• Bowman, A. W., , and A. Azzalini, 1997: Applied Smoothing Techniques for Data Analysis: The Kernel Approach with S-Plus Illustrations. Oxford University Press, 193 pp.

  • Chan, J. C. L., 2000: Tropical cyclone activity over the western North Pacific associated with El Niño and La Niña events. J. Climate, 13, 29602972.

    • Search Google Scholar
    • Export Citation
  • Chan, J. C. L., 2005: The physics of tropical cyclone motion. Annu. Rev. Fluid Mech., 37, 99128.

  • Chand, S. S., , and K. J. E. Walsh, 2010: The influence of the Madden–Julian oscillation on tropical cyclone activity in the Fiji region. J. Climate, 23, 868886.

    • Search Google Scholar
    • Export Citation
  • Chen, T. C., , S. Y. Wang, , M. C. Yen, , and A. J. Clark, 2009: Impact of the intraseasonal variability of the western North Pacific large-scale circulation on tropical cyclone tracks. Wea. Forecasting, 24, 646666.

    • Search Google Scholar
    • Export Citation
  • Chia, H. H., , and C. F. Ropelewski, 2002: The interannual variability in the genesis location of tropical cyclones in the northwest Pacific. J. Climate, 15, 29342944.

    • Search Google Scholar
    • Export Citation
  • Gray, W. M., 1979: Hurricanes: Their formation, structure and likely role in the tropical circulation. Meteorology over the Tropical Oceans, D. B. Shaw, Ed., Royal Meteorological Society, 155–218.

  • Hall, J. D., , A. J. Matthews, , and D. J. Karoly, 2001: The modulation of tropical cyclone activity in the Australian region by the Madden–Julian oscillation. Mon. Wea. Rev., 129, 29702982.

    • Search Google Scholar
    • Export Citation
  • Harr, P. A., , and R. L. Elsberry, 1991: Tropical cyclone track characteristics as a function of large-scale circulation anomalies. Mon. Wea. Rev., 119, 14481468.

    • Search Google Scholar
    • Export Citation
  • Harr, P. A., , and R. L. Elsberry, 1995a: Large-scale circulation variability over the tropical western North Pacific. Part I: Spatial patterns and tropical cyclone characteristics. Mon. Wea. Rev., 123, 12251246.

    • Search Google Scholar
    • Export Citation
  • Harr, P. A., , and R. L. Elsberry, 1995b: Large-scale circulation variability over the tropical western North Pacific. Part II: Persistence and transition characteristics. Mon. Wea. Rev., 123, 12471268.

    • Search Google Scholar
    • Export Citation
  • Ho, C.-H., , J.-J. Baik, , J.-H. Kim, , D.-Y. Gong, , and C.-H. Sui, 2004: Interdecadal changes in summertime typhoon tracks. J. Climate, 17, 17671776.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471.

  • Kim, H. M., , P. J. Webster, , and J. A. Curry, 2011: Modulation of North Pacific tropical cyclone activity by three phases of ENSO. J. Climate, 24, 18391849.

    • Search Google Scholar
    • Export Citation
  • Kim, J., , C. Ho, , H. Kim, , C. Sui, , and S. K. Park, 2008: Systematic variation of summertime tropical cyclone activity in the western North Pacific in relation to the Madden–Julian oscillation. J. Climate, 21, 11711191.

    • Search Google Scholar
    • Export Citation
  • Li, R., , and W. Zhou, 2012: Changes in western Pacific tropical cyclones associated with the El Niño–Southern Oscillation cycle. J. Climate, 25, 58645878.

    • Search Google Scholar
    • Export Citation
  • Li, R., , and W. Zhou, 2013: Modulation of western North Pacific tropical cyclone activities by the ISO. Part I: Genesis and intensity. J. Climate, in press.

    • Search Google Scholar
    • Export Citation
  • Li, R., , W. Zhou, , J. Chan, , and P. Huang, 2012: Asymmetric modulation of western North Pacific cyclogenesis by the Madden–Julian oscillation under ENSO conditions. J. Climate, 25, 53745385.

    • Search Google Scholar
    • Export Citation
  • Liebmann, B., , and C. A. Smith, 1996: Description of a complete (interpolated) outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc., 77, 12751277.

    • Search Google Scholar
    • Export Citation
  • Madden, R. A., , and P. R. Julian, 1971: Detection of a 40–50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28, 702708.

    • Search Google Scholar
    • Export Citation
  • Mao, J. Y., , and J. C. L. Chan, 2005: Intraseasonal variability of the South China Sea summer monsoon. J. Climate, 18, 23882402.

  • Pohl, B., , and A. J. Matthews, 2007: Observed changes in the lifetime and amplitude of the Madden–Julian oscillation associated with interannual ENSO sea surface temperature anomalies. J. Climate, 20, 26592674.

    • Search Google Scholar
    • Export Citation
  • Ramsay, H. A., , L. M. Leslie, , P. J. Lamb, , M. B. Richman, , and M. Leplastrier, 2008: Interannual variability of tropical cyclones in the Australian region: Role of large-scale environment. J. Climate, 21, 10831103.

    • Search Google Scholar
    • Export Citation
  • Wang, B., , and J. C. L. Chan, 2002: How strong ENSO events affect tropical storm activity over the western North Pacific. J. Climate, 15, 16431658.

    • Search Google Scholar
    • Export Citation
  • Wu, M. C., , W. L. Chang, , and W. M. Leung, 2004: Impacts of El Niño–Southern Oscillation events on tropical cyclone landfalling activity in the western North Pacific. J. Climate, 17, 14191428.

