• 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., , and C. Li, 2004: The East Asian winter monsoon. East Asian Monsoon, C.-P. Chang, Ed., Series on Meteorology of East Asia, Vol. 1, World Scientific, 54–106.

  • Chang, C.-P., , Y. Zhang, , and T. Li, 2000a: Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: Roles of subtropical ridge. J. Climate, 13, 43104325.

    • Search Google Scholar
    • Export Citation
  • Chang, C.-P., , Y. Zhang, , and T. Li, 2000b: Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part II: Meridional structure of the monsoon. J. Climate, 13, 43264340.

    • Search Google Scholar
    • Export Citation
  • Chang, C.-P., , Z. Wang, , and H. Hendon, 2006: The Asian winter monsoon. The Asian Monsoon, B. Wang, Ed., Springer/Praxis, 89–128.

  • Chen, C.-S., , and Y.-L. Chen, 2003: The rainfall characteristics of Taiwan. Mon. Wea. Rev., 131, 13231341.

  • Chen, C.-S., , Y.-L. Chen, , C.-L. Liu, , P.-L. Lin, , and W.-C. Chen, 2007: Statistics of heavy rainfall occurrences in Taiwan. Wea. Forecasting, 22, 9811002.

    • Search Google Scholar
    • Export Citation
  • Chen, G. T. J., 1994: Large-scale circulation associated with the East Asian summer monsoon and the mei-yu over South China and Taiwan. J. Meteor. Soc. Japan, 72, 959983.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , and H.-S. Chen, 2011: Interdecadal variability of summer rainfall in Taiwan associated with tropical cyclones and monsoon. J. Climate, 24, 57865798.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , F.-C. Lu, , S.-L. Kuo, , and C.-F. Shih, 2005: Summer climate variability in Taiwan and associated large-scale processes. J. Meteor. Soc. Japan, 83, 499516.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , T. Li, , and C.-F. Shih, 2007: Fall persistence barrier of sea surface temperature in the South China Sea associated with ENSO. J. Climate, 20, 158172.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , T. Li, , and C.-F. Shih, 2008a: Asymmetry of the El Niño-spring rainfall relationship in Taiwan. J. Meteor. Soc. Japan, 86, 297312.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , F.-C. Lu, , and C.-F. Shih, 2008b: The decadal oscillation of fall temperature in Taiwan. Terr. Atmos. Oceanic Sci., 19, 497504.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , B. Wang, , J.-W. Hwu, , and C.-F. Shih, 2009: Potential predictability of tropical low-level circulation in CWB GFS ensemble hindcast. J. Meteor. Soc. Japan, 87, 171188.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , J.-L. Chu, , C.-F. Shih, , and Y.-C. Tung, 2010a: Interannual variability of circulation-rainfall relationship in Taiwan during the mei-yu season. Int. J. Climatol., 30, 22642276.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , T. Li, , and C.-F. Shih, 2010b: Tropical cyclone–and monsoon-induced rainfall variability in Taiwan. J. Climate, 23, 41074120.

    • Search Google Scholar
    • Export Citation
  • Chen, T.-C., , S.-P. Weng, , N. Yamazaki, , and S. Kiehne, 1998: Interannual variation in the tropical cyclone formation over the western North Pacific. Mon. Wea. Rev., 126, 10801090.

    • Search Google Scholar
    • Export Citation
  • Chen, T.-C., , S.-Y. Wang, , M.-C. Yen, , and A. 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
  • Chen, W. Y., 1982: Fluctuation in Northern Hemisphere 700 mb height field associated with the Southern Oscillation. Mon. Wea. Rev., 110, 808823.

    • Search Google Scholar
    • Export Citation
  • Chu, P.-S., 2004: ENSO and tropical cyclone activity. Hurricanes and Typhoons: Past, Present, and Potential, R. J. Murnane and K.-B. Liu, Eds., Columbia University Press, 279–332.

  • Chu, P.-S., , X. Zhao, , C.-T. Lee, , and M.-M. Lu, 2007: Climate prediction of tropical cyclone activity in the vicinity of Taiwan using the multivariate least absolute deviation regression method. Terr. Atmos. Oceanic Sci., 18, 805825.

    • Search Google Scholar
    • Export Citation
  • Clark, J. D., , and P.-S. Chu, 2002: Interannual variation of tropical cyclone activity over the central North Pacific. J. Meteor. Soc. Japan, 80, 403418.

    • Search Google Scholar
    • Export Citation
  • Davis, R. E., 1976: Predictability of sea-surface temperature and sea-level pressure anomalies over the North Pacific Ocean. J. Phys. Oceanogr., 6, 249266.

    • Search Google Scholar
    • Export Citation
  • Deser, C., , and M. L. Blackmon, 1995: On the relationship between tropical and North Pacific sea surface temperature variations. J. Climate, 8, 16771680.

    • Search Google Scholar
    • Export Citation
  • Ding, Y., 1994: Monsoon over China. Kluwer Academic, 419 pp.

  • Ding, Y., 2004: Seasonal march of the East-Asian summer monsoon. East Asian Monsoon, C.-P. Chang, Ed., Series on Meteorology of East Asia, Vol. 1, World Scientific, 3–53.

  • Frank, W. M., 1987: Tropical cyclone formation. A Global View of Tropical Cyclones, R. L. Elsberry, Ed., Office of Naval Research, 53–90.

  • Gill, A. E., 1980: Some simple resolutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447462.

  • Goswami, B. N., 2006: The Asian monsoon: Interdecadal variability. The Asian Monsoon, B. Wang, Ed., Springer/Praxis, 295–327.

  • Gray, W. M., 1975: Tropical cyclone genesis. Colorado State University Atmospheric Sciience Paper 323, 121 pp.

  • Harr, P. A., , and R. L. Elsberry, 1995: 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
  • Ho, C.-H., , J.-J. Baik, , J.-H. Kim, , and D.-Y. Gong, 2004: Interdecadal changes in summertime typhoon tracks. J. Climate, 17, 17671776.

  • Hsu, H.-H., 2005: East Asian and western North Pacific summer monsoon region. Intraseasonal Variability in the Atmosphere-Ocean System, K.-M. Lau and D. Waliser, Eds., Springer-Praxis, 65–98.

  • Jiang, H., , and E. J. Zipser, 2010: Contribution of tropical cyclones to the global precipitation from eight seasons of TRMM data: Regional, seasonal, and interannual variations. J. Climate, 23, 15261543.

    • Search Google Scholar
    • Export Citation
  • Jiang, Z., , G. T.-J. Chen, , and M.-C. Wu, 2003: Large-scale circulation patterns associated with heavy spring rain events over Taiwan in strong and non-ENSO years. Mon. Wea. Rev., 131, 17691782.

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

  • Klein, S. A., , B. J. Soden, , and N.-C. Lau, 1999: Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge. J. Climate, 12, 917932.

    • Search Google Scholar
    • Export Citation
  • Ko, K.-C., , and H.-H. Hsu, 2006: Sub-monthly circulation features associated with tropical cyclone track over the East Asian monsoon area during July-August season. J. Meteor. Soc. Japan, 84, 871889.

    • Search Google Scholar
    • Export Citation
  • Ko, K.-C., , and H.-H. Hsu, 2009: ISO modulation on the submonthly wave pattern and recurving tropical cyclones in the tropical western North Pacific. J. Climate, 22, 582599.

    • Search Google Scholar
    • Export Citation
  • Kubota, H., , and B. Wang, 2009: How much do tropical cyclones affect seasonal and interannual rainfall variability over the western North Pacific? J. Climate, 22, 54955510.

    • Search Google Scholar
    • Export Citation
  • Lander, M., 1994: An exploratory analysis of the relationship between tropical storm formation in the western North Pacific and ENSO. Mon. Wea. Rev., 122, 636651.

    • Search Google Scholar
    • Export Citation
  • Lau, K.-M., and Coauthors, 2000: A report of the field operations and early results of the South China Sea Monsoon Experiment (SCSMEX). Bull. Amer. Meteor. Soc., 81, 12611270.

    • Search Google Scholar
    • Export Citation
  • Lau, N.-C., , and M. J. Nath, 2000: Impacts of ENSO on the variability of the Asian–Australian monsoons as simulated in GCM experiments. J. Climate, 13, 42874309.

