, though higher moments, skewness and Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

11 WINTERTIME TEMPERATURE SOUTH KOREA /87-/1 71/72-8/86 Relative Frequency. % Day Temperature Difference, C Figure 7. Relative frequency distributions of 1 day temperature differences during the period from to 2 1 (full line), and the period from to (dotted line with circles) kurtosis are evident. This robustness of distributional characteristics of 1 day temperature differences during the two different periods would be an aspect distinguishable from the distribution of daily temperatures. The occurrence of cold surges over South Korea was defined as a day when the 2 day minimum temperature fall is larger than 7. C and the 1-day maximum temperature fall of the previous day or of that day is larger than 1. C. The circles in Figure 2 indicate the days determined to be cold surges in this study. The expected number of occurrences of cold surge events during one winter season in South Korea is slightly less than two. The sparseness of such events may be due to our severe criterion for cold surges or to an aspect of the temporal distribution of cold surges in the East Asian region. Also, although the build-up and maintenance of the strong Siberian high is one of the necessary conditions for a cold surge outbreak, the cold surge frequency does not increase monotonically with the strengthening of the Siberian high. In December and January the intensity of the Siberian high commonly reaches a peak and the cold air is abundant. Nevertheless, the occurrence of cold surges in those months is not favoured by a strong Siberian high and an intense jet stream (Zhang et al., 1997), under which the short waves tend to move rapidly without causing any disturbances (Boyle, 1986). In the transition periods of November and March, however, a blocking over the Asian continent and a deep trough along the coast yield a strong northwesterly over Lake Baikal, and in these highly unstable conditions the potential energy is more ready to be released when triggered by a short wave (Zhang et al., 1997). According to the investigation of Zhang et al. (1997), the cold surges in China are, therefore, most active during November and March. The total number of cold surge events during the period to 2 1 was 27, which is exactly the same number as during the period to This property of the frequency of cold surges is consistent with the lack of change in the distributional characteristics of 1 day temperature differences for the two different periods. On a seasonal basis, the frequencies of cold surges are shown by a vertical bar in the lower panel of Figure. The dominant feature gives no indication of a trend in the frequency of cold surges. The highest frequency year was , with a value of 7, and the years with no cold surge were , , and The widespread occurrence in is consistent not only with the level of the seasonal mean temperature but also with the number of days with an extreme low daily minimum temperature below. C. The seasonal mean temperature and the number of cold days in were 2.7 C (the normalized value of.7) and 26 days respectively. These values indicate a moderate cold, rather than an Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

12 16 S.-B. RYOO, W.-T. KWON AND J.-G. JHUN extremely cold, winter for Overall, the frequency of cold surges in South Korea does not appear to relate to the level of seasonal mean temperatures Trend analysis To validate recent changes in the level of seasonal mean temperature, and in the frequency of cold days and cold surges identified in the prior analyses, the trends of each time series were assessed using Spearman rank statistics (WMO, 1966). This test is useful when several terms of the time series happen to be equal in value and the ranks of these terms are tied. In the case of cold surges, there are many tied values. By convention, the ranks of tied values are each set equal to the average rank. Then, after calculating the test statistics of the three variables mentioned above, we compared this with the probability points of Student s t distribution for 41 degrees of freedom. A two-tailed test with a significance level of. was recommended by WMO (1966) and is commonly chosen in climate analysis. For a more detailed explanation the reader is referred to WMO (1966), which includes the equations for Spearman rank statistics and for its test statistics. The results of the trend analyses for the three variables are summarized in Table III. An asterisk in the table indicates a significant trend with α =. level. Table III shows that, during the to 2 1 time period, there was a significant positive trend in the level of the seasonal mean minimum temperatures and a significant negative trend in the occurrence of very low daily minimum temperatures or cold days. However, on a regional basis, no significant trend in the occurrence of cold surges was observed during this period. These results are highly consistent with those of the preceding sections. 4. DISCUSSION To examine the planetary-scale circulations contributing to abrupt changes in wintertime temperature over South Korea since the mid-198s, the differences in wintertime Northern Hemisphere geopotential height and temperature fields between the periods to 2 1 and to were computed over the domain poleward of 2 N based on the NCEP NCAR reanalysis data and are shown in Figure 8. This analytical method is more descriptive and more explicit for extracting patterns related to the dynamical mode of variability than any other. Focusing first on the differences in height fields, the most dominant feature in Figure 8 is a substantial decrease throughout the troposphere over the polar region. And in the upper level an annular mode in the mid-latitudes becomes more wavelike, with eddies embedded between meanders. Three negative-difference cores are located over central Asia, Alaska, and the North Atlantic, regions that are climatologically associated with blocking in mid-tropospheric circulation. This is consistent with the result presented in Thompson and Wallace (21: Figure 3). According to them, a decreased incidence of blocking is a pronounced aspect at a height of hpa under a high index of the Northern Hemisphere annular mode, and has favoured a declining frequency of occurrence of cold events over North America, Europe, Siberia, and East Asia. The pattern of differences at a height of 1 hpa is remarkably similar to an aspect of the Arctic oscillation (AO; Thompson and Wallace, 1998), with negative values in the high latitudes and positive values in Table III. Summary of test statistics for trends of winter-to-winter seasonal mean minimum temperatures, frequency of cold days, and frequency of cold surges in South Korea during the period from to 2 1. An asterisk indicates that this trend is significant at the % level Seasonal Cold day Cold surge Test statistics Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

