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Keywords:

  • typhoon;
  • long-term change;
  • Korea;
  • Japan;
  • landfall

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] Long-term changes in tropical cyclones (TCs) that made landfall in Korea and Japan during the TC seasons (June–October) are examined for the period 1977–2008. The TC activity is characterized by four parameters: power dissipation index (PDI), TC-induced rainfall, number of landfall TCs, and TC duration. The analysis period is divided into 2 decades (1977–1988 and 1997–2008). The PDI and TC-induced rainfall increase significantly in the later decade. This enhancement in the TC activity is because of the increase in the number of landfall TCs and the longer duration of the TCs over the two countries. The increase in the number of landfall TCs is associated with the enhanced northward steering flows over the East China Sea. The longer TC duration is mainly due to the high intensity of the approaching TCs prior to landfall. The other factors (i.e., tracks, translational speeds, mean drift lengths, and weakening rates of TCs) could also affect the TC duration, but they are found to be not significant. The results of our study reveal that the recent intensification of TCs is attributable to the changes observed in the later decade in the large-scale environments in the vicinity of the two countries. These changes include warmer sea surface temperature, highly humid midtroposphere, and weaker vertical wind shear over the region. In addition, another responsible factor is the anomalous upward motion driven by the relocation of secondary circulation near the jet entrance, which is highly related with weaker upper tropospheric jet stream in the recent decade.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Tropical cyclone (TC) landfall results in strong wind gust and heavy rainfall, and thus it causes major disasters (e.g., floods, landslides, and storm surge). Climatologically, in the western North Pacific (WNP) basin, around four TCs make landfall in Korea and Japan during each TC season (June through October) [Wu et al., 2004; Kim et al., 2005a, 2005b; Choi and Kim, 2007]. More intense TCs sustain their destructiveness at midlatitudes, and such cyclones have caused considerable destruction in these countries in recent decades (e.g., Rusa in 2002, Maemi in 2003, Nabi in 2005, and Melor in 2009). The resultant economic losses incurred in the last 3 decades in Korea and Japan amount to more than 60 billion United States (U.S.) dollars (http://www.emdat.be/database). Furthermore, in the 2000s, the averaged economic losses resulting from one landfall TC amounted to almost 1 billion U.S. dollars, which is 3 times higher than those in the 1980s. Pielke et al. [2008] showed that increasing value of social infrastructure is one of possible reasons for a large increase in the economic losses in the case of the United States. However, they also designated that intense TCs (≥category 3, ∼24% of the U.S. landfall TCs) account for about 85% of the total damages. Thus, the development of human society is not the only factor for the increase in the economic losses caused by TCs, but long-term change in the TC activity can be another possible factor. Leaving the change in human society for the topic of other research areas, to examine any changes in TC activity becomes an important task in a meteorological perspective.

[3] The following topic has been in debate for the last few years: is the increased emission of greenhouse gases responsible for the notable change in the TC activity? However, no conclusions have been drawn in this regard, because continuous modifications have been made to the techniques used for evaluating the change in the TC activity related with the increased greenhouse gases [e.g., Emanuel, 2005; Landsea, 2005; Webster et al., 2005; Chan, 2006]. Recently the dispute is temporarily reconciled by summarizing that an uncertainty remains due to time changes in observing technologies [Landsea et al., 2006, 2010; Knutson et al., 2010]. According to some studies, there has been a significant increase in the destructive power of TCs over the WNP basin arising from human-induced global warming [Emanuel, 2005; Webster et al., 2005]. On the other hand, other studies have shown that the changes of TC intensity induced by global warming are not detectable and that the natural variability is dominant in the past changes [Landsea, 2005; Chan, 2006]. The uncertainty surrounding the effects of human-induced increase in greenhouse gases in the WNP basin is greater than that in the North Atlantic basin because very little effort has gone into homogenizing the TC data.

