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Negative relationship between Korea landfalling tropical cyclone activity and Pacific Decadal Oscillation
Dynamics of Atmospheres and Oceans 87 (2019) 101100
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Negative relationship between Korea landfalling tropical cyclone activity and Pacific Decadal Oscillation Jae-Won Choia, Hae-Dong Kimb, a b
T
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College of Atmospheric Sciences, Nanjing University of Information Science and Technology, Nanjing, 210044, China Department of Global Environment, Keimyung University, Daegu, Republic of Korea
A R T IC LE I N F O
ABS TRA CT
Keywords: Korea Tropical cyclone Pacific Decadal Oscillation
The present study discovered a strong negative correlation between Korea-landfalling tropical cyclone (TC) frequency and Pacific Decadal Oscillation (PDO) in the summer. Thus, the present study selected years that had the highest PDO index (positive PDO years) and years that had the lowest PDO index (negative PDO years) to analyze a mean difference between the two phases in order to determine the reason for the strong negative correlation between the two variables. In the positive PDO years, TCs were mainly generated in the southeastern part of the western North Pacific, and lower TC passage frequency was found in most regions in the mid-latitude in East Asia. Moreover, a slightly weaker TC intensity than that in the negative PDO years was revealed. In order to determine the cause of the TC activity revealed in the positive PDO years, 850 hPa and 500 hPa stream flows were analyzed first. In the mid-latitude region in East Asia, anomalous huge cyclonic circulations were strengthened, while anomalous anticyclonic circulations were strengthened in the low-latitude region. Accordingly, Korea was being influenced by anomalous northwesterlies, which played a role in blocking TCs from moving northward to Korea. The results of analysis on 850 hPa air temperature, precipitation, 600 hPa relative humidity, and sea surface temperature (SST) showed that negative anomalies were strengthened in the northwest region in the western North Pacific while positive anomalies were strengthened in the southeast region. The atmospheric and oceanic environments were related to frequent occurrences of TCs in the southeast region in the western North Pacific during the positive PDO years. All factors of air temperature, precipitation, 600 hPa relative humidity, and SST revealed negative (positive for vertical wind shear) anomalies near Korea, so that atmospheric and oceanic environments were formed that could rapidly weaken TC intensity, even if the TCs moved northward to Korea in the positive PDO years.
1. Introduction Climatologically, tropical cyclones (TCs) have a higher frequency in the western North Pacific than in any other typhoon basin (Neumann, 1993). As a result, nations surrounding the western North Pacific experience considerable loss of life and property as a result of TCs landing every year. A large number of studies have been conducted to understand the characteristics of TCs and to predict their occurrence. Until now, the most remarkable variable to understanding the characteristics of TC genesis is El Niño–Southern Oscillation (ENSO) (Wang and Chan, 2002; Chu, 2004; Wu et al., 2004). In general, TCs in El Niño years occur mainly in the southeast sea area in the western North Pacific. They then change direction towards the eastern sea of China or land in Korea and ⁎
Corresponding author. E-mail address: [email protected] (H.-D. Kim).
https://doi.org/10.1016/j.dynatmoce.2019.101100 Received 28 November 2018; Received in revised form 10 June 2019; Accepted 11 July 2019 Available online 12 July 2019 0377-0265/ © 2019 Published by Elsevier B.V.
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Japan, which are located in the mid-latitude in East Asia. By contrast, TCs in La Niña years occur in the northwest sea area in the western North Pacific, followed by a change of direction to the west or northwest so that TCs land mainly in the Philippines, South China, or the Indochina Peninsula. Therefore, TCs in El Niño years exhibit a longer life and stronger intensity than TCs during La Niña years. Gray (Gray, 1975, 1984; Gray et al., 1992, 1993; Gray et al., 1994) discovered six important physical parameters for TC occurrence. These are (i) lower-level relative vorticity, (ii) local or planetary vorticity (Coriolis parameter), (iii) inverse of the vertical shear of the horizontal wind between the lower and upper troposphere, (iv) ocean thermal energy due to temperatures above 26.8 °C to a depth of 60 m, (v) vertical gradient of equivalent potential temperature between the surface and 500 hPa, and (vi) middle troposphere relative humidity. Because Gray’s parameters reflect the seasonal characteristics of TC activity, they have been used in statistical models for seasonal prediction of TC genesis. Using Gray’s troposphere parameters in the TC genesis region and ENSO, the characteristics of TC genesis have been studied successfully in a number of studies. However, TC activity in lower and mid-latitude regions is affected not only by environmental factors in the tropical region, but also by interaction with various teleconnection patterns that are present outside the tropical region. As a result, it is critically important to disclose a relationship between teleconnection patterns and TC activity by finding a signal of patterns that affect TC activity. Chan et al. (1998) and Chen et al. (2001) focused on factors of teleconnection patterns rather than Gray’s parameters. That is, they discussed the characteristics of TC activity by using (i) sea surface temperature anomalies in the central and eastern Pacific, (ii) index of characteristics of atmospheric circulations in the western Pacific and Asia from April in the previous year to March in the current year, and (iii) atmosphere and oceanic parameters, including an index that represents atmospheric circulations in Australia and the South Pacific as well as annual trends of changes in TC activity (climatology and persistence). Ho et al. (2005) proved an increase in TC activity near the East China Sea as an anomalous anticyclone was strengthened at the mid-latitude in both hemispheres in the positive Antarctic Oscillation (AAO) phase, which was generated in the Southern Hemisphere. Wang and Fan (2007) discovered that negative correlations exist between the AAO from June to September and the TC genesis frequency in the western North Pacific. They demonstrated through this study that, in the positive AAO phase, an environment that is unfavorable to the occurrence of TCs is created in the subtropical western Pacific, such as increased vertical wind shear, the development of an atmospheric vertical structure of high pressure in the lower layers and low pressure in the upper layers, and lowered sea surface temperature. Meanwhile, Wang et al. (2007) investigated the relations between the North Pacific Oscillation (NPO) and the TC genesis frequency in the western North Pacific and the subtropical Atlantic during the June-September period. As a result, they suggested that the TC genesis frequencies in the former and latter seas exhibit positive and negative correlations with the NPO respectively, and changes in TC genesis frequency between the two seas are realized through teleconnection patterns. Choi et al., 2010a, 2010b showed a high frequency of TC genesis as the monsoon trough was strengthened in the subtropical western Pacific whereas an anomalous anticyclone was strengthened in the eastern sea of Japan during a positive Pacific-Japan teleconnection pattern. They also reported a trend of increased TC activity in the mid-latitude region in East Asia and a decrease in TC activity near the Philippines and the South China Sea due to the anomalous pressure system patterns. Furthermore, Choi et al. (2012) claimed that the anomalous anticyclone was strengthened in the mid-latitude region in East Asia in the positive Arctic Oscillation (AO) phases whereas the monsoon trough was developed in the western North Pacific, thereby increasing TC activities in the mid-latitude region in East Asia as well as TC genesis frequency. Choi et al. (2013) demonstrated the influence of the Arctic Oscillation on the TC activity around Taiwan and Choi et al. (2017) showed interdecadal variation of tropical cyclone genesis frequency in late season over the western North Pacific. As for studies on Korea-landfalling or affecting TC activity, Choi and Kim (2007) showed that the frequency of Korea-landfalling TCs has been increasing since the mid-1990s, and that the increase in the affecting frequency of strong TCs is particularly noticeable. They deduced that this is because the frequency of TCs passing Mainland China before landing on Korea has decreased as the TC track shifts eastward due to the eastward retreat of western North Pacific subtropical high (WNPSH). In addition, Choi et al., 2010a, 2010b applied the statistical change-point analysis which was used in the study of Ho et al. (2004) to the frequency change of Korealandfalling TCs. Their results showed that the analysis period of 54 years (1951–2004) could be divided into three periods, and emphasized that the affecting frequency of strong TCs was highest in the recent period. The recent increasing of the affecting frequency of strong TCs is also related to the eastward retreat of WNPSH. Park et al. (2006) showed that this increasing trend is also noticeable for the frequency of Korea-affecting TCs. In particular, Choi and Kim (2011) applied statistical change-point analysis to the frequency of TCs affecting Korea, and showed that their frequency has been increasing since the early 1980s. These two studies also stressed that the recent increasing of the affecting or landfalling frequency of TCs is associated with the zonal movement of WNPSH. Choi et al. (2015a) showed the possible relationship between SST in the equatorial eastern Pacific and TC frequency that affects Korea and Choi et al. (2015b) also showed the relationship between east Indian Ocean SST and tropical cyclone affecting Korea. The former study demonstrated that Korea affecting TC frequency is related to northeastward movement of Madden-Julian Oscillation (MJO) and the latter study stressed that Korea affecting TC frequency is associated with the zonal movement of WNPSH according to the development of ENSO. Pacific Decadal Oscillation (PDO), an important signal at decadal time scales, shifted from its negative to positive phase after the early 1980s. The intensity and frequency of ENSO events vary with respect to PDO, which controls the heat and dynamic state of the ocean and atmosphere on a multidimensional scale. The warm (cold) PDO phase can enhance (weak) El Nino phenomenon, increasing (reducing) the effects of heated water pools on the equatorial Pacific in the TC season through local diabatic heating. El Nino phenomenon accompanies stronger equatorial Walker circulation at the warm PDO phase than in the cold PDO phase. In contrast, the Walker circulation pattern associated with the La Nina event is less affected by the other PDO phase (Dong and Xiao (2016). PDO can be regarded as a leading factor that results in the decadal variation of TC frequency. Yoon and Yeh (2010) found that when ENSO and 2