13 WINTERTIME TEMPERATURE SOUTH KOREA 17 Figure 8. Differences between the composite means of geopotential height for the period to 2 1 and for the period to at 1 hpa, 8 hpa, hpa and 3 hpa (left), and between the composite means of temperatures (right). Areas with temperature differences greater than 1 C are shaded Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

14 18 S.-B. RYOO, W.-T. KWON AND J.-G. JHUN mid-latitudes, although the centre of action over the North Atlantic is shifted toward western Europe and differences over the North Pacific are relatively weaker than those over polar regions. This shift appears to be consistent with a northeastward displacement of the northern centre of the North Atlantic oscillation, in a greenhouse-gas scenario run of Ulbrich and Christoph (1999), when the anthropogenic forcing exceeds 3Wm 2. The patterns of the differences in the Northern Hemisphere temperature fields between the two periods shown in the right panel of Figure 8 exhibit the annular mode documented by Thompson and Wallace (1998), with positive values over much of northern Eurasia and the eastern USA and negative values over eastern Canada and northern Africa. The centre of the warming over the Eurasian continent, in particular, is located just north of the Korean peninsula. This consistency in geopotential height and temperature fields suggests that the recent positive phase of the AO may contribute to these abrupt changes in wintertime daily minimum temperatures over South Korea. The El Niño southern oscillation (ENSO) phenomenon has important effects on the interannual variation of the winter monsoon and cold surges over East Asia (e.g. Lau and Chang, 1987; Zhang et al., 1997). Thus, we also examined the difference in the frequencies of cold surges between the El Niño and La Niña episodes to check whether there is also a relationship between the winter-to-winter variation of cold surge frequency in South Korea and ENSO. The difference between two episodes gives an estimate of ENSO s contribution to the occurrences of cold surges in South Korea when linearity is assumed. The El Niño and La Niña episodes from October to December are denoted by the characters in the lower panel of Figure. Weak periods were designated as E or L, moderate strength periods as E or L, and strong periods as E + or L +. Three years with no cold surge, namely , , and , all belong to the La Niña episodes. On the other hand, the highest frequency year of is an El Niño event year, and other years with more than four cases of cold surges, namely and , are also El Niño event years. This result is slightly different from that of Zhang et al. (1997), in which the frequency of cold surges in China reached a maximum a year before the El Niño events and a minimum a year after the El Niño events.. SUMMARY AND CONCLUDING REMARKS Changes and trends in wintertime daily minimum temperatures over South Korea have been examined with particular emphasis on substantial positive anomalies since the mid-198s. We identified abrupt changes in the level of seasonal mean minimum temperatures, and in the number of days with extreme low temperature over South Korea, between the periods to 2 1 and to Since the mid-198s, the occurrence of days with extreme low temperature tends to be less than in the previous period. On the other hand, the difference between days with an extreme warm minimum temperature for two periods is not as strong as for extreme cold days. Consequently, South Korea has experienced more moderate daily minimum temperatures since the mid-198s. This finding for South Korea is highly consistent with those over the northeastern USA (DeGaetano, 1996), Canada (Bonsal et al., 21), Australia and New Zealand (Plummer et al., 1999), and China (Zhai et al., 1999): significant decreases in days with extreme low daily temperature but no significant increase in days with extreme warm temperature. However, we did not find a significant difference between the occurrence of cold surges for the two periods. This result conflicts with a recent review by Easterling et al. (2), in which it is said that the occurrence of cold waves in China is diminishing. During the period from to 2 1, there are significant trends toward warmer seasonal mean temperatures and toward fewer extreme cold days, but there is no significant trend in the wintertime occurrence of cold surges. This study suggests that the recent aspects of the AO may have contributed to the abrupt change in wintertime daily minimum temperatures over South Korea. This supports the earlier studies of Fyfe et al. (1999) and Thompson et al. (2), that the trend of many atmospheric variables in the Northern Hemisphere during the winter season, including a warming of the surface temperature in mid and high latitudes, is related to the variability of the AO. Thompson et al. (2) have matched about % of the recent wintertime Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

