• Ashok, K., S. K. Behera, S. A. Rao, H. Weng, and T. Yamagata, 2007: El Niño Modoki and its possible teleconnection. J. Geophys. Res., 112, C11007, https://doi.org/10.1029/2006JC003798.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Balmaseda, M. A., K. Mogensen, and A. Weaver, 2013: Evaluation of the ECMWF Ocean Reanalysis ORAS4. Quart. J. Roy. Meteor. Soc., 139, 11321161, https://doi.org/10.1002/qj.2063.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Berry, G., and M. J. Reeder, 2014: Objective identification of the intertropical convergence zone: Climatology and trends from the ERA-Interim. J. Climate, 27, 18941909, https://doi.org/10.1175/JCLI-D-13-00339.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Briegel, L. M., and W. M. Frank, 1997: Large-scale influences on tropical cyclogenesis in the western North Pacific. Mon. Wea. Rev., 125, 13971413, https://doi.org/10.1175/1520-0493(1997)125<1397:LSIOTC>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Cai, W., and Coauthors, 2014: Increasing frequency of extreme El Niño events due to greenhouse warming. Nat. Climate Change, 4, 111116, https://doi.org/10.1038/nclimate2100.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Camargo, S. J., and A. H. Sobel, 2005: Western North Pacific tropical cyclone intensity and ENSO. J. Climate, 18, 29963006, https://doi.org/10.1175/JCLI3457.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Camargo, S. J., K. A. Emanuel, and A. H. Sobel, 2007: Use of a genesis potential index to diagnose ENSO effects on tropical cyclone genesis. J. Climate, 20, 48194834, https://doi.org/10.1175/JCLI4282.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Chan, J. C. L., 1985: Tropical cyclone activity in the northwest Pacific in relation to the El Niño/Southern Oscillation phenomenon. Mon. Wea. Rev., 113, 599606, https://doi.org/10.1175/1520-0493(1985)113<0599:TCAITN>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Chan, J. C. L., 2000: Tropical cyclone activity over the western North Pacific associated with El Nino and La Nina events. J. Climate, 13, 29602972, https://doi.org/10.1175/1520-0442(2000)013<2960:TCAOTW>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Chen, T.-C., S.-Y. Wang, M.-C. Yen, and W. A. Gallus Jr., 2004: Role of the monsoon gyre in the interannual variation of tropical cyclone formation over the western North Pacific. Wea. Forecasting, 19, 776785, https://doi.org/10.1175/1520-0434(2004)019<0776:ROTMGI>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Elsner, J. B., A. A. Tsonis, and T. H. Jagger, 2006: High-frequency variability in hurricane power dissipation and its relationship to global temperature. Bull. Amer. Meteor. Soc., 87, 763768, https://doi.org/10.1175/BAMS-87-6-763.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Feng, J., W. Chen, C.-Y. Tam, and W. Zhou, 2011: Different impacts of El Niño and El Niño Modoki on China rainfall in the decaying phases. Int. J. Climatol., 31, 20912101, https://doi.org/10.1002/joc.2217.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447462, https://doi.org/10.1002/qj.49710644905.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hong, C. C., Y. H. Li, T. Li, and M. Y. Lee, 2011: Impacts of central Pacific and eastern Pacific El Niños on tropical cyclone tracks over the western North Pacific. Geophys. Res. Lett., 38, L16712, https://doi.org/10.1029/2011GL048821.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471, https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kim, H.-M., P. J. Webster, and J. A. C. Curry, 2011: Modulation of North Pacific tropical cyclone activity by three phases of ENSO. J. Climate, 24, 18391849, https://doi.org/10.1175/2010JCLI3939.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Krishnamurthy, L., G. A. Vecchi, R. Msadek, H. Murakami, A. Wittenberg, and F. Zeng, 2016: Impact of strong ENSO on regional tropical cyclone activity in a high-resolution climate model in the North Pacific and North Atlantic Oceans. J. Climate, 29, 23752394, https://doi.org/10.1175/JCLI-D-15-0468.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lee, T., and M. J. McPhaden, 2010: Increasing intensity of El Niño in the central-equatorial Pacific. Geophys. Res. Lett., 37, L14603, https://doi.org/10.1029/2010GL044007.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., and W. Zhou, 2012: Changes in western Pacific tropical cyclones associated with the El Niño–Southern Oscillation cycle. J. Climate, 25, 58645878, https://doi.org/10.1175/JCLI-D-11-00430.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., and W. Zhou, 2013a: Modulation of western North Pacific tropical cyclone activities by the ISO. Part I: Genesis and intensity. J. Climate, 26, 29042918, https://doi.org/10.1175/JCLI-D-12-00210.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., and W. Zhou, 2013b: Modulation of western North Pacific tropical cyclone activities by the ISO. Part II: Tracks and landfalls. J. Climate, 26, 29192930, https://doi.org/10.1175/JCLI-D-12-00211.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., W. Zhou, J. C. L. Chan, and P. Huang, 2012: Asymmetric modulation of the western North Pacific cyclogenesis by the Madden–Julian oscillation under ENSO conditions. J. Climate, 25, 53745385, https://doi.org/10.1175/JCLI-D-11-00337.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, X., W. Zhou, D. L. Chen, C. Y. Li, and J. Song, 2014: Water vapor transport and moisture budget over eastern China: Remote forcing from the two types of El Niño. J. Climate, 27, 87788792, https://doi.org/10.1175/JCLI-D-14-00049.1.

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

  • Murakami, H., and Coauthors, 2017: Dominant role of subtropical Pacific warming in extreme eastern Pacific hurricane seasons: 2015 and the future. J. Climate, 30, 243264, https://doi.org/10.1175/JCLI-D-16-0424.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Paek, H., J. Y. Yu, and C. Qian, 2017: Why were the 2015/2016 and 1997/1998 extreme El Niños different? Geophys. Res. Lett., 44, 18481856, https://doi.org/10.1002/2016GL071515.

    • Search Google Scholar
    • Export Citation
  • Patricola, C. M., P. Chang, and R. Saravanan, 2016: Degree of simulated suppression of Atlantic tropical cyclones modulated by flavour of El Niño. Nat. Geosci., 9, 155160, https://doi.org/10.1038/ngeo2624.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pradhan, P., B. Preethi, K. Ashok, R. Krishnan, and A. Sahai, 2011: Modoki, Indian Ocean dipole, and western North Pacific typhoons: Possible implications for extreme events. J. Geophys. Res., 116, D18108, https://doi.org/10.1029/2011JD015666.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108, 4407, https://doi.org/10.1029/2002JD002670.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ritchie, E. A., 1995: Mesoscale aspects of tropical cyclone formation. Ph.D. dissertation, Monash University, 167 pp. [Available from Monash University, Wellington Rd., Clayton, VIC 3168, Australia.]

  • Ritchie, E. A., and G. J. Holland, 1999: Large-scale patterns associated with tropical cyclogenesis in the western Pacific. Mon. Wea. Rev., 127, 20272043, https://doi.org/10.1175/1520-0493(1999)127<2027:LSPAWT>2.0.CO;2.

    • Crossref
    • 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, https://doi.org/10.1175/1520-0442(2002)015<1643:HSEEAT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., and B. Wang, 2004: Assessing impacts of global warming on tropical cyclone tracks. J. Climate, 17, 16861698, https://doi.org/10.1175/1520-0442(2004)017<1686:AIOGWO>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, and R. Huang, 2011: A primary study of the correlation between the net air–sea heat flux and the interannual variation of western North Pacific tropical cyclone track and intensity. Acta Oceanol. Sin., 30, 2735, https://doi.org/10.1007/s13131-011-0158-8.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, R. Huang, and R. Wu, 2012: Possible linkage between the monsoon trough variability and the tropical cyclone activity over the western North Pacific. Mon. Wea. Rev., 140, 140150, https://doi.org/10.1175/MWR-D-11-00078.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, T. Li, and R. Huang, 2014a: ENSO-phase dependent TD and MRG wave activity in the western North Pacific. Climate Dyn., 42, 12171227, https://doi.org/10.1007/s00382-013-1754-4.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., and Coauthors, 2014b: Simulations of the present and late-twenty-first-century western North Pacific tropical cyclone activity using a regional model. J. Climate, 27, 34053424, https://doi.org/10.1175/JCLI-D-12-00830.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, and R. Wu, 2015a: Influence of the monsoon trough on westward-propagating tropical waves over the western North Pacific. Part I: Observations. J. Climate, 28, 71087127, https://doi.org/10.1175/JCLI-D-14-00806.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, and R. Wu, 2015b: Influence of the monsoon trough on westward-propagating tropical waves over the western North Pacific. Part II: Energetics and numerical experiments. J. Climate, 28, 93329349, https://doi.org/10.1175/JCLI-D-14-00807.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yanai, M., and R. H. Johnson, 1993: Impacts of cumulus convection on thermodynamic fields. Representation of Cumulus Convection in Numerical Models of the Atmosphere, Meteor. Monogr., No. 46, Amer. Meteor. Soc., 39–62.