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PDO are in (out of) phase, the Eurasian-like pattern acts to enhance (reduce) extratropic-related rainfall over northeast Asia, resulting in the strengthening (weakening) of the northeast Asian summer monsoon. Feng et al. (2014) showed that when ENSO is in (out of) phase with PDO, an anomalous tripolar (dipole) rainfall pattern exists in East China, while the WNPSH experiences a one-time (twotime) northward shift. They further found that, due to the phase transition of PDO, the life cycle of ENSO has changed, resulting in the different influence of ENSO on East Asian summer monsoons (EASMs) in different PDO phases. However, it is not easy to find studies on the relationship between Korea-affecting TC activity and PDO. Therefore, this study examined the relationship between the two under different PDO phases. PDO is an important climatic oscillation that has a significant influence on the Pacific climate (Mantua et al., 1997) and TC activity (Wang et al., 2010; Aiyyer and Thorncroft, 2011; Liu and Chan, 2013). Thus, it would be worthwhile to analyze a relationship between the frequency of Korea-landfalling TCs and PDO. In the present study, Section 0 describes the data and analysis method. Section 0 analyzes the relationship between Korealandfalling TC frequency and PDO. Section 0 identifies the relationship between the two variables, and Section 0 summarizes the study results. 2. Data and methods 2.1. Data For information on TC activities, the best-track data provided by the Regional Specialized Meteorological Center (RSMC)–Tokyo Typhoon Center were used. These data included the names of TCs, their latitudes and longitudes, central pressure, and maximum sustained wind speed (MSWS) at six-hour intervals. TCs are largely divided into four grades based on MSWS: tropical depression (TD, MSWS < 17 ms−1), tropical storm (TS, 17 ms−1s ≤ MSWS ≤ 24 ms−1), severe tropical storm (STS, 25 ms−1 ≤ MSWS ≤ 32 ms−1), and typhoon (TY, MSWS ≥ 33 ms−1). The analysis of this study also included those TCs that turned into extratropical cyclones, given the fact that TCs inflict substantial damage on the mid-latitude region of East Asia even after turning into extratropical cyclones. This study used the reanalysis data provided by the National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) which has the following variables: geopotential height (gpm), zonal and meridional wind (ms−1), air temperature (°C), relative humidity (%), precipitation (mmday−1), total cloud cover (%), and velocity potential (m2s−110-6) (Kalnay et al., 1996; Kistler et al., 2001). These data have a spatial resolution of 2.5° × 2.5° for latitude and longitude and 17 vertical layers (8 layers for relative humidity and 1 for precipitation). This data are available from 1948 to the present. The National Oceanic and Atmospheric Administration’s (NOAA) Extended Reconstructed Sea Surface Temperature (SST) (Reynolds et al., 2002), available from the forenamed organization, was also used. The data have a horizontal resolution of 2.0° × 2.0° latitude-longitude and are available for the years 1854 to the present day. Also, the NOAA interpolated outgoing longwave radiation (OLR) data were retrieved from the NOAA satellite series. These data start from June 1974 and are available from NOAA’s Climate Diagnosis Center (CDC). However, the data are incomplete: they are missing the period from March to December of 1978. Detailed information about this OLR data can be found on the CDC’s website (http://www.cdc.noaa.gov) and in the study by Liebmann and Smith (1996). 2.2. Methods In order to calculate TC passage frequencies, each TC was calculated after being relocated within a 5° × 5° grid. Even if a TC passed over the same grid multiple times, it was regarded as a single passage. TC genesis frequencies were also calculated in the above manner. The independent two-sample t-test was used for the comparison of significance between two means in this study (Wilks, 1995). A Korea-landfalling TC is defined as one in which the TC’s center encounters the coastline of the Korean Peninsula, as reported on the surface weather chart in the 6 -hly RSMC-Tokyo Typhoon Center’s best-track dataset. Furthermore, the present study focused on Korea-landfalling TCs from July to September, so average data from July to September were used in the analysis. This is because most TCs that landed in Korea happened during these months, which accounts for three-quarters of the total, as presented in a study by Choi et al., 2010a, 2010b. In the present study, July to September is defined as the summer period. 3. Time series analysis of korea-landfalling TC frequencies and PDO Fig. 1 shows a time series of Korea-landfalling TC frequencies and average PDO index during the summer. The Korea-landfalling TC frequency showed a distinctive interannual variation, and the PDO index revealed distinctive interannual and interdecadal variations. In particular, a time series of the latter showed a rapid decrease in the index since the late 1990s, thereby revealing a climate regime shift after the late 1990s. An out-of-phase was also distinctively revealed between the two variables. Thus, -0.51 of a high negative correlation existed between the two variables. This correlation is statistically significant at the 99% confidence level. However, if a trend is removed from the two time series, a correlation between the two variables could be different. The Korealandfalling TC frequency had little change in trend, whereas the mean summer PDO index showed a rapidly decreasing trend. Therefore, a correlation was analyzed after removing the trend from the two variables. However, a high negative correlation had not changed between the two variables, even though the trend was removed from the two time series (r = -0.49, significant at the 99% confidence level). The PDO is significantly affected by the ENSO. Therefore, a partial correlation analysis was conducted to see 3
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Fig. 1. Time series of Korea landfalling TC frequency (solid line with a closed circle) and PDO index (dotted line with an open circle) for JAS and their trends. Table 1 Partial correlation analysis among Korea landfalling TC frequency, PDO index, and Niño-3.4 index for July-September (JAS). Control Variables None
a
Variables
Correlation/ Significance
TC freq.
PDO
Niño-3.4
TC freq.
Correlation Significance Correlation Significance Correlation Significance Correlation Significance Correlation Significance
1.00 · −0.51 0.002 −0.07 0.695 1.00 · −0.53 0.001
−0.51 0.002 1.00 · 0.45 0.007 −0.53 0.001 1.00 ·
−0.07 0.695 0.45 0.007 1.00 · · · · ·
PDO Niño-3.4 Niño-3.4
TC freq. PDO
a. Cells contain zero-order (Pearson) correlations. Table 2 Korea landfalling TC frequency in positive and negative PDO years. Red and blue years denote El Niño and La Niña years, respectively. Positive PDO years Year 1983 1987 1992 1993 1995 1997 2008 2009 2014 2015 Average
Negative PDO years TC freq. 0 1 2 0 1 1 0 0 1 1 0.7
Year 1994 1998 1999 2000 2002 2004 2010 2011 2012 2013 Average
TC freq. 4 1 3 3 2 2 2 2 3 0 2.2
whether a high negative correlation between two variables was due to the effect of the ENSO (Table 1). As explained above, a high positive correlation exists between mean summer PDO index and mean summer Niño-3.4 index. This correlation is statistically significant at the 99% confidence level. When a mean summer Niño-3.4 index was defined with control variables, a high negative correlation (-0.53) existed between Korea-landfalling TC frequency and mean summer PDO index. This correlation is rather higher than the previously analyzed correlation and it is significant at the 99% confidence level. However, the Pearson method is sensitive to outliers and thus this study performed Spearman correlation analysis again. As a result, we got a similar result with Pearson method (not shown). In order to determine the reason for the high negative correlation between the two variables, a 10-year period whose mean summer PDO index was the largest among 35 analysis years (hereafter referred to as positive PDO years), and 10-year period whose mean summer PDO index was the least (hereafter referred to as negative PDO years) were selected. A mean difference between the two phases was then analyzed (Table 2). These selected 20 years account for 60% of the total analysis period. The mean summer PDO index in the positive PDO years showed 0.7 or higher, while that in the negative PDO years showed -0.4 or lower. All years except for 1992 showed 1 TC or less in the positive PDO years, whereas all years except for 1998 and 2013 showed 2 TCs or more in the negative PDO years. Thus, a mean Korea-landfalling TC frequency in the positive PDO years was 0.7 TC, whereas that in the negative PDO years was 2.2 TCs, resulting in a 1.5-TC difference between the two phases. This difference was statistically significant at the 95%
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Fig. 2. Differences of (a) TC genesis frequency and (b) TC passage frequency between positive PDO years and negative PDO years in 5° × 5° latitude and longitude grid box for JAS. Small solid rectangles indicate that the differences are significant at the 95% confidence level. In (b), solid and dotted lines denote western North Pacific subtropical highs in positive and negative PDO years, respectively. Solid and dashed lines denoted WNPSH (5,875 gpm contour) in positive and negative PDO years, respectively.
Fig. 3. (a) TC central pressure and (b) TC maximum sustained wind speed. The boxes show the 25th and 75th percentiles, the lines in the boxes mark the median and the circles are values below (above) the 25th (75th) percentiles of distribution. The numbers to the right and left sides of the figure represent average values (cross marks) for positive PDO years and for negative PDO years, respectively.
