Circulation features associated with the record-breaking typhoon landfall on Japan in 2004

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L14713, doi: /2005gl022494, 2005 Circulation features associated with the record-breaking typhoon landfall on Japan in 2004 Joo-Hong Kim, 1,2 Chang-Hoi Ho, 1 and Chung-Hsiung Sui 2 Received 20 January 2005; revised 24 May 2005; accepted 8 June 2005; published 30 July [1] Ten typhoons struck Japan in 2004, which was an alltime high although the total number of typhoons formed over the western North Pacific was slightly above normal. The characteristics of typhoon activity are the unusually high number of typhoons approaching Japan in the early summer (June) and fall (September and October) and the frequent landfalls in the middle summer (July and August). Seasonal mean large-scale circulation in 2004 was characterized by a split of the North Pacific subtropical high (NPSH) east of Taiwan and persistent anticyclonic anomalies to the southeast of Japan, enabling typhoons to penetrate the weakened NPSH and move to Japan. Two possible causes are suggested here to maintain the persistent anticyclonic anomalies near Japan: one is positive feedback between typhoons moving northward and midlatitude circulation near Japan, the other is response to the broadscale tropical deep convection. A further modeling study is required to substantiate these arguments. Citation: Kim, J.-H., C.-H. Ho, and C.-H. Sui (2005), Circulation features associated with the record-breaking typhoon landfall on Japan in 2004, Geophys. Res. Lett., 32, L14713, doi: /2005gl Introduction 1 School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea. 2 Institute of Hydrological Sciences, National Central University, Jhongli, Taiwan. Copyright 2005 by the American Geophysical Union /05/2005GL [2] In the western North Pacific (WNP), about threefourths of the annual number (27) of tropical cyclones (maximum sustained wind speed 17 m s 1, locally known as typhoons, a convention we shall follow hereafter) occur during the typhoon season (June to October) [Ho et al., 2004]. During this season, the WNP warm pool is expanded to the north and the axis of the monsoon trough also migrates further north than in other periods, giving rise to favorable conditions for typhoon development over the vast area of the tropical WNP basin. The North Pacific subtropical high (NPSH) bounded to the north of the monsoon trough also experiences seasonal migration showing its annual northernmost position in August. All of these seasonal migratory features discussed above are associated with the variability of typhoon tracks as well as their formation locations [Lander, 1994a]. [3] During the typhoon season, climatologically, the prevailing middle- and upper-tropospheric easterlies extend into 20 N west of 150 E [Miller et al., 1988]. This is associated with the development of the Tibetan High in the upper-troposphere and the monsoon trough in the lowertroposphere. The middle- and upper-tropospheric zonal flow around a typhoon is recognized as one of the important factors for determining whether typhoons recurve or preserve their moving direction [Hodanish and Gray, 1993]. Because the easterlies intrude into northern Philippine Sea and the East China Sea, many typhoons are able to march into these areas preserving their initial northwestward movement and then recurve to the northeast as they meet the midlatitude westerlies. Accordingly, many typhoons move to Korea and Japan, causing severe damage [Saunders et al., 2000]. [4] The statistics of typhoons released by the Korean Meteorological Administration indicate that 3 4 typhoons have nationwide influence every typhoon season, but less than one typhoon actually strikes the nation. For Japan, on the other hand, 10.7 typhoons approached within a 300 km distance from the coastline, and 2.8 typhoons among them actually landed on Japan during the typhoon season for the period A statistical survey of the historical record (Table 1) shows that the number of typhoons making landfall on Japan during June through October is in the range of 2 4 with a standard deviation 1.4 and an extreme number 6 in one year only, i.e The year 2004, however, encountered a record-breaking 10 typhoon landfalls, which is about four times as many as indicated by the climatology (2.8). It is an extraordinary year from a statistical point of view. In this study, we attempt to find causal evidence for the events of this unusual year. 2. Data and Methodology [5] In this study, we use a typhoon best track dataset archived by the Regional Specialized Meteorological Centers (RSMC) Tokyo Tropical Cyclone Center which covers 54 years from 1951 to In addition, we use the daily mean