[4] In addition to long-term basin-wide changes in the TC activity, regional changes are a matter of concern for countries located in TC-prone coastal regions. While a significant number of studies have investigated the long-term regional changes [Kubota and Chan, 2009; Tu et al., 2009; Goh and Chan, 2010], very few studies have focused on the landfall TCs in midlatitude countries. In fact, long-term changes in the TC activity over Korea and Japan would be considerably different from basin-wide changes. This is because distinct large-scale environments (e.g., upper tropospheric jet stream, strong vertical wind shear, and Arctic and Antarctic influences) influence the TC activity over the midlatitude [Ho et al., 2005, 2009; Kim et al., 2005a, 2005b; Choi and Byun, 2010; Kim et al., 2010]. Archer and Caldeira [2008] noted that the weakened jet stream over the midlatitudes might enhance the TC activity. Kim et al. [2006] found that since the late 1970s, there has been an increase in the TC-induced heavy rainfall in Korea. Chan [2008] stated that the TCs headed for the two countries in recent years had high intensities; however, Chan [2008] did not carry out detailed analyses of landfalling TCs in the two countries. Thus, a further investigation is required to reveal the aspect and the mechanism of the recent changes in the landfalling TC activity over the midlatitude, especially in Korea and Japan.

[5] We begin with the description of the data sets and the analysis methods in section 2. In section 3, the temporal changes observed in the landfall TC activity are presented. The large-scale environments related to the change in the TC activity are discussed in section 4. Finally, section 5 provides a summary of the study and discussions.

2. Data and Method

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

[6] In this study, four best-track data sets available for WNP TCs are used together in order to ensure that the obtained results are accurate since some inconsistencies exist among the data sets [Wu et al., 2006; Kossin et al., 2007; Knapp and Kruk, 2010]. These data sets are obtained from the Regional Specialized Meteorological Center (RSMC)–Tokyo Typhoon Center, Joint Typhoon Warning Center (JTWC), Hong Kong Observatory (HKO), and China Meteorological Administration–Shanghai Typhoon Institute (CMASTI). Details of the all data sets are summarized in Table 1. The data sets have some nontrivial inconsistencies; especially for the definition of maximum sustained wind. The consistency of the definition of maximum sustained wind is essential because it is necessary for calculating power dissipation index (PDI) which is one of the indices for examining the TC activity. To analyze the TC activity fairly in those four best-track data sets, the maximum sustained wind should be adjusted beforehand. In this study, 10 min averaged maximum sustained wind speed is adopted as a reference. The correction factors for the JTWC and CMASTI data sets are 0.88 and 0.871, respectively [Knapp and Kruk, 2010]. In addition, the unit is expressed in the International System of Units (m s−1) using the multiplicative factor, 0.514 for the RSMC, JTWC, and HKO data sets.

Table 1. Characteristics of Four Best-Track Data Sets for TC Over the WNP
 RSMCJTWCHKOCMASTI
Record period1951 to present1945 to present1961 to present1949 to present
Time for maximum sustained wind10 min1 min10 min2 min
Unit for maximum sustained windknotknotknotm s−1
Time interval of record6 h (3 h near Japan)6 h6 h6 h
Multiplicative factor for 10 min maximum sustained wind0.880.871
Multiplicative factor for SI units0.5140.5140.514

[7] The analyses are limited to the period 1977–2008; this is because of the following reasons: (1) operation of the Geostationary Meteorological Satellite launched by Japan Meteorological Agency commenced in 1977 and (2) the RSMC data for the maximum sustained wind speed are available for the period after 1977. Only the TC activity during the TC season (June–October), when 90% of TCs approach Korea and Japan [Kim et al., 2005b], is determined in these analyses. A TC makes landfall in Korea and Japan when its center is at a distance of 1° from the coastal lines of these two countries (Figure 1). A major problem faced in the study is that the landfall area is not sufficiently vast to enable accurate examination of the TC activity from 3- or 6-hourly records. Therefore, the latitude-longitude location and wind speed data are linearly interpolated at time intervals of 1 h. TCs are categorized into four types on the basis of the 10 min averaged maximum sustained wind speeds (vmax): tropical depressions (TDs, vmax < 17 m s−1), tropical storms (TSs, 17 m s−1vmax < 25 m s−1), severe tropical storms (STSs, 25 m s−1vmax < 33 m s−1), and typhoons (TYs, vmax ≥ 33 m s−1). This categorization is based on the definitions provided by the World Meteorological Organization. In this paper, we will consider only tropical storms, severe tropical storms, and typhoons.