    • Search Google Scholar
    • Export Citation
  • Lau, N.-C., , and M. J. Nath, 2003: Atmosphere–ocean variations in the Indo-Pacific sector during ENSO episodes. J. Climate, 16, 320.

    • Search Google Scholar
    • Export Citation
  • Lee, M.-H., , C.-H. Ho, , and J.-H. Kim, 2010: Influence of tropical cyclone landfalls on spatiotemporal variations in typhoon season rainfall over South China. Adv. Atmos. Sci., 27, 443454.

    • Search Google Scholar
    • Export Citation
  • Matsuno, T., 1966: Quasi-geostrophic motions in equatorial areas. J. Meteor. Soc. Japan, 44, 2543.

  • McBride, J. J., 1995: Tropical cyclone formation. Global perspectives on tropical cyclones, R. L. Elsberry, Ed., World Meteorological Organization Tech. Doc. WMO/TD-693, 63–105.

  • Ninomiya, K., , and T. Murakami, 1987: The early summer rainy season (Baiu) over Japan. Monsoon Meteorology, C.-P. Chang and T. K. Krishnamurti, Eds., Oxford University Press, 93–121.

  • Nitta, T., , and S. Yamada, 1989: Recent warming of tropical sea surface temperature and its relationship to the Northern Hemisphere circulation. J. Meteor. Soc. Japan, 67, 375383.

    • Search Google Scholar
    • Export Citation
  • Simmonds, I., , D. Bi, , and P. Hope, 1999: Atmospheric water vapor flux and its association with rainfall over China in summer. J. Climate, 12, 13531367.

    • Search Google Scholar
    • Export Citation
  • Smith, T. M., , and R. W. Reynolds, 2003: Extended reconstruction of global sea surface temperature based on COADS data (1854–1997). J. Climate, 16, 16011612.

    • Search Google Scholar
    • Export Citation
  • Smith, T. M., , and R. W. Reynolds, 2004: Improved extended reconstruction of SST (1854–1997). J. Climate, 17, 24662477.

  • Tao, S., , and L. Chen, 1987: A review of recent research on the East Asian summer monsoon over China. Monsoon Meteorology, C.-P. Chang and T. K. Krishnamurti, Eds., Oxford University Press, 50–92.

  • Tu, J.-Y., , C. Chou, , and P.-S. Chu, 2009: The abrupt shift of typhoon activity in the vicinity of Taiwan and its association with western North Pacific–East Asian climate change. J. Climate, 22, 36173628.

    • Search Google Scholar
    • Export Citation
  • Wang, B., 1992: The vertical structure and development of the ENSO anomaly mode during 1979–1989. J. Atmos. Sci., 49, 698712.

  • Wang, B., 1994: Climatic regimes of tropical convection and rainfall. J. Climate, 7, 11091118.

  • 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
  • Wang, B., , and LinHo, 2002: Rainy season of the Asian–Pacific summer monsoon. J. Climate, 15, 386398.

  • Wang, B., , and Q. Zhang, 2002: Pacific–East Asian teleconnection. Part II: How the Philippine Sea anomalous anticyclone is established during El Niño development. J. Climate, 15, 32523265.

    • Search Google Scholar
    • Export Citation
  • Wu, L., , and B. Wang, 2004: Assessing impacts of global warming on tropical cyclone tracks. J. Climate, 17, 16861698.

  • Wu, L., , B. Wang, , and S. Geng, 2005: Growing typhoon influence on East Asia. Geophys. Res. Lett., 32, L18703, doi:10.1029/2005GL022937.

  • Wu, R., 2002: Processes for the northeastward advance of the summer monsoon over the western North Pacific. J. Meteor. Soc. Japan, 80, 6783.

    • Search Google Scholar
    • Export Citation
  • Zhao, X., , and J. Li, 2009: Possible causes for the persistence barrier of SSTA in the South China Sea and the vicinity of Indonesia. Adv. Atmos. Sci., 26, 11251136.

    • Search Google Scholar
    • Export Citation
  • Zhao, X., , and P.-S. Chu, 2010: Bayesian changepoint analysis for extreme events (typhoons, heavy rainfall, and heat waves): An RJMCMC approach. J. Climate, 23, 10341046.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    Topography and 10 major meteorological stations in Taiwan.

  • View in gallery

    The 1950–2008 climatological means of (a) 850-hPa streamfunction, (b) 10-m winds, (c) precipitable water, and (d) vertically integrated moisture flux averaged for the September–October season. Contour intervals are 20 × 105 m2 s−1 for (a) and 5 kg m−2 for (c). Positive values in (a) and values larger than 40 kg m−2 in (c) are shaded.

  • View in gallery

    The 1950–2008 climatological means of tropical cyclone rainfall PTC and seasonal rainfall PSN in Taiwan: (a) September PTC, (b) September PSN, (c) October PTC, and (d) October PSN. The unit is millimeters.

  • View in gallery

    Time series of interdecadal components of PTC and PSN averaged from 10 Taiwan stations for (a) September and (b) October.

  • View in gallery

    The first joint interdecadal mode of TC rainfall and seasonal rainfall in Taiwan: (a) the first eigenvector of TC rainfall, (b) the first eigenvector of seasonal rainfall, and (c) their first principal components. This mode accounts for 71% of total interdecadal variance of TC rainfall and seasonal rainfall.

  • View in gallery

    Anomalous patterns correlated with C1 time series of the first mode in October: (a) sea surface temperature, (b) 850-hPa velocity potential to depict low-level large-scale divergent circulation, and (c) 850-hPa streamfunction to illustrate low-level large-scale circulation. In (a)–(c), contour intervals are 0.1 and the zero contour is suppressed. Correlation patterns significant at the 0.05 level are indicated by shading with consideration given for effective degrees of freedom.

  • View in gallery

    As in Fig. 6, but for anomalous correlation patterns of (a) vertically integrated moisture flux, (b) precipitable water, and (c) divergence of moisture flux. Contour intervals are 0.1 for (b) and 0.2 for (c). In (b) and (c), the zero contour is suppressed. Correlation patterns significant at the 0.05 level are indicated by shading with consideration given for effective degrees of freedom.

  • View in gallery

    As in Fig. 6, but for correlation patterns of (a) TC frequency, (b) 500-hPa vertical motion, and (c) 1000-hPa relative vorticity. Contour intervals are 0.2 and the zero contour is suppressed. Correlation patterns significant at the 0.05 level are indicated by shading with consideration given for effective degrees of freedom.

  • View in gallery

    Schematic diagrams for the major processes regulating the interdecadal mode of TC rainfall and seasonal rainfall in Taiwan during (a) October and (b) summer. The symbols are as follows: T+ for increased TC rainfall, S+ for increased seasonal rainfall, S− for decreased seasonal rainfall, gray arrow for moisture flux, dark arrow for TC track, rings for TC genesis, DIV for large-scale divergence, CON for large-scale convergence, AC for anticyclonic circulation, and C for cyclonic circulation. Details of these processes are described in the text.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 0 0 0
PDF Downloads 0 0 0

Coherent Interdecadal Variability of Tropical Cyclone Rainfall and Seasonal Rainfall in Taiwan during October

View More View Less
  • 1 Institute of Maritime Information and Technology, National Kaohsiung Marine University, Kaohsiung, Taiwan
© Get Permissions
Full access

Abstract

By separating total rainfall into tropical cyclone (TC) rainfall caused by TC passage and seasonal rainfall associated with moisture transport of prevailing seasonal flows, this study examines the interdecadal variability of these two rainfall components in Taiwan during fall. It is found that interdecadal variability of TC rainfall and seasonal rainfall tends to vary coherently in October. This coherent interdecadal rainfall mode features islandwide patterns in Taiwan and evident interdecadal oscillations. The associated large-scale regulating processes for the positive phase are characterized by warm sea surface temperature (SST) anomalies in the northern South China Sea and the western North Pacific, which in turn induce an anomalous anticyclonic circulation over the subtropical western Pacific to the east of Taiwan. On the western boundary of this anomalous anticyclone, anomalous southeasterly flows enhance mean moisture transport from the tropical western Pacific into Taiwan. Seasonal rainfall increases in Taiwan with stronger anomalies on the windward side over eastern Taiwan. Warm SST anomalies and accompanying large-scale convergence, ascendance, and positive relative vorticity anomalies provide favorable conditions for more TC formation in the Philippine Sea. More TCs tend to move along the western boundary of the anomalous anticyclone to cross the northern Philippines toward eastern Taiwan or toward the open oceans southwest of Taiwan. Taiwan is influenced by enhanced TC activity to have more TC rainfall with the largest anomalies being in the eastern parts.