    • Crossref
    • Export Citation
  • Yanai, M., S. Esbensen, and J.-H. Chu, 1973: Determination of bulk properties of tropical cloud clusters from large-scale heat and moisture budgets. J. Atmos. Sci., 30, 611627, https://doi.org/10.1175/1520-0469(1973)030<0611:DOBPOT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., J.-S. Kug, B. Dewitte, M.-H. Kwon, B. P. Kirtman, and F.-F. Jin, 2009: El Niño in a changing climate. Nature, 461, 511514, https://doi.org/10.1038/nature08316.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yu, J.-Y., and S. T. Kim, 2013: Identifying the types of major El Niño events since 1870. Int. J. Climatol., 33, 21052112, https://doi.org/10.1002/joc.3575.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yuan, Y., W. Zhou, H. Yang, and C. Y. Li, 2008: Warming in the northwestern Indian Ocean associated with the El Niño event. Adv. Atmos. Sci., 25, 246252, https://doi.org/10.1007/s00376-008-0246-7.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhang, W., H.-F. Graf, Y. Leung, and M. Herzog, 2012: Different El Niño types and tropical cyclone landfall in East Asia. J. Climate, 25, 65106523, https://doi.org/10.1175/JCLI-D-11-00488.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhang, W., G. A. Vecchi, H. Murakami, G. Villarini, and L. Jia, 2016: The Pacific meridional mode and the occurrence of tropical cyclones in the western North Pacific. J. Climate, 29, 381398, https://doi.org/10.1175/JCLI-D-15-0282.1.

    • Crossref
    • 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, https://doi.org/10.1002/joc.1380.

  • View in gallery

    Scatterplots for (a) Niño-3 and EMI indices and (b) EP SST and CP SST, where the red, blue, orange, and gray denote EP El Niño, La Niña, CP El Niño, and neutral years, respectively. The red lines are used to divide the four new Pacific warming regimes. The horizontal (vertical) dashed line in (b) is the climatological mean SST in the CP (EP) region, 28.4°C (25.2°C), over 1948–2015.

  • View in gallery

    Composites of SST anomalies during the period from June to November for years associated with (a) EP El Niño, (b) CP El Niño, and (c) La Niña, and (d) the difference between CP El Niño and EP El Niño (CP El Niño minus EP El Niño). Thick solid contours show mean 28°C SST values. Shading in (d) indicates areas where the difference is significant at the 95% level based on the Student’s t test. Boxes denote the Niño-3 region in (a) and (c), core regions of EMI in (b), and the CP region in (d).

  • View in gallery

    (top) Composites of SST anomalies (°C) and (bottom) a longitude–ocean depth cross section of potential temperature anomalies (°C) averaged over the latitudes of 5°S–5°N for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years. The dotted regions indicate statistical significance at the 95% level. The thick green line is the corresponding thermocline (20°C isotherm), and the dashed black lines represent the climatological thermocline.

  • View in gallery

    Mean numbers of TC (black) and TY (red) genesis per year associated with different Pacific warming regimes over (a) the WNP and (b) the SE region (0°–20°N, 150°E–180°). The red shading is the contribution from only those TCs reaching TY strength (33 m s−1). (b) Mean TC lifetimes per year that form in the WNP (blue) and the SE region (green).

  • View in gallery

    Mean (contours) and anomalies (color shaded) spatial distribution of (top) TC genesis number, (middle) TC occurrence, and (bottom) ACE (104 m2 s−2) in 5° × 5° boxes during June–November for (a),(e),(i) CEPW; (b),(f),(j) CPW; (c),(g),(k) EPW; and (d),(h),(l) WEPW years. Small (large) black dots indicate that the differences are significant at the 80% (90%) confidence level using the nonparametric Mann–Whitney test.

  • View in gallery

    Spatial distribution of correlation of TC genesis with (a) EP SST and (b) CP SST, (c) partial correlation of TC genesis and EP SST with CP SST influence removed, and (d) partial correlation of TC genesis and CP SST with EP SST influence removed. The solid (dashed) black lines denote the positive (negative) correlation significant at the 80% confidence level contours.

  • View in gallery

    Scatterplot of TC number over the region SE against SST in the (a) EP region and (b) CP region. The red, orange, green, blue, and gray colors correspond to CEPW, CPW, EPW, WEPW, and other years, respectively. The solid line is the linear regression, with the deviation of regression coefficient denoted by R2 (the percent of the fitting degree of linear regression).

  • View in gallery

    Composites of sea level pressure anomalies (hPa; contours), 850-hPa wind anomalies (m s−1; vectors; significant values at 95% confidence level are shown with red arrows), and OLR anomalies (W m−2; color shading) during June–November for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years.

  • View in gallery

    (left) Vertical–longitudinal sections of composite zonal and vertical velocity (vectors; m s−1 for u and 10−2 Pa s−1 for −ω) and apparent heat source (Q1; 10−2 W m−2) along the equator and (right) vertical–latitudinal sections of regressed meridional and vertical velocity (vectors; m s−1 for υ and 10−2 Pa s−1 for −ω) and apparent heat source (Q1; 10−2 W m−2) along 160°E for (a),(e) CEPW, (b),(f) CPW, (c),(g) EPW, and (d),(h) WEPW years.

  • View in gallery

    Scatterplot of (a),(c) OLR and (b),(d) 850-hPa zonal wind anomaly against (top) CP SST and (bottom) EP SST. OLR is averaged over the equatorial central Pacific region (OLREQ; 5°S–5°N, 150°E–180°). The 850-hPa zonal wind anomalies are averaged over the northern region (UN; 5°–15°N, 140°E–180°) in (b) and over the equatorial region (UEQ; 5°S–5°N, 150°E–180°) in (d) of the central Pacific. SSTs are averaged over the CP in (a) and (b) and over the EP in (c) and (d). The red, orange, blue, and gray colors corespnd to EP El Niño, CP El Niño, La Niña, and neutral years, respectively. The solid line is the linear regression.

  • View in gallery

    The 850-hPa streamline charts with superimposed 850-hPa relative vorticity (10−6 s−1; values in dotted regions are significant at the 95% confidence level) for average of (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years during the TC season. The green solid lines indicate the 500-hPa geopotential height (gpm). The 850-hPa monsoon trough (MT) is denoted by a thick-dashed black line.

  • View in gallery

    (left) The anomalous mean vertical wind shear (VWS; m s−1) between 200 and 850 hPa, (middle) 500–700-hPa relative humidity (RH; %), and (right) 850-hPa EKE (m2 s−2) during July–November for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years. The dotted regions indicate areas exceeding a 95% confidence level using the Student’s t test.

  • View in gallery

    Schematic illustrating the processes by which the location and intensity of Pacific warming affect TC activity over the WNP for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW. The red lines denote contours of positive SST anomalies, and blue lines indicate convection; the solid black and dashed gray arrows indicate anomalous low-level winds and an anomalous Walker cell, respectively; green shading indicates the MT region; TC genesis and tracks are marked by typhoon symbols and orange arrows, respectively.

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Impact of Two Types of El Niño on Tropical Cyclones over the Western North Pacific: Sensitivity to Location and Intensity of Pacific Warming

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  • 1 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • 2 Department of Maritime Information and Technology, National Kaohsiung Marine University, Kaohsiung, Taiwan
  • 3 Institute for Climate and Global Change Research, Jiangsu Collaborative Innovation Center for Climate Change, School of Atmospheric Sciences, Nanjing University, Nanjing, China
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Abstract

The present study investigates the impact of various central Pacific (CP) and eastern Pacific (EP) warming on tropical cyclones (TCs) over the western North Pacific (WNP) for the period 1948–2015 based on observational and reanalysis data. Four distinctly different forms of tropical Pacific warming are identified to examine different impacts of locations and intensity of tropical Pacific warming on the WNP TCs. It is shown that WNP TC activity related to ENSO shows stronger sensitivity to the intensity of CP SST warming. The locations of TC genesis in an extreme EP El Niño featuring concurrent strong CP and EP warming (CEPW) display a notable southeastward shift that is generally similar to the CP El Niño featuring CP warming alone (CPW). These influences are clearly different from the effects of moderate EP El Niño associated with EP warming alone (EPW). The above influences of Pacific warming on TCs possibly occur via atmospheric circulation variability. Anomalous convection associated with CP SST warming drives anomalous low-level westerlies away from the equator as a result of a Gill-type Rossby wave response, leading to an enhanced broad-zone, eastward-extending monsoon trough (MT). An anomalous Walker circulation in response to EP SST warming drives an increase in anomalous equatorial westerlies over the WNP, leading to a narrow-zone, slightly equatorward shift of the eastward-extending MT. These changes in the MT coincide with a shift in large-scale environments and synoptic-scale perturbations, which favor TC genesis and development. In addition, during weaker EP SST warming (WEPW) with similar intensity to CPW, local SST forcing exhibits primary control on WNP TCs and atmospheric circulation.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Dr. Liang Wu, wul@mail.iap.ac.cn