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Figure 1. Landfall domain indicated by the shaded area.

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[8] In the present study, four parameters are used to characterize the TC activity: PDI, TC-induced rainfall, number of landfall TCs, and TC duration. These parameters represent the strength of the TC activity for each year, and they have been used in many studies in the past [Emanuel, 2005, 2007; Kamahori et al., 2006; Kim et al., 2006; Englehart et al., 2008; Lee et al., 2010]. In this study, we determine the “accumulated” and “mean” values of the parameters separately. The “accumulated” value is defined as the sum of each parameter for all landfall TCs during a TC season and the “mean” is the ratio of the accumulated value to the total number of landfall TCs. Therefore, the “mean” value is the averaged potential impact of one TC.

[9] The accumulated TC duration is the duration for which the landfall TC remains in the target region. The accumulated PDI is defined as

  • equation image

where N is the total number of landfall TCs every year, and T is the duration for which each TC remains in the target region. This definition of the accumulated PDI is obtained from Emanuel [2005]. The accumulated TC-induced rainfall is calculated by the summation of the amounts of rainfall caused by landfall TCs. We use the 0.5° × 0.5° grid gauge pixel data pertaining to daily precipitation for the period 1979–2008, obtained from the Climate Prediction Center, the National Oceanic and Atmospheric Administration (NOAA) [Xie et al., 2007]. Since this data set is only based on the rain gauge data recorded at the weather stations in the target region, which includes a sufficient number of stations, the data set is more homogeneous in time than satellite-based rainfall data sets. TC-induced rainfall is defined as the rainfall occurring when the center of a landfall TC is within 5° of the grid points in the landfall domain [e.g., Kim et al., 2006; Lee et al., 2010]. To examine long-term changes in the TC activity, each parameter is smoothed by using a five-weight filter [1–3–4–3–1] in order to exclude interannual variations in the TC activity [Emanuel, 2007]. The filter is defined as

  • equation image

where xi′ is a representative value of the parameters for a target year, xi is a raw value for the target year, and xi± integers is a raw value for the target year ± the number of years.

[10] To characterize large-scale environments related to the TC activity, we analyze horizontal wind velocity, specific humidity, and the divergence, which were reanalyzed by the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) [Kalnay et al., 1996] and by NOAA [Xue et al., 2003]. The NCEP/NCAR reanalysis data and the NOAA monthly extended reconstructed sea surface temperature (SST) version 3 data have horizontal resolutions of 2.5° × 2.5° and 2° × 2°, respectively.

3. Changes in Landfall TC Activity Over Korea and Japan

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

[11] Figure 2 shows the time series of the number of landfall TCs in Korea and Japan, the total number of TC genesis events in the WNP, and the ratio of the number of landfall TCs in Korea and Japan to the total number of TC genesis events in the WNP. While the interannual variations are large, change-point analysis reveals statistically significant shifts in the time series of the number of landfall TCs and WNP TCs (Table 2) [Chu, 2002]. Table 2 represents the most significant years for shifts in the time series of the number of landfall TCs and WNP TCs in each best-track data set. For the WNP TC (the landfall TC), the significant years for the all data sets (the all data sets except HKO data set) are near the late 1990s (the late 1980s). Based on Table 2, we analyze the long-term change in the landfall TC activity by dividing the analysis period into 2 decades (i.e., 1977–1988 and 1997–2008) and excluding the transition decade. The number of landfall TCs has increased notably since the early 1990s (Figure 2a); however, there was a decrease in the number of landfall TCs in the mid-to-late 1990s. On the other hand, the total number of TC genesis events in the WNP increased gradually until the mid-1990s but decreased rapidly afterward and stabilized at a number same as that in the 1970s (Figure 2b). Consequently, the ratio of the total number of landfall TCs (Figure 2a) to the total number of WNP TCs (Figure 2b) (i.e., a/b × 100%) shows a significant increase at the 95% confidence level (Figure 2c). While 17% of the WNP TCs influence the target region in the earlier decade, 25% of the WNP TCs influence the target region in the later decade. This increased landfall frequency has been reported in previous studies [Choi and Kim, 2007; Liu and Chan, 2008; Tu et al., 2009].

image

Figure 2. Time series of (a) the number of landfall TCs over Korea and Japan, (b) the number of TC genesis events over the WNP, and (c) the ratio of number of landfall TCs over Korea and Japan to the number of TC genesis events over the WNP. The gray and black lines indicate the raw and low-pass filtered values, respectively.

