• Adler, R. F., and Coauthors, 2003: The version-2 Global Precipitation Climatology Project (GPCP) Monthly Precipitation Analysis (1979–present). J. Hydrometeor., 4, 11471167, doi:10.1175/1525-7541(2003)004<1147:TVGPCP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • An, S.-I., F. Jin, and I.-S. Kang, 1999: The role of zonal advection feedback in phase transition and growth of ENSO in the Cane-Zebiak model. J. Meteor. Soc. Japan, 77, 11511160.

    • Search Google Scholar
    • Export Citation
  • 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, doi:10.1029/2006JC003798.

    • Search Google Scholar
    • Export Citation
  • Behringer, D., and Y. Xue, 2004: Evaluation of the global ocean data assimilation system at NCEP: The Pacific Ocean. Eighth Symp. on Integrated Observing and Assimilation Systems for Atmosphere, Oceans, and Land Surface, Seattle, WA, Amer. Meteor. Soc., 2.3. [Available online at https://ams.confex.com/ams/pdfpapers/70720.pdf.]

  • Bratcher, A. J., and B. S. Giese, 2002: Tropical Pacific decadal variability and global warming. Geophys. Res. Lett., 29, 1918, doi:10.1029/2002GL015191.

    • Search Google Scholar
    • Export Citation
  • Cai, W. J., and T. Cowan, 2009: La Niño Modoki impacts Australia autumn rainfall variability. Geophys. Res. Lett., 36, L12805, doi:10.1029/2009GL037885.

    • Search Google Scholar
    • Export Citation
  • Carton, J. A., and B. S. Giese, 2008: A reanalysis of ocean climate using Simple Ocean Data Assimilation (SODA). Mon. Wea. Rev., 136, 29993017, doi:10.1175/2007MWR1978.1.

    • Search Google Scholar
    • Export Citation
  • Chang, P., and S. G. Philander, 1994: A coupled ocean–atmosphere instability of relevance to the seasonal cycle. J. Atmos. Sci., 51, 36273648, doi:10.1175/1520-0469(1994)051<3627:ACOIOR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Chen, Z., Z. Wen, R. Wu, P. Zhao, and J. Cao, 2014: Influence of two types of El Niños on the East Asian climate during boreal summer: A numerical study. Climate Dyn., 43, 469481, doi:10.1007/s00382-013-1943-1.

    • Search Google Scholar
    • Export Citation
  • Chung, P.-H., and T. Li, 2013: Interdecadal relationship between the mean state and El Niño types. J. Climate, 26, 361379, doi:10.1175/JCLI-D-12-00106.1.

    • Search Google Scholar
    • Export Citation
  • Dai, A., 2013: The influence of the inter-decadal Pacific oscillation on U.S. precipitation during 1923–2010. Climate Dyn., 41, 633646, doi:10.1007/s00382-012-1446-5.

    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597, doi:10.1002/qj.828.

    • Search Google Scholar
    • Export Citation
  • Ding, Q., E. J. Steig, D. S. Battisti, and M. Kuttel, 2011: Winter warming in West Antarctica caused by central tropical Pacific warming. Nat. Geosci., 4, 398403, doi:10.1038/ngeo1129.

    • Search Google Scholar
    • Export Citation
  • Feng, J., and J. Li, 2011: Influence of El Niño Modoki on spring rainfall over south China. J. Geophys. Res., 116, D13102, doi:10.1029/2010JD015160.

    • Search Google Scholar
    • Export Citation
  • Feng, J., and J. Li, 2013: Contrasting impacts of two types of ENSO on the boreal spring Hadley circulation. J. Climate, 26, 47734789, doi:10.1175/JCLI-D-12-00298.1.

    • Search Google Scholar
    • Export Citation
  • Feng, J., L. Wang, W. Chen, S. K. Fong, and K. C. Leong, 2010: Different impacts of two types of Pacific Ocean warming on Southeast Asian rainfall during boreal winter. J. Geophys. Res., 115, D24122, doi:10.1029/2010JD014761.

    • 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, doi:10.1002/joc.2217.

    • Search Google Scholar
    • Export Citation
  • Graf, H.-F., and D. Zanchettin, 2012: Central Pacific El Niño, the “subtropical bridge,” and Eurasian climate. J. Geophys. Res., 117, D01102, doi:10.1029/2011JD016493.

    • Search Google Scholar
    • Export Citation
  • Guo, Y., Z. Wen, R. Wu, R. Lu, and Z. Chen, 2015: Impact of tropical Pacific precipitation anomaly on the East Asian upper-tropospheric westerly jet during the boreal winter. J. Climate, 28, 64576474, doi:10.1175/JCLI-D-14-00674.1.

    • Search Google Scholar
    • Export Citation
  • Jin, F., and S.-I. An, 1999: Thermocline and zonal advective feedback within the equatorial ocean recharge oscillator model for ENSO. Geophys. Res. Lett., 26, 29892992, doi:10.1029/1999GL002297.

    • Search Google Scholar
    • Export Citation
  • Jin, F., S. T. Kim, and L. Bejarano, 2006: Acoupled-stability index for ENSO. Geophys. Res. Lett., 33, L23708, doi:10.1029/2006GL027221.

    • Search Google Scholar
    • Export Citation
  • Kang, I.-S., S.-I. An, and F. Jin, 2001: A systematic approximation of the SST anomaly equation for ENSO. J. Meteor. Soc. Japan, 79, 110, doi:10.2151/jmsj.79.1.

    • Search Google Scholar
    • Export Citation
  • Kao, H.-Y., and J.-Y. Yu, 2009: Contrasting eastern Pacific and central Pacific types of ENSO. J. Climate, 22, 615632, doi:10.1175/2008JCLI2309.1.

    • Search Google Scholar
    • Export Citation
  • Kim, H.-M., P. J. Webster, and J. A. Curry, 2009: Impact of shifting patterns of Pacific Ocean warming on North Atlantic tropical cyclones. Science, 325, 7780, doi:10.1126/science.1174062.

    • Search Google Scholar
    • Export Citation
  • Kim, J.-S., K.-Y. Kim, and S.-W. Yeh, 2012: Statistical evidence for the natural variation of the central Pacific El Niño. J. Geophys. Res., 117, C06014, doi:10.1029/2012JC008003.