Abstract

The present study investigates the impact of various central Pacific (CP) and eastern Pacific (EP) warming on tropical cyclones (TCs) over the western North Pacific (WNP) for the period 1948–2015 based on observational and reanalysis data. Four distinctly different forms of tropical Pacific warming are identified to examine different impacts of locations and intensity of tropical Pacific warming on the WNP TCs. It is shown that WNP TC activity related to ENSO shows stronger sensitivity to the intensity of CP SST warming. The locations of TC genesis in an extreme EP El Niño featuring concurrent strong CP and EP warming (CEPW) display a notable southeastward shift that is generally similar to the CP El Niño featuring CP warming alone (CPW). These influences are clearly different from the effects of moderate EP El Niño associated with EP warming alone (EPW). The above influences of Pacific warming on TCs possibly occur via atmospheric circulation variability. Anomalous convection associated with CP SST warming drives anomalous low-level westerlies away from the equator as a result of a Gill-type Rossby wave response, leading to an enhanced broad-zone, eastward-extending monsoon trough (MT). An anomalous Walker circulation in response to EP SST warming drives an increase in anomalous equatorial westerlies over the WNP, leading to a narrow-zone, slightly equatorward shift of the eastward-extending MT. These changes in the MT coincide with a shift in large-scale environments and synoptic-scale perturbations, which favor TC genesis and development. In addition, during weaker EP SST warming (WEPW) with similar intensity to CPW, local SST forcing exhibits primary control on WNP TCs and atmospheric circulation.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Dr. Liang Wu, wul@mail.iap.ac.cn

1. Introduction

Tropical cyclones (TCs) are extreme natural disasters. They affect nearly all coastal areas of the western North Pacific (WNP). It is well known that TC activity over the WNP is strongly influenced by El Niño–Southern Oscillation (ENSO) on an interannual time scale. During El Niño years when warming occurs in the equatorial eastern Pacific, TCs tend to form in the southeast quadrant of the WNP. These TCs are of great intensity and form farther eastward (e.g., Camargo and Sobel 2005; Camargo et al. 2007; Chan 1985, 2000; Wang and Chan 2002; Wu et al. 2011, 2012; Li et al. 2012; Li and Zhou 2012, 2013a,b). Recent research shows that different ENSO types (flavors) have been identified such as ENSOs of the equatorial central Pacific [central Pacific (CP) El Niño] and those of the eastern Pacific [eastern Pacific (EP) El Niño]. TC activity over the WNP shows different characteristics depending on the El Niño type. For example, during CP El Niño events, TC activity tends to shift westward and extends through the northwestern part of the western Pacific (Kim et al. 2011). This is accompanied by a northward shift in genesis (Zhang et al. 2012, Hong et al. 2011). These differences have been explained by the modulation of atmospheric circulations over the WNP forced by Pacific warming–induced heating at different locations. However, these studies have not fully considered the possible impacts of different intensity of SST warming during different El Niño types.

Some modeling studies have shown that both the location and intensity of SST warming during El Niño events can modulate WNP TC activity. Based on model simulations, Krishnamurthy et al. (2016) show that TCs in the WNP have a quasi-linear response to the amplitude of eastern Pacific ENSO events and exhibit a farther eastward shift during strong El Niño events. Note that SST warming during CP El Niño events is not as pronounced as during EP El Niño events. From this it might be assumed that CP El Niños have less influence over TC activity globally. However, based on simulated doubled CP El Niño forcing, Patricola et al. (2016) found that teleconnections between the Pacific and Atlantic Oceans meant that for El Niño events of similar intensity, CP El Niño events are about 1.5 times more likely than EP El Niño events to affect Atlantic TC activity due to the influence of tropical Pacific convection. Although TC activity in the WNP differs from that in the Atlantic, greater tropical Pacific convection is expected to affect WNP TC activity during CP El Niño events more greatly than during similar intensity EP El Niño events. Generally, the above modeling supports the idea that the intensity and location of Pacific warming strongly influence TC activity; however, these effects have not been fully observed.

Several climate change studies suggest that the location and intensity of Pacific warming events are changing and will continue to change into the future. Some studies project increasingly frequent extreme EP El Niño events as the climate warms (Cai et al. 2014). Warm SST anomalies have expanded westward into the central Pacific and cover a broad range of longitudes during past extreme EP El Niño events (e.g., 1982, 1997, and 2015 events). In addition, CP El Niño events of greater intensity and frequency have been observed in recent decades (Lee and McPhaden 2010; Yeh et al. 2009) with projections for a warming climate indicating potential further increases (Yeh et al. 2009). It is interesting to note that the warming trend in the CP may be primarily due to stronger El Niño events. Extreme EP El Niño events tend to correspond to higher SST anomalies in the CP region. Hence, investigating the relative roles of central Pacific warming (CPW) and eastern Pacific warming (EPW) in relation to El Niño types is important. This knowledge would help answer the following questions: What are the differences between the influence of two types of El Niño on large-scale circulation and TC activity over the WNP? How does the intensity of ENSO affect WNP TCs? Do these two influences of the CPW and EPW over TCs have similar spatial patterns and magnitudes in response to the same intensity of warming?

To answer these questions, the present study focuses on the relationship between the different ENSO types of Pacific warming and TC activity in the WNP. Our objective is to advance our understanding of the impact of location and magnitude of Pacific warming on TC activity. For the purposes of this study, El Niño events are classified by location of tropical Pacific warming as pure EPW, pure CPW, and coexisting CPW and EPW (CEPW). The effects of weak EPW (WEPW) are used for comparison with the effects of CPW of the same intensity. Results suggest that WNP TC activity is strongly correlated with the intensity of SST warming in the CP, and the effect of EP SST warming is secondary. This paper is arranged as follows: Section 2 describes data sources and processing methods. Section 3 describes different Pacific warming regimes utilizing CP and EP SST only, and their corresponding influences over TC activity in the WNP. Section 4 gives detailed discussions on potential governing mechanisms. Section 5 presents a summary of findings.

2. Data and methods

The analyses in this study use different datasets extending from 1948 to 2015. Analyses use the maximum number of years available for each dataset and only data from June to November, which is the main TC season in the WNP. The best-track dataset is from the Joint Typhoon Warning Center (JTWC) for the period 1948–2015. These data are used to determine the position and intensity of each TC. The time of TC genesis is defined as the time when it appears on track records. Further, atmospheric field data come from the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis dataset (Kalnay et al. 1996) and SST field data are from the Hadley Centre (Rayner et al. 2003) for the period 1948–2015. In addition, oceanic subsurface conditions are from reanalysis data (ORAS4) of the European Centre for Medium-Range Weather Forecasts (Balmaseda et al. 2013) for the period 1958–2015, and outgoing longwave radiation (OLR) field data are from National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites (Liebmann and Smith 1996) for the period 1974–2015. The apparent heat source Q1 (e.g., Yanai et al. 1973; Yanai and Johnson 1993) is computed from NCEP-1.

The ENSO regimes are classified based on SST anomalies averaged over June–November. Many methods have been proposed to identify the types of major ENSO events (e.g., Kim et al. 2011; Pradhan et al. 2011; Zhang et al. 2012; Yu and Kim 2013; Feng et al. 2011; Zhou and Chan 2007; Yuan et al. 2008; Li et al. 2014). Here we use two traditionally simple indices to define the EP Niño and CP Niño. The EP El Niño (La Niña) years are defined as positive (negative) SST anomalies in the equatorial eastern Pacific (the Niño-3 region: 5°S–5°N, 150°–90°W) that are warmer (cooler) than 0.7 standard deviations, averaged over June–November. The CP El Niño years are determined as in the work by Ashok et al. (2007) using the El Niño Modoki index (EMI), defined as SSTC − 0.5SSTW 0.5SSTE based on June–November mean SST anomalies in central (SSTC; 10°S–10°N, 165°E–140°W), western (SSTW; 15°S–5°N, 110°–70°W), and eastern (SSTE; 10°S–20°N, 125°–145°E) regions of the tropical Pacific. The EMI values are greater than 0.3 standard deviations for the CP El Niño. Figure 1a shows each year in phase space defined by the EMI and Niño-3. The two types of El Niño are well separated using these two indices. A total of 12 EP El Niño years (1951, 1957, 1963, 1965, 1969, 1972, 1976, 1982, 1987, 1997, 2009, and 2015), 12 CP El Niño years (1948, 1966, 1967, 1977, 1986, 1990, 1991, 1992, 1993, 1994, 2002, and 2004) and 16 La Niña years (1949, 1950, 1954, 1955, 1956, 1964, 1970, 1971, 1973, 1975, 1984, 1985, 1988, 1999, 2007, and 2010) are selected. The other 27 years are defined as neutral years. In addition, the statistical significance of anomalies is estimated by the Student’s t test.

Fig. 1.
Fig. 1.