    • Search Google Scholar
    • Export Citation
  • Zhang, C., , and J. Gottschalck, 2002: SST anomalies of ENSO and the Madden–Julian oscillation in the equatorial Pacific. J. Climate, 15, 24292445.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , and J. C. L. Chan, 2005: Intraseasonal oscillations and the South China Sea summer monsoon onset. Int. J. Climatol., 25, 15851609.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , and J. C. L. Chan, 2007: ENSO and South China Sea summer monsoon onset. Int. J. Climatol., 27, 157167.

  • Zhou, W., , C. Y. Li, , and X. Wang, 2007a: Possible connection between Pacific oceanic interdecadal pathway and East Asian winter monsoon. Geophys. Res. Lett., 34, L01701, doi:10.1029/2006GL027809.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , X. Wang, , T. J. Zhou, , C. Li, , and J. C. L. Chan, 2007b: Interdecadal variability of the relationship between the East Asian winter monsoon and ENSO. Meteor. Atmos. Phys., 98, 283293.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    (a) Meridional average (5°–20°N) of 30–60-day filtered OLR anomalies (shading; W m−2) together with the cyclogenesis locations for a complete MJO cycle. (b) Meridional average (5°–20°N) of 10–20-day filtered OLR anomalies (shading; W m−2) together with the cyclogenesis locations for a complete QBWO cycle.

  • View in gallery

    (a) Five common TC landfall areas: China, Japan, the Philippines, South Korea, and Vietnam. (b) Climatological TC activity (10−2 counts day−1) during the TC season in the WNP.

  • View in gallery

    (left) TC tracks, (middle) TC activity anomalies, and (right) the probability density estimate (PDE) of TC occurrences for different MJO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8. The numbers in parentheses denote the total number of TCs during different phases. Solid (dashed) contours in the middle panel represent positive (negative) anomalies of TC activity, while regions statistically significant at 90% confidence are shaded. Contours in the right panel give the percentages of TC occurrences (i.e., the contour labeled 75 means that 75% of the TC occurrences are within the contour).

  • View in gallery

    TC tracks and landfall locations for different MJO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8. The numbers in parentheses denote the total number of landfalls, while the colors of the triangles represent different landfall locations (red—China, orange—the Philippines, yellow—South Korea, green—Vietnam, and purple—Japan).

  • View in gallery

    (left) TC tracks, (middle) 850-hPa streamlines and 30–60-day filtered vorticity anomalies (10−6 s−1), and (right) 500-hPa geopotential height anomalies (m) for different MJO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8. Dashed purple contours in the right panel denote the climatological 5870 geopotential height, while solid black contours denote the 5870 geopotential height for different MJO phases. Only anomalies exceeding 95% confidence based on the Student’s t test are shown.

  • View in gallery

    As in Fig. 3, but showing the (left) TC tracks, (middle) TC activity anomalies, and (right) PDE of TC occurrences for different QBWO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8.

  • View in gallery

    As in Fig. 4, but showing the TC tracks and landfall locations for different QBWO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8.

  • View in gallery

    As in Fig. 5, but showing the (left) TC tracks, (middle) 850-hPa streamlines and 10–20-day filtered vorticity anomalies (10−6 s−1), and (right) 500-hPa geopotential height anomalies (m) for different QBWO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8.

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Modulation of Western North Pacific Tropical Cyclone Activity by the ISO. Part II: Tracks and Landfalls

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  • 1 Guy Carpenter Asia–Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong, China
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Abstract

This study investigates how tropical cyclone (TC) tracks and landfalls are modulated by the two major components of the intraseasonal oscillation (ISO), the 30–60-day Madden–Julian oscillation (MJO) and the 10–20-day quasi-biweekly oscillation (QBWO). In the convective phases of the MJO (phases 7 + 8 and 1 + 2), the western North Pacific Ocean (WNP) is mainly clustered with westward- and northwestward-moving TCs. The strong easterlies (southeasterlies) in the southern flank of the subtropical high lead to an increase in TC activity and landfalls in the Philippines and Vietnam (China and Japan) in phase 7 + 8 (phase 1 + 2). In the nonconvective phases (phases 3 + 4 and 5 + 6), TCs change from the original straight-moving type to the recurving type, such that the tendency for landfalls is significantly reduced. The QBWO, on the other hand, has a significant influence on TC landfalls in the Philippines and Japan. The strengthening of the subtropical high in phase 1 + 2 favors the development of westward-moving TCs and results in an increase in landfalls in the Philippines, while in phase 3 + 4 (phase 5 + 6), there is an increase (decrease) in TC activity and landfalls in Japan because of changes in genesis locations and large-scale circulations. The results herein suggest that both the MJO and QBWO exert distinctive impacts on TCs in the WNP.

Corresponding author address: Dr. Wen Zhou, Guy Carpenter Asia–Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong, China. E-mail: wenzhou@cityu.edu.hk

Abstract

This study investigates how tropical cyclone (TC) tracks and landfalls are modulated by the two major components of the intraseasonal oscillation (ISO), the 30–60-day Madden–Julian oscillation (MJO) and the 10–20-day quasi-biweekly oscillation (QBWO). In the convective phases of the MJO (phases 7 + 8 and 1 + 2), the western North Pacific Ocean (WNP) is mainly clustered with westward- and northwestward-moving TCs. The strong easterlies (southeasterlies) in the southern flank of the subtropical high lead to an increase in TC activity and landfalls in the Philippines and Vietnam (China and Japan) in phase 7 + 8 (phase 1 + 2). In the nonconvective phases (phases 3 + 4 and 5 + 6), TCs change from the original straight-moving type to the recurving type, such that the tendency for landfalls is significantly reduced. The QBWO, on the other hand, has a significant influence on TC landfalls in the Philippines and Japan. The strengthening of the subtropical high in phase 1 + 2 favors the development of westward-moving TCs and results in an increase in landfalls in the Philippines, while in phase 3 + 4 (phase 5 + 6), there is an increase (decrease) in TC activity and landfalls in Japan because of changes in genesis locations and large-scale circulations. The results herein suggest that both the MJO and QBWO exert distinctive impacts on TCs in the WNP.