    • Search Google Scholar
    • Export Citation
  • Kim, S. T., and J.-Y. Yu, 2012: The two types of ENSO in CMIP5 models. Geophys. Res. Lett., 39, L11704, doi:10.1029/2012GL052006.

  • Kug, J.-S., F.-F. Jin, and S.-I. An, 2009: Two types of El Niño events: Cold tongue El Niño and warm pool El Niño. J. Climate, 22, 14991515, doi:10.1175/2008JCLI2624.1.

    • Search Google Scholar
    • Export Citation
  • Kug, J.-S., J. Choi, S.-I. An, F.-F. Jin, and A. T. Wittenberg, 2010: Warm pool and cold tongue El Niño events as simulated by the GFDL 2.1 coupled GCM. J. Climate, 23, 12261239, doi:10.1175/2009JCLI3293.1.

    • Search Google Scholar
    • Export Citation
  • Lee, S.-K., C. Wang, and D. B. Enfield, 2010: On the impact of central Pacific warming events on Atlantic tropical storm activity. Geophys. Res. Lett., 37, L17702, doi:10.1029/2010GL044459.

    • 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, doi:10.1029/2010GL044007.

    • Search Google Scholar
    • Export Citation
  • Li, H. L., H. J. Wang, and Y. Z. Yin, 2012: Interdecadal variation of the West African summer monsoon during 1979-2010 and associated variability. Climate Dyn., 39, 28832894, doi:10.1007/s00382-012-1426-9.

    • Search Google Scholar
    • Export Citation
  • Li, T., 1997: Phase transition of the El Niño–Southern Oscillation: A stationary SST mode. J. Atmos. Sci., 54, 28722887, doi:10.1175/1520-0469(1997)054<2872:PTOTEN>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • McPhaden, M. J., T. Lee, and D. McClurg, 2011: El Niño and its relationship to changing background conditions in the tropical Pacific Ocean. Geophys. Res. Lett., 38, L15709, doi:10.1029/2011GL048275.

    • Search Google Scholar
    • Export Citation
  • North, G. R., T. L. Bell, R. F. Cahalan, and F. J. Moeng, 1982: Sampling errors in the estimation of empirical orthogonal functions. Mon. Wea. Rev., 110, 699706, doi:10.1175/1520-0493(1982)110<0699:SEITEO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Praveen Kumar, B., J. Vialard, M. Lengaigne, V. S. N. Murty, M. J. McPhaden, M. F. Cronin, F. Pinsard, and K. Gopala Reddy, 2013: TropFlux wind stresses over the tropical oceans: Evaluation and comparison with other products. Climate Dyn., 40, 20492071, doi:10.1007/s00382-012-1455-4.

    • 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, doi:10.1029/2002JD002670.

    • Search Google Scholar
    • Export Citation
  • Ren, H.-L., and F.-F. Jin, 2013: Recharge oscillator mechanisms in two types of ENSO. J. Climate, 26, 65066523, doi:10.1175/JCLI-D-12-00601.1.

    • Search Google Scholar
    • Export Citation
  • Simpson, I. R., R. Seager, M. Ting, and T. A. Shaw, 2016: Causes of change in Northern Hemisphere winter meridional winds and regional hydroclimate. Nat. Climate Change, 6, 6570, doi:10.1038/nclimate2783.

    • Search Google Scholar
    • Export Citation
  • Su, J., R. Zhang, T. Li, X. Rong, J.-S. Kug, and C.-C. Hong, 2010: Causes of the El Niño and La Niña amplitude asymmetry in the equatorial eastern Pacific. J. Climate, 23, 605617, doi:10.1175/2009JCLI2894.1.

    • Search Google Scholar
    • Export Citation
  • Taschetto, A. S., and M. H. England, 2009: El Niño Modoki impacts on Australian rainfall. J. Climate, 22, 31673174, doi:10.1175/2008JCLI2589.1.

    • Search Google Scholar
    • Export Citation
  • Wang, C., and X. Wang, 2013: Classifying El Niño Modoki I and II by different impacts on rainfall in southern China and typhoon tracks. J. Climate, 26, 13221338, doi:10.1175/JCLI-D-12-00107.1.

    • Search Google Scholar
    • Export Citation
  • Wang, S.-Y., M. L’Heureux, and J.-H. Yoon, 2013: Are greenhouse gases changing ENSO precursors in the western North Pacific? J. Climate, 26, 63096322, doi:10.1175/JCLI-D-12-00360.1.

    • Search Google Scholar
    • Export Citation
  • Wang, X., and C. Wang, 2014: Different impacts of various El Niño events on the Indian Ocean Dipole. Climate Dyn., 42, 9911005, doi:10.1007/s00382-013-1711-2.

    • Search Google Scholar
    • Export Citation
  • Wen, C., A. Kumar, Y. Xue, and M. J. McPhaden, 2014: Changes in tropical Pacific thermocline depth and their relationship to ENSO after 1999. J. Climate, 27, 72307249, doi:10.1175/JCLI-D-13-00518.1.

    • Search Google Scholar
    • Export Citation
  • Weng, H., S. K. Behera, and T. Yamagata, 2009: Anomalous winter climate conditions in the Pacific Rim during recent El Niño Modoki and El Niño events. Climate Dyn., 32, 663674, doi:10.1007/s00382-008-0394-6.

    • Search Google Scholar
    • Export Citation
  • Wu, B., T. Zhou, and T. Li, 2012: Two distinct modes of tropical Indian Ocean precipitation in boreal winter and their impacts on equatorial western Pacific. J. Climate, 25, 921938, doi:10.1175/JCLI-D-11-00065.1.

    • Search Google Scholar
    • Export Citation
  • Wu, R., B. P. Kirtman, and V. Krishnamurthy, 2008: An asymmetric mode of tropical Indian Ocean rainfall variability in boreal spring. J. Geophys. Res., 113, D05104, doi:10.1029/2007JD008931.

    • Search Google Scholar
    • Export Citation
  • Wu, R., Z. Wen, S. Yang, and Y. Li, 2010: An interdecadal change in southern China summer rainfall around 1992/93. J. Climate, 23, 23892403, doi:10.1175/2009JCLI3336.1.