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Table 2. Significant Years for Shifts in the Time Series of the Number of Landfall TCs and WNP TCs in Each Best-Track Data Set
TC Data SetChange Points (year)Confidence Level (%)
Landfall TC
RSMC199095
JTWC198695
HKO
CMASTI198699
 
WNP TC
RSMC199695
JTWC199995
HKO199999
CMASTI199999

[12] Figure 3 represents the time series of the accumulated TC duration, PDI, and TC-induced rainfall within the landfall domain covering the two countries as seen in Figure 1. These TC-related parameters are considered the potential impacts of landfall TCs over the region. A considerable increase in all the parameters is observed after the 1990s; however, there are inconsistencies in the TC duration and PDI among the data sets. Nevertheless, the mean amplitudes of the TC duration and the PDI nearly doubled in the later decade in each TC data set. An increase in the accumulated TC-induced rainfall was observed as well; this is in good agreement with the rain gauge observation in Korea [see Kim et al., 2006, Figure 2]. The impact of the landfall TCs is well correlated with the number of TCs shown in Figure 2a; the correlation coefficients of the three parameters are significant at 99% and are higher than 0.8. It is very likely that the increased total impacts of the landfall TCs are highly related with the increase in the landfall frequency.

image

Figure 3. Time series of (a) the accumulated TC duration, (b) the accumulated PDI, and (c) the accumulated TC-induced rainfall for the landfall TCs.

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[13] The mean or normalized values (obtained by dividing the accumulated values by the number of landfall TCs) used to investigate the impacts of each TC are shown in Figure 4. Unlike the number of landfall TCs (Figure 2a), the mean TC duration within the landfall domain (Figure 1) shows a gradual increase (Figure 4a), mainly because the sunken values in the 1990s flattens due to the normalization. The mean TC durations in the earlier and later decades are 11 h and 19 h, respectively; this indicates that, after its landfall, a TC sustains its intensity, which is higher than or equal to the TS intensity, for a longer period in the later decade. Table 3 shows the numbers of landfall TCs with a duration shorter or longer than 12 h for each TC best-track data set; the data are presented for the periods 1977–1988 and 1997–2008, and the difference between the data is also presented. Based on the Table 3 values, it is confirmed that there is a dramatic increase in the relative frequencies of the TCs sustained for longer durations after their landfall. The average number of long-lived TCs in the later decade (i.e., 43.5) is 1.6 times that in the earlier decade (i.e., 27). In contrast, there is a slight decrease in the average number of short-lived TCs.

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Figure 4. Time series of the mean values of (a) the TC duration, (b) the PDI, and (c) the TC-induced rainfall for the landfall TCs.

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Table 3. The Number of TCs Sustained Less Than or Equal To or More Than 12 h Over Korea and Japan for Two Periods, 1977–1988 and 1997–2008, and the Difference Between the Two Periods
 TotalLess Than or Equal to 12 hMore Than 12 h
1977–19881997–2008Difference1977–19881997–2008Difference1977–19881997–2008Difference
RSMC3954+15128−42746+19
JTWC3446+12106−42440+16
HKO3956+17912+33044+14
CMASTI3853+15119−22744+17

[14] As in the case of the mean TC duration, a significant increase is observed in the mean PDI at the 95% confidence level, despite the large inconsistencies among the data sets (Figure 4b). According to Emanuel [2007], the accumulated PDI shows the cumulative impact of the number of TCs, TC duration, and maximum wind speed. Here, the mean PDI is strongly correlated with the mean TC duration (correlation coefficient r = 0.75). The potential destructive power of a TC increases because of the long duration of the TC over the countries. The mean TC-induced rainfall showed a substantial increase in the late 1990s, followed by a decrease in the late 2000s (Figure 4c). Further, there was a large amount of TC-induced rainfall in the late 1970s. The increased TC durations partially affect the temporal variability of the TC-induced rainfall over the region (r = 0.43, significant at the 95% confidence level). The time series of the mean TC-induced rainfall showed a decadal variability, unlike the TC duration or PDI. This discrepancy does not imply that uncertainty exists in the rainfall data set because the rainfall data over land area would definitely be accurate. This discrepancy is due to the strong influence of various factors, such as synoptic structure, topography, and moisture distribution, on the TC-induced rainfall [e.g., Kim et al., 2006; Park and Lee, 2007].