horizontal wind and geopotential height at standard pressure levels derived from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data [Kalnay et al., 1996]. [6] Though the typhoon track is largely dependent upon the transient synoptic flow, there is an apparent signal in the longer timescale climatic fluctuations, i.e. low-frequency variability of the atmospheric state. This kind of consideration is very common and has already been applied to many previous studies of variations of typhoon activity in association with the El Niño/Southern Oscillation (ENSO) [Lander, 1994b; Chen et al., 1998; Chan, 2000; Wang and Chan, 2002], the Madden-Julian Oscillation [Maloney and Hartmann, 2001], and so on. [7] In the present study, we first examine the slowly evolving circulation pattern that determines the typhoon L of5

2 Table 1. Historical Record of the Number of Typhoon Landfalls on Japan During the Typhoon Season for the Period Number of Landfalls Number of Years Total Sum 146 Mean 2.8 Standard deviation 1.4 movement toward Japan and the associated climate forcing mechanisms. Then, the impact of typhoons on the midlatitude circulation is further examined through a composite study of the atmospheric fields associated with those typhoons striking Japan. 3. Characteristics in Typhoon Activity [8] Figure 1 depicts the RSMC best tracks of 21 typhoons formed during the typhoon season in Ten typhoons among them made landfall on Japan. These landfalling typhoons, with the exception of Namtheun (0410) and Malou (0411), all underwent a recurving path in the region N, E and approached Japan with a northeastward movement. Interestingly, none of them landed on Korea even though some of them recurved in relatively higher latitudes. Also, all typhoons except Ma-on (0422) landed on the western territory of Japan. [9] As seen in the figure, the genesis locations (starting points) of the 10 typhoons are scattered, consisting of nine in the North Pacific Ocean, and one in the South China Sea. According to the ENSO monitoring by the Climate Prediction Center/NCEP, positive sea surface temperature anomalies (SSTA) persisted in the NINO4 region during the typhoon season of 2004, which is representative of the early stage of a warm (El Niño) episode. As a result, equatorial westerlies extended further east so that the relative vorticity in the southeastern region of WNP was stronger (Figure 1). This provided a higher potential for typhoon formation in the WNP. [10] Table 2 shows the monthly distribution of typhoon landfalls and approaches to Japan, relative to total formation during the typhoon season. According to the variation of the large-scale mean flows, we divide the typhoon season into three periods, i.e. early summer (June), middle summer (July and August), and fall (September and October). In the early summer, five typhoons formed in the WNP, three of them approached Japan, and two made landfall on Japan. All these numbers were regarded extremely rare compared to the climatology. During the middle summer, while total formation was near normal, the number approaching Japan was more frequent than the climatology, resulting in an unusually high number of landfalls (4 vs. the mean number 1.5). During the fall, again Japan experienced an unusually high number of typhoon landfalls (4) even though the typhoon formation was relatively suppressed in the WNP. In fact, most typhoons formed during the fall of 2004 headed for Japan. Summing up, the characteristics of typhoon activity in 2004 are as follows: a higher number of typhoons approaching Japan at all times, record-breaking typhoon formation and landfall in the early summer, and an unusually high number of landfall in spite of suppressed typhoon activity in the fall. Then, why did many typhoons approach Japan in 2004? To give a possible answer to this question, the large-scale environments which steer typhoons will be discussed henceforth Large-Scale Environments [11] To find the reason why most typhoons approached Japan in 2004, we examine the total fields of tropospheric layer-mean winds between 850 hpa and 300 hpa which are generally regarded as steering flows controlling typhoon movement (Figure 2). Also shown in Figure 2 are anomalous tropospheric layer-mean winds and 500-hPa geopotential heights to indicate where major changes appeared. Characteristic differences in the early summer (Figure 2a) from the climatology were a split of the NPSH east of Taiwan and a strengthened ridge around Japan. The former enabled typhoons to move northward and the latter prevented typhoons from recurving sharply, i.e. the chance Figure 1. RSMC best tracks of 21 typhoons formed during the typhoon season in Bold lines and black dots indicate tracks