Scatterplots for (a) Niño-3 and EMI indices and (b) EP SST and CP SST, where the red, blue, orange, and gray denote EP El Niño, La Niña, CP El Niño, and neutral years, respectively. The red lines are used to divide the four new Pacific warming regimes. The horizontal (vertical) dashed line in (b) is the climatological mean SST in the CP (EP) region, 28.4°C (25.2°C), over 1948–2015.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

3. Different Pacific warming regimes and TC activity in the WNP

a. Pacific warming regimes

Figure 2 displays composite charts of SST anomalies for EP El Niño, CP El Niño, and EP La Niña, respectively. The maximum SST anomalies are observed in the central (eastern) tropical Pacific during EP El Niño (CP El Niño) years, and the amplitude of EP El Niño is stronger than CP El Niño, in agreement with the findings of previous studies. It is interesting to note that although the maximum of SST anomalies in CP El Niño shifts to the central Pacific, the amplitude of tropical central Pacific SST anomalies (~0.40°C) in CP El Niño is still smaller than EP El Niño (~0.50°C). Figure 2d shows the difference in SST between the two types of years. A large area of negative and statistically significant difference in SST covers the tropical eastern–central Pacific west of the date line between CP El Niño and EP El Niño years, accompanied by weaker positive differences in a small area east of the date line. When compared to EP El Niño years, these results may seem unexpected in that the intensity of SST anomalies in the central Pacific have no tendency to increase, and may significantly decrease, even when the maximum SST anomaly shifts to the central Pacific during CP El Niño years. In fact, stronger CP tropical atmospheric heating has been included in the effects of EP El Niño events. However, for the most part, climatological understanding of the effects of different phases of ENSO has not fully considered the role of warming in different Pacific regions.

Fig. 2.
Fig. 2.

Composites of SST anomalies during the period from June to November for years associated with (a) EP El Niño, (b) CP El Niño, and (c) La Niña, and (d) the difference between CP El Niño and EP El Niño (CP El Niño minus EP El Niño). Thick solid contours show mean 28°C SST values. Shading in (d) indicates areas where the difference is significant at the 95% level based on the Student’s t test. Boxes denote the Niño-3 region in (a) and (c), core regions of EMI in (b), and the CP region in (d).

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

To help identify the role of regional SST forcing under different ENSO types, tropical Pacific warming is separated into four Pacific warming regimes (Fig. 1b and Table 1) based on SSTs in the equatorial central Pacific (10°S–10°N, 165°E–140°W; Fig. 2a) and eastern Pacific (5°S–5°N, 150°–90°W; Fig. 2d). As seen in Fig. 1b, between EP SST and CP SST, SSTs in these two regions are highly correlated (simultaneous correlation = 0.81), implying that high CP SSTs correspond to high EP SSTs. An extreme EP El Niño is always associated with concurrently strong EP and CP warming influences. Those coexisting extreme CP and EP warmings are termed CEPW and defined as strong SST anomalies of ≥28.7°C in the CP and >26.3°C in the EP. To examine the influence of pure CP warming (CPW) or EP warming (EPW), CPW is defined as a CP El Niño year with anomalously warm CP SST ≥ 28.7°C and cool EP SST of <25.6°C. For EPW, a moderate EP El Niño of 25.9°–26.3°C occurs in concert with anomalously cool CP SST of <28.7°C. To compare the influence of similar intensity SSTs in CP and EP, weak EP SST warming (WEPW) is defined as a positive to neutral EP SST of 25.5°–25.9°C and a cool CP SST of <28.7°C. Anomalous SST warming in WEPW and CPW events has the same intensity (SST anomaly ~ 0.3°–0.7°C) but at different locations. The results of categorizing years according to the above are listed in Table 2.

Table 1.

Definition of the Pacific warming regimes used in Fig. 1b. The numbers in parentheses are the anomaly of SST.

Table 1.
Table 2.

Categorization of years into the different ENSO regimes, as shown in Fig. 1b.

Table 2.

Figure 3 shows composite SST anomalies and the vertical distribution of subsurface temperature anomalies averaged between 5°S and 5°N during the period from June to November for CEPW, CPW, EPW, and WEPW years. These four Pacific warming regimes identified here show distinct SST anomaly patterns. The CEPW corresponds to warmer SST anomalies in the tropical central and eastern Pacific and cooler SST anomalies in the western Pacific. Compared with CEPW events, CPW is confined to the central Pacific with a warm SST anomaly whereas EPW is in the equatorial eastern Pacific but extends less meridionally. The warm SST anomaly for the WEPW is moved farther eastward to 60°–140°W. Anomalies corresponding to tropical Pacific warming are also evident in the vertical distribution of subsurface temperature anomalies averaged along 5°S–5°N. Maximum positive anomalies during CEPW and EPW years occur at similar depth in the eastern Pacific and expand eastward to the central Pacific, where the thermocline deepens. However, the EPW (CEPW) corresponds to anomalous warm (cold) subsurface temperatures distributed in the upper 100 m in the central Pacific with a shallow (deep) negative anomaly down to 80 m (from the surface to 300 m) in depth east of 175°W (165°E) where the thermocline is deeper (shallower). During CPW years, the location of the maximum positive anomaly occurs between 160°E and 100°W to a depth of 150 m with little deepening of thermocline depth. During WEPW years, subsurface temperature anomalies are characterized by a broad strong negative region at a depth below 50 m in the tropical Pacific and a weak positive region at a depth above 50 m with a central maximum in the eastern Pacific.

Fig. 3.
Fig. 3.

(top) Composites of SST anomalies (°C) and (bottom) a longitude–ocean depth cross section of potential temperature anomalies (°C) averaged over the latitudes of 5°S–5°N for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years. The dotted regions indicate statistical significance at the 95% level. The thick green line is the corresponding thermocline (20°C isotherm), and the dashed black lines represent the climatological thermocline.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

The above results show that there are significant differences in the magnitude and spatial extent of SST and subsurface temperature anomalies between the different regions of the Pacific. The patterns of SST anomalies are very similar to patterns of subsurface temperature anomalies, indicating that SST warming in different regions of the equatorial Pacific is due to subsurface thermal structure.

b. Variability of WNP TC activity associated with different Pacific warming regimes

Figure 4 shows the mean number and lifetimes of TCs in the WNP for different Pacific warming regimes. There is no significant difference in the total number of TCs and typhoons (TYs) over the WNP except for EPW years (Fig. 4a). There are fewer TCs and TYs in EPW years than in other warming regimes. There is a small shift toward longer lifetimes in CEPW and CPW years than in EPW and WEPW years (Fig. 4c).

Fig. 4.
Fig. 4.

Mean numbers of TC (black) and TY (red) genesis per year associated with different Pacific warming regimes over (a) the WNP and (b) the SE region (0°–20°N, 150°E–180°). The red shading is the contribution from only those TCs reaching TY strength (33 m s−1). (b) Mean TC lifetimes per year that form in the WNP (blue) and the SE region (green).

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

Figure 5 presents the effect of different Pacific warming regimes on the spatial distribution of TC activity. The figure shows climatology, anomalous spatial distribution of TC genesis, occurrence densities, and accumulated cyclone energy (ACE) values for different Pacific warming regimes. Previous studies have noted that an anomalous increase SST in the tropical eastern Pacific (EP El Niño) leads to a dramatic eastward shift in TC genesis (Camargo and Sobel 2005; Camargo et al. 2007; Chan 1985, 2000; Wang and Chan 2002; Wu et al. 2012). Consistent with most previous studies, the frequency of TC genesis in CEPW years is enhanced in the eastern region (140°E–180°) and suppressed in the western region (120°–140°E). The TC genesis in CPW years is also displaced eastward with a maximum positive anomaly centered around 150°E–180° but extending westward with a secondary maximum east of the Philippines. However, unlike CEPW (extreme EP El Niño) years, TCs in EPW (moderate EP El Niño) years mainly form between 10° and 20°N with weakly positive anomalies in the same region and maximum values near 150°–160°E. TC genesis during WEPW years has a maximum positive anomaly between 130° and 160°E. It is important to note that in CEPW and CPW years, TCs form in the southeast quadrant of the WNP (SE; 0°–20°N, 150°E–180°) at a rate approximately twice that of EPW or WEPW years (Figs. 4b and 5).

Fig. 5.
Fig. 5.

Mean (contours) and anomalies (color shaded) spatial distribution of (top) TC genesis number, (middle) TC occurrence, and (bottom) ACE (104 m2 s−2) in 5° × 5° boxes during June–November for (a),(e),(i) CEPW; (b),(f),(j) CPW; (c),(g),(k) EPW; and (d),(h),(l) WEPW years. Small (large) black dots indicate that the differences are significant at the 80% (90%) confidence level using the nonparametric Mann–Whitney test.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

Corresponding to an increase in TC genesis in the SE region during CEPW and CPW years, TCs also tend to have longer lifetimes (Fig. 4c) and more frequency of TC occurrences (tracks) in most of the WNP basin (Figs. 5e,f). However, there is more chance of TCs recurving northward in CEPW years. This is due an anomalous low-level anticyclone locating over the South China Sea (10°–30°N, 100°–130°E) during CEPW years from September to November. During EPW years, more TCs form between 150° and 160°E (Fig. 5c) and these tend to track northwestward toward the Korea Peninsula and parts of Japan (Fig. 5g). During WEPW years, TCs tend to recurve from a northwestward to northward or westward in farther west longitudes due to the westward extension of TC genesis (Figs. 5d,h).