Corresponding author address: Jau-Ming Chen, Institute of Maritime Information and Technology, National Kaohsiung Marine University, No. 482, Jhongjhou 3rd Rd., Kaohsiung, 805, Taiwan. E-mail: cjming@mail.nkmu.edu.tw

Abstract

By separating total rainfall into tropical cyclone (TC) rainfall caused by TC passage and seasonal rainfall associated with moisture transport of prevailing seasonal flows, this study examines the interdecadal variability of these two rainfall components in Taiwan during fall. It is found that interdecadal variability of TC rainfall and seasonal rainfall tends to vary coherently in October. This coherent interdecadal rainfall mode features islandwide patterns in Taiwan and evident interdecadal oscillations. The associated large-scale regulating processes for the positive phase are characterized by warm sea surface temperature (SST) anomalies in the northern South China Sea and the western North Pacific, which in turn induce an anomalous anticyclonic circulation over the subtropical western Pacific to the east of Taiwan. On the western boundary of this anomalous anticyclone, anomalous southeasterly flows enhance mean moisture transport from the tropical western Pacific into Taiwan. Seasonal rainfall increases in Taiwan with stronger anomalies on the windward side over eastern Taiwan. Warm SST anomalies and accompanying large-scale convergence, ascendance, and positive relative vorticity anomalies provide favorable conditions for more TC formation in the Philippine Sea. More TCs tend to move along the western boundary of the anomalous anticyclone to cross the northern Philippines toward eastern Taiwan or toward the open oceans southwest of Taiwan. Taiwan is influenced by enhanced TC activity to have more TC rainfall with the largest anomalies being in the eastern parts.

Corresponding author address: Jau-Ming Chen, Institute of Maritime Information and Technology, National Kaohsiung Marine University, No. 482, Jhongjhou 3rd Rd., Kaohsiung, 805, Taiwan. E-mail: cjming@mail.nkmu.edu.tw

1. Introduction

Taiwan is located in the conjunction zone between East Asia and the western North Pacific (WNP). Its major water resources come from rainfall induced by the East Asian (EA) monsoon and tropical cyclones (TCs). TC activity is most vigorous in the WNP from June to October (e.g., Gray 1975; Frank 1987; McBride 1995; Chu 2004; Chu et al. 2007). According to official statistics compiled by the Central Weather Bureau of Taiwan of the TCs affecting Taiwan during the period 1897–2004, 7.9% occur in June, 23.3% in July, 29.3% in August, 22.6% in September, and 8.8% occur in October. Notable TC activity covers both the summer and fall seasons. A TC normally has a life cycle between 1 and 2 weeks. Its passage may cause torrential rainfall within 1–2 days. TC-induced rainfall is generally regarded as a transient process.

The land–sea contrast between the Asian landmass and the Pacific Ocean results in a significant EA monsoon climate, featuring a southwesterly monsoon in summer and a northeasterly monsoon in fall and winter (e.g., Ding 2004; Chan and Li 2004; Chang et al. 2006). The prevailing northeasterly flows originate from the Siberian–Mongolian high and transport cold and dry air southward. By contrast, the southwesterly flows of summer carry abundant moisture from tropical oceans onto the EA landmass, yielding evident monsoon rainfall in southern and eastern China, Taiwan, and Japan (e.g., Ninomiya and Murakami 1987; Tao and Chen 1987; Ding 1994; Wang and LinHo 2002; Chen and Chen 2003; Hsu 2005). The monsoon rainfall activity in these regions usually lasts for months and is regarded as a seasonal-integration process (e.g., Chen 1994; Wang 1994; Wu 2002).

In summer, TC activity and major monsoon circulations exhibit close dynamic connections on various time scales. Climatologically, most TCs are formed in monsoon-trough regions around the Philippine Sea and the western Pacific (e.g., Chen et al. 1998). On an intraseasonal time scale, the intensification of the monsoon trough and the westerly phase of the monsoon intraseasonal oscillation (ISO) provide favorable conditions for TCs to recurve northward, while the strengthening of the Pacific subtropical high and the easterly phase of the ISO tend to steer TCs in a straight direction (e.g., T.-C. Chen et al. 2009; Ko and Hsu 2006, 2009). In terms of interannual variability, El Niño causes the WNP monsoon trough to shift eastward, which corresponds with an eastward displacement of the location of TC genesis (e.g., Lander 1994; Chen et al. 1998; Wang and Chan 2002; Wang and Zhang 2002). The eastward displacement of the monsoon trough and the Pacific subtropical high can be as high as 60° in longitude in El Niño years when compared with La Niña years (e.g., Clark and Chu, 2002). In general, TC activity tends to be enhanced (weakened) in regions beneath anomalous cyclonic (anticyclonic) flows (e.g., Harr and Elsberry 1995; Chan 2000). In the case of interdecadal variation, a detectable shift in WNP TC activity has been noted for the past several decades (e.g., Ho et al. 2004; Wu et al. 2005; Zhao and Chu 2010). This shift corresponds to increased TC frequency in the regions north of Taiwan in association with weakening of the Pacific subtropical high under the effects of global warming (e.g., Wu and Wang 2004; Tu et al. 2009).

TCs and monsoon circulations are two dominant factors determining local rainfall in the EA–WNP region. In view of these two different mechanisms, rainfall in this region can be broadly separated into TC rainfall and seasonal (or non-TC) rainfall (e.g., Chen et al. 2005; C.-S. Chen et al. 2007; Kubota and Wang 2009; Chen et al. 2010a). Chen et al. (2010b) demonstrated that, in Taiwan, TC rainfall and seasonal rainfall during summer tend to vary inversely in terms of interannual variability. Two major interannual modes appear to lead to excessive TC rainfall but suppressed seasonal rainfall and vice versa. Chen and Chen (2011) further demonstrated that the dominant interdecadal rainfall mode in Taiwan features increasing TC rainfall but decreasing seasonal rainfall. The major large-scale regulatory process is characterized by the weakening and eastward retreat of the Pacific subtropical high. The anomalous northeasterly flows on its western boundary suppress the intensity of prevailing monsoon southwesterly flows from the South China Sea (SCS) into Taiwan, leading to decreased moisture supply and seasonal rainfall. The weakened Pacific subtropical high is accompanied by weakened easterly trade winds and vertical wind shears on its southern boundary, facilitating more TC formation in the open ocean southeast of Taiwan. These TCs are steered by southerly/southeasterly flows on the western boundary of the eastward-retreating Pacific subtropical high to recurve northward toward Taiwan, leading to increased TC rainfall. These processes reveal a clear interaction between TCs and seasonal monsoon circulations in modulating local rainfall variability.

From summer into fall, TC activity is still active in September and October in the EA–WNP region. However, the prevailing seasonal circulation changes from southwesterly to northeasterly flows. With this circulation change, interactions between seasonal circulations and TCs are expected to differ from summer to fall. To examine this hypothesis, the interdecadal variability of TC rainfall and seasonal rainfall in Taiwan during fall is examined in this study. Results of this study will be compared with the summer analysis conducted by Chen and Chen (2011). The major questions raised in this study are as follows:

  • What are the relative contributions of TC rainfall and seasonal rainfall to total rainfall in Taiwan during fall?
  • Is there any systematic relationship between interdecadal variability of TC rainfall and seasonal rainfall in Taiwan during fall? If yes, what are the salient features of such an interdecadal rainfall mode? What are the associated large-scale regulatory processes?
  • What are the differences between the interdecadal rainfall modes of summer and fall?

Climate variability in the EA–WNP region has been intensively examined for summer and winter but much less studied for fall. In fact, evident climate variability features have been found in this region, such as a fall persistence barrier in the SCS sea surface temperature (e.g., J.-M Chen et al. 2007; Zhao and Li 2009) and a quasi-11-yr oscillation in fall temperature for Taiwan (e.g., Chen et al. 2008b). Results of this study should help us to better understand the modulating processes for fall rainfall in Taiwan and the surrounding EA region.