and formation locations of 10 typhoons striking Japan. Black contours indicate the 850-hPa relative vorticity anomalies during June October. Contour interval is s 1. Positive values are shaded. Table 2. Monthly Number of Typhoon Landfalls on Japan and Approaches Within a 300 km Distance From the Coastline of Japan, and Total Formation During the Typhoon Season a Landfall/Approach/Formation Early summer, Jun 2/3/5 0.2 ± 0.4/0.9 ± 0.9/1.7 ± 0.9 Middle summer Jul 1/2/2 0.5 ± 0.5/2.2 ± 1.4/3.9 ± 1.8 Aug 3/6/8 1.0 ± 0.7/3.5 ± 1.8/5.6 ± 1.9 Mid-summer total 4/8/ ± 1.1/5.6 ± 2.3/9.6 ± 2.8 Fall Sep 2/3/3 0.9 ± 0.8/2.7 ± 1.4/5.0 ± 1.6 Oct 2/3/3 0.2 ± 0.3/1.5 ± 0.8/3.9 ± 1.4 Fall total 4/6/6 1.1 ± 1.1/4.2 ± 1.6/8.8 ± 2.0 Total 10/17/ ± 1.4/10.7 ± 3.1/20.2 ± 4.3 a The values for represent the climatological means and standard deviations. 2of5

3 Figure 2. Total (thin solid lines) and anomalous (thick solid lines) fields of tropospheric layer-mean streamlines (between 850 hpa and 300 hpa) and anomalous 500-hPa geopotential heights (shaded; unit is gpm) during June (a), July through August (b), September through October (c), and June through October (d) in of hitting Japan increased. During the middle summer (Figure 2b), the NPSH seems to intrude more northward, so the geopotential height anomalies slightly increased near Japan. Typhoon movement toward Japan typically increases in this period, so four typhoon landfalls are not surprising even though the number is higher than normal. The flow pattern in the fall also showed a split of the subtropical high near 20 N, 130 E with an anticyclone over south China which typically forms in fall when the NPSH retreats eastward (Figure 2c). However, the NPSH near Japan was significantly stronger as compared with the climatology. This prevented typhoons from recurving sharply, resulting in a higher likelihood of a typhoon hitting Japan in the fall. Integrating over the season, the prominent characteristics of the seasonal mean pattern in 2004 were also a split of the subtropical high east of Taiwan and a strengthened northern fringe of the NPSH around Japan (Figure 2d), which made a strong southwesterly confluent flow toward Japan so that the typhoons frequently headed for Japan. 4. Discussion: Possible Mechanisms [12] What caused the circulation features shown in Figure 2? These may be partly attributed to the gross tropical heat sources in the WNP which generate a wave response in the midlatitudes crossing Japan as suggested by Nitta [1987]. Figure 3 shows anomalous fields of SST, 850-hPa winds, and outgoing longwave radiation (OLR) as a proxy for deep convection for three periods of the typhoon season. As expected, SSTA were positive in the tropical central Pacific and tropical convection was suppressed over the maritime continent throughout the typhoon season of 2004 with an enhanced suppression toward the late season, indicating the developing stage of a warm episode. In the early summer (Figure 3a), SSTA over 10 N 20 N, 140 E 170 E were also positive and deep convective activity was enhanced in the tropical western Pacific through the central Pacific. Correspondingly, strong low-level cyclonic circulation was found over the Philippine Sea. In response to the broadscale tropical convective forcing, suppressed convection and anticyclonic circulation anomalies developed in the midlatitudes where warm SSTA were evident due to more downward shortwave radiation [Yoo et al., 2004]. This kind of north-south pattern is a well-known Pacific-Japan (PJ) pattern [Nitta, 1987]. Record-breaking events of typhoon formation during this period are attributable to the strong cyclonic anomalies accompanied by widely organized deep convection in the WNP which provided a favorable condition for typhoon formation. In the middle summer (Figure 3b), warm SSTA developed near the equatorial central Pacific. As an equatorial wave response to the warm SSTA, low-level cyclonic circulation anomalies developed in the tropical WNP which provided favorable conditions for cyclogenesis. Indeed, in August, the formation of super typhoons Chaba (0416) and Songda (0418) east of 160 E seems to be influenced by El Niño. These two typhoons showed a long trail before recurving over the East China Sea and finally landed on Kyushu Island. A midlatitude wave response was also clear near Japan, i.e. the PJ pattern was still evident. Convective activity in the tropical WNP became weaker than that in the early summer, which was mainly due to the suppressed convective activity during July Figure 3. Anomalous fields of SST (contour), OLR (shaded), and 850-hPa winds (vector; unit is m s 1 ) during June (a), July through August (b), and September through October (c) in The SST and OLR contour intervals are 0.5 C and 10 W m 2, respectively. 3of5