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Fig. 4. Difference in (a) 850 hPa geopotential height and (b) sea surface temperature between positive and negative PDO years. Units are gpm in (a) and °C in (b). Hatched lines are significant at the 95% confidence level except for land areas in (b).
confidence level. 4. Differences between positive and negative PDO years 4.1. TC activity The present study investigated a difference in TC activity between the two phases to determine the cause of the high negative correlation between the two variables. TCs were generated mainly in the southeastern part of the western North Pacific during the positive PDO years overall, whereas TCs were generated in the northwestern part of the western North Pacific in the negative PDO years regarding TC genesis frequency in summer (Fig. 2a). This can be verified from the analysis result on means of TC genesis location between the two phases. A mean TC genesis location during the positive PDO years was 14.6 °N and 142.4 °E, while that in the negative PDO years was 16.4 °N and 135.0 °E, indicating that TCs in the positive (negative) PDO years were mainly generated in 6
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Fig. 5. Same as in Fig. 4, but for (a) outgoing longwave radiation, (b) total cloud cover, and (c) precipitation. Units are W m−2 in (a), % in (b) and mm day-1 in (c). Hatched lines are significant at the 95% confidence level.
the southeastern (northwestern) part of the western North Pacific. A latitude difference between the two phases is significant at the 90% confidence level and a longitude difference is significant at the 95% confidence level. A difference in TC passage frequency in summer between the two phases showed that TCs tended to land in the Indochina peninsula via the South China Sea or move northward toward the far eastern sea in Japan during the positive PDO phases (Fig. 2b). On the other hand, TCs tended to move north toward Korea via the East China Sea from the eastern sea in Taiwan during the negative PDO years. As analyzed above, a distinctive negative correlation exists between the frequency of Korea-landfalling TCs and mean 7
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Fig. 6. Same as in Fig. 4, but for (a) 850 hPa and (b) 500 hPa stream flows. Shaded areas are significant at the 95% confidence level.
summer PDO index. Negative values were also revealed in the eastern regions of China and Taiwan, in which such negative correlations may also be found in those regions. Regarding this, a study done by Chan et al. (2012) proved that there was a strong negative correlation between PDO index and frequency of TCs landing on the eastern coast of China. TC genesis location and TC passage frequency may influence TC intensity. Thus, TC intensity between the two phases was analyzed (Fig. 3). Here, TC intensity is defined as a TC central pressure (Fig. 3a) and TC MSWS (Fig. 3b) at the time of the TC landing in Korea in the 6 -h interval RSMC-Tokyo best-track dataset. Overall, TC intensity in the positive PDO years seems weaker than that in the negative PDO years. A mean TC central pressure in the positive PDO years was 7 hPa larger than in the negative PDO years, and TC MSWS was 8 knots slower on average in the positive PDO years than in the negative PDO years. Mean differences in TC central pressure and TC MSWS between the two phases are significant at the 95% confidence level. This result means that landed TC intensity in Korea was stronger (weaker) in the positive (negative) PDO years than in the negative (positive) PDO years.
4.2. Large-scale environments If a typical positive PDO phase is not revealed in a difference between the two phases prior to determining the reason for the high negative correlation between the two variables, then subsequent analysis becomes meaningless. Thus, the present study analyzed a difference in 850 hPa geopotential height and SST between the two phases (Fig. 4). The analysis results showed a positive anomaly in the Arctic regions, while a negative anomaly was found in the mid-latitude regions. A positive anomaly was also located in the lowlatitude region (Fig. 4a). In particular, a strong negative anomaly was formed in the North Pacific. This was a typical anomalous pressure system pattern that was revealed in the positive PDO phase (Mantua et al., 1997). A difference in SST between the two phases showed that a strong warm anomaly was revealed from the equatorial eastern Pacific to the equatorial central Pacific, whereas a strong cold anomaly was exhibited in the North Pacific. This was a typical spatial distribution of SST that was revealed in the positive PDO phase (Mantua et al., 1997). A difference in OLR between the two phases was analyzed to determine a level of convective activity (Fig. 5a). A negative anomaly was developed in the east-west direction from the eastern sea of China to the far eastern sea of Japan via Korea. In contrast, a positive anomaly was revealed in a latitude zone of 10°-20 °N. The positive anomaly was stronger in the northwest area of the subtropical western North Pacific than in other areas. A negative anomaly was again located in a latitude zone of 0°-10 °N. The negative anomaly was more distinctive in the southeast area of the subtropical western North Pacific than in other areas. Because OLR is larger (smaller), it means convection is weaker (stronger). Thus, the above result showed that convective activity was stronger (weaker) in the southeast (northwest) area of the subtropical western North Pacific in the positive PDO years. This result revealed a trend that TCs were generated mainly in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years as analyzed above. A difference in total cloud cover between two the phases showed an opposite of spatial distribution in the OLR result (Fig. 5b). That is, a negative anomaly was revealed in the northwest area of the subtropical western North Pacific in the 8
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Fig. 7. Same as in Fig. 4, but for (a) 850 hPa and (b) 200 hPa horizontal divergences. Contour interval is 2s−1*107. Shaded areas are significant at the 95% confidence level.
positive PDO years, whereas a positive anomaly was seen in the southeast area. A difference in precipitation between the two phases based on the above results of OLR and total cloud cover showed that precipitation was small in the northwest area of the subtropical western North Pacific in the positive PDO years, whereas precipitation was large in the southeast area (Fig. 5c). A difference in 850 hPa and 500 hPa stream flows between the two phases was analyzed to determine the status of atmospheric circulations that affected a negative correlation between the Korea-landfalling TC frequency and mean summer PDO (Fig. 6). The analysis results on the 850 hPa stream flows showed that anomalous huge cyclonic circulations were strengthened between 30 °N and 50 °N, whereas anomalous huge anticyclonic circulations were strengthened in the zone south of 30 °N (Fig. 6a). A strong cold SST anomaly was located in the North Pacific because of the anomalous huge cyclonic circulations strengthened in the 30°-50 °N zone as analyzed above. Due to the anomalous pressure system pattern in the south-north direction, the Korean Peninsula was affected by anomalous northwesterlies. These northwesterlies played a role as anomalous steering flows that blocked the TCs from moving north toward Korea from low latitudes. Thus, the above result explains the reason for the negative correlation between the Korea-landfalling TC frequency and mean summer PDO index. The anomalous northwesterlies affected not only Korea but also the eastern sea of China and the southwestern region in Japan so that lower TC passage frequency was revealed in those regions in the positive PDO years. The anomalous westerlies were strengthened from the eastern Philippines Sea to the South China Sea so that TC passage frequency in those regions became high in the positive PDO years. The anomalous pressure system pattern that was revealed in the analysis result on 500 hPa stream flows was similar to the spatial distribution revealed in the analysis on 850 hPa stream flows (Fig. 6b). The anomalous huge cyclonic circulations were strengthened in a latitude zone of 30°-50 °N while the anomalous anticyclonic circulations were strengthened in a latitude zone of 10°-20 °N. The anomalous anticyclonic circulations that were strengthened in the east-west direction in the subtropical western North Pacific meant that the WNPSH was strengthened in the west direction. Therefore, the present study analyzed the condition of the WNPSH between the two phases (Fig. 2b). Here, the WNPSH was defined as a region which has more than 5875 gpm at 500 hPa. The WNPSH 9
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Fig. 8. Same as in Fig. 4, but for (a) 200–850 hPa vertical wind shear, (b) 850 hPa relative vorticity, and (c) 600 hPa relative humidity. Contour intervals are 1 ms−1 for vertical wind shear, 2-6s-1 for 850 hPa relative vorticity, and 2% for 600 hPa relative humidity. Shaded areas are significant at the 95% confidence level.
in the positive PDO years was not developed in the north but rather developed in the west up to the South China Sea as well as in the equatorial direction (solid line in Fig. 2b). By contrast, the WNPSH in the negative PDO years was weakened further east than in the positive PDO years (dashed line in Fig. 2b). In general, TCs tended to move along the western periphery in the WNPSH. Thus, TCs in 10
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Fig. 9. Same as in Fig. 4, but for (a) 850 hPa and (b) 500 hPa air temperatures. Contour interval is 0.2 °C. Shaded areas are significant at the 95% confidence level.
each phase moved along the west side of the WNPSH so that TC track and west boundary of the WNPSH were matched well. As discussed above, a difference in 850 hPa and 200 hPa horizontal divergences between the two phases was analyzed to determine the cause of the difference in TC genesis location between the two phases (Fig. 7). A positive anomaly was exhibited in the northwest area of the western North Pacific in the 850 hPa horizontal divergence, whereas a negative anomaly was revealed in the southeast area (Fig. 7a). This means that anomalous divergence was strengthened in the northwest area of the western North Pacific, whereas anomalous convergence was strengthened in the southeast area in the positive PDO years. A negative anomaly was exhibited in the northwest area of the western North Pacific in the 200 hPa horizontal divergence, whereas a positive anomaly was revealed in the southeast area (Fig. 7b). This means that anomalous convergence was strengthened in the northwest area of the western North Pacific whereas anomalous divergence was strengthened in the southeast area in the positive PDO years. Thus, anomalous downward flows were strengthened in the northwest area of the western North Pacific whereas anomalous upward flows were strengthened in the positive PDO years considering both the upper and lower troposphere. This result revealed the reason why TCs were generated mainly in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years as analyzed above. A difference in 200–850 hPa vertical wind shear between the two phases showed that a strong positive anomaly was located in the latitude zone of 30°-40 °N (Fig. 8a). In general, as the value of vertical wind shear becomes larger, a more unfavorable environment against TC development is provided. This strong positive anomaly at the mid-latitude in East Asia during the positive PDO years makes TC intensity weaker when TCs land in Korea. A difference in 850 hPa relative vorticity between the two phases revealed that a negative anomaly was strengthened in the northwest area of the western North Pacific while a positive anomaly was strengthened in the southeast area (Fig. 8b). This result revealed why TCs were generated mainly in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years as analyzed above. A difference in 600 hPa relative humidity between the two phases showed that a negative anomaly was strengthened from Korea up to the far eastern sea of Japan (Fig. 8c). This is the reason why TC intensity at the time of landing in Korea became weak in the positive PDO years.