Compared to TC occurrences (tracks), the distribution of ACE values, which are used to evaluate the aggregate activity of TCs, shows many characteristics similar to TC track patterns; however, this distribution is shifted slightly westward and northward (Figs. 5i–l). This shift in ACE is affected by increased TC intensity as cyclones track over warm oceans. In aggregate, the distribution of ACE values per year for CEPW and CPW years is higher; however, TCs during CPW years versus CEPW are of markedly less strength. This result indicates that there is a substantial decrease in number and proportion of intense typhoons in CPW years compared to CEPW years, while the number of TCs and cyclone days has slightly increased. During EPW and WEPW years, relative ACE values are not high and more TC activity tends to occur east of the Philippines than in other regions.

The above analyses reveal similar pattern between CEPW and CPW but obvious differences between CEPW and EPW in TC genesis, intensity, track, and ACE. TC activity is insensitive to the intensity of moderate El Niño (EPW) and positive-neutral events (WEPW). These results suggest that TC activity may be insensitive to the intensity of EP SST. Previous studies (Wu and Wang 2004; Wu et al. 2012) have shown that southeastward shifts in TC genesis may be key to TCs of longer lifetimes, greater frequency, and stronger intensity, thus determining the spatial distribution of TC activity. SST in both EP and CP is more positively correlated with TC genesis in the SE region of the basin (Figs. 6a,b). The correlation between region-averaged TC genesis in the SE region and EP SST is 0.50, and the correlation between TC genesis and CP SST is 0.63. These two correlations all exceed a 95% confidence level, but correlation for CP SST is somewhat larger than for EP SST. To contrast the importance of SST in these two regions, Fig. 7 shows scatterplots of TC numbers over the SE region against SST in EP and CP regions. TC genesis in the SE region exhibits a strong linear response to CP SST (with deviation of the regression coefficient being 0.40) but a weak response to EP SST (with deviation of regression coefficient being 0.26). These results can help in our understanding of TCs in periods of weak EP SST (<26.3°C). It has been noted that the influences of EP SST and CP SST on TCs are not independent because EP SST is highly correlated with CP SST with a correlation coefficient of 0.81 (Fig. 1b). To determine the relationship between TCs and SST in EP and CP regions, partial correlation analyses (Elsner et al. 2006) are employed to isolate influences due to different area-averaged SSTs. As seen in Fig. 6d, when the effect of EP SST is removed, partial correlation analysis shows a small, but not negligible, positive relationship between CP SST and TC genesis in the SE region. However, after removing the effect of CP SST (Fig. 6c), correlation decreases to almost no direct relationship existing between EP SST and TC genesis in the SE region. This result is more significant for area-averaged TC genesis. After removing the effect of EP SST, correlation between CP SST and TC genesis in the SE region still exceeds the 95% significance level (r = 0.44). However, if the effect of CP SST is removed, correlation between EP SST and TC genesis in the SE region is nearly zero. This result suggests that TC formation in the SE region is more dependent on CP SST than EP SST. Possible physical explanations for this dependence are discussed in the next section.

Fig. 6.
Fig. 6.

Spatial distribution of correlation of TC genesis with (a) EP SST and (b) CP SST, (c) partial correlation of TC genesis and EP SST with CP SST influence removed, and (d) partial correlation of TC genesis and CP SST with EP SST influence removed. The solid (dashed) black lines denote the positive (negative) correlation significant at the 80% confidence level contours.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

Fig. 7.
Fig. 7.

Scatterplot of TC number over the region SE against SST in the (a) EP region and (b) CP region. The red, orange, green, blue, and gray colors correspond to CEPW, CPW, EPW, WEPW, and other years, respectively. The solid line is the linear regression, with the deviation of regression coefficient denoted by R2 (the percent of the fitting degree of linear regression).

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

4. Possible physical mechanisms

a. The response of large-scale circulation to different Pacific warming regimes

Previous studies have documented that the large-scale circulation is very sensitive to the tropical Pacific SST forcing. Figure 8 shows composite charts of sea level pressure (SLP) anomalies, 850-hPa wind anomalies, and OLR anomalies during June–November with respect to different Pacific warming regimes. During CEPW years, over the tropical central and eastern (western) Pacific, positive (negative) SST anomalies induce above (below) normal SLP, creating an anomalous zonal pressure gradient. This strong negative pressure gradient over the tropical Pacific can produce significant anomalous low-level westerlies centered over the equator between 130°E and 120°W. Such anomalous low-level westerlies can lead to an anomalous one-cell Walker circulation over the equatorial Pacific (Fig. 9a). Strong anomalous ascending (descending) motion throughout the troposphere east of the date line (west of 155°E) accompanied by low-level convergence (divergence) of zonal winds tends to produce a positive (negative) heating anomaly (Q1), suggesting strong convection over the region of ascending branch. In the west of the central Pacific (along 160°E; Fig. 9e), strong Q1 heating and cooling are located at the midtroposphere (near 200–700 hPa), and significant upward (downward) motion penetrates throughout the whole troposphere along with Q1 heating (cooling) in between 0° and 15°N (20°–40°N). As expected, the Q1 anomaly is consistent with the OLR anomaly (Fig. 8a) as well as anomalous lower-level westerly convergence and upward motion, which can contribute to further enhanced anomalous Walker cell and its anomalous equatorial westerlies. On the other hand, in a Gill-type Rossby wave mechanism (Gill 1980), the convective heating can trigger a low-level anomalous cyclonic flow to the west of this heating area via excitation of tropical waves. This enhanced diabatic and convection heating in the central and eastern equatorial Pacific induces strong anomalous low-level westerlies to its northwestern and southwestern flanks (north of 5°N) with anomalous cyclonic circulation as a Gill-type Rossby wave response. In other words, anomalous heating and increased deep convection in the central and eastern tropical Pacific associated with the anomalous Walker circulations may further enhance anomalous tropical westerlies and extend poleward to higher latitudes (10°S–10°N) via a Gill-type Rossby wave response mechanism.

Fig. 8.
Fig. 8.

Composites of sea level pressure anomalies (hPa; contours), 850-hPa wind anomalies (m s−1; vectors; significant values at 95% confidence level are shown with red arrows), and OLR anomalies (W m−2; color shading) during June–November for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

Fig. 9.
Fig. 9.

(left) Vertical–longitudinal sections of composite zonal and vertical velocity (vectors; m s−1 for u and 10−2 Pa s−1 for −ω) and apparent heat source (Q1; 10−2 W m−2) along the equator and (right) vertical–latitudinal sections of regressed meridional and vertical velocity (vectors; m s−1 for υ and 10−2 Pa s−1 for −ω) and apparent heat source (Q1; 10−2 W m−2) along 160°E for (a),(e) CEPW, (b),(f) CPW, (c),(g) EPW, and (d),(h) WEPW years.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

During CPW years (Fig. 8b), weakening and westward shifting of warming SST anomalies lead to weak positive SLP anomalies in the equatorial region between 180° and 130°W, which are flanked by negative SLP anomalies to the west and east. Compared to CEPW years, these phenomena weaken low-level westerlies in the equatorial central and eastern Pacific (150°E–120°W). This change leads to a weakening and westward shift of an anomalous two-cell Walker circulation, with anomalous ascending motion over a region 140°E–150°W (Fig. 9b). It is noted that the relatively weaker Q1 heating associated with maximum vertical motion is weaker, and its maximum is centered in the upper troposphere over the region 140°–160°E, generating a westward shift (over 150°E–180°) in anomalous convection (Fig. 8b). West of the central Pacific (along 160°E; Fig. 9f), weak Q1 heating occurs in the midtroposphere (above 600 hPa), and significant upward motion penetrates the whole troposphere along with Q1 heating between 5°S and 10°N; however, this upward motion is not accompanied by downward motion farther north. Although CPW has a relatively weak Walker circulation, the increased Q1 heating and deep convection locate in the CP region because of ascending caused by local CP SST and Walker circulations. This anomalous convection over the central Pacific may lead to anomalous cyclonic activity to the northwest, inducing significant anomalous westerlies centered over a region 10°N because of a Gill-type Rossby wave response. Therefore, the Gill-type Rossby wave response overwhelms an anomalous Walker circulation during CPW years, leading to the poleward migration of the center of westerlies.

For EPW years (Fig. 8c), a narrow band of anomalous westerlies extends across the central and eastern Pacific, centered at the equator, with narrow anomalous warm SSTs. This leads to an anomalous one-cell Walker circulation with anomalous ascending (descending) motion and strong Q1 heating (cooling) over 180°–140°W (130°–150°E) (Fig. 9c). In the west of the central Pacific (along 160°E; Fig. 9g), strong downward motion penetrates the whole troposphere over equatorial regions, accompanied by upward motion and Q1 heating between 5° and 15°N. A relatively narrow convection anomaly occurs in this location over a warmer EP ocean and extends toward the central Pacific. This westward retreating narrow anomalous convection heating along the equator also contributes to anomalous equatorial westerlies related to an anomalous Walker circulation as a clear southwestward shift of heat-induced cyclonic circulation anomalies (Fig. 8c). Compared to CEPW and CPW years, anomalous equatorial westerlies, as part of an anomalous Walker circulation, can be mostly attributed to the effect of EP SST.