4 to Japan, and its heating further intensifies/maintains the ridge, which guides the next typhoon to Japan and so on. This hypothesis needs further substantiation in the modeling study. Figure 4. Composite fields of the 500-hPa geopotential height anomalies for the 146 typhoons striking Japan for the period (top panels) and for the 10 typhoons striking Japan in 2004 (bottom panels) when typhoons pass 20 N (left panels) and 30 N (right panels). Shaded areas are statistically significant at the 95% confidence level. The contour interval is 5 gpm. (figure not shown). In the fall (Figure 3c), the SSTA in the equatorial central Pacific and Indian Ocean became warmer. Low-level divergence developed over the tropical WNP and the maritime continent accompanied by suppressed convective activity. The strengthened anticyclonic cell near south China shown in Figure 2c suppressed typhoon formation and also blocked the approach of typhoons which had developed over the Philippine Sea. Instead, convection was enhanced around 20 N, 130 E, indicating a frequent passage of typhoons there. There was also a wave train-like signal in Figure 3c represented by an anomalous cyclonic circulation near (20 N, 130 E), a strengthened ridge near Japan (35 N, 150 E) and a cyclonic center at (30 N, 170 E). [13] While the above discussions explain the unusual tracks of typhoons from the viewpoint of slow processes, the following discussions dwell on the possible effect of typhoons in maintaining the slow processes. In Figure 4, we present the composite patterns of the 500-hPa geopotential height anomalies when the typhoons pass 20 N and 30 N for all 146 and 10 typhoons which made landfall on Japan for the period (top panels) and 2004 (bottom panels), respectively. Interestingly, the plot shows that as typhoons move northward, they make a strong wave signal. This is easily understood in that the strong convective heating related to typhoons induces a thermal response of the atmosphere so that a Rossby wavetrain is produced [Hoskins and Karoly, 1981; Nitta, 1987]. Considering the location near Japan at (40 N, 140 E) a nodal point where positive geopotential height anomalies are preferentially intensified when typhoons reach 20 N 30 N, the sustained anomalous anticyclone near Japan shown in Figures 2 and 3 may be partly due to the frequent typhoon recurving in the region 20 N 30 N and approaching Japan. In this sense, we prudentially suggest a hypothesis of a mechanism of positive feedback between typhoon and midlatitude circulation here, i.e. a strong ridge near Japan guides a typhoon 5. Concluding Remarks [14] In the 2004 typhoon season, five typhoons formed in the early summer, which was remarkably high, but only six typhoons formed in the fall, which was below average. The two numbers offset each other, so the number of formations during the season was near normal. However, ten of the total number of typhoons made landfall on Japan, which is record-breaking. The prominent characteristics in the largescale circulation were a split of the NPSH near Taiwan and the intensified northern fringe of the NPSH near Japan. While the former allowed typhoons to go through the break in the NPSH and to move northward, the latter prevented them from recurving sharply so that they had a better chance of striking Japan. [15] We suggest two possible mechanisms for maintaining the sustained anomalous high near Japan at all times: one is thermal forcing related to typhoons, and the other is broad-scale tropical deep convection. Typhoons passing 20 N 30 N before striking Japan generate a strong wave response in the midlatitudes (Figure 4) that includes a sustained anticyclone near Japan. This mechanism appears more relevant in the fall since broad-scale tropical convection was rather suppressed. On the other hand, broad-scale deep convection looks more important in the early summer. A very strong low-level cyclonic cell prevailed over the tropical region, i.e. tropical WNP was in the convective phase of a tropical intraseasonal oscillation. Thus a remarkable number of typhoons formed in the WNP and the strengthened anticyclone in the midlatitudes in the early summer originated mainly from the widely organized deep tropical convection in the WNP. A further modeling study is necessary to determine conclusively what portion of the anomalous flow in 2004 was due to large-scale convective heating versus the influence of the typhoons themselves. [16] Acknowledgments. This study was performed for the Research and Development programs on Meteorology and Seismology funded by the Korean Meteorological Administration. 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