[15] The increased number and duration of landfall TCs are key indicators of the enhanced TC impact over Korea and Japan. Why did TCs live longer over the land in a later decade? Several factors, such as intensity, weakening rate, track, and translation speed of the TC, can be responsible. All possible factors are investigated whether they are major or minor roles related with the increased TC duration. The TC translation speed decreases slightly in the 2000s. The TC tracks tend to follow the Japanese islands more in the later decade (figure not shown). Partially related with the change in the TC tracks, the mean drift length of TCs over the landfall domain has also increased since the late 1980s (not shown). However, these changes in the TC translation speed and TC tracks are not significant, indicating that they may be minor factors for the increased TC duration in the domain.

[16] Intensity is identified as the main factor that induces the increase in the TC duration. Table 4 presents the number of TCs in categories 1 (i.e., typhoon) and 2 (i.e., intense typhoon) for each decade (1977–1988 and 1997–2008) under consideration. The difference between the number of typhoons in the 2 decades ranges from 4 to 13 depending on the best-track data sets used, while that between the number of intense typhoons ranges from 0 to 7. The exact numbers of typhoons and intense typhoons vary markedly among the data sets. However, a consistent increase in the number of typhoons and intense typhoons is revealed in all the data sets, except the number of intense typhoons in CMASTI, which has no record of any intense landfall typhoon occurring during the analysis period. In addition, the average of the maximum sustained wind speed when TCs make landfall has been larger in the later decade (figure not shown). These results are consistent with those reported by Choi and Kim [2007] and Chan [2008]. Choi and Kim [2007] reported an increase in the intensity of landfall TCs over the Korean peninsula. Chan [2008] showed that the number of intense TCs heading for the midlatitude region has increased.

Table 4. The Number of TCs Classified by Saffir-Simpson's Categories 1 and 2 Over Korea and Japan for Two Periods, and the Difference Between the Two Periods
 Category 1 (≥32 m s−1)Category 2 (≥42 m s−1)
1977–19881997–2008Difference1977–19881997–2008Difference
RSMC2226+423+1
JTWC1220+816+5
HKO1528+1307+7
CMASTI714+700

[17] Because the TC weakening rates are similar (approximately −4.4 m s−1 per 10 h in the earlier decade and −5.0 m s−1 per 10 h in the later decade), the more intense TCs in the later decade could sustain for a longer duration after landfall. Figure 5 shows the ratios of the sum of the number of TSs, STSs, and TYs to the landfall number of TSs, STSs, and TYs occurring in the earlier decade (gray lines) and later decade (black lines). To minimize the effect of a single long-lived TC, we consider the ratios only for the cases where the total number of TSs, STSs, and TYs exceeds five. Since TCs have a strong intensity in the later decade, it is found that they have a remarkably long lifetime (Figure 5).

image

Figure 5. The ratios of the sum of the number of TSs, STSs, and TYs to the landfall number of TSs, STSs, and TYs occurring after TC landfall in Korea and Japan. Here “0” in the abscissa indicates the first-time entry of a TC in the domain shown in Figure 1. The gray and black lines indicate the ratio for the periods 1977–1988 and 1997–2008, respectively.

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4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

[18] Figure 6 shows the changes in the mean winds (i.e., steering flows) in the troposphere (i.e., between 850 and 200 hPa) in the later decade and the averaged track density of the landfall TCs for the entire period 1977–2008. The track density is defined as the number of TCs in a 10° × 10° grid window, which is shifted by 1° in latitude and longitude, leading to the moving averaged track frequency at a 1° × 1° resolution. The recent change in the mean wind velocity of the troposphere is characterized by a large anomalous cyclonic circulation that extends from 20°N to 40°N to the west of 130°E. Significant anomalous easterlies that are associated with this anomalous cyclone blow over North China and the Yellow Sea; however, these easterlies have a minor influence on the tracks since most TCs recurve in subtropics, approximately 25°N. On the other hand, the significant anomalous westerlies over the South China Sea turn to the northeastern region of Taiwan and play a critical role in interpreting the change in the TC tracks. Few TCs heading toward the South China Sea and a larger number of recurving TCs heading toward Korea and Japan have been observed over the WNP [Tu et al., 2009]; this observation is consistent with the change in the steering flows.