For WEPW years, the maximum SST anomaly is at 60°–140°W; in addition, a slightly warm SST anomaly extends from the central and eastern tropical Pacific to the western tropical Pacific. Although the intensity (location) of SST warming during WEPW years is similar to that of during CPW (EPW) years, unlike CPW and EPW, both anomalous wind and SLP are weakly distributed over the central and western Pacific (Fig. 8d). These results suggest that a large-scale circulation response to warming in the EP requires greater warming. This may be due to the region having a higher SST threshold for deep convection over the EP. In addition, it is interesting to note that WEPW enhances convective heating associated with the relative anomalous upward motion and Q1 heating west of the date line over equatorial western–central Pacific regions but it is of weak magnitude (Figs. 9d,h). These anomalies are linked to local changes only in sea surface warming in the WNP. During years when weaker SST warming of the same intensity as CPW shifts to the EP, atmospheric circulations induced by local SST forcing exhibit primary control over the effects on TCs over the WNP.

It is noteworthy that OLR and westerlies over the tropical central and western Pacific forced by differential SST warming show differences among Pacific warming regimes. Anomalous warming of CP SST has more effect on anomalous equatorial convection and low-level zonal winds north of the equator while anomalous warming in the EP mainly enhances an anomalous Walker circulation and equatorial lower-level westerlies. These relationships are examined for anomalous 850-hPa zonal winds of the north (5°–15°N, 140°E–180°; representing the UN) and equatorial (5°S–5°N, 140°E–180°; representing the UEQ) regions of the eastern tropical WNP, as well as equatorial convection (5°S–5°N, 150°E–180°; representing OLREQ). Scatterplots of both OLREQ and UN versus CP SST are shown in Figs. 10a and 10b, where red, orange, blue, and gray dots indicate EP El Niño, CP El Niño, La Niña, and neutral years, respectively, to better understand the effect of different phases of conventional two-type ENSO events. Both OLREQ and UN show clearly linear responses to CP SST, indicating that UN and OLREQ are associated with the warming of CP because of a Gill-type Rossby wave response. However, there is no apparent relationship between EP SST and OLREQ for positive EP SST anomaly. In this case, OLREQ is nearly independent of EP SST for positive EP SST anomaly (Fig. 10c); hence, there is a weaker UN response in warm EP SST events compared to warm CP SST events (not shown). These findings indicate that strong warming of CP tends to cause low-level westerlies in the north of the equatorial WNP with convective anomalies in the equatorial WNP. Further, there is a relationship between UEQ and EP SST for both positive and negative EP SST (Fig. 10d), suggesting that anomalous low-level equatorial westerlies from east of the WNP to the CP are a result of a change in Walker circulation associated with EP SST warming. In addition, note that separations in the two conventional warming phases of ENSO are much less pronounced in Figs. 10a and 10b (red and orange dots), indicating that CP SST relationships with equatorial convection and anomalous zonal winds of the north do not correspond well with separating the two types of conventional ENSO.

Fig. 10.
Fig. 10.

Scatterplot of (a),(c) OLR and (b),(d) 850-hPa zonal wind anomaly against (top) CP SST and (bottom) EP SST. OLR is averaged over the equatorial central Pacific region (OLREQ; 5°S–5°N, 150°E–180°). The 850-hPa zonal wind anomalies are averaged over the northern region (UN; 5°–15°N, 140°E–180°) in (b) and over the equatorial region (UEQ; 5°S–5°N, 150°E–180°) in (d) of the central Pacific. SSTs are averaged over the CP in (a) and (b) and over the EP in (c) and (d). The red, orange, blue, and gray colors corespnd to EP El Niño, CP El Niño, La Niña, and neutral years, respectively. The solid line is the linear regression.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

Any changes in central and eastern Pacific warming can cause related shifts in positions of low-level westerlies over the WNP, not only via a change of the Walker cell, but also via a Gill-type Rossby wave response. Generally, there are anomalous low-level westerlies over the WNP as a part of the Walker circulation induced by EPW. The equatorial westerlies are sensitive to the intensity of Pacific warming in the EP. Specifically, the westerlies response to SST warming in the EP is insensitive to a weaker EP SST warming. However, the Gill-type Rossby wave response may be a more important control on the poleward migration of low-level westerlies during CPW years. For CEPW years, the combined contributions of CP and EP SST warming drive both the Walker circulation and Gill-type Rossby wave response, causing the WNP westerlies development in broad latitudes (10°S–10°N).

b. Monsoon trough and large-scale environmental conditions associated with TC activity

It has long been recognized that large-scale oceanic and atmospheric environmental conditions influence TC activity. Although local SST forcing plays an essential role in driving TC activity, most of the tropical WNP reaches the threshold (26°C) of favorable TC genesis and development during the TC season. Dynamic and thermodynamic environmental conditions associated with vast atmospheric circulation systems are very important governors of TC activity in the WNP. Many previous studies have demonstrated that TC activity is closely related to monsoon troughs (MTs). As observed by Briegel and Frank (1997), Chen et al. (2004), Ritchie and Holland (1999), and Ritchie (1995), more than 70% of WNP TC genesis is linked to MTs. Thus, any mechanism that can affect the location and intensity of MTs would cause variation in the frequency and location of TC genesis. The WNP MT is a near-equatorial confluence zone between low-level easterly trade winds and westerly monsoonal winds, which is strongly influenced by anomalous low-level zonal winds. As shown in the previous section, there is a clear change in anomalous 850-hPa zonal winds for different Pacific warming events, suggesting that different Pacific warming events may have different impacts on MTs.

Figure 11 shows composite charts of the 850-hPa streamline and anomalous relative vorticity and 500-hPa geopotential height for each Pacific warming type, respectively. Within these different regimes, MT location is shifted in longitude with a slight shift in latitude. As seen in Fig. 8, enhanced anomalous westerlies extend poleward to higher latitudes (10°N) during CEPW years. On the other hand, following the southwestward extension of an enhanced subtropical high (Fig. 11a) in response to anomalous descending motion of Hadley circulation (Fig. 9a), easterly trade winds have a stronger intensity nearer the southern subtropical high. These lead to an increase in confluence between enhanced low-level easterly trade winds and enhanced westerly monsoonal winds, producing an enhanced and eastward extension of the MT (Fig. 11a). This eastward extension of the MT acts to increase cyclonic vorticity (Fig. 11a) and reduce vertical wind shear (Fig. 12a) north of the mean MT position in its eastern part (east of 150°E). The anomaly in 500–700-hPa relative humidity (Fig. 12a) is similar to those for OLR (Fig. 8a), with increased relative humidity and enhanced convection in the SE region. This is essentially in the extended region of the MT. These large-scale conditions associated with the eastern extension of the MT are favorable for TC genesis. In addition, such an MT favors synoptic-scale disturbances gaining energy from mean flows through barotropic energy conversion, resulting in the growth of perturbations as a possible forcing mechanism for TC genesis (Wu et al. 2014a, 2015a,b). Following Wu et al. (2012, 2014b), disturbance activity is characterized by the variance of 2–8-day-filtered eddy kinetic energy (EKE) at 850 hPa. A large area of positive and statistically significant anomalies in synoptic-scale disturbances covers the extended region of the MT from the tropical central Pacific to the WNP during CEPW years (Fig. 12a). As a result, perturbations and large-scale environmental conditions associated with the eastern extension of the MT are favorable for TC genesis in the southeastern part of the WNP during CEPW years.

Fig. 11.
Fig. 11.

The 850-hPa streamline charts with superimposed 850-hPa relative vorticity (10−6 s−1; values in dotted regions are significant at the 95% confidence level) for average of (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years during the TC season. The green solid lines indicate the 500-hPa geopotential height (gpm). The 850-hPa monsoon trough (MT) is denoted by a thick-dashed black line.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

Fig. 12.
Fig. 12.

(left) The anomalous mean vertical wind shear (VWS; m s−1) between 200 and 850 hPa, (middle) 500–700-hPa relative humidity (RH; %), and (right) 850-hPa EKE (m2 s−2) during July–November for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW years. The dotted regions indicate areas exceeding a 95% confidence level using the Student’s t test.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

Compared to CEPW, EPW alone (moderate El Niño) results in only narrow weak anomalous westerlies near the equator (Fig. 8c) and a weaker subtropical high, which in turn result in a westward retreat in the location of the MT (Fig. 11c). Accompanying this narrow MT development, increased low-level anomalous cyclonic vorticity and perturbations appear in the eastern part (130°–160°E) of the MT along with reduced vertical shear in the north of the MT (Fig. 12c). In terms of TC genesis (Fig. 5c), anomalous TC genesis associated with EPW occurs between 10° and 20°N along the anomalous cyclonic vorticity of the MT axis. In WEPW years, the MT retreats farther westward to the west of 150°E (Fig. 11d). The intensity and location of the MT are similar to that of the climatological mean, indicating that the weaker atmospheric response over the WNP to WEPW does not modify the MT. On the other hand, weak local SST warming in the equatorial western Pacific is followed by no significant enhanced convection, increased relative humidity, and reduced vertical wind shear east of the MT region (130°–160°E) (Fig. 12d), which are conditions favorable for TC genesis in this region (Fig. 5d).