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Fig. 10. (a) Distribution of 200 hPa jet streak in positive (solid line) and negative PDO years (dashed line) and (b) a difference in 200 hPa zonal wind speed between positive and negative PDO years. Jet streaks are defined as areas that 200 hPa zonal wind speed is greater than 25ms−1. In (b) contour interval is 1 ms−1 and shaded areas are significant at the 95% confidence level.
A difference in SST between the two phases showed that a cold anomaly that developed strongly in the North Pacific became an important oceanic environment that weakened TC intensity when TCs landed in Korea (Fig. 4b). Furthermore, a cold anomaly was strengthened in the northwest area of the western North Pacific and a warm anomaly was strengthened in the southeast area. A difference in 850 hPa and 500 hPa air temperatures between the two phases revealed a cold anomaly in the north region of 30 °N including Korea (Fig. 9). A difference in 200 hPa air temperature between the two phases also showed a cold anomaly in the East Asia continent including Korea (not shown). Thus, a cold anomaly was strengthened in all layers in the troposphere over Korea so that TC intensity can be weakened during TCs landing in Korea. A positive vertical wind shear anomaly, which was developed strongly in the mid-latitude region in East Asia in the positive PDO years, meant that a zonal wind speed is stronger in the upper layer than in the lower layer in the troposphere. Thus, a 200 hPa jet streak during the two phases was analyzed (Fig. 10a). Here, a jet streak was defined as a region where 200 hPa zonal wind speed was larger than 25 ms−1. A jet streak in the upper troposphere in the positive PDO years was developed strongly in a latitude zone of 35°45 °N, whereas a jet streak was only revealed in some parts in the west and east of the mid-latitude in East Asia among the analysis regions in the negative PDO years. A jet in the upper troposphere makes the vertical structure of a TC unstable, thereby weakening TC intensity. Therefore, a jet streak in the upper troposphere, which was strengthened in the mid-latitude in East Asia in the positive PDO years, can not only weaken Korea-landfalling TC intensity but also lower their frequency by rapidly moving TCs in the northeast (Japan) direction. In order to determine the characteristics of global-scale atmospheric circulation, a difference in 850 hPa and 200 hPa divergent winds and velocity potentials between the two phases was analyzed (Fig. 11). In the 850 hPa divergent wind and velocity potential, an anomalous divergent wind was strengthened in the northwest area of the western North Pacific, whereas an anomalous convergent wind was strengthened in the northeast area (Fig. 11a). In the 200 hPa divergent wind and velocity potential, a spatial distribution opposite to the result of the 850 hPa divergent wind and velocity potential was exhibited (Fig. 11b). Therefore, when considering the 12
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Fig. 11. Same as in Fig. 4, but for (a) 850 hPa and (b) 200 hPa divergent wind and velocity potential. Shaded areas denote negative anomalies. Contour interval is 3 m2s−110-6.
upper and lower troposphere, anomalous atmospheric circulations were developed, in which air that was converged and elevated in the southeast area of the western North Pacific in the positive PDO years was converged and descended in the southeast area of the western North Pacific. 5. Summary and conclusion The present study analyzed a correlation between Korea-landfalling TC frequency in summer and PDO and presented a strong negative correlation between the two variables. For this purpose, the study selected 10 years that had the highest PDO index (positive PDO years) and 10 years that had the lowest PDO index (negative PDO years) to analyze a mean difference between the two phases in order to determine the reason for the strong negative correlation between the two variables. TCs were mainly generated in the southeast area of the western North Pacific in the positive PDO years, and slightly lower TC passage frequency was revealed in the mid-latitude region in East Asia including Korea than in other regions, whereas a high frequency was exhibited from the eastern sea of the Philippines to the South China Sea. Moreover, a slightly weaker TC intensity than that in the negative PDO years was revealed. In order to determine the cause of the TC activity revealed in the positive PDO years, 850 hPa and 500 hPa stream flows were analyzed first. In the mid-latitude region in East Asia, anomalous huge cyclonic circulations were strengthened while anomalous anticyclonic circulations were strengthened in the low-latitude region. Due to the anomalous pressure system pattern, Korea was influenced by anomalous northwesterlies, which played a role as anomalous steering flows that blocked TCs from moving northward to Korea. In the analysis on the 850 hPa and 200 hPa horizontal divergence, anomalous downward flows were strengthened in the northwest area of the western North Pacific while anomalous upward flows were strengthened in the southeast area. Furthermore, the analysis result on 850 hPa air temperature, precipitation, 600 hPa relative humidity, and SST showed that negative anomalies were strengthened in the northwest region in the western North Pacific while positive anomalies were strengthened in the southeast 13
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region. The atmospheric and oceanic environments were related to frequent occurrence of TCs in the southeast region in the western North Pacific during the positive PDO years. All data on air temperature, precipitation, 600 hPa relative humidity, and SST revealed negative (positive for 200–850 hPa vertical wind shear) anomalies near Korea. This means that atmospheric and oceanic environments were formed that can rapidly weaken TC intensity even if TCs moved northward to Korea in the positive PDO years. The analysis result on the 200 hPa jet streak showed an environment that made it difficult for TCs to enter Korea, due to the strong jet effect that was formed in the positive PDO years. TC intensity can be rapidly weakened due to the strong jet even if TCs entered the Peninsula. To determine the characteristics of the global-scale atmospheric circulations, a difference in 850 hPa divergent wind between the two phases was analyzed. The analysis found that an anomalous convergent wind was strengthened in the southeast area of the western North Pacific while an anomalous divergent wind was strengthened in the northwest area. This result can be related to more frequent TCs in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years than in the negative (positive) PDO years. For future study, PDO will be investigated on whether it can act as a main predictor in the statistical model for seasonal prediction of Korea-landfalling TC activity. Acknowledgement This paper is supported by The Startup Foundation for Introducing Talent of NUIST (Grant/Award number: 2018r059). References Aiyyer, A., Thorncroft, C., 2011. Interannual-to-multidecadal variability of vertical shear and tropical cyclone activity. J. Clim. 24, 2949–2962. https://doi.org/10.1175/ 2010JCLI3698.1. Chan, J.C.L., Liu, K.S., Xu, M., Yang, Q., 2012. Variations of frequency of landfalling typhoons in East China, 1450-1949. J. Int. Climatol. 32, 1946–1950. https://doi.org/10. 1002/joc.2410. Chan, J.C.L., Shi, J.E., Lam, C.M., 1998. Seasonal forecasting of tropical cyclone activity over the western North Pacific and the South China Sea. Wea. Forecasting 13, 997–1004. Chen, J.E., Shi, J.E., Liu, K.S., 2001. Improvements in the seasonal forecasting of tropical cyclone activity over the western North Pacific. Wea. Forecasting 16, 491–498. Choi, K.S., Cha, Y.M., Kang, S.D., Kim, H.D., 2015a. Relationship between sst in the equatorial eastern pacific and TC frequency that affects Korea. Theor. Appl. Climatol. 121, 243–252. Choi, K.S., Cha, Y.M., Kang, S.D., Kim, H.D., Kim, B.J., 2013. Influence of the arctic oscillation on the TC activity around Taiwan. Theor. Appl. Climatol. 116, 695–706. Choi, J.W., Cha, Y., Kim, T., Kim, H.D., 2017. Interdecadal variation of tropical cyclone genesis frequency in late season over the western North Pacific. Int. J. Climatol. 37, 4335–4346. Choi, K.S., Kim, B.J., 2007. Climatological characteristics of tropical cyclones making landfall over the Korean Peninsula. Asia-Pacific J. Atmos. Sci. 43, 97–109. Choi, K.S., Kim, T.R., 2011. Regime shift of the early 1980s in the characteristics of the tropical cyclone affecting Korea. J. Korean Earth Sci. Soc. 32, 453–460. Choi, K.S., Kim, B.J., Km, D.W., Byun, H.R., 2010b. Interdecadal variation of tropical cyclone making landfall over the Korean Peninsula. Int. J. Climatol. 30, 1472–1483. Choi, K.S., Park, S., Chang, K.H., Lee, J.H., 2015b. A possible relationship between east Indian Ocean SST and tropical cyclone affecting Korea. Nat. Hazards 76, 283–301. Choi, K.S., Wu, C.C., Cha, E.J., 2010a. Change of tropical cyclone activity by Pacific-Japan teleconnection pattern in the western North Pacific. J. Geophys. Res. 115, D19114. https://doi.org/10.1029/2010JD013866. Chu, P.S., 2004. In: Murnane, R.J., Liu, K.