    • Search Google Scholar
    • Export Citation
  • Xiang, B., B. Wang, and T. Li, 2013: A new paradigm for the predominance of standing central Pacific warming after the late 1990s. Climate Dyn., 41, 327340, doi:10.1007/s00382-012-1427-8.

    • Search Google Scholar
    • Export Citation
  • Xie, Z., Y. Du, and S. Yang, 2015: Zonal extension and retraction of the subtropical westerly jet stream and evolution of precipitation over East Asia and the western Pacific. J. Climate, 28, 67836798, doi:10.1175/JCLI-D-14-00649.1.

    • 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, doi:10.1038/nature08316.

    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., B. P. Kirtman, J.-S. Kug, W. Park, and M. Latif, 2011: Natural variability of the central Pacific El Niño event on multi-centennial timescales. Geophys. Res. Lett., 38, L02704, doi:10.1029/2010GL045886.

    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., J.-S. Kug, and S.-I. An, 2014: Recent progresses on two types of El Niño: Observations, dynamics, and future changes. Asia-Pac. J. Atmos. Sci., 50, 6981, doi:10.1007/s13143-014-0028-3.

    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., X. Wang, C. Wang, and B. Dewitte, 2015: On the relationship between the North Pacific climate variability and the central Pacific El Niño. J. Climate, 28, 663677, doi:10.1175/JCLI-D-14-00137.1.

    • Search Google Scholar
    • Export Citation
  • Yu, J.-Y., H.-Y. Kao, and T. Lee, 2010: Subtropics-related interannual sea surface temperature variability in the central equatorial Pacific. J. Climate, 23, 28692884, doi:10.1175/2010JCLI3171.1.

    • Search Google Scholar
    • Export Citation
  • Yu, J.-Y., M.-M. Lu, and S.-T. Kim, 2012: A change in the relationship between tropical central Pacific SST variability and the extratropical atmosphere around 1990. Environ. Res. Lett., 7, 034025, doi:10.1088/1748-9326/7/3/034025.

    • Search Google Scholar
    • Export Citation
  • Zhu, Y. L., H. J. Wang, W. Zhou, and J. H. Ma, 2011: Recent changes in the summer precipitation pattern in East China and the background circulation. Climate Dyn., 36, 14631473, doi:10.1007/s00382-010-0852-9.

    • Search Google Scholar
    • Export Citation
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    Spatial pattern of (a),(b) the first two EOF modes and (c) the corresponding principal component (PC1 and PC2) of the precipitation anomalies over the tropical Pacific in boreal spring for the period of 1980–2013. (d),(e),(f) As in (c), but for boreal summer, autumn, and winter, respectively. The red (blue) line denotes PC1 (PC2).

  • View in gallery

    Spatial distribution of (a),(b) the first two EOF modes and (c) the corresponding PCs of the precipitation anomalies over the tropical Pacific in spring for the period of 1980–98. The red (blue) line denotes PC1 (PC2).

  • View in gallery

    As in Fig. 2, but for the period of 1999–2013.

  • View in gallery

    Regression maps of the SST (shading; unit: °C) and 1000-hPa horizontal wind anomalies (vector; unit: m s−1) onto the (a) pre-1998 PC1 and (b) post-1999 PC1. (c) The selected domains that are used to investigate the variability of SST and zonal wind anomaly over the equatorial central Pacific and eastern Pacific. The blue boxes denote the areas for zonal wind (CP: 2°S–2°N, 170°E–140°W; EP: 2°S–2°N, 120°–80°W) and the red boxes (SST_east: 2°S–2°N, 165°–125°W; SST_west: 2°S–2°N, 155°E–165°W) are used to represent the zonal SST gradient.

  • View in gallery

    Patterns of the leading SVD mode between (a),(d) the precipitation (20°S–20°N, 100°E–80°W) and (b),(e) the SST anomalies in the tropical Pacific (30°S–30°N, 100°E–80°W) in MAM for (left) before 1998 and (right) after 1998. (c),(f) The associated time series for both periods.

  • View in gallery

    Time series of three positive feedback oceanic processes (left) in the CP and (right) the EP at (top) 10 m and (bottom) 40 m. The unit is °C month−1. The blue, red, and black line denotes the Ekman pumping (EK) feedback, zonal advection (ZA) feedback, and thermocline (TH) feedback, respectively. The gray bar denotes the summation of the three positive feedback processes.

  • View in gallery

    Area-averaged SST in the (left) east, west box in the vicinity of the CP (outlined in Fig. 4c) and (right) their difference (east minus west). The unit is °C. The red bar in each panel represents the SST value during the pre-1998 period. The blue bar denotes the similarity during the post-1999 period.

  • View in gallery

    Relationship between the zonal wind at 1000 hPa (the black dashed line; unit: m s−1), surface wind stress from Tropflux data (the red line; unit: dyn cm−2), zonal wind stress from SODA data (the green line; unit: dyn cm−2), and zonal current at 5 m (the blue line; unit: m s−1) during 1980–2013 in the (a) CP and (b) EP. The scale of zonal wind at 1000 hPa is given on the right y axis.

  • View in gallery

    Relationship between zonal current anomalies at the upper ocean derived from GODAS (the black dashed line; unit: m s−1), geostrophic current (the red dashed line; unit: m s−1), and Ekman current anomalies (the blue line; unit: m s−1) from Tropflux dataset in the (a) CP and (b) EP.

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    Spatial distribution of (a),(c) the first two EOF modes and (b),(d) the corresponding principal component (PC1 and PC2) of the thermocline depth anomalies in the Pacific in boreal spring for the period of 1980–2013, derived from the GODAS reanalysis dataset. In (a), the contour represents the climatological thermocline depth. The unit is m and the interval is 50 m.

  • View in gallery

    SST anomaly tendency (contour in 0.15°C month−1) and SST anomaly (shading in °C) along the equatorial (2°S–2°N) for (a),(b),(c) the three strong rainfall years and (d) their composite result during the pre-1998 period. The zeros lines are not displayed.

  • View in gallery

    As in Fig. 11, but for the post-1999 period.

  • View in gallery

    Composite map of (left) the TH feedback anomalies, (middle) the ZA feedback anomalies, and (right) the EK feedback anomalies based on strong rainfall years of (upper) pre-1998 and (bottom) post-1999 period. The unit is °C month−1.