image

Figure 6. Difference in mean wind fields in the troposphere (m s−1) between 850 and 200 hPa for the periods 1997–2008 and 1977–1988. Only significant vectors at the 90% confidence level are plotted. The gray contours are the climatological characteristics of TC tracks for the TCs that made landfall in Korea and Japan during 1977–2008.

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[19] The significant anomalous southerlies to the east of Taiwan (20°N–30°N, 125°E–130°E) are located at the main recurving point of landfall TC tracks. These anomalous southerlies are related with the change in the monsoon trough and WNP subtropical high. The monsoon trough has strengthened in the recent decade, which is indicated by the negative anomalies of 850 and 500 hPa geopotential heights and the cyclonic circulation around South China and Taiwan (figure not shown). Meanwhile, the WNP subtropical high has slightly retreated eastward and expanded north and southward, forming the positive anomalies of geopotential heights and the anticyclonic circulation over the southeast of Japan. This northeast-high and southwest-low type of anomalies lead to the significant northward steering flows over the east of Taiwan (Figure 6). Naturally, a TC migrated from the southeast is sensitive to changes in the steering flows near Taiwan region. It is highly likely that these northward steering flow anomalies cause an increase in the landfall frequency in Korea and Japan (Figure 2a). In addition, Tu et al. [2009] showed the eastward retreat of the WNP subtropical high during 2000–2006, which is a good indicator of long-term changes in the TC tracks [e.g., Ho et al., 2004; Wu et al., 2005], leading a greater (lesser) number of TCs to Taiwan (the South China Sea). This also explains the increase in the landfall frequency in Korea and Japan.

[20] Figure 7 shows the changes in the atmospheric/oceanic environments that are physically associated with the TC activity over the midlatitude. There is a significant increase in the SST over the basin, particularly near the Korean peninsula and the Japanese islands (Figure 7a). The warmer sea surface at midlatitudes transfers a large amount of energy to TCs before they make landfall; as a result, these TCs can sustain their intensities for longer durations [e.g., Emanuel, 2005; Webster et al., 2005]. Interestingly, between the Philippine Sea and the South Sea of the Korean peninsula, there exists a channel in which the degree of SST warming is unexpectedly low. The exact reason for the formation of such a channel is unclear at present. However, the occurrence of recurving TCs with high intensity and high landfall frequency is thought to be a possible reason for the existence of this channel. An intense TC cools the sea surface by several degrees by mixing ocean mixed layer [e.g., Zedler et al., 2002; Wu et al., 2008]. If the weaker-warming channel were considered as a footprint of the cumulative TC impact, it would be a good example of the TC impact on the climate in the given region. Figure 7b shows the changes in the tropospheric moisture and divergence in the moisture, averaged over layers between surface and 300 hPa. There is a large moisture convergence toward the Japanese islands resulting in a more humid atmosphere there. The humid environment may help in sustaining the TC intensity for a longer duration and thus increase the TC-induced rainfall over Japan [e.g., Powell, 1987; Jiang et al., 2008; Hill and Lackmann, 2009]. The change in magnitude of the vertical shear of the horizontal winds between 850 and 200 hPa is displayed in Figure 7c. The weaker shear observed in the later decade spreads over the wide region at midlatitudes (30°N–40°N) where the climatological jet is located. The weakened shear also helps in sustaining the intensity of a landfall TC because the strong vertical wind shear over the region destroys the TC structure and accelerates the extratropical transition of the TC [e.g., Baik and Paek, 2001; Kaplan and DeMaria, 2001; Paterson et al., 2005]. Overall, all the three environments (i.e., SST, moisture, and vertical wind shear) over Korea and Japan have changed such that the conditions are favorable for TC activity.