2. Data and methodology

Local rainfall characteristics in Taiwan are depicted by daily rainfall records from 10 major meteorological stations surrounding the coasts of Taiwan (Fig. 1). These stations are located in the plains at altitudes lower than 35 m. The regulatory processes of governing large-scale ocean–atmosphere system for rainfall in Taiwan are delineated in terms of monthly data of the extended reconstruction sea surface temperature (ERSST; e.g., Smith and Reynolds 2003, 2004) and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data (Kalnay et al. 1996). The NCEP–NCAR data are hereafter referred to as the reanalysis data. TC activity is portrayed by the 6-h WNP TC best-track data from the Joint Typhoon Warning Center (JTWC). In this study, analyses span from 1950 to 2008.

Fig. 1.
Fig. 1.

Topography and 10 major meteorological stations in Taiwan.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

Also, the interdecadal component of field A is estimated by its 9-yr running mean and is hereafter denoted as . The application of the running process ought to greatly reduce the degrees of freedom of the analyzed field. To take this effect into account, a measure of the effective degrees of freedom used by Davis (1976), Chen (1982), and Zhao and Li (2009) is employed in the statistical significance test. The effective degree of freedom N is computed as n/T, where n is the number of sample observations and between two different fields or for a single field. Here and are the autocorrelation coefficients of and , respectively, with a time lag of . The maximum of the integer K corresponds to n/2.

3. Large-scale climate features

The climate patterns affecting Taiwan during fall are depicted by climatological (1950–2008) patterns averaged from September and October (i.e., TC active period) extracted from the reanalysis data (Fig. 2). The large-scale circulations are represented by 850-hPa streamfunction S850 and 10-m winds V10. The moisture processes are illustrated by vertically integrated moisture flux VQ and precipitable water W, where , is the horizontal wind vector, q is the specific humidity, and the vertical integral is from a given pressure level to = 1000 hPa. As shown in Fig. 2a, salient circulation features include a subtropical high in the North Pacific and a thermal low in Asia. The discrepancy between summer and fall climate patterns features in a manner where the Pacific subtropical high is weakened and displaced eastward as its western tip shifts from over the open oceans on the southwestern side of Japan in summer (e.g., Chen et al. 2010b) to the eastern side in fall. This weakening is parts of the seasonal transition processes from summer to winter. Another transition feature is the emergence of a weak and separated high north of Taiwan, a precursor of the establishment of the Siberian–Mongolian high over Asia in winter. As revealed by V10 vectors (Fig. 2b), the separated high induces northeasterly flows toward Taiwan. These northeasterly flows are dry and cold: that is, they are not the acting mechanism for moisture transport. As indicted by W field (Fig. 2c), major moisture contents are constrained in the tropics. The moisture flux from the tropics into Taiwan, as shown by VQ (Fig. 2d), is mainly from the tropical western Pacific along the southwestern boundary of the Pacific subtropical high. It is of interest to find that, during fall, Taiwan is under the influence of northeasterly flows from the north but its moisture sources are from the tropical western Pacific to the east of Taiwan.

Fig. 2.
Fig. 2.

The 1950–2008 climatological means of (a) 850-hPa streamfunction, (b) 10-m winds, (c) precipitable water, and (d) vertically integrated moisture flux averaged for the September–October season. Contour intervals are 20 × 105 m2 s−1 for (a) and 5 kg m−2 for (c). Positive values in (a) and values larger than 40 kg m−2 in (c) are shaded.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

4. Fall rainfall characteristics in Taiwan

Local rainfall in Taiwan during fall consists of two major components: TC rainfall caused by TC passage and seasonal rainfall induced by local thunderstorms and moisture supply from the tropical western Pacific. TC rainfall is commonly interpreted as rainfall occurring within a spatial range from the TC center; however, so far, no consensus has been reached with respect to the range. The employed distance criterion varies from about 250–300 km (e.g., Chen et al. 2005; C.-S. Chen et al. 2007) to 500 (e.g., Jiang and Zipser 2010; Lee et al. 2010) and 1000 km (e.g., Kubota and Wang 2009). In Taiwan, the Central Weather Bureau will issue an official warning for a coming TC given the condition that the TC’s surrounding flows (with sustained wind speed exceeding 18 kt) will influence the coasts of Taiwan within 18 h. TCs generally move with a speed of 10–15 km h−1, which is equal to a distance of 180–270 km in 18 h. Kubota and Wang (2009) showed that the majority of TC rainfall occurs within about 250 km of the storm. Rainfall occurring in Taiwan during the TC warning period is naturally considered as TC rainfall. As such, Chen et al. (2010b) suggested that a distance criterion of 2.5° in longitude and latitude (approximate 250 km) as the impact zone to categorize TC rainfall. Chen and Chen (2011) demonstrated that such a definition helped to clearly portray a significant interdecadal mode of summer rainfall in Taiwan, featuring increasing TC rainfall but decreasing seasonal monsoon rainfall. This study employs Chen et al.’s (2010b) approaches to categorize TC rainfall in September and October. For the days with a TC’s center in a region close to Taiwan within 2.5° in latitude and longitude (19.5°–27.5°N, 117.5°–124.5°E; referred to as the impact zone), rainfall occurring in Taiwan during these days is considered to be caused by a TC’s surrounding flows and defined as TC rainfall PTC. TC cases analyzed in this study include both tropical storms and typhoons with maximum sustained wind speeds exceeding 34 kt. The remaining parts of local rainfall primarily result from moisture supply carried by seasonal flows, which are sorted together as seasonal rainfall PSN.

The spatial distribution of climatological (1950–2008) means of PTC and PSN in Taiwan for September and October are shown in Fig. 3. In September, the 10-station average is 154 mm for PTC and 132 mm for PSN, accounting for 53.8% and 46.2% of total rainfall, respectively. In October, the average is 43 mm for PTC and 116 mm for PSN, contributing to 27% and 73% of total rainfall, respectively. Overall, rainfall during fall is larger in eastern Taiwan than in western Taiwan. This is because eastern Taiwan is on the windward side of major rainfall mechanisms originating from the tropical western Pacific: moisture transport by seasonal flows and TC passage. In eastern Taiwan, a significant decrease of PTC from September to October reflects large-scale background features becoming less favorable for TC activity. It is associated with a southward shift of major TC tracks in the northern SCS–Philippine Sea regions from the latitudes around 20°N in September to 15°N in October. A TC closer to Taiwan should induce more TC rainfall than a remote one. By contrast, PSN in eastern Taiwan is even larger in October than in September. The northeasterly flow becomes more intense in October than in September in accordance with the progression of seasonal cooling. The moist southeasterly flow from the tropical western Pacific and the intensified northeasterly flows from the north may converge over eastern Taiwan. The confluent flows later interact with the 2–3-km-high Central Mountain Range (see Fig. 1) to result in more seasonal rainfall in October, yielding the largest records of PSN in the northeastern parts of Taiwan (i.e., 314 mm at Ilan station).

Fig. 3.
Fig. 3.

The 1950–2008 climatological means of tropical cyclone rainfall PTC and seasonal rainfall PSN in Taiwan: (a) September PTC, (b) September PSN, (c) October PTC, and (d) October PSN. The unit is millimeters.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

5. Interdecadal variability of TC rainfall and seasonal rainfall

Interdecadal rainfall variability in Taiwan is depicted by the 9-yr running means of PTC (denoted as ) and PSN (denoted as ) averaged from 10 stations in Fig. 1. As shown in Fig. 4a, and time series for September exhibit relatively different variability features. The standard deviation (SD) is 42 mm for and 17 mm for . Here, varies with a much larger magnitude than . For the variability phase, and tend to evolve independently. The correlation coefficient between these two time series is 0.14, insignificant at the 0.1 level with an effective degree of freedom around 19. The long-term trend, as estimated by the linear regression method, is negative for (−0.034 mm yr−1) but positive for (0.163 mm yr−1). For October rainfall (Fig. 4b), and tend to fluctuate rather coherently. Both time series exhibit a positive trend, 0.744 mm yr−1 for and 0.189 mm yr−1 for . Their correlation coefficient is as high as 0.68, significant at the 0.01 level with an effective degree of freedom around 14. The SD is 26 mm for and 27 mm for , showing a comparable magnitude between their interdecadal variability.