  • View in gallery

    Annual variability of anomalous zonal current at upper ocean for (top) the pre-1998 and (middle) post-1999 period. The red lines represent the strong rainfall years and the gray lines the other years. The black thick line denotes the averaged anomalous zonal current for different epochs. (bottom) The annual variation of the zonal mean SST gradient. The scale for the purple line is given on the right y axis.

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Interdecadal Change in the Tropical Pacific Precipitation Anomaly Pattern around the Late 1990s during Boreal Spring

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  • 1 Center for Monsoon and Environment Research and Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, China
  • | 2 Center for Monsoon and Environment Research and Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, and Jiangsu Collaborative Innovation Center for Climate Change, Jiangsu, China
  • | 3 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
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Abstract

The leading mode of boreal spring precipitation variability over the tropical Pacific experienced a pronounced interdecadal change around the late 1990s. The pattern before 1998 features positive precipitation anomalies over the equatorial eastern Pacific (EP) with positive principle component years. The counterpart after 1998 exhibits a westward shift of the positive center to the equatorial central Pacific (CP). Observational evidence shows that this interdecadal change in the leading mode of precipitation variability is closely associated with a distinctive sea surface temperature (SST) anomaly pattern. The westward shift of the anomalous precipitation center after 1998 is in tandem with a similar shift of maximum warming from the EP to CP. Diagnostic analyses based on a linear equation of the mixed layer temperature anomaly exhibit that an interdecadal enhancement of zonal advection (ZA) feedback process plays a vital role in the shift in the leading mode of both the tropical Pacific SST and the precipitation anomaly during spring. Moreover, the variability of the anomalous zonal current at the upper ocean dominates the ZA feedback change, while the mean zonal SST gradient associated with a La Niña–like pattern of the mean state only accounts for a relatively trivial proportion of the ZA feedback change. It was found that both the relatively rapid decaying of the SST anomalies in the EP and the La Niña–like mean state make it conceivable that the shift of the leading mode of the tropical precipitation anomaly only occurs in spring. In addition, the largest variance of the anomalous zonal current in spring might contribute to the unique interdecadal change in the tropical spring precipitation anomaly pattern.

Corresponding author address: Zhiping Wen, Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275, China. E-mail: eeswzp@mail.sysu.edu.cn

Abstract

The leading mode of boreal spring precipitation variability over the tropical Pacific experienced a pronounced interdecadal change around the late 1990s. The pattern before 1998 features positive precipitation anomalies over the equatorial eastern Pacific (EP) with positive principle component years. The counterpart after 1998 exhibits a westward shift of the positive center to the equatorial central Pacific (CP). Observational evidence shows that this interdecadal change in the leading mode of precipitation variability is closely associated with a distinctive sea surface temperature (SST) anomaly pattern. The westward shift of the anomalous precipitation center after 1998 is in tandem with a similar shift of maximum warming from the EP to CP. Diagnostic analyses based on a linear equation of the mixed layer temperature anomaly exhibit that an interdecadal enhancement of zonal advection (ZA) feedback process plays a vital role in the shift in the leading mode of both the tropical Pacific SST and the precipitation anomaly during spring. Moreover, the variability of the anomalous zonal current at the upper ocean dominates the ZA feedback change, while the mean zonal SST gradient associated with a La Niña–like pattern of the mean state only accounts for a relatively trivial proportion of the ZA feedback change. It was found that both the relatively rapid decaying of the SST anomalies in the EP and the La Niña–like mean state make it conceivable that the shift of the leading mode of the tropical precipitation anomaly only occurs in spring. In addition, the largest variance of the anomalous zonal current in spring might contribute to the unique interdecadal change in the tropical spring precipitation anomaly pattern.

Corresponding author address: Zhiping Wen, Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275, China. E-mail: eeswzp@mail.sysu.edu.cn

1. Introduction

An interdecadal change of global climate was observed around 1990s in many recent studies (Bratcher and Giese 2002; Wu et al. 2010; Ding et al. 2011; Zhu et al. 2011; Li et al. 2012; Chung and Li 2013; Dai 2013; Xiang et al. 2013; Wen et al. 2014; Yeh et al. 2015). Several studies presented observational evidence that the properties of El Niño–Southern Oscillation (ENSO) experienced a pronounced interdecadal change around 1990s. It has suggested that a different type of El Niño, the so-called date line El Niño, El Niño Modoki, warm pool El Niño, or central Pacific El Niño has occurred more frequently after 1990 (Ashok et al. 2007; Kao and Yu 2009; Kug et al. 2009; Yeh et al. 2009; Lee and McPhaden 2010; Yeh et al. 2011; Kim et al. 2012; Yu et al. 2012; Yeh et al. 2015). This new type of El Niño has different characteristics in the location of the maximum sea surface temperature (SST) anomalies in comparison to the conventional El Niño that typically features warming in the equatorial eastern Pacific (EP). Following previous studies, we will refer to the new type of El Niño as the central Pacific (CP) El Niño and the conventional El Niño as the EP El Niño. In recent decades, the CP El Niño received a lot of attention in terms of its mechanism (Kug et al. 2009; Chung and Li 2013; Xiang et al. 2013; Yeh et al. 2015), its impact on worldwide climate variability (Kim et al. 2009; Ding et al. 2011; Lee et al. 2010; Graf and Zanchettin 2012; Feng and Li 2013; Chen et al. 2014), the associated global teleconnection pattern from the tropics to the mid–high latitude (Weng et al. 2009; Wang and Wang 2013; Wang and Wang 2014), and its future change under global warming (Yeh et al. 2009; Kim and Yu 2012).

Many studies examined the tropical Pacific precipitation anomaly pattern associated with the two types of El Niño (e.g., Ashok et al. 2007; Taschetto and England 2009; Weng et al. 2009; Chung and Li 2013; Xiang et al. 2013). In association with the EP El Niño, maximum the precipitation anomaly is located to the east of the date line. In association with the CP El Niño, the positive precipitation anomaly shifts westward and appears around 160°E. The amplitude of the precipitation anomalies is smaller in the CP El Niño than EP El Niño, as the SST anomalies are smaller (Kao and Yu 2009; Kug et al. 2009). The close relationship between CP El Niño and its related precipitation pattern mentioned above leads us to postulate that the associated precipitation anomaly pattern in the CP El Niño mature winter should exhibit a significant variability on the interdecadal time scale since the CP El Niño has occurred more frequently after 1990. Yet, no predominant interdecadal change has been observed in the variability of precipitation anomalies in the boreal winter over the tropical Pacific based on the results from Guo et al. (2015). Accordingly, there is need to further look into the variability of the precipitation anomaly over the tropical Pacific on various time scales.