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Figure 7. Differences in (a) the SST (°C), (b) moisture (10−3 kg kg−1) and moisture flux (m s−1) averaged from surface to 300 hPa, and (c) the magnitude of vertical wind shear (m s−1) between 1997–2008 and 1977–1988. Vectors significant at the 90% confidence level are shown. Light and dark shadings are the significant regions at the 90% and 95% confidence levels, respectively.

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[21] Another factor responsible for the intensified landfall TCs over the two countries is the local change in the three-dimensional circulation patterns. Figure 8a shows maximum areas of the 200 hPa jet stream (>30 m s−1) for the earlier and later decades and the difference of 200 hPa zonal wind between the 2 decades. Figures 8b, 8c, and 8d show the recent changes in the 300 hPa divergence, 850 hPa divergence, and 500 hPa vertical wind, respectively. For calculating these variables, we take into consideration “no-TC days,” which are defined as the days when there are no TCs above 30°N in all the best-track data sets; thus, the direct circulation of TCs at midlatitudes can be ignored. It is observed that the jet stream weakens considerably over North China (Figure 8a), as has been reported by Kwon et al. [2007], Archer and Caldeira [2008], and Zhu et al. [2011]. Kwon et al. [2007] argued that this weakening of the jet stream arises from the Rossby wave response to the heat source (i.e., increased precipitation) over South China. Archer and Caldeira [2008] attributed the weakening of the summertime East Asian jet to the warming environment. Zhu et al. [2011] suggested that the negative phase of the Pacific decadal oscillation in 2000–2008 could make the westerly upper tropospheric jet weaker. As shown in Figure 8a, the weakening of the jet stream over the Asian continent results in the formation of the relative jet entrance around the Korean peninsula, leading to a significant anomalous divergence (convergence) around the Korean peninsula at the upper (lower) troposphere (Figures 8b and 8c). Further, there is a significant anomalous ascending motion of the TCs around Korea toward the East China Sea (Figure 8d), which is dynamically consistent with the anomalous low-level convergence and upper-level divergence. These responses in the secondary circulation due to the relocation of the jet entrance are likely to give rise to conditions that are favorable for enhancing the influence of the landfall TCs; thus, under these conditions, the intensities of the TCs [Holland and Merrill, 1984; Bosart et al., 2000] and the TC-induced rainfall increase [Shi et al., 1997; Kim et al., 2006] over the Korean peninsula. Thus, we suggest that the large change in the jet stream is one of the factors that contribute to the high intensity of the landfall TCs.

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Figure 8. Differences in (a) the 200 hPa zonal wind (m s−1) (thick solid (dashed) line indicates 200 hPa zonal wind exceeding 30 m s−1 in 1997–2008 (1977–1988)), (b) 300 hPa divergence (10−6 s−1), (c) 850 hPa divergence (10−6 s−1), and (d) 500 hPa vertical wind (10−2 Pa s−1, positive upward) between 1997–2008 and 1977–1988. Light and dark shadings indicate the significant regions at the 90% and 95% confidence levels, respectively.

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5. Summary and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

[22] In this study, we investigated the changes in the activities of landfall TCs over Korea and Japan for the period 1977–2008 by using four parameters: the number of landfall TCs, TC duration, PDI, and TC-induced rainfall. We used the four available best-track TC data sets (RSMC, JTWC, HKO, and CMASTI) together to increase the confidence level of the results. Although there are nontrivial discrepancies in the amplitude of the changes, the long-term changes given by the four data sets are consistent with one another. The number of TC genesis events in the WNP increased until the mid-1990s and decreased thereafter (Figure 2b), whereas the number of landfall TCs since the 1990s is higher than that before 1990 (Figure 2a), resulting in a gradual increase in the ratio of the number of landfall TCs to the total number WNP TCs (Figure 2c). This increase in the number of landfall TCs over Korea and Japan is a result of the enhanced southerly steering flows over the East China Sea (Figure 6). The accumulated TC duration, PDI, and TC-induced rainfall within the landfall domain (Figure 1) also increase and this increase correlates well with the landfall frequency (Figure 3).