Fig. 4.
Fig. 4.

Time series of interdecadal components of PTC and PSN averaged from 10 Taiwan stations for (a) September and (b) October.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

The above analyses demonstrate that interdecadal rainfall mode in Taiwan is dominated by in September. On the other hand, and tend to vary coherently with a comparable magnitude in October. The coherent mutual relationship in October differs from the reverse phase relationship between TC rainfall and seasonal monsoon rainfall in summer reported by Chen and Chen (2011). Thus, it is of interest to examine rainfall characteristics and regulatory processes for the newly found coherent interdecadal rainfall mode in Taiwan during October. Their characteristics will be compared with those of the summer interdecadal rainfall mode.

To illustrate the spatiotemporal characteristics of the coherent interdecadal rainfall mode, both and from 10 major Taiwan stations in October are simultaneously subject to empirical orthogonal function (EOF) analysis. This analysis acts as a combined EOF with the input of 10 anomalies and 10 anomalies. The first eigenmode accounts for 71% of interdecadal rainfall variance from these stations. Its spatial variability [the first eigenvector (E1)] features positive anomalies for (Fig. 5a) and (Fig. 5b) throughout the island. The coherent mode exhibits an islandwide pattern. Rainfall anomalies are much larger in eastern Taiwan than in western Taiwan in both components. The corresponding temporal variability [the first principal component (C1)] contains noticeable interdecadal oscillations (Fig. 5c). It exhibits minimum phases around the late 1950s, the late 1970s, and the early 2000s and maximum phases in the early 1970s and the mid-1990s. Power spectral analysis of C1 (not shown) time series reveals two significant interdecadal modes with periods of 26 and 17 yr, suggesting the occurrence of a near-20-yr fluctuation. The correlation coefficients between C1 and Taiwan-averaged and time series in Fig. 4b are 0.95 and 0.86, respectively. In fact, the first EOF mode of () accounts for 82% (88%) of total interdecadal variance (not shown). Its principal components are highly correlated with that of the first joint EOF mode with a correlation coefficient of 0.86 (0.94). Moreover, its eigenvectors exhibit positive anomalies over all Taiwan stations, featuring stronger anomalies in eastern Taiwan than in western Taiwan. These results indicate that the first joint EOF mode is representative of dominant interdecadal variability for both TC rainfall and seasonal rainfall in Taiwan during October. The islandwide climate variability patterns in Taiwan are normally regulated by large-scale processes associated with variations in oceanic boundary conditions and low-level circulations (e.g., Jiang et al. 2003; Chen et al. 2008a,b). The large-scale regulating processes for the first interdecadal rainfall mode are thus analyzed.

Fig. 5.
Fig. 5.

The first joint interdecadal mode of TC rainfall and seasonal rainfall in Taiwan: (a) the first eigenvector of TC rainfall, (b) the first eigenvector of seasonal rainfall, and (c) their first principal components. This mode accounts for 71% of total interdecadal variance of TC rainfall and seasonal rainfall.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

6. Large-scale regulatory processes

Large-scale processes regulating the first interdecadal rainfall mode are investigated from simultaneous correlation patterns corresponding to the C1 time series in Fig. 5c. Hereafter, the correlation patterns significant at the 0.05 level, with consideration given for effective degrees of freedom, are indicated by shading. Note that the first eigenvectors of rainfall in Taiwan in Fig. 5 show positive anomalies over all stations. A positive correlation value between C1 time series and the large-scale fields indicates that positive anomalies of the large-scale fields correspond to increased rainfall in Taiwan. Correlated patterns in Fig. 6a exhibit significant positive values in the northern SCS–tropical western Pacific region and negative values in the tropical central Pacific. This east–west contrast of correlation patterns reflects a change in tropical heating which in turn causes changes in tropical Walker circulation to modulate the overlying atmospheric circulation (e.g., Wang 1992; Klein et al. 1999; Lau and Nath 2000, 2003; J.-M. Chen et al. 2009). Tropical warm SST anomalies act as a heating source, while cold anomalies act as a heating sink. The impacts of tropical heating on atmospheric circulation can be suitably depicted by the 850-hPa velocity potential (X850). The correlated patterns (Fig. 6b) exhibit a significant and dominant convergent center (positive value) in the SCS–Maritime Continent region and a divergent zone in the farthermost eastern Pacific. The convergent center spatially corresponds with the positive correlation patterns of SST (or warm SST anomalies). In association with the appearance of the dominant tropical convergent center, the low-level circulation (represented by the 850-hPa streamfunction S850) should change accordingly through a Rossby wave–like response (e.g., Matsuno 1966; Gill 1980). Under this response, the correlated values (Fig. 6c) exhibit a positive pattern in the Northern Hemisphere and a negative pattern in the Southern Hemisphere on the eastern side of the convergent center, pointing to the existence of a pair of anomalous anticyclonic circulations straddling the equator. This indicates that Taiwan is influenced by a significant anomalous anticyclonic circulation elongated in the regions east of Taiwan.

Fig. 6.
Fig. 6.

Anomalous patterns correlated with C1 time series of the first mode in October: (a) sea surface temperature, (b) 850-hPa velocity potential to depict low-level large-scale divergent circulation, and (c) 850-hPa streamfunction to illustrate low-level large-scale circulation. In (a)–(c), contour intervals are 0.1 and the zero contour is suppressed. Correlation patterns significant at the 0.05 level are indicated by shading with consideration given for effective degrees of freedom.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

Local rainfall variability in the EA region is strongly affected by moisture transport of the large-scale circulation anomalies (e.g., Simmonds et al. 1999; Lau et al. 2000). Variability of seasonal monsoon rainfall in Taiwan during summer was found to be closely controlled by moisture flux from monsoon southwesterly flows originating from the SCS (e.g., Chen et al. 2005, 2010b). Similarly, it is reasonably to infer that in Taiwan during October is likely to be modulated by the significant anomalous anticyclonic circulation to the east of Taiwan, where seasonal moisture flux is most evident (see Fig. 2d). To justify this inference, moisture processes depicted by moisture flux VQ, precipitable water W, and divergence of moisture flux (denoted as DIVQ) are analyzed. Their correlation patterns with respect to C1 time series are shown in Fig. 7.

Fig. 7.
Fig. 7.

As in Fig. 6, but for anomalous correlation patterns of (a) vertically integrated moisture flux, (b) precipitable water, and (c) divergence of moisture flux. Contour intervals are 0.1 for (b) and 0.2 for (c). In (b) and (c), the zero contour is suppressed. Correlation patterns significant at the 0.05 level are indicated by shading with consideration given for effective degrees of freedom.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

As shown in Fig. 7a, the correlation patterns feature an anticyclonic circulation to the east of Taiwan. This indicates that an anomalous anticyclonic circulation to the east of Taiwan induces significant southeasterly VQ anomalies along its southern boundary. These anomalies act to enhance moisture transport from the tropical ocean, across Taiwan, toward eastern China. This moisture transport increases moisture contents in the atmosphere. Thus, positive correlation patters of stretch from the open oceans south of Japan southwestward across Taiwan toward the SCS (Fig. 7b), indicating increased moisture content in these regions. Moreover, patterns in Fig. 6b show that the vicinity of Taiwan is within the large-scale convergence region. Moisture convergence following increased moisture supply thus appears in the regions from the Philippine Sea across Taiwan toward southeastern China, as revealed by significant negative correlation patterns in Fig. 7c. The excessive moisture transport by the significant anomalous anticyclonic circulation causes increased moisture content and moisture convergence over Taiwan. These factors provide favorable conditions for more seasonal rainfall on the windward side of the moisture transport. As such, positive anomalies occur in Taiwan with particularly large anomalies over eastern Taiwan (see Fig. 5b).