In retrospect, many studies have focused on the tropical precipitation anomaly pattern associated with different types of El Niño in the mature winter (e.g., Ashok et al. 2007; Taschetto and England 2009; Weng et al. 2009; Feng et al. 2010; Chung and Li 2013). However, recent researches have suggested the importance of the spring season (e.g., Cai and Cowan 2009; Taschetto and England 2009; Feng et al. 2011; Feng and Li 2011). For instance, Taschetto and England (2009) examined the influence of CP El Niño on rainfall variability during the Australian autumn [March–May (MAM)], indicating that MAM is the time of maximum rainfall anomalies induced by CP El Niño over Australia. Cai and Cowan (2009) showed that the difference between the influence of the two types of ENSO events on Australian rainfall is most conspicuous during MAM. Consequently, the above results suggest that the boreal spring (MAM) should be considered as an important season for the CP El Niño and its associated tropical precipitation variability. We note that most studies investigated the tropical precipitation anomaly pattern associated with two types of El Niño with a focus on the mature winter. Under the background of the frequent occurrence of CP El Niño after the 1990s, the variation of tropical rainfall anomaly in different seasons from year to year needs to be investigated further. To unravel the uniqueness of the spring precipitation change, we examine the spatiotemporal characteristics of the tropical Pacific precipitation variability in different seasons. Based on the empirical orthogonal function (EOF) analysis, the leading patterns of precipitation variability over the tropical Pacific have been presented in Fig. 1. It is interesting to note that the first two modes of tropical precipitation anomalies in boreal winter are dominant by an interannual component (Fig. 1f), which is consistent with previous studies (e.g., Guo et al. 2015). Similar features are observed in the corresponding principle component (PC) in the boreal summer and fall (Figs. 1d and 1e). The most striking difference is found in the variability of precipitation anomalies in spring (Fig. 1c). A major change in the PC1 time series (denoted as the red line in Fig. 1c) is the sudden flatness after 1998. This result is indicative of an interdecadal change in spring rainfall variability over the tropical Pacific around the late 1990s. The EOF1 (Fig. 1a) shows a zonal dipole pattern with anomalous warming in the equatorial central and eastern Pacific and cooling in the western Pacific. EOF2 (Fig. 1b) shows a zonal tripole pattern with the positive center in the equatorial central Pacific and the negative center in the western and southeastern Pacific. The regressions of SST anomalies against the PC1 and PC2 of rainfall anomalies during the period of 1980–2013 in spring are not shown here. The results show that during MAM, the SST anomalies associated with EOF1 exhibits a traditional El Niño–like pattern with warming in the equatorial central and eastern Pacific and cooling in the western Pacific. The counterpart associated with EOF2 shows a CP El Niño–like pattern with the warming center in the equatorial central Pacific and the cooling center in the southeastern Pacific and the Philippine Sea. In addition, the SST anomaly evolution associated with the first two EOF modes was also examined through regressing SST anomaly in different seasons (boreal autumn of year −1, winter, spring and summer of year 0, in which year 0 represents the present year and −1 the year before). The results show that both CP and EP El Niño–like patterns are at a decaying stage of these two different SST warming events. It was worthwhile to mention that the different rainfall pattern is corresponding to a different atmospheric circulation teleconnection and regional climate from the low to mid–high latitudes (e.g., Wu et al. 2008; Wu et al. 2012; Xie et al. 2015). For instance, the tropical heating field associated with different rainfall patterns can significantly affect extratropical precipitation patterns and circumglobal teleconnection at the mid–high latitudes (e.g., Guo et al. 2015; Simpson et al. 2016; Xie et al. 2015). Consequently, it is of great importance to study this interdecadal change in the spring rainfall anomaly pattern.

Fig. 1.
Fig. 1.

Spatial pattern of (a),(b) the first two EOF modes and (c) the corresponding principal component (PC1 and PC2) of the precipitation anomalies over the tropical Pacific in boreal spring for the period of 1980–2013. (d),(e),(f) As in (c), but for boreal summer, autumn, and winter, respectively. The red (blue) line denotes PC1 (PC2).

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

The questions we intend to address here are as follows: 1) Why did the leading mode of tropical spring rainfall variation experience a pronounced interdecadal change around the late 1990s? 2) Could the interdecadal shifts in the tropical Pacific precipitation anomaly during spring be attributed to a frequent occurrence of CP El Niño after 1990s? 3) Why did such an interdecadal change occur only in boreal spring? The rest of the paper is organized as follows. Datasets are described in section 2. In section 3, we present the interdecadal change in the precipitation anomaly over the tropical Pacific during boreal spring as background information. The probable causes are investigated in section 4. Section 5 explores the reasons why the interdecadal change in the tropical precipitation anomaly pattern occurred only in spring. Section 6 gives a summary and a discussion.

2. Datasets

The datasets used in the present study consist of 1) monthly quantities derived from the global European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis dataset (ERA-Interim; Dee et al. 2011) with a horizontal resolution of 2.5° in both zonal and meridional directions, 2) monthly precipitation data from the Global Precipitation Climatology Project version 2.2 (GPCPv2.2; Adler et al. 2003) that combines observations and satellite precipitation data onto 2.5° × 2.5° global grids as a proxy of precipitation during spring, 3) monthly mean Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) with horizontal resolution of 1° longitude × 1° latitude (Rayner et al. 2003), 4) monthly multilevel ocean analysis from National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS), provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, from their website at http://www.esrl.noaa.gov/psd/, with horizontal resolution of 1° longitude × ⅓° latitude (Behringer and Xue 2004), 5) monthly Simple Ocean Data Assimilation (SODA) reanalysis version 2.0.2-4 for 1980–2001 (Carton and Giese 2008), which is available at http://sodaserver.tamu.edu/, and 6) monthly wind stress from Tropflux reanalysis products for 1980–2013 (Praveen Kumar et al. 2013). The TropFlux data is produced under a collaboration between Laboratoire d’Océanographie: Expérimentation et Approches Numériques (LOCEAN) from Institut Pierre Simon Laplace (IPSL, Paris, France) and National Institute of Oceanography/CSIR (NIO, Goa, India) and supported by Institut de Recherche pour le Développement (IRD, France). TropFlux relies on data provided by the ERA-Interim and ISCCP projects. The dataset of SODA and Tropflux was used to discuss the relationship between the wind stress and oceanic current in section 3b. Before performing our analysis, the climatological mean for the period 1980–2013 was removed, and the mean for spring was calculated as average of values from MAM.