15 WINTERTIME TEMPERATURE SOUTH KOREA 19 warming over the Eurasian continent with month-to-month variations in the AO index. It is still not clear, however, whether this atmospheric mode has been excited by increasing greenhouse gas concentrations or not. Finally, in comparing the frequencies of cold surges for a given ENSO state, the ENSO phenomena appear to contribute to the interannual variation of winter cold surge occurrences in South Korea. Years with no cold surges belong to the La Niña episodes, but all years with more than four cases of cold surges are El Niño event years. ACKNOWLEDGEMENTS This research was performed for the project Development of forecast techniques for the East Asian monsoon Changma circulation system, one of the Meteorological and Earthquake R&D programs funded by the Korea Meteorological Administration (KMA). We would like to thank Mr Won-Hee Park and Ms Gi- Ho Ahn for their helpful data handling and figure drafting and two anonymous reviewers for their valuable comments. REFERENCES Bonsal BR, Zhang X, Vincent LA, Hogg WD. 21. Characteristics of daily and extreme temperatures over Canada. Journal of Climate 14: Boyle JS Comparison of the synoptic conditions in midlatitude accompanying cold surges over eastern Asia for the months of December 1974 and Part I: monthly mean fields and individual events. Monthly Weather Review 114: Boyle JS, Chen TJ Synoptic aspects of the wintertime East Asian monsoon. In Monsoon Meteorology, Chang CP, Krishnamurti TN (eds). Oxford University Press: Chang CP, Lau KM Northeasterly cold surges and near-equatorial disturbances, Part II: planetary-scale aspects. Monthly Weather Review 18: DeGaetano AT Recent trends in maximum and minimum temperature threshold exceedences in the northeastern United States. Journal of Climate 9: Ding YH Monsoon over China. Kluwer Academic Publishers. Easterling DR, Evans JL, Groisman PY, Karl TR, Kunkel KE, Ambenje P. 2. Observed variability and trends in extreme climate events: a brief review. Bulletin of the American Meteorological Society 81: Fyfe JC, Boer GJ, Flato GM The Arctic and Antarctic oscillation and their projected changes under global warming. Geophysical Research Letters 26: Gong DY, Ho CH. 22. The Siberian high and climate change over middle to high latitude Asia. Theoretical and Applied Climatology 72: 1 9. Gong DY, Ho CH. 24. Intra-seasonal variability of wintertime temperature over East Asia. International Journal of Climatology 24: Houghton. 21. Climate Change 21: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. Kunkel KE, Pielke Jr RA, Changnon SA Temporal fluctuations in weather and climate extremes that cause economic and human health impacts: a review. Bulletin of the American Meteorological Society 8: Lau KM, Chang CP Planetary scale aspects of the winter monsoon and atmospheric teleconnections. In Monsoon Meteorology, Chang CP and Krishnamurti TN (eds). Oxford University Press: Lau KM, Li MT The monsoon of East Asia and its global association a survey. Bulletin of the American Meteorological Society 6: Plummer N, Salinger MJ, Nicholls N, Suppiah R, Hennessy KJ, Leighton RM, Trewin BC, Page CM, Lough JM Changes in climate extremes over the Australian region and New Zealand during the twentieth century. Climatic Change 42: Ryoo SB, Moon SE Detection of warming effects due to industrialization: an accumulated intervention model with an application in Pohang, Korea. Journal of Applied Meteorology 34: Ryoo SB, Moon SE, Cho BG Air temperature change due to urbanization in South Korea. Journal of the Korean Meteorological Society 29: (in Korean). Ryoo SB, Jhun JG, Kwon WT, Min SK. 22. Climatological aspects of warm and cold winters in South Korea. Korean Journal of Atmospheric Sciences : Storch HV, Zwiers FW Statistical Analysis in Climate Research. Cambridge University Press. Thompson DWJ, Wallace JM The Arctic oscillation signature in the wintertime geopotential height and temperature fields. Geophysical Research Letters 2: Thompson DWJ, Wallace JM. 21. Regional climate impacts of the Northern Hemisphere annular mode. Science 293: Thompson DWJ, Wallace JM, Hegerl G. 2. Annular modes in the extratropical circulation. Part II: trends. Journal of Climate 13: Ulbrich U, Christoph M A shift of the NAO and increasing storm track activity over Europe due to anthropogenic greenhouse gas forcing. Climate Dynamics 1: 1 9. Walsh JE, Phillips AS, Portis DH, Chapman WL. 21. Extreme cold outbreaks in the United States and Europe, Journal of Climate 14: Copyright 24 Royal Meteorological Society Int. J. Climatol. 24: (24)

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