Corresponding author address: Dr. Wen Zhou, Guy Carpenter Asia–Pacific Climate Impact Center, School of Energy and Environment, City University of Hong Kong, Hong Kong, China. E-mail: wenzhou@cityu.edu.hk

1. Introduction

In the first part of this study (Li and Zhou 2013, hereafter Part I), the basic characteristics of the two major modes of the intraseasonal oscillation (ISO) were revealed and their respective impacts on tropical cyclone (TC) frequency and intensity were studied. Results show that both the Madden–Julian oscillation (MJO; Madden and Julian 1971) and the quasi-biweekly oscillation (QBWO) exert distinctive TC modulation in the western North Pacific Ocean (WNP). The MJO controls the basinwide TC frequency and results in a northeastward shift in TC genesis positions, whereas the QBWO demonstrates localized TC modulations with a northwestward propagation of cyclogenesis locations (Fig. 1). In the present study, we continue to investigate other TC parameters, including TC tracks and landfalls, to see how they are modulated under different ISO conditions.

Fig. 1.
Fig. 1.

(a) Meridional average (5°–20°N) of 30–60-day filtered OLR anomalies (shading; W m−2) together with the cyclogenesis locations for a complete MJO cycle. (b) Meridional average (5°–20°N) of 10–20-day filtered OLR anomalies (shading; W m−2) together with the cyclogenesis locations for a complete QBWO cycle.

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

Several studies have identified the intraseasonal variability of TC tracks. For example, Harr and Elsberry (1991) found that TC tracks alternate between straight and recurving clusters with an intraseasonal scale. Kim et al. (2008) looked briefly into the relationship between TC tracks and landfalls with the MJO. They showed that a dense area of tracks migrates eastward (westward) when the MJO-related convection is located near the equatorial Indian Ocean (tropical WNP). They also found a significant MJO modulation of TC landfalls in southern China, South Korea, and Japan. More recently, Chen et al. (2009) linked the intraseasonal variation of TC tracks with that of the monsoon trough and the subtropical high. They suggested that the straight-moving TCs are generally linked to an intensified subtropical anticyclone, while the recurving TCs are usually associated with a deepened monsoon trough. In spite of this, the above studies focus only on the impacts of the MJO on TCs, without considering the influence of the QBWO. As shown in Part I, the QBWO and the MJO play equally important roles in TC modulation, and the impacts of the QBWO should not be neglected. In addition, similar to Mao and Chan (2005) and Zhou and Chan (2005), our results in Part I also revealed significant changes in large-scale circulations, including the monsoon trough and the subtropical high, under different phases of the MJO and QBWO, which are believed to be closely related to the movement and landfall of TCs. Therefore, as an extension of previous studies, we examine here the intraseasonal variability of TC tracks and landfalls in association with these two dominant ISO modes.

The data and methodology are described in section 2. Sections 3 and 4 examine the respective impacts of the MJO and QBWO on TC tracks and landfalls. Finally, a discussion and summary are presented in section 5.

2. Data and methodology

a. Data

As in Part I, TC data are based on the Joint Typhoon Warning Center (http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/best_tracks/wpindex.html) at 6-h intervals. Atmospheric data, including wind and geopotential height, were obtained from the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis (Kalnay et al. 1996). To determine the ISO mode, daily mean outgoing longwave radiation (OLR) was taken from the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites on a 2.5° latitude–longitude grid (Liebmann and Smith 1996). The study period is confined to the TC season (June–November) during 1975–2010, as in Part I.

b. Determination of ISO phase and amplitude

The phase and amplitude of the QBWO and the MJO are determined respectively by the following:
eq1
where PC1 and PC2 are the two leading principal components of the MJO/QBWO modes, as described in detail in Part I. Each of the four phases, labeled “1 + 2,” “3 + 4,” “5 + 6,” and “7 + 8,” covers a quarter of the MJO/QBWO cycle specified by the respective PC1 and PC2. The amplitude, whenever it is greater than one, indicates that the MJO/QBWO is active at that particular time. Table 1 summarizes the number and percentage of TCs associated with different ISO conditions (same as Fig. 5 in Part I). When considering individually the contribution of QBWO and MJO, the percentage of TCs associated with active QBWO (20%) is indeed comparable to that of active MJO (23%), indicating the importance of QBWO, in addition to MJO, in TC modulation. Further details of the phase and amplitude classification can be found in section 2 of Part I.
Table 1.

The number (and percentage) of TCs in the WNP associated with different ISO conditions.

Table 1.

c. Analysis of TC tracks

To characterize how TC tracks vary in different ISO phases, TC activity is examined by counting the frequency of TC occurrences in each 5° × 5° latitude–longitude grid and then dividing by the number of ISO days. A TC staying in the same grid box for consecutive periods is counted only once to avoid the domination of long-lasting TCs. Thus, a large value in a particular region corresponds to high TC activity, which can be brought about either by an enhancement in TC genesis or TC movement. As in previous studies (Chan 2000; Ho et al. 2004; Kim et al. 2008), a Student’s t test is utilized to ascertain whether the TC activity and the circulation anomalies in a particular ISO phase is statistically different from that of the climatology.