-B. (Eds.), ENSO and Tropical Cyclone Activity. Hurricanes and Typhoons: Past, Present, and Potential. Columbia University Press, pp. 297–332. Dong, Xiao, 2016. Influences of the pacific decadal oscillation on the east asian summer monsoon in non-ENSO years. Atm. Sci. Lett 17, 115–120. Feng, J., Wang, L., Chen, W., 2014. How does the East Asian summer monsoon behave in the decaying phase of El Niño during different PDO phases? J. Clim. 27, 2682–2698. Gray, W.M., 1975. Tropical Cyclone Genesis. Dept. Of Atmospheric Science Paper 234. Colorado State University, Fort Collins, CO 121 pp. Gray, W.M., 1984. Atlantic seasonal hurricane frequency. Part II: forecasting its variability. Mon. Wea. Rev. 112, 1669–1683. Gray, W.M., Landsea, C.W., Mielke, Jr.P.W., Berry, K.J., 1992. Predicting Atlantic seasonal hurricane activity 6–11 months in advance. Wea. Forecasting 7, 440–455. Gray, W.M., Landsea, C.W., Mielke, Jr.P.W., Berry, K.J., 1993. Predicting Atlantic basin seasonal tropical cyclone activity by 1 August. Wea. Forecasting 8, 73–86. Gray, W.M., Landsea, C.W., Mielke Jr., P.W., Berry, K.J., 1994. Predicting Atlantic basin seasonal tropical cyclone activity by 1 June. Wea. Forecasting 9, 103–115. Ho, C.H., Baik, J.J., Kim, J.H., Gong, D.Y., 2004. Interdecadal changes in summertime typhoon tracks. J. Clim. 17, 1767–1776. Ho, C.H., Kim, J.H., Kim, H.S., Sui, C.H., Gong, D.Y., 2005. Possible influence of the Antarctic Oscillation on tropical cyclone activity in the western North Pacific. J. Geophys. Res. 110, D19104. Kalnay, E., Kanamitsu, M., Kistler, R., et al., 1996. The NCEP/NCAR 40-year reanalysis project. Bull. Amer Meteor. Soc. 77, 437–471. Kistler, R., Kalnay, E., Collins, W., et al., 2001. The NCEP–NCAR 50-year reanalysis: monthly means CD-ROM and documentation. Bull. Amer Meteor. Soc. 82, 247–267. Liebmann, B., Smith, C.A., 1996. Description of a complete (interpolated) outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc. 77, 1275–1277. Liu, K.S., Chan, J.C.L., 2013. Inactive period of western North Pacific tropical cyclone activity in 1998-2011. J. Clim. 26, 2614–2630. https://doi.org/10.1175/JCLI-D-1200053.1. Mantua, N.J., Hare, S.R., Zhang, Y., Wallace, J.M., Francis, R.C., 1997. A Pacific interdecadal climate oscillation with impacts on salmon production. Bull. Amer. Meteoro. Soc. 78, 1069–1079. Neumann, C.J., 1993. global overview, in global guide to tropical cyclone forecasting. In: Holland, G.J. (Ed.), 1.1–1.56. Technical Document WMO/TC-No. 560, Report No. TCP31. World Meteorological Organization., Geneva. Park, J.K., Kim, B.S., Jung, W.S., Kim, E.B., Lee, D.G., 2006. Change in statistical characteristics of typhoon affecting the Korean Peninsula. Atmosphere 16, 1–17. Reynolds, R.W., Rayner, N.A., Smith, T.M., Stokes, D.C., Wang, W., 2002. An improved in situ and satellite SST analysis for climate. J. Clim. 15, 1609–1625. Wang, B., Chan, J.C.L., 2002. How strong ENSO events affect tropical storm activity over the western North Pacific. J. Clim. 15, 1643–1658. Wang, H.J., Fan, K., 2007. Relationship between the Antarctic Oscillation in the western North Pacific typhoon frequency. Chin. Sci. Bull. 52, 561–565. Wang, H.J., Sun, J.Q., Fan, K., 2007. Relationships between the North Pacific Oscillation and the typhoon/hurricane frequencies. Sci. China Ser. D Earth Sci. 50, 1409–1416. Wang, B., Yang, Y., Ding, Q.H., Murakami, H., Huang, F., 2010. Climate control of the global tropical storm days (1965–2008). Geophys. Res. Lett. 37, L07704. https://doi.org/ 10.1029/2010GL042487. Wilks, D.S., 1995. Statistical Methods in the Atmospheric Sciences. Academic Press 467 pp. Wu, M.C., Chan, W.L., Leung, W.M., 2004. Impacts of El Niño-Southern Oscillation events on tropical cyclone landfalling activity in the western North Pacific. J. 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Negative relationship between Korea landfalling tropical cyclone activity and Pacific Decadal Oscillation Jae-Won Choia, Hae-Dong Kimb, a b
T
⁎
College of Atmospheric Sciences, Nanjing University of Information Science and Technology, Nanjing, 210044, China Department of Global Environment, Keimyung University, Daegu, Republic of Korea
A R T IC LE I N F O
ABS TRA CT
Keywords: Korea Tropical cyclone Pacific Decadal Oscillation
The present study discovered a strong negative correlation between Korea-landfalling tropical cyclone (TC) frequency and Pacific Decadal Oscillation (PDO) in the summer. Thus, the present study selected years that had the highest PDO index (positive PDO years) and years that had the lowest PDO index (negative PDO years) to analyze a mean difference between the two phases in order to determine the reason for the strong negative correlation between the two variables. In the positive PDO years, TCs were mainly generated in the southeastern part of the western North Pacific, and lower TC passage frequency was found in most regions in the mid-latitude in East Asia. Moreover, a slightly weaker TC intensity than that in the negative PDO years was revealed. In order to determine the cause of the TC activity revealed in the positive PDO years, 850 hPa and 500 hPa stream flows were analyzed first. In the mid-latitude region in East Asia, anomalous huge cyclonic circulations were strengthened, while anomalous anticyclonic circulations were strengthened in the low-latitude region. Accordingly, Korea was being influenced by anomalous northwesterlies, which played a role in blocking TCs from moving northward to Korea. The results of analysis on 850 hPa air temperature, precipitation, 600 hPa relative humidity, and sea surface temperature (SST) showed that negative anomalies were strengthened in the northwest region in the western North Pacific while positive anomalies were strengthened in the southeast region. The atmospheric and oceanic environments were related to frequent occurrences of TCs in the southeast region in the western North Pacific during the positive PDO years. All factors of air temperature, precipitation, 600 hPa relative humidity, and SST revealed negative (positive for vertical wind shear) anomalies near Korea, so that atmospheric and oceanic environments were formed that could rapidly weaken TC intensity, even if the TCs moved northward to Korea in the positive PDO years.
1. Introduction Climatologically, tropical cyclones (TCs) have a higher frequency in the western North Pacific than in any other typhoon basin (Neumann, 1993). As a result, nations surrounding the western North Pacific experience considerable loss of life and property as a result of TCs landing every year. A large number of studies have been conducted to understand the characteristics of TCs and to predict their occurrence. Until now, the most remarkable variable to understanding the characteristics of TC genesis is El Niño–Southern Oscillation (ENSO) (Wang and Chan, 2002; Chu, 2004; Wu et al., 2004). In general, TCs in El Niño years occur mainly in the southeast sea area in the western North Pacific. They then change direction towards the eastern sea of China or land in Korea and ⁎
Corresponding author. E-mail address: [email protected] (H.-D. Kim).
https://doi.org/10.1016/j.dynatmoce.2019.101100 Received 28 November 2018; Received in revised form 10 June 2019; Accepted 11 July 2019 Available online 12 July 2019 0377-0265/ © 2019 Published by Elsevier B.V.
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Japan, which are located in the mid-latitude in East Asia. By contrast, TCs in La Niña years occur in the northwest sea area in the western North Pacific, followed by a change of direction to the west or northwest so that TCs land mainly in the Philippines, South China, or the Indochina Peninsula. Therefore, TCs in El Niño years exhibit a longer life and stronger intensity than TCs during La Niña years. Gray (Gray, 1975, 1984; Gray et al., 1992, 1993; Gray et al., 1994) discovered six important physical parameters for TC occurrence. These are (i) lower-level relative vorticity, (ii) local or planetary vorticity (Coriolis parameter), (iii) inverse of the vertical shear of the horizontal wind between the lower and upper troposphere, (iv) ocean thermal energy due to temperatures above 26.8 °C to a depth of 60 m, (v) vertical gradient of equivalent potential temperature between the surface and 500 hPa, and (vi) middle troposphere relative humidity. Because Gray’s parameters reflect the seasonal characteristics of TC activity, they have been used in statistical models for seasonal prediction of TC genesis. Using Gray’s troposphere parameters in the TC genesis region and ENSO, the characteristics of TC genesis have been studied successfully in a number of studies. However, TC activity in lower and mid-latitude regions is affected not only by environmental factors in the tropical region, but also by interaction with various teleconnection patterns that are present outside the tropical region. As a result, it is critically important to disclose a relationship between teleconnection patterns and TC activity by finding a signal of patterns that affect TC activity. Chan et al. (1998) and Chen et al. (2001) focused on factors of teleconnection patterns rather than Gray’s parameters. That is, they discussed the characteristics of TC activity by using (i) sea surface temperature anomalies in the central and eastern Pacific, (ii) index of characteristics of atmospheric circulations in the western Pacific and Asia from April in the previous year to March in the current year, and (iii) atmosphere and oceanic parameters, including an index that represents atmospheric circulations in Australia and the South Pacific as well as annual trends of changes in TC activity (climatology and persistence). Ho et al. (2005) proved an increase in TC activity near the East China Sea as an anomalous anticyclone was strengthened at the mid-latitude in both hemispheres in the positive Antarctic Oscillation (AAO) phase, which was generated in the Southern Hemisphere. Wang and Fan (2007) discovered that negative correlations exist between the AAO from June to September and the TC genesis frequency in the western North Pacific. They demonstrated through this study that, in the positive AAO phase, an environment that is unfavorable to the occurrence of TCs is created in the subtropical western Pacific, such as increased vertical wind shear, the development of an atmospheric vertical structure of high pressure in the lower layers and low pressure in the upper layers, and lowered sea surface temperature. Meanwhile, Wang et al. (2007) investigated the relations between the North Pacific Oscillation (NPO) and the TC genesis frequency in the western North Pacific and the subtropical Atlantic during the June-September period. As a result, they suggested that the TC genesis frequencies in the former and latter seas exhibit positive and negative correlations with the NPO respectively, and changes in TC genesis frequency between the two seas are realized through teleconnection patterns. Choi et al., 2010a, 2010b showed a high frequency of TC genesis as the monsoon trough was strengthened in the subtropical western Pacific whereas an anomalous anticyclone was strengthened