Interestingly, the location of MT does not show significant variation in longitude between CPW and EPW years. Although the southern edge of the subtropical high is close to the same position during EPW and CPW years, the subtropical high during CPW shifts eastward to around 130°E and is more intense than that during EPW (Fig. 11b). CPW’s anomalous westerly winds (Fig. 8b) strengthen the mean westerly winds, enlarging the strength of the MT, leading to an anomalously broad MT zone shifted slightly northward. This well-established broad MT is followed by increased reduced vertical wind shear, greater low-level cyclonic vorticity, lower OLR, and higher 500–700-hPa relative humidity over the broad MT region (Figs. 12b and 8b), which are favorable conditions for TC genesis, while a weak and narrow MT during EPW is not. This finding supports the idea that broad convergence zones in the moist tropics are likely to be associated with deep convection and large-scale ascent (Berry and Reeder 2014).

5. Summary

This study investigates the influence of different types of Pacific warming on TCs over the WNP during July–November for the period 1948–2015. First, we compared SST characteristics between two types of El Niño. Extreme EP El Niño events are associated with concurrent strong SST warming in both CP and EP regions, while EPW alone is associated with moderate EP El Niño. Weak SST warming (from positive to neutral) in the EP (WEPW) has a similar intensity (SSTA ~ 0.3°–0.7°C) but in different locations compared to the SST warming in the CP (CPW). Tropical Pacific warming is separated into four modes based on the location and magnitude of SSTs in the tropical CP and EP: CEPW, CPW, EPW, and WEPW.

During CEPW years, TC genesis position shifts southeastward, and TCs have longer lifetimes and increased frequency of occurrences (tracks) in most of the WNP basin region in response to stronger SST warming in both the central and eastern Pacific Ocean. During CPW years, TC genesis and activity displaces similar changes, but maximum centers extend westward to east of the Philippines compared to CEPW years. Unlike CEPW (extreme EP El Niño) years, TC genesis during EPW (moderate EP El Niño) and WEPW does not show a dramatic southeastward shift with weak positive anomalies east of the Philippines. This suggests that TCs forming in the SE region may be more dependent on CP SST than EP SST. Furthermore, when the effect of EP SST is removed, CP SST still exhibits a relatively high relationship with TC genesis in the SE region; however, correlation between the EP SST and TC genesis in the SE region is reduced to almost zero after removal of the effect of CP SST. This southeastward shift of TC genesis position might be key to determining the spatial distribution of TC activity (Wu and Wang 2004; Wu et al. 2012). In addition, because of higher threshold behavior of atmospheric responses to colder oceans, in weaker SST warming periods in the EP (WEPW), WNP TCs and atmospheric circulations in response to SST forcing are more dependent on local SST in the WNP.

Possible reasons for modulation of different modes of Pacific warming on TC genesis have also been investigated through atmospheric processes. Figure 13 depicts schematically the salient features of the atmospheric response to the different Pacific warming. Anomalous low-level westerlies in western and central Pacific are associated with different SST warmings of the tropical Pacific and may be important contributors to TC genesis. Anomalous low-level equatorial westerlies are a result of anomalous Walker circulation associated with SST warming in the EP (Figs. 13a,c). These equatorial westerlies show greater intensity and eastward extension with an increase in the intensity of EP SST warming (Fig. 13a). On the other hand, anomalous convection associated with CP SST warming possibly leads to anomalous cyclonic activity over a more northwest region with significantly increased anomalous westerlies centered at 10°N because of a Gill-type Rossby wave response (Figs. 13a,b).

Fig. 13.
Fig. 13.

Schematic illustrating the processes by which the location and intensity of Pacific warming affect TC activity over the WNP for (a) CEPW, (b) CPW, (c) EPW, and (d) WEPW. The red lines denote contours of positive SST anomalies, and blue lines indicate convection; the solid black and dashed gray arrows indicate anomalous low-level winds and an anomalous Walker cell, respectively; green shading indicates the MT region; TC genesis and tracks are marked by typhoon symbols and orange arrows, respectively.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0298.1

These changes in location and intensity of low-level westerlies further lead to a shift in the MT and primary environmental conditions to favor TC genesis and development. During CPW (EPW) years when SST warming occurs in the CP (EP) alone, anomalous westerlies away from (along and/or near) the equator lead to an anomalously broad (narrow) zone of the eastward-extending MT with a slightly northward (southward) shift (Figs. 13b,c). During CEPW years when SST warming occurs both in the CP and EP, enhanced anomalous westerlies extend poleward to a higher latitude (10°N), and thus there is an enhanced farther eastward extension of MTs (Fig. 13a). Accompanied with this eastward shift in MTs, the primary environmental conditions (such as weaker vertical wind shear and stronger low-level cyclonic vorticity, and more perturbations) are favorable for TC genesis in the SE region during CEPW and CPW years. However, these conditions associated with a weaker and southward shift of a narrow MT are not significant changes during EPW years when compared to CEPW and CPW years. Moreover, during phases of weaker EP SST warming (WEPW) of similar intensity to CPW, WNP TCs and weaker MT in response to SST forcing are more dependent on local SST in the WNP (Fig. 13d) because of a higher SST threshold for deep convection development in the EP region.

This study indicates that SST warming in the CP exhibits primary control over the effects of ENSO on TCs over the WNP. The results explain why strong EP El Niño events or CP El Niño can affect TCs over the WNP, while moderate EP El Niño events do not. These findings are important for understanding WNP TC activity at present and into the future as the intensity and frequency of extreme El Niño and CP El Niño events are projected to increase under a warming climate. However, these results are obtained based on a limited sample size in terms of observations. Future study would be useful in confirming the effects of intensity and location of Pacific warming on WNP TCs now and into the future through greater sampling and additional modeling. In addition, some studies have found that enhanced SST warming in the subtropical ENP favors the occurrence of TCs in the WNP, especially in the southeastern WNP, through forcing the Pacific meridional mode (PMM) (Murakami et al. 2017; Zhang et al. 2016). Such subtropical EP SST warming occurs in CPW and CEPW years (Figs. 3a,b). Its influence is not independent of CP SST’s influence because subtropical ENP SST is highly correlated with CP SST with a correlation coefficient of 0.78. This suggests that the influence of the tropical Pacific on TC activity may involve additionally the subtropical influence. Observational studies have showed that the different SST distribution in the CP and subtropical ENP tends to produce different impacts on climate between the 1997 and 2015 strongest extreme El Niño events (Paek et al. 2017; Zhang et al. 2016; Murakami et al. 2017) and might cause variations in TC activity. These need further investigation.

Acknowledgments

This work is jointly supported by the National Natural Science Foundation of China Grants 41475077, 41461164005, and 41230527, the National Basic Research Program of China under Grant 2014CB953902, and the Youth Innovation Promotion Association CAS 2017106.