3. Interdecadal change in tropical precipitation anomaly pattern in spring

It has been found that the leading mode of spring precipitation variability shows a significant change around the late 1990s. For the purpose of contrasting the spring precipitation variability over the tropical Pacific before and after 1998, the EOF analysis is applied to GPCP precipitation anomalies during the pre-1998 (1980–98) and post-1999 (1999–2013) period, respectively (shown as Figs. 2 and 3). A Monte Carlo method is used to examine the significance of the interdecadal change in the rainfall anomaly pattern during spring. The MAM-mean rainfall anomaly for the period of 1980–2013 was randomly separated into a 19-yr set and a 15-yr set. Next the EOF analysis was applied to the two datasets. Then the same approach was repeated for 100 times. The results show that the spatial pattern of the EOF1s based on the two random rainfall anomaly sets can be significantly distinguished from that based on the GPCP rainfall anomalies before and after 1998. Consequently, the observed change in the rainfall anomaly pattern is likely owing to its interdecadal shift around 1998. The pre-1998 EOF1 (Fig. 2a), which explains 41.8% of the total precipitation variance, exhibits a zonal dipole pattern with a positive center over the CP and EP and a negative center over the western North Pacific. The positive anomaly center of pre-1998 EOF2 (Fig. 2b) is located over the CP. According to the location of the precipitation anomaly, the pre-1998 EOF1 could be considered as an EP-type pattern and the EOF2 is considered a CP-type pattern. During the post-1999 period, the EOF1 is characterized by a zonal triple pattern with a positive center over the CP and two negative centers over the western North Pacific and EP (Fig. 3a). The spatial correlation coefficient between Fig. 2b and Fig. 3a is about 0.80. This indicates that the leading mode shifts from an EP-type pattern before 1998 to a CP-type after 1998. Furthermore, the explained variance in the pre-1998 EOF2 was 19.9% and increased to 33.3% in the post-1999 EOF1. The result is suggestive of an interdecadal shift in the leading mode of precipitation anomaly during boreal spring before and after 1998. In addition, the standard deviation of tropical precipitation in spring during the pre-1998 and post-1999 period also shows distinctive discrepancies (figure not shown). During the pre-1998 period, the largest variability of spring precipitation is located over the CP and EP. The counterpart during the post-1999 period appears over the equatorial western Pacific and to the north of equatorial central and eastern Pacific. Overall, the interannual variability of anomalous spring rainfall over tropical Pacific experiences a significant change during the pre-1998 and post-1999 period based on the observations.

Fig. 2.
Fig. 2.

Spatial distribution of (a),(b) the first two EOF modes and (c) the corresponding PCs of the precipitation anomalies over the tropical Pacific in spring for the period of 1980–98. The red (blue) line denotes PC1 (PC2).

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

Fig. 3.
Fig. 3.

As in Fig. 2, but for the period of 1999–2013.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

According to the leading mode shift in spring precipitation anomaly, it is clear that its interannual variability is quite different before and after 1998. In an attempt to detect the atmospheric circulation and SST anomaly associated with a different main rainfall mode, the regression maps of SST and 1000-hPa wind anomalies onto the pre-1998 and post-1999 PC1 are presented in Fig. 4. During the pre-1998 period, the positive SST anomaly center extends from the coast of South America to the CP and is confined along the equator, while the negative anomalies in the CP show a horseshoe pattern that extends toward the subtropics with the positive rainfall anomaly over the EP (Fig. 4a). The anomalous westerly appears over the EP and easterly appears over the equatorial western Pacific. During the post-1999 period, the positive SST anomalies are located in the CP, extending toward the northeastern subtropical ocean off Mexico and Central America (Fig. 4b). The anomalous westerly is observed over the CP, which distinguishes from that during the pre-1998 period.

Fig. 4.
Fig. 4.

Regression maps of the SST (shading; unit: °C) and 1000-hPa horizontal wind anomalies (vector; unit: m s−1) onto the (a) pre-1998 PC1 and (b) post-1999 PC1. (c) The selected domains that are used to investigate the variability of SST and zonal wind anomaly over the equatorial central Pacific and eastern Pacific. The blue boxes denote the areas for zonal wind (CP: 2°S–2°N, 170°E–140°W; EP: 2°S–2°N, 120°–80°W) and the red boxes (SST_east: 2°S–2°N, 165°–125°W; SST_west: 2°S–2°N, 155°E–165°W) are used to represent the zonal SST gradient.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

An EOF analysis is also applied to the MAM-mean SST anomaly in tropical Pacific during the pre-1998 and post-1999 period (figure not shown). The first mode during the first (second) epoch is analogous to the regression maps seen in Fig. 4a (Fig. 4b), which is consistent with Xiang et al. (2013). Hence, the interannual variability of spring rainfall before and after 1998 is related to different SST anomaly pattern. To confirm the relationship between the rainfall anomaly pattern and the SST anomaly distribution, a singular value decomposition (SVD) analysis was applied during two different epochs (Fig. 5). The spatial structure of the first SVD modes before 1998 (left panel in Fig. 5) is similar to the EOF1 pattern of anomalous precipitation during the pre-1998 period (Fig. 2a) and the tropical Pacific SST regression (Fig. 4a), respectively. It is also true that the spatial structure of the first SVD modes after 1998 (right panel in Fig. 5) is similar to EOF1 pattern of anomalous precipitation during the post-1999 period (Fig. 3a) and the tropical Pacific SST regression (Fig. 4b), respectively. Hence, the EP-type rainfall pattern before 1998 is closely associated with an EP warming SST structure, while the CP-type rainfall pattern after 1998 is strongly related to a CP warming SST structure.