in the eastern sea of Japan during a positive Pacific-Japan teleconnection pattern. They also reported a trend of increased TC activity in the mid-latitude region in East Asia and a decrease in TC activity near the Philippines and the South China Sea due to the anomalous pressure system patterns. Furthermore, Choi et al. (2012) claimed that the anomalous anticyclone was strengthened in the mid-latitude region in East Asia in the positive Arctic Oscillation (AO) phases whereas the monsoon trough was developed in the western North Pacific, thereby increasing TC activities in the mid-latitude region in East Asia as well as TC genesis frequency. Choi et al. (2013) demonstrated the influence of the Arctic Oscillation on the TC activity around Taiwan and Choi et al. (2017) showed interdecadal variation of tropical cyclone genesis frequency in late season over the western North Pacific. As for studies on Korea-landfalling or affecting TC activity, Choi and Kim (2007) showed that the frequency of Korea-landfalling TCs has been increasing since the mid-1990s, and that the increase in the affecting frequency of strong TCs is particularly noticeable. They deduced that this is because the frequency of TCs passing Mainland China before landing on Korea has decreased as the TC track shifts eastward due to the eastward retreat of western North Pacific subtropical high (WNPSH). In addition, Choi et al., 2010a, 2010b applied the statistical change-point analysis which was used in the study of Ho et al. (2004) to the frequency change of Korealandfalling TCs. Their results showed that the analysis period of 54 years (1951–2004) could be divided into three periods, and emphasized that the affecting frequency of strong TCs was highest in the recent period. The recent increasing of the affecting frequency of strong TCs is also related to the eastward retreat of WNPSH. Park et al. (2006) showed that this increasing trend is also noticeable for the frequency of Korea-affecting TCs. In particular, Choi and Kim (2011) applied statistical change-point analysis to the frequency of TCs affecting Korea, and showed that their frequency has been increasing since the early 1980s. These two studies also stressed that the recent increasing of the affecting or landfalling frequency of TCs is associated with the zonal movement of WNPSH. Choi et al. (2015a) showed the possible relationship between SST in the equatorial eastern Pacific and TC frequency that affects Korea and Choi et al. (2015b) also showed the relationship between east Indian Ocean SST and tropical cyclone affecting Korea. The former study demonstrated that Korea affecting TC frequency is related to northeastward movement of Madden-Julian Oscillation (MJO) and the latter study stressed that Korea affecting TC frequency is associated with the zonal movement of WNPSH according to the development of ENSO. Pacific Decadal Oscillation (PDO), an important signal at decadal time scales, shifted from its negative to positive phase after the early 1980s. The intensity and frequency of ENSO events vary with respect to PDO, which controls the heat and dynamic state of the ocean and atmosphere on a multidimensional scale. The warm (cold) PDO phase can enhance (weak) El Nino phenomenon, increasing (reducing) the effects of heated water pools on the equatorial Pacific in the TC season through local diabatic heating. El Nino phenomenon accompanies stronger equatorial Walker circulation at the warm PDO phase than in the cold PDO phase. In contrast, the Walker circulation pattern associated with the La Nina event is less affected by the other PDO phase (Dong and Xiao (2016). PDO can be regarded as a leading factor that results in the decadal variation of TC frequency. Yoon and Yeh (2010) found that when ENSO and 2
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PDO are in (out of) phase, the Eurasian-like pattern acts to enhance (reduce) extratropic-related rainfall over northeast Asia, resulting in the strengthening (weakening) of the northeast Asian summer monsoon. Feng et al. (2014) showed that when ENSO is in (out of) phase with PDO, an anomalous tripolar (dipole) rainfall pattern exists in East China, while the WNPSH experiences a one-time (twotime) northward shift. They further found that, due to the phase transition of PDO, the life cycle of ENSO has changed, resulting in the different influence of ENSO on East Asian summer monsoons (EASMs) in different PDO phases. However, it is not easy to find studies on the relationship between Korea-affecting TC activity and PDO. Therefore, this study examined the relationship between the two under different PDO phases. PDO is an important climatic oscillation that has a significant influence on the Pacific climate (Mantua et al., 1997) and TC activity (Wang et al., 2010; Aiyyer and Thorncroft, 2011; Liu and Chan, 2013). Thus, it would be worthwhile to analyze a relationship between the frequency of Korea-landfalling TCs and PDO. In the present study, Section 0 describes the data and analysis method. Section 0 analyzes the relationship between Korealandfalling TC frequency and PDO. Section 0 identifies the relationship between the two variables, and Section 0 summarizes the study results. 2. Data and methods 2.1. Data For information on TC activities, the best-track data provided by the Regional Specialized Meteorological Center (RSMC)–Tokyo Typhoon Center were used. These data included the names of TCs, their latitudes and longitudes, central pressure, and maximum sustained wind speed (MSWS) at six-hour intervals. TCs are largely divided into four grades based on MSWS: tropical depression (TD, MSWS < 17 ms−1), tropical storm (TS, 17 ms−1s ≤ MSWS ≤ 24 ms−1), severe tropical storm (STS, 25 ms−1 ≤ MSWS ≤ 32 ms−1), and typhoon (TY, MSWS ≥ 33 ms−1). The analysis of this study also included those TCs that turned into extratropical cyclones, given the fact that TCs inflict substantial damage on the mid-latitude region of East Asia even after turning into extratropical cyclones. This study used the reanalysis data provided by the National Center for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) which has the following variables: geopotential height (gpm), zonal and meridional wind (ms−1), air temperature (°C), relative humidity (%), precipitation (mmday−1), total cloud cover (%), and velocity potential (m2s−110-6) (Kalnay et al., 1996; Kistler et al., 2001). These data have a spatial resolution of 2.5° × 2.5° for latitude and longitude and 17 vertical layers (8 layers for relative humidity and 1 for precipitation). This data are available from 1948 to the present. The National Oceanic and Atmospheric Administration’s (NOAA) Extended Reconstructed Sea Surface Temperature (SST) (Reynolds et al., 2002), available from the forenamed organization, was also used. The data have a horizontal resolution of 2.0° × 2.0° latitude-longitude and are available for the years 1854 to the present day. Also, the NOAA interpolated outgoing longwave radiation (OLR) data were retrieved from the NOAA satellite series. These data start from June 1974 and are available from NOAA’s Climate Diagnosis Center (CDC). However, the data are incomplete: they are missing the period from March to December of 1978. Detailed information about this OLR data can be found on the CDC’s website (http://www.cdc.noaa.gov) and in the study by Liebmann and Smith (1996). 2.2. Methods In order to calculate TC passage frequencies, each TC was calculated after being relocated within a 5° × 5° grid. Even if a TC passed over the same grid multiple times, it was regarded as a single passage. TC genesis frequencies were also calculated in the above manner. The independent two-sample t-test was used for the comparison of significance between two means in this study (Wilks, 1995). A Korea-landfalling TC is defined as one in which the TC’s center encounters the coastline of the Korean Peninsula, as reported on the surface weather chart in the 6 -hly RSMC-Tokyo Typhoon Center’s best-track dataset. Furthermore, the present study focused on Korea-landfalling TCs from July to September, so average data from July to September were used in the analysis. This is because most TCs that landed in Korea happened during these months, which accounts for three-quarters of the total, as presented in a study by Choi et al., 2010a, 2010b. In the present study, July to September is defined as the summer period. 3. Time series analysis of korea-landfalling TC frequencies and PDO Fig. 1 shows a time series of Korea-landfalling TC frequencies and average PDO index during the summer. The Korea-landfalling TC frequency showed a distinctive interannual variation, and the PDO index revealed distinctive interannual and interdecadal variations. In particular, a time series of the latter showed a rapid decrease in the index since the late 1990s, thereby revealing a climate regime shift after the late 1990s. An out-of-phase was also distinctively revealed between the two variables. Thus, -0.51 of a high negative correlation existed between the two variables. This correlation is statistically significant at the 99% confidence level. However, if a trend is removed from the two time series, a correlation between the two variables could be different. The Korealandfalling TC frequency had little change in trend, whereas the mean summer PDO index showed a rapidly decreasing trend. Therefore, a correlation was analyzed after removing the trend from the two variables. However, a high negative correlation had not changed between the two variables, even though the trend was removed from the two time series (r = -0.49, significant at the 99% confidence level). The PDO is significantly affected by the ENSO. Therefore, a partial correlation analysis was conducted to see 3
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Fig. 1. Time series of Korea landfalling TC frequency (solid line with a closed circle) and PDO index (dotted line with an open circle) for JAS and their trends. Table 1 Partial correlation analysis among Korea landfalling TC frequency, PDO index, and Niño-3.4 index for July-September (JAS). Control Variables None
a
Variables
Correlation/ Significance
TC freq.
PDO
Niño-3.4
TC freq.