7. Interdecadal variability of TC activity

The PTC defined in this study is largely determined by TC activity in the vicinity of Taiwan. Chen and Chen (2011) pointed out that there are two key factors affecting the resultant TC rainfall: TC formation and movement track. The gross picture of these two factors can be well illustrated in terms of TC frequency , which is represented by the total count of TC appearance in every 3° × 3° box in October computed from the 6-h JTWC best-track data. Correlation patterns of with respect to C1 time series are shown in Fig. 8a. As revealed by significant patterns, positive values appear in the tropical oceans to the east of the Philippines, which is the major TC genesis region (e.g., Gray 1975; McBride 1995). Positive correlation values extend from the Philippine Sea northwestward across the northern Philippines toward open oceans southwest of Taiwan, while a minor positive pattern recurves from the northern SCS northeastward/northward toward the eastern coasts of Taiwan. The above positive patterns reflect that on an interdecadal time scale increase of TC rainfall in Taiwan is associated with more TC formation in the Philippine Sea and enhanced TC activity in the vicinity of Taiwan. Composite anomalies of two maximum (1969–74 and 1993–98) and two minimum (1954–59 and 1978–83) phases of time series in Fig. 4b reveal that more TC rainfall in Taiwan concurs with enhanced TC activity in the surrounding regions (not shown). Moreover, their difference anomalies (maximum minus minimum) exhibit spatial patterns highly resembling the correlation patterns in Fig. 8a, suggesting that enhanced TC activity in the SCS–Taiwan regions is the major reason for increased TC rainfall in Taiwan. The eastern parts of Taiwan are on the windward side of TCs coming from the northern Philippines, leading to stronger positive anomalies in eastern Taiwan than in western Taiwan in Fig. 5a.

Fig. 8.
Fig. 8.

As in Fig. 6, but for correlation patterns of (a) TC frequency, (b) 500-hPa vertical motion, and (c) 1000-hPa relative vorticity. Contour intervals are 0.2 and the zero contour is suppressed. Correlation patterns significant at the 0.05 level are indicated by shading with consideration given for effective degrees of freedom.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

Large-scale processes connected with the above TC formation and track features are discussed in this section. In terms of the thermal condition, correlated patterns (see Fig. 6a) reveal that significant warm SST anomalies furnish favorable conditions for more TC formation in the Philippine Sea. Warm anomalies are accompanied by large-scale low-level convergent anomalies across the SCS and Philippine Sea (see Fig. 6b), which in turn induce a low-level anomalous anticyclone in the subtropical western Pacific east of Taiwan (see Fig. 6c). Warm SST and convergent anomalies should facilitate TC activity via the enhancements of upward motion and relative vorticity. Correlation patterns of 500-hPa vertical motion ω500 and 1000-hPa relative vorticity ʒ1000 with respect to C1 time series are shown in Figs. 8b,c, respectively. The correlation patterns feature significant negative values of vertical motion and positive values of relative vorticity in the vicinity of Taiwan with a southwest–northeast orientation. Moreover, positive correlation values of relative vorticity exist in the Philippine Sea in concurrence with more TC formation. These results indicate that warm SSTs (see Fig. 6a) combine with enhanced ascendance and relative vorticity to facilitate more TC formation in the Philippine Sea (e.g., McBride 1995) and enhanced TC activity in the SCS and open oceans near Taiwan. The existence of an anomalous anticyclone over the northwestern Pacific induces anomalous southeasterly flows along its western boundary (see Fig. 7a), which tend to guide TCs formed in the Philippine Sea toward Taiwan. In brief, warm SST, large-scale convergence, and enhanced ascendance and relative vorticity set up advantageous conditions for increased TC formation in the Philippine Sea. These TCs are later steered by an anomalous anticyclone east of Taiwan to recurve toward Taiwan, yielding increases in TC frequency and TC rainfall in Taiwan.

8. Concluding remarks

In Taiwan, rainfall is generally separated into two major components in accordance with different generating mechanisms: TC rainfall caused by the passage of TCs and seasonal rainfall associated with moisture transport of the prevailing seasonal flows. The active TC season is prolonged, covering June–October in concurrence with monsoon southwesterly flows in summer but northeasterly flows in fall. Chen and Chen (2011) found a reverse phase relationship between the interdecadal variability of TC rainfall and seasonal rainfall in Taiwan during summer. From summer to fall, characteristics of TC activity and background flows vary markedly. The interdecadal features of rainfall variability in Taiwan during fall are likely to behave differently from those of summer and are thus of interest to this study. The main purpose of this study is to examine the mutual relationship between TC rainfall and seasonal rainfall on an interdecadal time scale in Taiwan during fall. Large-scale processes regulating the salient interdecadal rainfall mode are discussed.

This study adopts Chen et al.’s (2010b) definition to categorize TC rainfall PTC as the parts occurring in Taiwan during days when a TC’s center is within 2.5° in latitude and longitude (19.5°–27.5°N, 117.5°–124.5°E) of Taiwan. The remaining parts of rainfall in Taiwan are categorized as seasonal rainfall PSN, which is largely associated with local thunderstorms and moisture supply from the tropical western Pacific. Climatologically, PTC and PSN averaged from 10 Taiwan stations are relatively comparable in September, 53.8% versus 46.2% of total rainfall. In October, PSN dominates PTC by contributing 73% of total rainfall. The 9-yr running means of PTC and PSN (denoted as and , respectively) are employed to delineate their interdecadal components. The and time series averaged from 10 stations in Taiwan exhibit a correlation coefficient of only 0.14 in September and 0.68 in October. These two rainfall components tend to vary independently in September but coherently in October. The coherent interdecadal rainfall mode in October is in contrast to the reverse interdecadal mode in summer. This study focuses on the salient variability features of the coherent mode in October.

As revealed by EOF analysis, the dominant interdecadal rainfall mode in October exhibit the same sign for both and anomalies throughout all analyzed stations in Taiwan, reflecting coherent spatial variability features. The and anomalies are comparable in their magnitudes, and both are stronger in eastern Taiwan than in western Taiwan. Their associated temporal variability characterizes notable interdecadal fluctuations, while the long-term trend is indiscernible. The primary large-scale processes regulating this coherent interdecadal rainfall mode are illustrated by the schematic diagrams in Fig. 9a. Significant warm SST anomalies appear in the northern SCS–WNP region to force strong low-level convergence and ascending motion anomalies. Through a Rossby wave–like response, an anomalous low-level anticyclonic circulation exists to the northeast of the anomalous convergent center. Anomalous southeasterly flows on the western boundary of the anomalous anticyclone enhance mean moisture transport from the tropical western Pacific into Taiwan, leading to increased (S+) in Taiwan with stronger magnitude on the windward side over eastern Taiwan. Meanwhile, anomalies of warm SSTs, low-level convergence, and positive relative vorticity facilitate more TC formation in the Philippine Sea. Later, more TCs tend to move northwestward along the western boundary of the anomalous anticyclone to cross the northern Philippines toward the open oceans southwest of Taiwan or toward eastern Taiwan. Taiwan is influenced by enhanced TC activity to result in increased (T+) over all of Taiwan with larger anomalies in its eastern parts.

Fig. 9.
Fig. 9.

Schematic diagrams for the major processes regulating the interdecadal mode of TC rainfall and seasonal rainfall in Taiwan during (a) October and (b) summer. The symbols are as follows: T+ for increased TC rainfall, S+ for increased seasonal rainfall, S− for decreased seasonal rainfall, gray arrow for moisture flux, dark arrow for TC track, rings for TC genesis, DIV for large-scale divergence, CON for large-scale convergence, AC for anticyclonic circulation, and C for cyclonic circulation. Details of these processes are described in the text.

Citation: Journal of Climate 26, 1; 10.1175/JCLI-D-11-00697.1

The interdecadal relationship between TC rainfall and seasonal rainfall in Taiwan is found to vary coherently in October but inversely in summer (e.g., Chen and Chen 2011). To compare these two modes, salient regulating features for the summer interdecadal rainfall mode analyzed by Chen and Chen (2011) are illustrated schematically in Fig. 9b. Warm summer SST anomalies exist in the tropics, with strong warming in the central–eastern Pacific and the Indian Ocean and minor warming in the western Pacific. This east–west contrast induces a low-level divergent anomaly in the western Pacific, which in turn induces an anomalous low-level cyclone along the subtropical WNP as a Rossby wave–like response. Anomalous northeasterly flows on the western boundary of the anomalous cyclone suppress moisture transport that would otherwise be carried on by southwesterly flows from the SCS into Taiwan, resulting in decreased seasonal rainfall (S−). The appearance of an anomalous cyclone reflects weakening in the Pacific subtropical high and the easterly trade winds on its southern boundary. As such, weakened vertical wind shear occurs over the Philippine Sea to facilitate more TC formation over there. The formed TCs tend to be guided by the anomalous cyclone to recurve northward toward Taiwan. More TC frequency thus occurs along the eastern coasts of Taiwan, leading to increased TC rainfall (T+) in Taiwan. As a result, the reverse in the interdecadal phase relationship in summer features increased TC rainfall and decreased seasonal rainfall.