Fig. 5.
Fig. 5.

Patterns of the leading SVD mode between (a),(d) the precipitation (20°S–20°N, 100°E–80°W) and (b),(e) the SST anomalies in the tropical Pacific (30°S–30°N, 100°E–80°W) in MAM for (left) before 1998 and (right) after 1998. (c),(f) The associated time series for both periods.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

4. Possible reasons for the interdecadal change in the precipitation anomaly pattern

a. Probable causes

The leading mode of rainfall variability and the corresponding SST anomaly pattern before and after 1998 have so far been described herein. The probable reasons that cause this interdecadal change in spring precipitation variability over tropical Pacific will then be discussed in the rest of this section from the perspective of SST variability there.

To quantitatively diagnose which oceanic process dominantly contributes to the striking difference of SST warming structure associated with different leading mode of spring precipitation variability around 1998 (Figs. 2, 3, and 4), a linear equation for the mixed layer temperature anomaly was used in the present paper. Li (1997) and Jin et al. (2006) suggested that three feedbacks are mainly responsible for the SST variability during the ENSO development. They are the thermocline (TH) feedback (mean vertical advection of anomalous vertical temperature gradient), zonal advection (ZA) feedback (anomalous zonal advection of mean zonal SST gradient), and Ekman pumping (EK) feedback (anomalous vertical advection of mean vertical SST gradient), respectively. Consistent with previous works, the main dynamic coupling terms that we discuss here could be described by the following equation:
e1
where is SST, is subsurface temperature, is the zonal current, and is the vertical upwelling. For a given variable, an overbar indicates a time mean and a prime indicates a deviation from the climatological mean. The first three terms on the right-hand side (rhs) of Eq. (1) represent the ZA, EK, and TH effect, respectively. The last term R designates a residual that consists of physical processes that have been neglected. The contribution of other terms in the linear equation for the mixed layer temperature anomaly is also calculated. The values of these terms are approximately one order of magnitude smaller than that of the ZA, EK, and TH feedback processes. Hence, we mainly discussed these three feedbacks in this paper.

To explicitly show the contribution of each term on the rhs (neglecting the residual) to the variation of SST anomaly in the EP and CP, the domains chosen for calculating the different feedback processes are 2°S–2°N, 170°E–140°W for the CP and 2°S–2°N, 120°–80°W for the EP (outlined in Fig. 4c). Examination of the GODAS reanalysis data adds support to explaining the contrasting SST anomaly tendency associated with the different leading mode of rainfall before and after 1998. The time series of three feedback processes are shown in Fig. 6. The oceanic variables in the 10- and 40-m depths are used here to examine the contributions of different oceanic processes. Similar results are obtained when oceanic variables at other depths are used to calculate these terms. The ZA feedback (red line in Figs. 6a and 6c) and the summation of the three feedbacks (gray bar in Figs. 6a and 6c) in the CP exhibit an upward trend after 1998, which is statically significant at the 90% confidence level according to the running t test. It indicates that the interdecadal change of the leading precipitation mode might be attributed to the interdecadal intensification of the ZA feedback over the CP around the late 1990s. By comparison, there is no significant variation of the feedback processes over the EP on the interdecadal time scale (Figs. 6b and 6d). It is found in Fig. 6 that the summation of the three feedback processes is governed by the ZA feedback in the CP and the TH feedback in the EP. It indicates a dominant role of the zonal advection effect in the formation of the CP-type pattern (Fig. 3a), which is in accordance with many previous studies (Kug et al. 2009; Chung and Li 2013; Xiang et al. 2013). However, the previous studies only investigated the major physical process controlling different types of El Niño. The variability of these physical processes on the interdecadal time scale was not examined. Our present study found that the ZA feedback, which mainly controls the CP warming structure associated with the CP-type rainfall pattern, experiences a conspicuous interdecadal change around 1998. Hence, we focus on the ZA feedback process in the CP in the following analysis.

Fig. 6.
Fig. 6.

Time series of three positive feedback oceanic processes (left) in the CP and (right) the EP at (top) 10 m and (bottom) 40 m. The unit is °C month−1. The blue, red, and black line denotes the Ekman pumping (EK) feedback, zonal advection (ZA) feedback, and thermocline (TH) feedback, respectively. The gray bar denotes the summation of the three positive feedback processes.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

Note that the ZA feedback term is a product of the anomalous zonal current and the mean zonal SST gradient . The relative contribution of these two variables on the change in ZA feedback term will be discussed. First, the mean zonal SST gradient is investigated here (Fig. 7). The zonal gradient of mean SST is calculated according to the difference between the east box (2°S–2°N,165°–125°W) and the west box (2°S–2°N, 155°E–165°W) in the vicinity of the CP, which is indicated in Fig. 4c. During the pre-1998 period, the SST value in the east box is approximately 27.5°C and that in the west box is about 28.8°C. The mean SST zonal gradient is about −1.4°C. During the post-1999 period, the SST in the east box became colder (approximately 26.9°C) and the SST value in the west box also decreased to 28.5°C. The difference between the two boxes is about −1.7°C. Thus, the mean SST zonal gradient is enhanced by a value of 0.3°C after 1998, whose contribution to the ZA term change is approximately 8%. The mean zonal SST gradient is closely controlled by the mean state variability. An interdacadal change of global mean state was observed with a La Niña–like pattern (McPhaden et al. 2011; Xiang et al. 2013), accompanying a phase transition of the interdecadal Pacific oscillation (IPO) from a warm phase to a cold one around the late 1990s (Dai 2013). Hence, the phase transition of IPO might enhance mean zonal SST gradient during the second epoch. However, our result suggests that the mean zonal SST gradient associated with the IPO phase transition might play a secondary important role in the interdecadal change in the ZA feedback.

Fig. 7.
Fig. 7.