Correlation Significance Correlation Significance Correlation Significance Correlation Significance Correlation Significance
1.00 · −0.51 0.002 −0.07 0.695 1.00 · −0.53 0.001
−0.51 0.002 1.00 · 0.45 0.007 −0.53 0.001 1.00 ·
−0.07 0.695 0.45 0.007 1.00 · · · · ·
PDO Niño-3.4 Niño-3.4
TC freq. PDO
a. Cells contain zero-order (Pearson) correlations. Table 2 Korea landfalling TC frequency in positive and negative PDO years. Red and blue years denote El Niño and La Niña years, respectively. Positive PDO years Year 1983 1987 1992 1993 1995 1997 2008 2009 2014 2015 Average
Negative PDO years TC freq. 0 1 2 0 1 1 0 0 1 1 0.7
Year 1994 1998 1999 2000 2002 2004 2010 2011 2012 2013 Average
TC freq. 4 1 3 3 2 2 2 2 3 0 2.2
whether a high negative correlation between two variables was due to the effect of the ENSO (Table 1). As explained above, a high positive correlation exists between mean summer PDO index and mean summer Niño-3.4 index. This correlation is statistically significant at the 99% confidence level. When a mean summer Niño-3.4 index was defined with control variables, a high negative correlation (-0.53) existed between Korea-landfalling TC frequency and mean summer PDO index. This correlation is rather higher than the previously analyzed correlation and it is significant at the 99% confidence level. However, the Pearson method is sensitive to outliers and thus this study performed Spearman correlation analysis again. As a result, we got a similar result with Pearson method (not shown). In order to determine the reason for the high negative correlation between the two variables, a 10-year period whose mean summer PDO index was the largest among 35 analysis years (hereafter referred to as positive PDO years), and 10-year period whose mean summer PDO index was the least (hereafter referred to as negative PDO years) were selected. A mean difference between the two phases was then analyzed (Table 2). These selected 20 years account for 60% of the total analysis period. The mean summer PDO index in the positive PDO years showed 0.7 or higher, while that in the negative PDO years showed -0.4 or lower. All years except for 1992 showed 1 TC or less in the positive PDO years, whereas all years except for 1998 and 2013 showed 2 TCs or more in the negative PDO years. Thus, a mean Korea-landfalling TC frequency in the positive PDO years was 0.7 TC, whereas that in the negative PDO years was 2.2 TCs, resulting in a 1.5-TC difference between the two phases. This difference was statistically significant at the 95%
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Fig. 2. Differences of (a) TC genesis frequency and (b) TC passage frequency between positive PDO years and negative PDO years in 5° × 5° latitude and longitude grid box for JAS. Small solid rectangles indicate that the differences are significant at the 95% confidence level. In (b), solid and dotted lines denote western North Pacific subtropical highs in positive and negative PDO years, respectively. Solid and dashed lines denoted WNPSH (5,875 gpm contour) in positive and negative PDO years, respectively.
Fig. 3. (a) TC central pressure and (b) TC maximum sustained wind speed. The boxes show the 25th and 75th percentiles, the lines in the boxes mark the median and the circles are values below (above) the 25th (75th) percentiles of distribution. The numbers to the right and left sides of the figure represent average values (cross marks) for positive PDO years and for negative PDO years, respectively.
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Fig. 4. Difference in (a) 850 hPa geopotential height and (b) sea surface temperature between positive and negative PDO years. Units are gpm in (a) and °C in (b). Hatched lines are significant at the 95% confidence level except for land areas in (b).
confidence level. 4. Differences between positive and negative PDO years 4.1. TC activity The present study investigated a difference in TC activity between the two phases to determine the cause of the high negative correlation between the two variables. TCs were generated mainly in the southeastern part of the western North Pacific during the positive PDO years overall, whereas TCs were generated in the northwestern part of the western North Pacific in the negative PDO years regarding TC genesis frequency in summer (Fig. 2a). This can be verified from the analysis result on means of TC genesis location between the two phases. A mean TC genesis location during the positive PDO years was 14.6 °N and 142.4 °E, while that in the negative PDO years was 16.4 °N and 135.0 °E, indicating that TCs in the positive (negative) PDO years were mainly generated in 6
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Fig. 5. Same as in Fig. 4, but for (a) outgoing longwave radiation, (b) total cloud cover, and (c) precipitation. Units are W m−2 in (a), % in (b) and mm day-1 in (c). Hatched lines are significant at the 95% confidence level.
the southeastern (northwestern) part of the western North Pacific. A latitude difference between the two phases is significant at the 90% confidence level and a longitude difference is significant at the 95% confidence level. A difference in TC passage frequency in summer between the two phases showed that TCs tended to land in the Indochina peninsula via the South China Sea or move northward toward the far eastern sea in Japan during the positive PDO phases (Fig. 2b). On the other hand, TCs tended to move north toward Korea via the East China Sea from the eastern sea in Taiwan during the negative PDO years. As analyzed above, a distinctive negative correlation exists between the frequency of Korea-landfalling TCs and mean 7
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Fig. 6. Same as in Fig. 4, but for (a) 850 hPa and (b) 500 hPa stream flows. Shaded areas are significant at the 95% confidence level.
summer PDO index. Negative values were also revealed in the eastern regions of China and Taiwan, in which such negative correlations may also be found in those regions. Regarding this, a study done by Chan et al. (2012) proved that there was a strong negative correlation between PDO index and frequency of TCs landing on the eastern coast of China. TC genesis location and TC passage frequency may influence TC intensity. Thus, TC intensity between the two phases was analyzed (Fig. 3). Here, TC intensity is defined as a TC central pressure (Fig. 3a) and TC MSWS (Fig. 3b) at the time of the TC landing in Korea in the 6 -h interval RSMC-Tokyo best-track dataset. Overall, TC intensity in the positive PDO years seems weaker than that in the negative PDO years. A mean TC central pressure in the positive PDO years was 7 hPa larger than in the negative PDO years, and TC MSWS was 8 knots slower on average in the positive PDO years than in the negative PDO years. Mean differences in TC central pressure and TC MSWS between the two phases are significant at the 95% confidence level. This result means that landed TC intensity in Korea was stronger (weaker) in the positive (negative) PDO years than in the negative (positive) PDO years.
4.2. Large-scale environments If a typical positive PDO phase is not revealed in a difference between the two phases prior to determining the reason for the high negative correlation between the two variables, then subsequent analysis becomes meaningless. Thus, the present study analyzed a difference in 850 hPa geopotential height and SST between the two phases (Fig. 4). The analysis results showed a positive anomaly in the Arctic regions, while a negative anomaly was found in the mid-latitude regions. A positive anomaly was also located in the lowlatitude region (Fig. 4a). In particular, a strong negative anomaly was formed in the North Pacific. This was a typical anomalous pressure system pattern that was revealed in the positive PDO phase (Mantua et al., 1997). A difference in SST between the two phases showed that a strong warm anomaly was revealed from the equatorial eastern Pacific to the equatorial central Pacific, whereas a strong cold anomaly was exhibited in the North Pacific. This was a typical spatial distribution of SST that was revealed in the positive PDO phase (Mantua et al., 1997). A difference in OLR between the two phases was analyzed to determine a level of convective activity (Fig. 5a). A negative anomaly was developed in the east-west direction from the eastern sea of China to the far eastern sea of Japan via Korea. In contrast, a positive anomaly was revealed in a latitude zone of 10°-20 °N. The positive anomaly was stronger in the northwest area of the subtropical western North Pacific than in other areas. A negative anomaly was again located in a latitude zone of 0°-10 °N. The negative anomaly was more distinctive in the southeast area of the subtropical western North Pacific than in other areas. Because OLR is larger (smaller), it means convection is weaker (stronger). Thus, the above result showed that convective activity was stronger (weaker) in the southeast (northwest) area of the subtropical western North Pacific in the positive PDO years. This result revealed a trend that TCs were generated mainly in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years as analyzed above. A difference in total cloud cover between two the phases showed an opposite of spatial distribution in the OLR result (Fig. 5b). That is, a negative anomaly was revealed in the northwest area of the subtropical western North Pacific in the 8
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Fig. 7. Same as in Fig. 4, but for (a) 850 hPa and (b) 200 hPa horizontal divergences. Contour interval is 2s−1*107. Shaded areas are significant at the 95% confidence level.
positive PDO years, whereas a positive anomaly was seen in the southeast area. A difference in precipitation between the two phases based on the above results of OLR and total cloud cover showed that precipitation was small in the northwest area of the subtropical western North Pacific in the positive PDO years, whereas precipitation was large in the southeast area (Fig. 5c). A difference in 850 hPa and 500 hPa stream flows between the two phases was analyzed to determine the status of atmospheric circulations that affected a negative correlation between the Korea-landfalling TC frequency and mean summer PDO (Fig. 6). The analysis results on the 850 hPa stream flows showed that anomalous huge cyclonic circulations were strengthened between 30 °N and 50 °N, whereas anomalous huge anticyclonic circulations were strengthened in the zone south of 30 °N (Fig. 6a). A strong cold SST anomaly was located in the North Pacific because of the anomalous huge cyclonic circulations strengthened in the 30°-50 °N zone as analyzed above. Due to the anomalous pressure system pattern in the south-north direction, the Korean Peninsula was affected by anomalous northwesterlies. These northwesterlies played a role as anomalous steering flows that blocked the TCs from moving north toward Korea from low latitudes. Thus, the above result explains the reason for the negative correlation between the Korea-landfalling TC frequency and mean summer PDO index. The anomalous northwesterlies affected not only Korea but also the eastern sea of China and the southwestern region in Japan so that lower TC passage frequency was revealed in those regions in the positive PDO years. The anomalous westerlies were strengthened from the eastern Philippines Sea to the South China Sea so that TC passage frequency in those regions became high in the positive PDO years. The anomalous pressure system pattern that was revealed in the analysis result on 500 hPa stream flows was similar to the spatial distribution revealed in the analysis on 850 hPa stream flows (Fig. 6b). The anomalous huge cyclonic circulations were strengthened in a latitude zone of 30°-50 °N while the anomalous anticyclonic circulations were strengthened in a latitude zone of 10°-20 °N. The anomalous anticyclonic circulations that were strengthened in the east-west direction in the subtropical western North Pacific meant that the WNPSH was strengthened in the west direction. Therefore, the present study analyzed the condition of the WNPSH between the two phases (Fig. 2b). Here, the WNPSH was defined as a region which has more than 5875 gpm at 500 hPa. The WNPSH 9
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Fig. 8. Same as in Fig. 4, but for (a) 200–850 hPa vertical wind shear, (b) 850 hPa relative vorticity, and (c) 600 hPa relative humidity. Contour intervals are 1 ms−1 for vertical wind shear, 2-6s-1 for 850 hPa relative vorticity, and 2% for 600 hPa relative humidity. Shaded areas are significant at the 95% confidence level.