Apart from TC activity, the probability density estimate (PDE) of TC occurrences is also constructed to examine the spatial variation of TCs using the kernel density estimation approach (Bowman and Azzalini 1997). This is a nonparametric method whereby a smoothed contour of the PDE of the observed data is constructed using a density function; it has been previously applied in studying TC genesis (Ramsay et al. 2008; Chand and Walsh 2010). In the present study, it serves as an objective measure of the probability of TC occurrence.

d. Division of landfall areas and quantification of landfall frequency

The coastal regions rimming the WNP are divided into five major landfall areas: China, Japan, the Philippines, Vietnam, and South Korea (Fig. 2a). Similar to previous studies (Wu et al. 2004; Kim et al. 2008), such a division is based on national boundaries as well as the climatology of TC activity in the WNP (Fig. 2b). A TC is considered to have made landfall when it crosses the coastline of one of these five regions. To measure the frequency of landfalls for a particular ISO phase, the daily landfall rate (DLR) is used. It is defined similarly to the daily genesis rate in Part I, which is the number of TC landfalls divided by the number of days for a particular ISO phase. As in Part I, a statistical test given by
eq2
is then used to examine whether the DLR in a particular ISO phase is significantly different from that of the climatology (Hall et al. 2001). Note that Pe here is the climatological DLR, while P is the DLR and N is the number of days of the ISO phase.
Fig. 2.
Fig. 2.

(a) Five common TC landfall areas: China, Japan, the Philippines, South Korea, and Vietnam. (b) Climatological TC activity (10−2 counts day−1) during the TC season in the WNP.

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

3. Modulation of tracks and landfalls by the MJO

a. TC tracks

Following Part I, we first examine how TC tracks are modulated under the condition in which the MJO is active but the QBWO is inactive. Figure 3 shows the TC tracks, TC activity anomalies, and the PDE of TC occurrences for different MJO phases. In phase 7 + 8 (Fig. 3d), the TC genesis locations are the farthest south of any of the four MJO phases, with the mean positions located at around 10°N, 140°E. This is the convective phase of MJO, when cyclogenesis in the WNP is statistically enhanced (refer to section 4a in Part I). As shown in the activity and PDE plots, TCs during this phase mainly take a westward track, with a significant increase in TC activity over the Philippine Sea and the South China Sea (SCS). Meanwhile, a decrease in TC activity in the ocean southeast of Japan is also observed, which implies that the recurving track is not favorable in this phase. That is, in phase 7 + 8, the WNP is dominated mainly by westward-moving TCs, and recurving TCs are relatively fewer.

Fig. 3.
Fig. 3.

(left) TC tracks, (middle) TC activity anomalies, and (right) the probability density estimate (PDE) of TC occurrences for different MJO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8. The numbers in parentheses denote the total number of TCs during different phases. Solid (dashed) contours in the middle panel represent positive (negative) anomalies of TC activity, while regions statistically significant at 90% confidence are shaded. Contours in the right panel give the percentages of TC occurrences (i.e., the contour labeled 75 means that 75% of the TC occurrences are within the contour).

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

In phase 1 + 2 (Fig. 3a), the WNP is dominated mainly by northwestward-moving TCs. The PDE contours shift northeastward, which is consistent with the northeastward propagation of the MJO as discussed in Part I. This is the phase that demonstrates the largest increase in TC number (refer to section 4a in Part I). Consistent with this, a significant increase in TC activity occurs in most parts of the WNP, especially along the paths toward China and southern Japan, although a reduction is found in the SCS. In other words, phase 1 + 2 favors the development of northwestward-moving TCs, which probably have large influences on China and Japan.

In the nonconvective phase 3 + 4 (Fig. 3b), the PDE contours are similar to those in phase 1 + 2, with a slight eastward shift. TCs formed in this phase mainly take a recurving track, with an increase in TC activity in the ocean southeast of Japan and a reduction near the Philippines and the SCS. The pattern of TC activity in this phase is approximately the reverse of that in phase 7 + 8, which indicates that TC tracks actually change from the straight-moving type to the recurving type in the nonconvective phases. Similarly, in phase 5 + 6 (Fig. 3c), recurving TCs also cluster in the WNP. This is the phase that reveals the strongest TC suppression (refer to section 4a in Part I). In contrast to the convective phases, which are mainly dominated by straight-moving TCs, an examination of the activity plot in phase 5 + 6 reveals an increase in TC activity over the ocean southeast of Japan, while a significant suppression is found along the westward and northwestward paths toward the Philippines and China, respectively.

From the above, it is clear that TC tracks alternate among westward, northwestward, and recurving types during the evolution of the MJO cycle. The convective phases of the MJO (phases 7 + 8 and 1 + 2) favor the development of westward and northwestward TCs, while the nonconvective phases (phases 3 + 4 and 5 + 6) favor the development of the recurving type. The associated environmental parameters and the possible modulation mechanism will be examined in detail in section 3c.

b. TC landfalls

In section 3a, we have shown that TC tracks exhibit distinctive differences in different phases of the MJO. It is thus anticipated that TC landfalls will also be different. Figure 4 shows TC landfall locations, while Table 2 summarizes the corresponding landfall statistics. According to Table 2, the DLR in phase 7 + 8 shows a significant increase in the Philippines and Vietnam, which is consistent with an increase in westward-moving TCs during this phase (Fig. 3d). The overall DLR is 15.5%, which is significantly higher than that of the climatology (10.85%). In phase 1 + 2, there is similarly a basinwide increase in the DLR (15.64%), which is mainly caused by an increase in TC landfalls in China and Japan. This agrees well with an increase in TC activity along the northwestward paths toward China and southern Japan (Fig. 3a). The results here suggest that TCs have a higher probability of landfall in the convective phases of the MJO, relative to the climatology.