[23] While the accumulated TC duration, PDI, and TC-induced rainfall characterize the total TC activity in the landfall domain, the mean values of the parameters (calculated by dividing the accumulated values by the number of landfall TCs per year) indicate the average impact of one TC per a year. The mean values also show a significant increase (Figure 4). Linear increases are observed in the mean TC duration and mean PDI, implying that they are well correlated with each other. In contrast, the TC-induced rainfall shows a dramatic increase in the late 1990s. Since rainfall can be affected by various factors apart from the TC intensity [e.g., Kim et al., 2006; Park and Lee, 2007], the increase in the rainfall does not necessarily depend on the increase in the TC duration. Thus, in addition to the increased landfall frequency, the increased mean TC duration is another important factor responsible for the recent enhancement in the TC activity over Korea and Japan.

[24] It can be said that the high-intensity TCs sustained their intensity for a longer duration after landfall because the dissipation time required for these intense TCs to weaken is expected to be longer than that required for moderate TCs. This is the case because more number of typhoons and intensive typhoons made landfall in the region in a later decade (Table 4). The other factors (i.e., tracks, translation speeds, mean drift lengths, and weakening rates of TCs) could be also important for TC duration. The TC translation speeds have been slightly slower since the 2000s. The TC tracks have been shifted toward Japan. The mean drift lengths of TCs over the landfall domain have also increased since the late 1980s. The weakening rates of TCs have been a little bit faster in the later decade. However, these changes in four factors are not significant, indicating that they might have a minor role in the increased TC duration in the domain.

[25] Figure 9 shows a schematic representation of all mechanisms for stronger landfall TCs over Korea and Japan in the recent decade. The summarized mechanisms are as follows. The increase in the intensity of TCs may be due to warmer SSTs, more humid troposphere, and weaker vertical wind shear in the midlatitude during the TC season (Figure 7). In addition, a significant anomalous ascending background motion is observed over and around the East China Sea (Figure 8d). Since the days when TCs exist north of 30°N were ignored in the calculation of the climatological changes in the upper- and lower-level divergences and 500 hPa vertical motion (Figure 8), it can be suggested that these changes are not because of the strong ascending motion of TCs, but are driven by the secondary circulation response to the formation of a jet entrance over the Korean peninsula (Figure 8a). The enhanced ascending motion of the background environment is a possible cause for the increase in the intensity of TCs over Korea in a later decade.

image

Figure 9. A schematic for the factors leading to enhanced typhoon activity over Korea and Japan in a recent decade.

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[26] In the present study, we only considered the TSs, STSs, and TYs occurring when a TC makes landfall; in other words, we mainly investigate the influence of relatively intense systems only. This is because the discrepancies in the TD data among the TC data sets are considerably larger than those in the TS, STS, and TY data; this makes it difficult to distinguish the most credible TD records from among the others. However, heavy rainfall is also caused by the weak TDs in the midlatitude [e.g., Lee et al., 1992; Park et al., 2006]. Therefore, it is necessary to evaluate the cumulative impact of the TCs by using an objective method to define TDs. Further, the observational analyses presented herein should be reproduced by performing climate-model experiments with different background states. This is because even though environmental factors such as increase in the SSTs and humidity, weakening of the vertical wind shear, and relocation of the jet stream are statistically associated with the recently enhanced TC activity over Korea and Japan, we cannot assess the relative importance of these factors. On the basis of the model experiments, we can identify the factors that are more crucial (e.g., higher SSTs versus weaker vertical wind shear) for enhancing the TC activity at midlatitudes.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

[27] This study was funded by the Korea Meteorological Administration Research and Development Program under the grant CATER 2006–4204. D.-S. R. Park and H.-S. Kim received support from the BK21 project of the Korean government. D.-S. R. Park specially acknowledges some helpful comments by Su-Jong Jeong. The authors are grateful to three anonymous reviewers.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Changes in Landfall TC Activity Over Korea and Japan
  6. 4. The Related Large-Scale Environments and Possible Mechanisms to TC Changes
  7. 5. Summary and Discussion
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
jgrd16767-sup-0001-t01.txtplain text document0KTab-delimited Table 1.
jgrd16767-sup-0002-t02.txtplain text document0KTab-delimited Table 2.
jgrd16767-sup-0003-t03.txtplain text document0KTab-delimited Table 3.
jgrd16767-sup-0004-t04.txtplain text document0KTab-delimited Table 4.

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