in the positive PDO years was not developed in the north but rather developed in the west up to the South China Sea as well as in the equatorial direction (solid line in Fig. 2b). By contrast, the WNPSH in the negative PDO years was weakened further east than in the positive PDO years (dashed line in Fig. 2b). In general, TCs tended to move along the western periphery in the WNPSH. Thus, TCs in 10
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Fig. 9. Same as in Fig. 4, but for (a) 850 hPa and (b) 500 hPa air temperatures. Contour interval is 0.2 °C. Shaded areas are significant at the 95% confidence level.
each phase moved along the west side of the WNPSH so that TC track and west boundary of the WNPSH were matched well. As discussed above, a difference in 850 hPa and 200 hPa horizontal divergences between the two phases was analyzed to determine the cause of the difference in TC genesis location between the two phases (Fig. 7). A positive anomaly was exhibited in the northwest area of the western North Pacific in the 850 hPa horizontal divergence, whereas a negative anomaly was revealed in the southeast area (Fig. 7a). This means that anomalous divergence was strengthened in the northwest area of the western North Pacific, whereas anomalous convergence was strengthened in the southeast area in the positive PDO years. A negative anomaly was exhibited in the northwest area of the western North Pacific in the 200 hPa horizontal divergence, whereas a positive anomaly was revealed in the southeast area (Fig. 7b). This means that anomalous convergence was strengthened in the northwest area of the western North Pacific whereas anomalous divergence was strengthened in the southeast area in the positive PDO years. Thus, anomalous downward flows were strengthened in the northwest area of the western North Pacific whereas anomalous upward flows were strengthened in the positive PDO years considering both the upper and lower troposphere. This result revealed the reason why TCs were generated mainly in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years as analyzed above. A difference in 200–850 hPa vertical wind shear between the two phases showed that a strong positive anomaly was located in the latitude zone of 30°-40 °N (Fig. 8a). In general, as the value of vertical wind shear becomes larger, a more unfavorable environment against TC development is provided. This strong positive anomaly at the mid-latitude in East Asia during the positive PDO years makes TC intensity weaker when TCs land in Korea. A difference in 850 hPa relative vorticity between the two phases revealed that a negative anomaly was strengthened in the northwest area of the western North Pacific while a positive anomaly was strengthened in the southeast area (Fig. 8b). This result revealed why TCs were generated mainly in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years as analyzed above. A difference in 600 hPa relative humidity between the two phases showed that a negative anomaly was strengthened from Korea up to the far eastern sea of Japan (Fig. 8c). This is the reason why TC intensity at the time of landing in Korea became weak in the positive PDO years.
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Fig. 10. (a) Distribution of 200 hPa jet streak in positive (solid line) and negative PDO years (dashed line) and (b) a difference in 200 hPa zonal wind speed between positive and negative PDO years. Jet streaks are defined as areas that 200 hPa zonal wind speed is greater than 25ms−1. In (b) contour interval is 1 ms−1 and shaded areas are significant at the 95% confidence level.
A difference in SST between the two phases showed that a cold anomaly that developed strongly in the North Pacific became an important oceanic environment that weakened TC intensity when TCs landed in Korea (Fig. 4b). Furthermore, a cold anomaly was strengthened in the northwest area of the western North Pacific and a warm anomaly was strengthened in the southeast area. A difference in 850 hPa and 500 hPa air temperatures between the two phases revealed a cold anomaly in the north region of 30 °N including Korea (Fig. 9). A difference in 200 hPa air temperature between the two phases also showed a cold anomaly in the East Asia continent including Korea (not shown). Thus, a cold anomaly was strengthened in all layers in the troposphere over Korea so that TC intensity can be weakened during TCs landing in Korea. A positive vertical wind shear anomaly, which was developed strongly in the mid-latitude region in East Asia in the positive PDO years, meant that a zonal wind speed is stronger in the upper layer than in the lower layer in the troposphere. Thus, a 200 hPa jet streak during the two phases was analyzed (Fig. 10a). Here, a jet streak was defined as a region where 200 hPa zonal wind speed was larger than 25 ms−1. A jet streak in the upper troposphere in the positive PDO years was developed strongly in a latitude zone of 35°45 °N, whereas a jet streak was only revealed in some parts in the west and east of the mid-latitude in East Asia among the analysis regions in the negative PDO years. A jet in the upper troposphere makes the vertical structure of a TC unstable, thereby weakening TC intensity. Therefore, a jet streak in the upper troposphere, which was strengthened in the mid-latitude in East Asia in the positive PDO years, can not only weaken Korea-landfalling TC intensity but also lower their frequency by rapidly moving TCs in the northeast (Japan) direction. In order to determine the characteristics of global-scale atmospheric circulation, a difference in 850 hPa and 200 hPa divergent winds and velocity potentials between the two phases was analyzed (Fig. 11). In the 850 hPa divergent wind and velocity potential, an anomalous divergent wind was strengthened in the northwest area of the western North Pacific, whereas an anomalous convergent wind was strengthened in the northeast area (Fig. 11a). In the 200 hPa divergent wind and velocity potential, a spatial distribution opposite to the result of the 850 hPa divergent wind and velocity potential was exhibited (Fig. 11b). Therefore, when considering the 12
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Fig. 11. Same as in Fig. 4, but for (a) 850 hPa and (b) 200 hPa divergent wind and velocity potential. Shaded areas denote negative anomalies. Contour interval is 3 m2s−110-6.
upper and lower troposphere, anomalous atmospheric circulations were developed, in which air that was converged and elevated in the southeast area of the western North Pacific in the positive PDO years was converged and descended in the southeast area of the western North Pacific. 5. Summary and conclusion The present study analyzed a correlation between Korea-landfalling TC frequency in summer and PDO and presented a strong negative correlation between the two variables. For this purpose, the study selected 10 years that had the highest PDO index (positive PDO years) and 10 years that had the lowest PDO index (negative PDO years) to analyze a mean difference between the two phases in order to determine the reason for the strong negative correlation between the two variables. TCs were mainly generated in the southeast area of the western North Pacific in the positive PDO years, and slightly lower TC passage frequency was revealed in the mid-latitude region in East Asia including Korea than in other regions, whereas a high frequency was exhibited from the eastern sea of the Philippines to the South China Sea. Moreover, a slightly weaker TC intensity than that in the negative PDO years was revealed. In order to determine the cause of the TC activity revealed in the positive PDO years, 850 hPa and 500 hPa stream flows were analyzed first. In the mid-latitude region in East Asia, anomalous huge cyclonic circulations were strengthened while anomalous anticyclonic circulations were strengthened in the low-latitude region. Due to the anomalous pressure system pattern, Korea was influenced by anomalous northwesterlies, which played a role as anomalous steering flows that blocked TCs from moving northward to Korea. In the analysis on the 850 hPa and 200 hPa horizontal divergence, anomalous downward flows were strengthened in the northwest area of the western North Pacific while anomalous upward flows were strengthened in the southeast area. Furthermore, the analysis result on 850 hPa air temperature, precipitation, 600 hPa relative humidity, and SST showed that negative anomalies were strengthened in the northwest region in the western North Pacific while positive anomalies were strengthened in the southeast 13
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region. The atmospheric and oceanic environments were related to frequent occurrence of TCs in the southeast region in the western North Pacific during the positive PDO years. All data on air temperature, precipitation, 600 hPa relative humidity, and SST revealed negative (positive for 200–850 hPa vertical wind shear) anomalies near Korea. This means that atmospheric and oceanic environments were formed that can rapidly weaken TC intensity even if TCs moved northward to Korea in the positive PDO years. The analysis result on the 200 hPa jet streak showed an environment that made it difficult for TCs to enter Korea, due to the strong jet effect that was formed in the positive PDO years. TC intensity can be rapidly weakened due to the strong jet even if TCs entered the Peninsula. To determine the characteristics of the global-scale atmospheric circulations, a difference in 850 hPa divergent wind between the two phases was analyzed. The analysis found that an anomalous convergent wind was strengthened in the southeast area of the western North Pacific while an anomalous divergent wind was strengthened in the northwest area. This result can be related to more frequent TCs in the southeast (northwest) area of the western North Pacific in the positive (negative) PDO years than in the negative (positive) PDO years. For future study, PDO will be investigated on whether it can act as a main predictor in the statistical model for seasonal prediction of Korea-landfalling TC activity. Acknowledgement This paper is supported by The Startup Foundation for Introducing Talent of NUIST (Grant/Award number: 2018r059). References Aiyyer, A., Thorncroft, C., 2011. Interannual-to-multidecadal variability of vertical shear and tropical cyclone activity. J. Clim. 24, 2949–2962. https://doi.org/10.1175/ 2010JCLI3698.1. Chan, J.C.L., Liu, K.S., Xu, M., Yang, Q., 2012. 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