Fig. 4.
Fig. 4.

TC tracks and landfall locations for different MJO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8. The numbers in parentheses denote the total number of landfalls, while the colors of the triangles represent different landfall locations (red—China, orange—the Philippines, yellow—South Korea, green—Vietnam, and purple—Japan).

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

Table 2.

The DLR for different MJO phases. Phases where TC landfalls are statistically enhanced (suppressed) at 90% and 95% confidence are indicated by + and ++ (* and **), respectively. Boldface numbers are statistically significant at the 90% or 95% confidence level.

Table 2.

In contrast to the convective phases, the overall landfall frequency shows a significant reduction in the nonconvective MJO phases, with a DLR of 6.72% and 3.88% in phases 3 + 4 and 5 + 6, respectively. In phase 3 + 4, there is a reduction in TC landfalls in the Philippines, which is consistent with a concomitant decrease in TC activity over this region (Fig. 2b), while in phase 5 + 6, significant suppressions in TC landfalls can be found in China, Vietnam, and the Philippines, leading to the smallest DLR of all the MJO phases. This is because the majority of TCs take a recurving track over the open ocean off the shore of Japan (Fig. 2c). Thus, in the nonconvective MJO phases, the probability of landfall is much lower relative to the convective phases and the climatology.

It is also worth mentioning that our results here differ somewhat from those of Kim et al. (2008). They suggested that the MJO modulation of the number of TC landfalls is significant only in southern China, South Korea, and Japan. By eliminating the QBWO impacts, we found here that the modulation of the MJO on TC landfalls is more widespread, influencing regions including the Philippines, Vietnam, China, and Japan.

c. Large-scale environmental parameters

TC tracks are predominantly controlled by genesis locations and the surrounding environmental flows (Gray 1979; Chan 2000, 2005). Therefore, the variability of TC tracks and movements in different MJO phases is likely caused by alternations in the steering flow as well as the shift in genesis positions associated with the MJO. Figure 5 shows the 850-hPa streamlines and the 500-hPa geopotential height anomalies for different MJO phases. As illustrated previously in section 3a, the convective MJO phases (phases 7 + 8 and 1 + 2) favor the development of westward and northwestward TCs. In phase 7 + 8 (Fig. 5d), the western North Pacific subtropical high (WNPSH; denoted by 5870 gpm) extends farther westward relative to the climatology. Nevertheless, there is a strengthening of the monsoon trough at around 10°N because of the presence of enhanced MJO convection. The strengthened monsoon trough provides a favorable environment for cyclogenesis. TCs that form are then steered westward by the strong easterlies in the southern flank of the WNPSH. As a result, there is an increase in TC activity and landfalls in the Philippines and Vietnam.

Fig. 5.
Fig. 5.

(left) TC tracks, (middle) 850-hPa streamlines and 30–60-day filtered vorticity anomalies (10−6 s−1), and (right) 500-hPa geopotential height anomalies (m) for different MJO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8. Dashed purple contours in the right panel denote the climatological 5870 geopotential height, while solid black contours denote the 5870 geopotential height for different MJO phases. Only anomalies exceeding 95% confidence based on the Student’s t test are shown.

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

In phase 1 + 2 (Fig. 5a), the WNPSH retreats abruptly while the monsoon trough continues to strengthen. The significant weakening of the WNPSH, together with the simultaneous deepening of the monsoon trough, strongly favors the genesis of TCs. Meanwhile, the prevailing southeasterly in the convergence zone of the monsoon trough steers TCs to the northwest, leading to an increase in TC activity and landfalls in China and Japan.

In the nonconvective phases, however, the WNP is dominated mainly by recurving TCs. During phases 3 + 4 and 5 + 6 (Figs. 5b,c), the monsoon trough in the WNP is significantly weakened by the presence of suppressed MJO convection. Apart from this, a remarkable westward extension of the WNPSH can be found in phase 5 + 6. The presence of these unfavorable conditions causes a basinwide reduction in TC frequency and an eastward shift in TC genesis positions. In addition, these unfavorable conditions over the SCS and the Philippines also inhibit TCs from moving westward or northwestward, resulting in reduced TC activity and landfalls over these regions. TCs formed during these phases mainly recurve over the open ocean, following the low-level steering flows.

Consistent with previous studies (Harr and Elsberry 1995a,b; Mao and Chan 2005; Zhou and Chan 2005; Chen et al. 2009), our results here clearly show that large-scale circulations, including the monsoon trough and the WNPSH, are significantly modulated by the MJO. The changes in these circulations in turn influence the frequency and locations of cyclogenesis, as well as the tracks and landfall properties of TCs in the WNP.

4. Modulation of tracks and landfalls by the QBWO

a. TC tracks

To examine the impacts of the QBWO on TC tracks and landfalls, a similar analysis is conducted under the condition in which the QBWO is active but the MJO is inactive. Figure 6 shows the TC tracks, TC activity anomalies, and the PDE of TC occurrences for different QBWO phases. In accordance with the northwestward propagation of the QBWO, the PDE contours of TC occurrences also depict a westward shift during the evolution of the QBWO from phase 7 + 8 to 5 + 6. In phase 7 + 8 (Fig. 6d), TCs form mainly eastward of 150°E and recurve in the ocean southeast of Japan, leading to an increase in TC activity over these regions. In contrast, a reduction in TC activity is observed near the East China Sea, although it appears to be less significant. In other words, phase 7 + 8 is mainly associated with an increase in activity of the recurving TCs. In phase 1 + 2 (Fig. 6a), however, the TC tracks exhibit distinctive differences relative to those in phase 7 + 8 (Fig. 6d). In this phase, the WNP is mainly clustered with westward-moving TCs, while recurving TCs are relatively fewer. The majority of TCs forms at about 150°E and then subsequently moves westward to the Philippines and the SCS. Consistent with this, examination of the activity plot reveals a significant increase in TC activity near the Philippines, whereas suppressed TC activity is found off the shore of Japan.