Based on the schematic diagrams of Fig. 9, two interesting differences between the large-scale features of summer and October interdecadal rainfall modes in Taiwan are noted herein. First, SST anomalies associated with the summer rainfall mode show strong warming in the central–eastern Pacific and minor warming in the western Pacific, resembling the major features of global warming (e.g., Nitta and Yamada 1989; Deser and Blackmon 1995). TC rainfall and seasonal rainfall exhibit increasing and decreasing trends, respectively. These trend features to some degree link to the trend in global warming (e.g., Chen and Chen 2011). SST anomalies associated with the October mode feature warming in the western Pacific and cooling in the central Pacific. This pattern seems to be independent from global warming patterns. Accordingly, both TC rainfall and seasonal rainfall in Taiwan only exhibit interdecadal fluctuation but no discernible trend. Second, the summer mode features weakening in the Pacific subtropical high and monsoon southwesterly flows, resulting in decreased seasonal rainfall. In October, an intensified Pacific subtropical high enhances moisture transport from the tropical western Pacific toward Taiwan, yielding increased seasonal rainfall.

On the other hand, the summer and October interdecadal rainfall modes in Taiwan exhibit coherent dynamics in their large-scale regulating processes. The interdecadal variability of the summer monsoon is considered a global coupled ocean–atmosphere mode (e.g., Goswami 2006). In the EA region, the ocean–atmosphere interactions are generally depicted as atmospheric circulation being effectively regulated by tropical SST anomalies via the processes of a Rossby wave response (e.g., Chang et al. 2000a,b). As shown in Fig. 9, similar Rossby wave responses of subtropical circulations to tropical SST anomalies in the Pacific and Indian Ocean are notable in both summer and October interdecadal rainfall modes. In fact, Chen et al. (2008b) found that tropical SST anomalies can systematically regulate subtropical circulation to induce a near-11-yr oscillation of fall temperature in Taiwan. Their regulating processes are coherent with that of the October interdecadal rainfall mode shown in Fig. 9a. The above features of the summer and October interdecadal rainfall modes should provide informative guidance for predicting future climate change patterns in Taiwan. The associated large-scale regulatory processes are potentially useful for diagnosing rainfall variability patterns in the EA–Pacific region.

In October, major TC tracks shift southward to cross the northern Philippines and the SCS. Some of the TCs may concur with rainfall in eastern Taiwan: even the spatial distance is about 500 km or more (e.g., Kubota and Wang 2009). This is a special case regarding a TC’s remote effect on local rainfall in Taiwan. The aforementioned rainfall is not directly induced by a TC’s surrounding flows. Instead, it results from a connection between a TC and a subtropical front. A TC’s low pressure system in the south may remotely interact with a preexisting subtropical front in the north, leading to the enhancement of seasonal northeasterly flows toward Taiwan. These flows merge with seasonal southeasterly flow in the south from the western boundary of the Pacific subtropical high. These confluent flows cause rainfall on the windward side in northeastern and eastern Taiwan, while rainfall on the lee side in western Taiwan is relatively weak. TCs Joe (1983) and Parma (2009) are two cases with this connection. Rainfall in Taiwan associated with such TC–front connection is primarily induced by the seasonal flows and thus categorized as seasonal rainfall. However, a TC’s remote effect is partially involved in this rainfall activity. It is of interest to ask how much rainfall in Taiwan during fall results from this type of TC–front connection. This is a challenging subject for a future study to explore.

Acknowledgments

The authors thank the anonymous reviewers for their valuable comments. This study was supported by the National Science Council, Taiwan, under Grants NSC 99-2628-M-022-001 and NSC 98-2745-M-001-005-MY3.

REFERENCES

  • 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., , and C. Li, 2004: The East Asian winter monsoon. East Asian Monsoon, C.-P. Chang, Ed., Series on Meteorology of East Asia, Vol. 1, World Scientific, 54–106.

  • Chang, C.-P., , Y. Zhang, , and T. Li, 2000a: Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: Roles of subtropical ridge. J. Climate, 13, 43104325.

    • Search Google Scholar
    • Export Citation
  • Chang, C.-P., , Y. Zhang, , and T. Li, 2000b: Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part II: Meridional structure of the monsoon. J. Climate, 13, 43264340.

    • Search Google Scholar
    • Export Citation
  • Chang, C.-P., , Z. Wang, , and H. Hendon, 2006: The Asian winter monsoon. The Asian Monsoon, B. Wang, Ed., Springer/Praxis, 89–128.

  • Chen, C.-S., , and Y.-L. Chen, 2003: The rainfall characteristics of Taiwan. Mon. Wea. Rev., 131, 13231341.

  • Chen, C.-S., , Y.-L. Chen, , C.-L. Liu, , P.-L. Lin, , and W.-C. Chen, 2007: Statistics of heavy rainfall occurrences in Taiwan. Wea. Forecasting, 22, 9811002.

    • Search Google Scholar
    • Export Citation
  • Chen, G. T. J., 1994: Large-scale circulation associated with the East Asian summer monsoon and the mei-yu over South China and Taiwan. J. Meteor. Soc. Japan, 72, 959983.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , and H.-S. Chen, 2011: Interdecadal variability of summer rainfall in Taiwan associated with tropical cyclones and monsoon. J. Climate, 24, 57865798.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , F.-C. Lu, , S.-L. Kuo, , and C.-F. Shih, 2005: Summer climate variability in Taiwan and associated large-scale processes. J. Meteor. Soc. Japan, 83, 499516.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , T. Li, , and C.-F. Shih, 2007: Fall persistence barrier of sea surface temperature in the South China Sea associated with ENSO. J. Climate, 20, 158172.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , T. Li, , and C.-F. Shih, 2008a: Asymmetry of the El Niño-spring rainfall relationship in Taiwan. J. Meteor. Soc. Japan, 86, 297312.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , F.-C. Lu, , and C.-F. Shih, 2008b: The decadal oscillation of fall temperature in Taiwan. Terr. Atmos. Oceanic Sci., 19, 497504.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , B. Wang, , J.-W. Hwu, , and C.-F. Shih, 2009: Potential predictability of tropical low-level circulation in CWB GFS ensemble hindcast. J. Meteor. Soc. Japan, 87, 171188.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , J.-L. Chu, , C.-F. Shih, , and Y.-C. Tung, 2010a: Interannual variability of circulation-rainfall relationship in Taiwan during the mei-yu season. Int. J. Climatol., 30, 22642276.

    • Search Google Scholar
    • Export Citation
  • Chen, J.-M., , T. Li, , and C.-F. Shih, 2010b: Tropical cyclone–and monsoon-induced rainfall variability in Taiwan. J. Climate, 23, 41074120.

    • Search Google Scholar
    • Export Citation
  • Chen, T.-C., , S.-P. Weng, , N. Yamazaki, , and S. Kiehne, 1998: Interannual variation in the tropical cyclone formation over the western North Pacific. Mon. Wea. Rev., 126, 10801090.

    • Search Google Scholar
    • Export Citation
  • Chen, T.-C., , S.-Y. Wang, , M.-C. Yen, , and A. 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
  • Chen, W. Y., 1982: Fluctuation in Northern Hemisphere 700 mb height field associated with the Southern Oscillation. Mon. Wea. Rev., 110, 808823.

    • Search Google Scholar
    • Export Citation
  • Chu, P.-S., 2004: ENSO and tropical cyclone activity. Hurricanes and Typhoons: Past, Present, and Potential, R. J. Murnane and K.-B. Liu, Eds., Columbia University Press, 279–332.

  • Chu, P.-S., , X. Zhao, , C.-T. Lee, , and M.-M. Lu, 2007: Climate prediction of tropical cyclone activity in the vicinity of Taiwan using the multivariate least absolute deviation regression method. Terr. Atmos. Oceanic Sci., 18, 805825.