REFERENCES

  • Ashok, K., S. K. Behera, S. A. Rao, H. Weng, and T. Yamagata, 2007: El Niño Modoki and its possible teleconnection. J. Geophys. Res., 112, C11007, https://doi.org/10.1029/2006JC003798.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Balmaseda, M. A., K. Mogensen, and A. Weaver, 2013: Evaluation of the ECMWF Ocean Reanalysis ORAS4. Quart. J. Roy. Meteor. Soc., 139, 11321161, https://doi.org/10.1002/qj.2063.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Berry, G., and M. J. Reeder, 2014: Objective identification of the intertropical convergence zone: Climatology and trends from the ERA-Interim. J. Climate, 27, 18941909, https://doi.org/10.1175/JCLI-D-13-00339.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Briegel, L. M., and W. M. Frank, 1997: Large-scale influences on tropical cyclogenesis in the western North Pacific. Mon. Wea. Rev., 125, 13971413, https://doi.org/10.1175/1520-0493(1997)125<1397:LSIOTC>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Cai, W., and Coauthors, 2014: Increasing frequency of extreme El Niño events due to greenhouse warming. Nat. Climate Change, 4, 111116, https://doi.org/10.1038/nclimate2100.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Camargo, S. J., and A. H. Sobel, 2005: Western North Pacific tropical cyclone intensity and ENSO. J. Climate, 18, 29963006, https://doi.org/10.1175/JCLI3457.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Camargo, S. J., K. A. Emanuel, and A. H. Sobel, 2007: Use of a genesis potential index to diagnose ENSO effects on tropical cyclone genesis. J. Climate, 20, 48194834, https://doi.org/10.1175/JCLI4282.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Chan, J. C. L., 1985: Tropical cyclone activity in the northwest Pacific in relation to the El Niño/Southern Oscillation phenomenon. Mon. Wea. Rev., 113, 599606, https://doi.org/10.1175/1520-0493(1985)113<0599:TCAITN>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Chan, J. C. L., 2000: Tropical cyclone activity over the western North Pacific associated with El Nino and La Nina events. J. Climate, 13, 29602972, https://doi.org/10.1175/1520-0442(2000)013<2960:TCAOTW>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Chen, T.-C., S.-Y. Wang, M.-C. Yen, and W. A. Gallus Jr., 2004: Role of the monsoon gyre in the interannual variation of tropical cyclone formation over the western North Pacific. Wea. Forecasting, 19, 776785, https://doi.org/10.1175/1520-0434(2004)019<0776:ROTMGI>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Elsner, J. B., A. A. Tsonis, and T. H. Jagger, 2006: High-frequency variability in hurricane power dissipation and its relationship to global temperature. Bull. Amer. Meteor. Soc., 87, 763768, https://doi.org/10.1175/BAMS-87-6-763.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Feng, J., W. Chen, C.-Y. Tam, and W. Zhou, 2011: Different impacts of El Niño and El Niño Modoki on China rainfall in the decaying phases. Int. J. Climatol., 31, 20912101, https://doi.org/10.1002/joc.2217.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447462, https://doi.org/10.1002/qj.49710644905.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Hong, C. C., Y. H. Li, T. Li, and M. Y. Lee, 2011: Impacts of central Pacific and eastern Pacific El Niños on tropical cyclone tracks over the western North Pacific. Geophys. Res. Lett., 38, L16712, https://doi.org/10.1029/2011GL048821.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437471, https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kim, H.-M., P. J. Webster, and J. A. C. Curry, 2011: Modulation of North Pacific tropical cyclone activity by three phases of ENSO. J. Climate, 24, 18391849, https://doi.org/10.1175/2010JCLI3939.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Krishnamurthy, L., G. A. Vecchi, R. Msadek, H. Murakami, A. Wittenberg, and F. Zeng, 2016: Impact of strong ENSO on regional tropical cyclone activity in a high-resolution climate model in the North Pacific and North Atlantic Oceans. J. Climate, 29, 23752394, https://doi.org/10.1175/JCLI-D-15-0468.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Lee, T., and M. J. McPhaden, 2010: Increasing intensity of El Niño in the central-equatorial Pacific. Geophys. Res. Lett., 37, L14603, https://doi.org/10.1029/2010GL044007.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., and W. Zhou, 2012: Changes in western Pacific tropical cyclones associated with the El Niño–Southern Oscillation cycle. J. Climate, 25, 58645878, https://doi.org/10.1175/JCLI-D-11-00430.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., and W. Zhou, 2013a: Modulation of western North Pacific tropical cyclone activities by the ISO. Part I: Genesis and intensity. J. Climate, 26, 29042918, https://doi.org/10.1175/JCLI-D-12-00210.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., and W. Zhou, 2013b: Modulation of western North Pacific tropical cyclone activities by the ISO. Part II: Tracks and landfalls. J. Climate, 26, 29192930, https://doi.org/10.1175/JCLI-D-12-00211.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, C. Y., W. Zhou, J. C. L. Chan, and P. Huang, 2012: Asymmetric modulation of the western North Pacific cyclogenesis by the Madden–Julian oscillation under ENSO conditions. J. Climate, 25, 53745385, https://doi.org/10.1175/JCLI-D-11-00337.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Li, X., W. Zhou, D. L. Chen, C. Y. Li, and J. Song, 2014: Water vapor transport and moisture budget over eastern China: Remote forcing from the two types of El Niño. J. Climate, 27, 87788792, https://doi.org/10.1175/JCLI-D-14-00049.1.

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

  • Murakami, H., and Coauthors, 2017: Dominant role of subtropical Pacific warming in extreme eastern Pacific hurricane seasons: 2015 and the future. J. Climate, 30, 243264, https://doi.org/10.1175/JCLI-D-16-0424.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Paek, H., J. Y. Yu, and C. Qian, 2017: Why were the 2015/2016 and 1997/1998 extreme El Niños different? Geophys. Res. Lett., 44, 18481856, https://doi.org/10.1002/2016GL071515.

    • Search Google Scholar
    • Export Citation
  • Patricola, C. M., P. Chang, and R. Saravanan, 2016: Degree of simulated suppression of Atlantic tropical cyclones modulated by flavour of El Niño. Nat. Geosci., 9, 155160, https://doi.org/10.1038/ngeo2624.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Pradhan, P., B. Preethi, K. Ashok, R. Krishnan, and A. Sahai, 2011: Modoki, Indian Ocean dipole, and western North Pacific typhoons: Possible implications for extreme events. J. Geophys. Res., 116, D18108, https://doi.org/10.1029/2011JD015666.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108, 4407, https://doi.org/10.1029/2002JD002670.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Ritchie, E. A., 1995: Mesoscale aspects of tropical cyclone formation. Ph.D. dissertation, Monash University, 167 pp. [Available from Monash University, Wellington Rd., Clayton, VIC 3168, Australia.]

  • Ritchie, E. A., and G. J. Holland, 1999: Large-scale patterns associated with tropical cyclogenesis in the western Pacific. Mon. Wea. Rev., 127, 20272043, https://doi.org/10.1175/1520-0493(1999)127<2027:LSPAWT>2.0.CO;2.

    • Crossref
    • 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, https://doi.org/10.1175/1520-0442(2002)015<1643:HSEEAT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., and B. Wang, 2004: Assessing impacts of global warming on tropical cyclone tracks. J. Climate, 17, 16861698, https://doi.org/10.1175/1520-0442(2004)017<1686:AIOGWO>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, and R. Huang, 2011: A primary study of the correlation between the net air–sea heat flux and the interannual variation of western North Pacific tropical cyclone track and intensity. Acta Oceanol. Sin., 30, 2735, https://doi.org/10.1007/s13131-011-0158-8.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, R. Huang, and R. Wu, 2012: Possible linkage between the monsoon trough variability and the tropical cyclone activity over the western North Pacific. Mon. Wea. Rev., 140, 140150, https://doi.org/10.1175/MWR-D-11-00078.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, T. Li, and R. Huang, 2014a: ENSO-phase dependent TD and MRG wave activity in the western North Pacific. Climate Dyn., 42, 12171227, https://doi.org/10.1007/s00382-013-1754-4.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., and Coauthors, 2014b: Simulations of the present and late-twenty-first-century western North Pacific tropical cyclone activity using a regional model. J. Climate, 27, 34053424, https://doi.org/10.1175/JCLI-D-12-00830.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, and R. Wu, 2015a: Influence of the monsoon trough on westward-propagating tropical waves over the western North Pacific. Part I: Observations. J. Climate, 28, 71087127, https://doi.org/10.1175/JCLI-D-14-00806.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wu, L., Z. Wen, and R. Wu, 2015b: Influence of the monsoon trough on westward-propagating tropical waves over the western North Pacific. Part II: Energetics and numerical experiments. J. Climate, 28, 93329349, https://doi.org/10.1175/JCLI-D-14-00807.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yanai, M., and R. H. Johnson, 1993: Impacts of cumulus convection on thermodynamic fields. Representation of Cumulus Convection in Numerical Models of the Atmosphere, Meteor. Monogr., No. 46, Amer. Meteor. Soc., 39–62.

    • Crossref
    • Export Citation
  • Yanai, M., S. Esbensen, and J.-H. Chu, 1973: Determination of bulk properties of tropical cloud clusters from large-scale heat and moisture budgets. J. Atmos. Sci., 30, 611627, https://doi.org/10.1175/1520-0469(1973)030<0611:DOBPOT>2.0.CO;2.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., J.-S. Kug, B. Dewitte, M.-H. Kwon, B. P. Kirtman, and F.-F. Jin, 2009: El Niño in a changing climate. Nature, 461, 511514, https://doi.org/10.1038/nature08316.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yu, J.-Y., and S. T. Kim, 2013: Identifying the types of major El Niño events since 1870. Int. J. Climatol., 33, 21052112, https://doi.org/10.1002/joc.3575.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Yuan, Y., W. Zhou, H. Yang, and C. Y. Li, 2008: Warming in the northwestern Indian Ocean associated with the El Niño event. Adv. Atmos. Sci., 25, 246252, https://doi.org/10.1007/s00376-008-0246-7.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhang, W., H.-F. Graf, Y. Leung, and M. Herzog, 2012: Different El Niño types and tropical cyclone landfall in East Asia. J. Climate, 25, 65106523, https://doi.org/10.1175/JCLI-D-11-00488.1.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Zhang, W., G. A. Vecchi, H. Murakami, G. Villarini, and L. Jia, 2016: The Pacific meridional mode and the occurrence of tropical cyclones in the western North Pacific. J. Climate, 29, 381398, https://doi.org/10.1175/JCLI-D-15-0282.1.

    • Crossref
    • 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, https://doi.org/10.1002/joc.1380.

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