Fig. 6.
Fig. 6.

As in Fig. 3, but showing the (left) TC tracks, (middle) TC activity anomalies, and (right) PDE of TC occurrences for different QBWO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8.

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

In phase 3 + 4 (Fig. 6b), both straight-moving and recurving TCs prevail over the WNP. TCs form mainly westward of 150°E in this phase in accordance with the northwestward propagation of the QBWO. There is an increase in TC activity near the SCS and Japan, while TCs are suppressed over the ocean eastward of 150°E. This is mainly caused by the westward shift in TC genesis positions, which causes a significant reduction in cyclogenesis east of 150°E (see section 4b in Part I). Similarly, in phase 5 + 6 (Fig. 6c), alternations in TC tracks also occur. TCs in this phase generally move westward, such that there is a significant decrease in TC activity over Japan. Here, we have clearly demonstrated that in addition to the MJO, TC tracks are also significantly modulated by the QBWO in the WNP.

b. TC landfalls

In addition to TC tracks, we continue to investigate how TC landfalls are affected by the QBWO. Figure 7 shows the landfall locations of TCs, while Table 3 summarizes the corresponding landfall statistics. As shown in Table 3, the QBWO mainly affects TC landfalls in the Philippines and Japan. In phase 1 + 2, a significant increase in landfall frequency is found in the Philippines, which is consistent with an increase in westward-moving TCs during this phase (Fig. 6a). In addition, the landfall frequencies in Vietnam and China are also slightly higher, although the increase is not significant relative to the climatology. The overall DLR in this phase is 13.60%, which is significantly larger than that of the climatology (10.58%). The results suggest that in phase 1 + 2, TCs in the WNP have a higher probability of landfall, especially in the Philippines.

Fig. 7.
Fig. 7.

As in Fig. 4, but showing the TC tracks and landfall locations for different QBWO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8.

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

Table 3.

As in Table 2, but showing the DLR for different QBWO phases.

Table 3.

In phase 3 + 4, there is a significant increase in DLR in Japan, which is consistent with the corresponding increase in TC activity shown in Fig. 6b. The overall DLR is 13.25%, which is comparable to that of phase 1 + 2 (13.60%). On the other hand, phase 5 + 6 reveals a significant reduction in TC landfalls in Japan, which is coincident with the suppressed TC activity over this region (Fig. 6c). Relative to the DLR of phases 1 + 2 (13.60%) and 3 + 4 (13.25%), the overall DLR in phase 5 + 6 (9.68%) is significantly reduced, implying that there is generally a lower probability of landfall during this phase. Finally, in phase 7 + 8, the overall DLR (8.75%) is the lowest among the QBWO phases, which can be attributed to the respective reduction in DLR in the Philippines, Vietnam, China, and South Korea. The decrease in DLR coincides well with the suppressed TC activity over these regions, as revealed previously in Fig. 6d. Although there is a higher DLR because of an increase in TC activity southeast of Japan (Fig. 6d), the increase is not significant. As a result, the probability of TC landfall is the lowest during this phase.

Overall, the results here suggest that the QBWO also plays a role in modulating the landfalls of TCs in the WNP. Relative to the MJO, which shows a basinwide modulation on landfall frequency, the QBWO mainly modulates TCs in the Philippines and Japan. This again illustrates that TCs respond differently to these two components of the ISO.

c. Large-scale environmental parameters

Figure 8 shows the 850-hPa streamlines and the 500-hPa geopotential height anomalies for different QBWO phases. In phase 7 + 8 (Fig. 8d), TCs form mainly eastward of 150°E. The WNPSH extends westward, with positive height anomalies and negative vorticity anomalies near the Philippines. These unfavorable conditions inhibit TCs from moving westward. Instead, the eastward-oriented TCs are more prone to take a recurvature route over the ocean, following the low-level steering flow. As a result, there is an increase in TC activity over the open ocean. Following phase 7 + 8, the WNPSH continues to strengthen and extends to the SCS in phase 1 + 2 (Fig. 8a). Influenced by the strong WNPSH, TCs that form are steered by the strong easterlies to the west and northwest, leading to an increase in TC activity near the Philippines (Fig. 6a).

Fig. 8.
Fig. 8.

As in Fig. 5, but showing the (left) TC tracks, (middle) 850-hPa streamlines and 10–20-day filtered vorticity anomalies (10−6 s−1), and (right) 500-hPa geopotential height anomalies (m) for different QBWO phases: (a) phase 1 + 2, (b) phase 3 + 4, (c) phase 5 + 6, and (d) phase 7 + 8.

Citation: Journal of Climate 26, 9; 10.1175/JCLI-D-12-00211.1

In phase 3 + 4 (Fig. 8b), the WNPSH starts to retreat eastward and becomes comparable to that of the climatology. There is a combination of straight-moving and recurving TCs in this phase, which is consistent with an increase in TC activity in the SCS and Japan (Fig. 6b). Finally, in phase 5 + 6 (Fig. 8c), although the WNPSH retreats farther eastward, TC tracks in this phase are predominantly controlled by the genesis positions. The northwestward shift in genesis positions induced by the QBWO suppresses the frequency of recurving TCs, leading to a general reduction in TC activity near Japan. Therefore, apart from the MJO, the environmental steering flows are also modulated by the QBWO, which leads to changes in TC activity and landfall features during different phases of the QBWO.