    • Search Google Scholar
    • Export Citation
  • Clark, J. D., , and P.-S. Chu, 2002: Interannual variation of tropical cyclone activity over the central North Pacific. J. Meteor. Soc. Japan, 80, 403418.

    • Search Google Scholar
    • Export Citation
  • Davis, R. E., 1976: Predictability of sea-surface temperature and sea-level pressure anomalies over the North Pacific Ocean. J. Phys. Oceanogr., 6, 249266.

    • Search Google Scholar
    • Export Citation
  • Deser, C., , and M. L. Blackmon, 1995: On the relationship between tropical and North Pacific sea surface temperature variations. J. Climate, 8, 16771680.

    • Search Google Scholar
    • Export Citation
  • Ding, Y., 1994: Monsoon over China. Kluwer Academic, 419 pp.

  • Ding, Y., 2004: Seasonal march of the East-Asian summer monsoon. East Asian Monsoon, C.-P. Chang, Ed., Series on Meteorology of East Asia, Vol. 1, World Scientific, 3–53.

  • Frank, W. M., 1987: Tropical cyclone formation. A Global View of Tropical Cyclones, R. L. Elsberry, Ed., Office of Naval Research, 53–90.

  • Gill, A. E., 1980: Some simple resolutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447462.

  • Goswami, B. N., 2006: The Asian monsoon: Interdecadal variability. The Asian Monsoon, B. Wang, Ed., Springer/Praxis, 295–327.

  • Gray, W. M., 1975: Tropical cyclone genesis. Colorado State University Atmospheric Sciience Paper 323, 121 pp.

  • Harr, P. A., , and R. L. Elsberry, 1995: 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
  • Ho, C.-H., , J.-J. Baik, , J.-H. Kim, , and D.-Y. Gong, 2004: Interdecadal changes in summertime typhoon tracks. J. Climate, 17, 17671776.

  • Hsu, H.-H., 2005: East Asian and western North Pacific summer monsoon region. Intraseasonal Variability in the Atmosphere-Ocean System, K.-M. Lau and D. Waliser, Eds., Springer-Praxis, 65–98.

  • Jiang, H., , and E. J. Zipser, 2010: Contribution of tropical cyclones to the global precipitation from eight seasons of TRMM data: Regional, seasonal, and interannual variations. J. Climate, 23, 15261543.

    • Search Google Scholar
    • Export Citation
  • Jiang, Z., , G. T.-J. Chen, , and M.-C. Wu, 2003: Large-scale circulation patterns associated with heavy spring rain events over Taiwan in strong and non-ENSO years. Mon. Wea. Rev., 131, 17691782.

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

  • Klein, S. A., , B. J. Soden, , and N.-C. Lau, 1999: Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge. J. Climate, 12, 917932.

    • Search Google Scholar
    • Export Citation
  • Ko, K.-C., , and H.-H. Hsu, 2006: Sub-monthly circulation features associated with tropical cyclone track over the East Asian monsoon area during July-August season. J. Meteor. Soc. Japan, 84, 871889.

    • Search Google Scholar
    • Export Citation
  • Ko, K.-C., , and H.-H. Hsu, 2009: ISO modulation on the submonthly wave pattern and recurving tropical cyclones in the tropical western North Pacific. J. Climate, 22, 582599.

    • Search Google Scholar
    • Export Citation
  • Kubota, H., , and B. Wang, 2009: How much do tropical cyclones affect seasonal and interannual rainfall variability over the western North Pacific? J. Climate, 22, 54955510.

    • Search Google Scholar
    • Export Citation
  • Lander, M., 1994: An exploratory analysis of the relationship between tropical storm formation in the western North Pacific and ENSO. Mon. Wea. Rev., 122, 636651.

    • Search Google Scholar
    • Export Citation
  • Lau, K.-M., and Coauthors, 2000: A report of the field operations and early results of the South China Sea Monsoon Experiment (SCSMEX). Bull. Amer. Meteor. Soc., 81, 12611270.

    • Search Google Scholar
    • Export Citation
  • Lau, N.-C., , and M. J. Nath, 2000: Impacts of ENSO on the variability of the Asian–Australian monsoons as simulated in GCM experiments. J. Climate, 13, 42874309.

    • Search Google Scholar
    • Export Citation
  • Lau, N.-C., , and M. J. Nath, 2003: Atmosphere–ocean variations in the Indo-Pacific sector during ENSO episodes. J. Climate, 16, 320.

    • Search Google Scholar
    • Export Citation
  • Lee, M.-H., , C.-H. Ho, , and J.-H. Kim, 2010: Influence of tropical cyclone landfalls on spatiotemporal variations in typhoon season rainfall over South China. Adv. Atmos. Sci., 27, 443454.

    • Search Google Scholar
    • Export Citation
  • Matsuno, T., 1966: Quasi-geostrophic motions in equatorial areas. J. Meteor. Soc. Japan, 44, 2543.

  • McBride, J. J., 1995: Tropical cyclone formation. Global perspectives on tropical cyclones, R. L. Elsberry, Ed., World Meteorological Organization Tech. Doc. WMO/TD-693, 63–105.

  • Ninomiya, K., , and T. Murakami, 1987: The early summer rainy season (Baiu) over Japan. Monsoon Meteorology, C.-P. Chang and T. K. Krishnamurti, Eds., Oxford University Press, 93–121.

  • Nitta, T., , and S. Yamada, 1989: Recent warming of tropical sea surface temperature and its relationship to the Northern Hemisphere circulation. J. Meteor. Soc. Japan, 67, 375383.

    • Search Google Scholar
    • Export Citation
  • Simmonds, I., , D. Bi, , and P. Hope, 1999: Atmospheric water vapor flux and its association with rainfall over China in summer. J. Climate, 12, 13531367.

    • Search Google Scholar
    • Export Citation
  • Smith, T. M., , and R. W. Reynolds, 2003: Extended reconstruction of global sea surface temperature based on COADS data (1854–1997). J. Climate, 16, 16011612.

    • Search Google Scholar
    • Export Citation
  • Smith, T. M., , and R. W. Reynolds, 2004: Improved extended reconstruction of SST (1854–1997). J. Climate, 17, 24662477.

  • Tao, S., , and L. Chen, 1987: A review of recent research on the East Asian summer monsoon over China. Monsoon Meteorology, C.-P. Chang and T. K. Krishnamurti, Eds., Oxford University Press, 50–92.

  • Tu, J.-Y., , C. Chou, , and P.-S. Chu, 2009: The abrupt shift of typhoon activity in the vicinity of Taiwan and its association with western North Pacific–East Asian climate change. J. Climate, 22, 36173628.

    • Search Google Scholar
    • Export Citation
  • Wang, B., 1992: The vertical structure and development of the ENSO anomaly mode during 1979–1989. J. Atmos. Sci., 49, 698712.

  • Wang, B., 1994: Climatic regimes of tropical convection and rainfall. J. Climate, 7, 11091118.

  • 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
  • Wang, B., , and LinHo, 2002: Rainy season of the Asian–Pacific summer monsoon. J. Climate, 15, 386398.

  • Wang, B., , and Q. Zhang, 2002: Pacific–East Asian teleconnection. Part II: How the Philippine Sea anomalous anticyclone is established during El Niño development. J. Climate, 15, 32523265.

    • Search Google Scholar
    • Export Citation
  • Wu, L., , and B. Wang, 2004: Assessing impacts of global warming on tropical cyclone tracks. J. Climate, 17, 16861698.

  • Wu, L., , B. Wang, , and S. Geng, 2005: Growing typhoon influence on East Asia. Geophys. Res. Lett., 32, L18703, doi:10.1029/2005GL022937.

  • Wu, R., 2002: Processes for the northeastward advance of the summer monsoon over the western North Pacific. J. Meteor. Soc. Japan, 80, 6783.

    • Search Google Scholar
    • Export Citation
  • Zhao, X., , and J. Li, 2009: Possible causes for the persistence barrier of SSTA in the South China Sea and the vicinity of Indonesia. Adv. Atmos. Sci., 26, 11251136.

    • Search Google Scholar
    • Export Citation
  • Zhao, X., , and P.-S. Chu, 2010: Bayesian changepoint analysis for extreme events (typhoons, heavy rainfall, and heat waves): An RJMCMC approach. J. Climate, 23, 10341046.

    • Search Google Scholar
    • Export Citation
Save