5. Discussion and summary

In this two-part paper, the intraseasonal variability of TCs in the WNP is investigated in association with the two major components of the ISO, the 30–60-day MJO and the 10–20-day QBWO. The general characteristics of these two modes, as well as their impacts on TC frequency and intensity, have been presented in Part I. In the present paper, we continue to look into how TC tracks and landfalls are modulated by these two dominant modes of the ISO.

In the convective phases of the MJO (phases 7 + 8 and 1 + 2), the WNP is mainly clustered with westward and northwestward TCs. The monsoon trough exhibits a remarkable strengthening, which causes a basinwide increase in TC frequency. Meanwhile, the strong easterlies and southeasterlies in the southern flank of the WNPSH steer TCs to the west and northwest in phases 7 + 8 and 1 + 2, respectively. As a result, TCs show an increase in activity and landfall probability in the Philippines and Vietnam (China and Japan) in phase 7 + 8 (phase 1 + 2). In other words, TCs that form during the convective phases of the MJO are more influential to the coastal regions because of an increase in TC frequency and landfall probability. In the nonconvective phases (phases 3 + 4 and 5 + 6), however, TCs change from the original straight-moving type to the recurving type. This is probably because of the eastward shift in TC genesis positions. The presence of unfavorable conditions over the SCS and the Philippines inhibits TCs from moving westward or northwestward, such that the majority of them recurve over the ocean southeast of Japan. This results in a much lower DLR, suggesting that the tendency for landfalls is lower during nonconvective MJO phases. It can thus be concluded that the MJO has significant impacts on TC activity and landfalls, influencing a wide area including the Philippines, Vietnam, China, and Japan.

Apart from the MJO, the QBWO also plays a role in modulating the tracks and landfalls of TCs in the WNP. In phase 7 + 8, unfavorable conditions over the Philippines inhibit TCs from moving to the west. TCs form mainly eastward of 150°E and recurve over the open ocean. Subsequently, the majority of TCs are steered westward by the strong easterlies in phase 1 + 2, as a result of the strengthening of the WNPSH. This leads to an increase in the probability of landfall in the Philippines. In phase 3 + 4, there is a combination of straight-moving and recurving TCs, contributing to an increase in TC activity in the SCS and Japan, while in phase 5 + 6, the northwestward shift in TC genesis positions induced by the QBWO suppresses the recurvature of TCs, leading to a reduction in TC activity near Japan. Clearly, we have demonstrated that the tracks and landfalls of TCs also possess quasi-biweekly variations that are distinct from those of the MJO. Relative to the basinwide modulation of the MJO, the QBWO is more likely to affect TC landfalls in the Philippines and Japan.

Overall, this two-part paper has shown that the MJO and QBWO exert distinctive impacts on different parameters of TCs in the WNP, including frequency, genesis location, intensity, track, and landfall. The next challenge is to investigate TC modulation under the coupled influences of these two modes, which has not been investigated extensively in the present study. A brief examination of the impacts of the coupled modes on cyclogenesis in Part I suggests that the QBWO generally exerts modulation upon the background MJO, and the modulation seems to vary under different MJO conditions. However, the mechanism involved is seemingly more complex and requires much further consideration. Apart from this, another important factor that should also be taken into account in the future is El Niño–Southern Oscillation (ENSO), which has been well documented to have significant impacts on WNP TCs (Chan 2000; Wang and Chan 2002; Chia and Ropelewski 2002; Kim et al. 2011; Li and Zhou 2012) and the East Asian monsoon (Zhou and Chan 2005, 2007; Zhou et al. 2007a,b). Previous studies have suggested that the ENSO and MJO are indeed related (Zhang and Gottschalck 2002; Pohl and Matthews 2007), and that MJO activity tends to be stronger during a developing El Niño event. More recently, Li et al. (2012) even found that the MJO modulation on cyclogenesis in the WNP is enhanced during El Niño events. Therefore, the interactions among TCs, ENSO, and these two dominant modes of the ISO are another interesting topic that deserves further investigation in the future. Nevertheless, the present two-part study sheds new light on the TC–ISO relationship and serves as an important step toward a better understanding of the intraseasonal variability of TCs in the WNP.

Acknowledgments

This research is supported by the 973 Basic Research Program Grant 2009CB421401, National Nature Science Foundation of China–Yunnan joint Grant U0833602, National Nature Science Foundation of China Grant 41175079, and CityU Strategic Research Grants 7002917 and 7002780.

REFERENCES

  • Bowman, A. W., , and A. Azzalini, 1997: Applied Smoothing Techniques for Data Analysis: The Kernel Approach with S-Plus Illustrations. Oxford University Press, 193 pp.

  • Chan, J. C. L., 2000: Tropical cyclone activity over the western North Pacific associated with El Niño and La Niña events. J. Climate, 13, 29602972.

    • Search Google Scholar
    • Export Citation
  • Chan, J. C. L., 2005: The physics of tropical cyclone motion. Annu. Rev. Fluid Mech., 37, 99128.

  • Chand, S. S., , and K. J. E. Walsh, 2010: The influence of the Madden–Julian oscillation on tropical cyclone activity in the Fiji region. J. Climate, 23, 868886.

    • Search Google Scholar
    • Export Citation
  • Chen, T. C., , S. Y. Wang, , M. C. Yen, , and A. J. Clark, 2009: Impact of the intraseasonal variability of the western North Pacific large-scale circulation on tropical cyclone tracks. Wea. Forecasting, 24, 646666.

    • Search Google Scholar
    • Export Citation
  • Chia, H. H., , and C. F. Ropelewski, 2002: The interannual variability in the genesis location of tropical cyclones in the northwest Pacific. J. Climate, 15, 29342944.

    • Search Google Scholar
    • Export Citation
  • Gray, W. M., 1979: Hurricanes: Their formation, structure and likely role in the tropical circulation. Meteorology over the Tropical Oceans, D. B. Shaw, Ed., Royal Meteorological Society, 155–218.

  • Hall, J. D., , A. J. Matthews, , and D. J. Karoly, 2001: The modulation of tropical cyclone activity in the Australian region by the Madden–Julian oscillation. Mon. Wea. Rev., 129, 29702982.

    • Search Google Scholar
    • Export Citation
  • Harr, P. A., , and R. L. Elsberry, 1991: Tropical cyclone track characteristics as a function of large-scale circulation anomalies. Mon. Wea. Rev., 119, 14481468.

    • Search Google Scholar
    • Export Citation
  • Harr, P. A., , and R. L. Elsberry, 1995a: Large-scale circulation variability over the tropical western North Pacific. Part I: Spatial patterns and tropical cyclone characteristics. Mon. Wea. Rev., 123, 12251246.

    • Search Google Scholar
    • Export Citation
  • Harr, P. A., , and R. L. Elsberry, 1995b: Large-scale circulation variability over the tropical western North Pacific. Part II: Persistence and transition characteristics. Mon. Wea. Rev., 123, 12471268.

    • Search Google Scholar
    • Export Citation
  • Ho, C.-H., , J.-J. Baik, , J.-H. Kim, , D.-Y. Gong, , and C.-H. Sui, 2004: Interdecadal changes in summertime typhoon tracks. J. Climate, 17, 17671776.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471.

  • Kim, H. M., , P. J. Webster, , and J. A. Curry, 2011: Modulation of North Pacific tropical cyclone activity by three phases of ENSO. J. Climate, 24, 18391849.

    • Search Google Scholar
    • Export Citation
  • Kim, J., , C. Ho, , H. Kim, , C. Sui, , and S. K. Park, 2008: Systematic variation of summertime tropical cyclone activity in the western North Pacific in relation to the Madden–Julian oscillation. J. Climate, 21, 11711191.

    • Search Google Scholar
    • Export Citation
  • Li, R., , and W. Zhou, 2012: Changes in western Pacific tropical cyclones associated with the El Niño–Southern Oscillation cycle. J. Climate, 25, 58645878.

    • Search Google Scholar
    • Export Citation
  • Li, R., , and W. Zhou, 2013: Modulation of western North Pacific tropical cyclone activities by the ISO. Part I: Genesis and intensity. J. Climate, in press.

    • Search Google Scholar
    • Export Citation
  • Li, R., , W. Zhou, , J. Chan, , and P. Huang, 2012: Asymmetric modulation of western North Pacific cyclogenesis by the Madden–Julian oscillation under ENSO conditions. J. Climate, 25, 53745385.

    • Search Google Scholar
    • Export Citation
  • Liebmann, B., , and C. A. Smith, 1996: Description of a complete (interpolated) outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc., 77, 12751277.

    • Search Google Scholar
    • Export Citation
  • Madden, R. A., , and P. R. Julian, 1971: Detection of a 40–50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28, 702708.

    • Search Google Scholar
    • Export Citation
  • Mao, J. Y., , and J. C. L. Chan, 2005: Intraseasonal variability of the South China Sea summer monsoon. J. Climate, 18, 23882402.

  • Pohl, B., , and A. J. Matthews, 2007: Observed changes in the lifetime and amplitude of the Madden–Julian oscillation associated with interannual ENSO sea surface temperature anomalies. J. Climate, 20, 26592674.

    • Search Google Scholar
    • Export Citation
  • Ramsay, H. A., , L. M. Leslie, , P. J. Lamb, , M. B. Richman, , and M. Leplastrier, 2008: Interannual variability of tropical cyclones in the Australian region: Role of large-scale environment. J. Climate, 21, 10831103.

    • Search Google Scholar
    • Export Citation
  • Wang, B., , and J. C. L. Chan, 2002: How strong ENSO events affect tropical storm activity over the western North Pacific. J. Climate, 15, 16431658.

    • Search Google Scholar
    • Export Citation
  • Wu, M. C., , W. L. Chang, , and W. M. Leung, 2004: Impacts of El Niño–Southern Oscillation events on tropical cyclone landfalling activity in the western North Pacific. J. Climate, 17, 14191428.

    • Search Google Scholar
    • Export Citation
  • Zhang, C., , and J. Gottschalck, 2002: SST anomalies of ENSO and the Madden–Julian oscillation in the equatorial Pacific. J. Climate, 15, 24292445.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , and J. C. L. Chan, 2005: Intraseasonal oscillations and the South China Sea summer monsoon onset. Int. J. Climatol., 25, 15851609.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , and J. C. L. Chan, 2007: ENSO and South China Sea summer monsoon onset. Int. J. Climatol., 27, 157167.

  • Zhou, W., , C. Y. Li, , and X. Wang, 2007a: Possible connection between Pacific oceanic interdecadal pathway and East Asian winter monsoon. Geophys. Res. Lett., 34, L01701, doi:10.1029/2006GL027809.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , X. Wang, , T. J. Zhou, , C. Li, , and J. C. L. Chan, 2007b: Interdecadal variability of the relationship between the East Asian winter monsoon and ENSO. Meteor. Atmos. Phys., 98, 283293.

    • Search Google Scholar
    • Export Citation
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