Area-averaged SST in the (left) east, west box in the vicinity of the CP (outlined in Fig. 4c) and (right) their difference (east minus west). The unit is °C. The red bar in each panel represents the SST value during the pre-1998 period. The blue bar denotes the similarity during the post-1999 period.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

The time series of the anomalous zonal wind and current, another crucial factor of the ZA feedback, are given in Fig. 8. The zonal wind stress obtained from the SODA 2.0.2 dataset is available from 1980 to 2001 (green line in Fig. 8) and that from Tropflux (red line in Fig. 8) is available from 1980 to 2013. The zonal wind at 1000 hPa (black dashed line in Fig. 8) is served as a supplementation of the zonal surface wind stress in virtue of their significant correlation (R = 0.96 in Fig. 8a and R = 0.92 in Fig. 8b). Both the anomalous zonal wind (red line in Fig. 8) and the zonal current (blue line in Fig. 8) in the CP are characterized by a prominent change before and after 1998, which is significant at the 95% confidence level based on the running t rest. It was found that the anomalous zonal current in the CP at the upper ocean is westward during the first epoch and eastward during the second epoch. The correlation coefficient between the anomalous zonal current and the variability of the ZA feedback in the CP is about 0.99. This indicates that the anomalous zonal current change might be considered to be a crucial factor responsible for the interdecadal change of the ZA feedback process.

Fig. 8.
Fig. 8.

Relationship between the zonal wind at 1000 hPa (the black dashed line; unit: m s−1), surface wind stress from Tropflux data (the red line; unit: dyn cm−2), zonal wind stress from SODA data (the green line; unit: dyn cm−2), and zonal current at 5 m (the blue line; unit: m s−1) during 1980–2013 in the (a) CP and (b) EP. The scale of zonal wind at 1000 hPa is given on the right y axis.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

It is worthwhile to mention that the anomalous zonal surface wind stress is opposite to anomalous zonal ocean current in both the CP and EP (Fig. 8). To further unravel this question, wind-induced Ekman currents and geostrophic currents are diagnosed in an equatorial -plane framework. The anomalous zonal geostrophic currents (Su et al. 2010) may be written as
e2
where is the reduced gravity, is the planetary vorticity gradient, and is the the anomalous thermocline depth. Following Su et al. (2010), the thermocline depth is defined as the depth of 18°C isotherm. In addition, the anomalous zonal Ekman current (Chang and Philander 1994; Su et al. 2010) is described as
e3
where is the seawater density, is the mean mixed layer depth, is the dissipation rate, and and the zonal and meridional wind stress derived from the Tropflux reanalysis, respectively. The mean mixed layer depth is 40 m here for CP (Xiang et al. 2013). The calculated area-averaged zonal geostrophic and Ekman current anomalies are presented in Fig. 9. Note that the zonal current anomalies in the EP (Fig. 9b) are primarily contributed by the geostrophic current (red dashed line in Fig. 9), and the wind-induced Ekman current (blue dashed line in Fig. 9) is one order of magnitude smaller and its direction is opposite to that of the geostrophic current. A similar feature is found in Fig. 9a apart from some subtle differences. The zonal current anomaly in the CP is in-phase with the geostrophic current and out-of-phase with the wind-induced Ekman current. Therefore, the dominant contribution of the geostrophic current, associated with the meridional variation of the thermocline depth, leads to the opposite behavior of the zonal wind stress and oceanic upper current.
Fig. 9.
Fig. 9.

Relationship between zonal current anomalies at the upper ocean derived from GODAS (the black dashed line; unit: m s−1), geostrophic current (the red dashed line; unit: m s−1), and Ekman current anomalies (the blue line; unit: m s−1) from Tropflux dataset in the (a) CP and (b) EP.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

To summarize, it was demonstrated that the anomalous zonal current in the CP shows a salient shift, changing from negative anomalies during the pre-1998 period to positive ones during the post-1999 period. The observed interdecadal change in the ZA feedback process is mainly determined by the anomalous zonal current. Accordingly, the positive contribution of ZA feedback to the SST variability in the CP is apparently enhanced during the post-1999 period. In addition, the intensified mean zonal SST gradient aids the interdecadal change in the ZA feedback though its effect is relatively trivial. Overall, the enhanced ZA feedback favors an effective warming of ocean water in the CP, which might cause the change in the leading mode of the spring precipitation anomalies around the late-1990s.

As mentioned above, the change of anomalous zonal current is closely related to distribution of anomalous thermocline. To detect the role of thermocline depth variability in the interdecadal change of the CP warming structure that is associated with the CP-type rainfall pattern, we investigate the thermocline depth change in tropical Pacific. The first two modes of the spring thermocline depth anomaly, which are significantly distinguished from the other modes based on the method proposed by North et al. (1982), are shown in Fig. 10. The anomalies in the EOF1 exhibit a zonal dipole pattern with positive values in the western Pacific and negative values in the EP. A meridional pattern with positive thermocline depth anomalies in the CP and negative anomalies straddling the equator appears between 180° and 120°W (Fig. 10a). The PC1 shows a significant upward linear trend (Fig. 10b). Note that the climatological thermocline depth, which is indicated in Fig. 10a as contours, is shallower to the north of the equator and deeper to its south around 160°W. Subsequently, the meridional contrast of thermocline depth becomes more and more striking along with the upward PC1. Hence, the meridional distribution of the thermocline depth during the post-1999 period may result in the change in the geostrophic current, which in turn affects the SST anomaly structure and the rainfall anomaly pattern over the CP. In addition, the EOF2 mode displays a spatially broad structure in the CP (Fig. 10c). The corresponding PC2 shows negative values around the late-1990s (Fig. 10d), suggesting a shallower thermocline in the CP, which promotes more effective warming there during the post-1999 period.

Fig. 10.
Fig. 10.

Spatial distribution of (a),(c) the first two EOF modes and (b),(d) the corresponding principal component (PC1 and PC2) of the thermocline depth anomalies in the Pacific in boreal spring for the period of 1980–2013, derived from the GODAS reanalysis dataset. In (a), the contour represents the climatological thermocline depth. The unit is m and the interval is 50 m.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

Some studies have shown that the relationship between the tropical Pacific thermocline depth and ENSO experienced an interdecadal change after 1998. Actually, there is still a debate about the role of the thermocline change in the recent ENSO events, namely, a frequent occurrence of the CP El Niño. Some researches pointed out that the thermocline variations are not very important for CP ENSOs (Kug et al. 2009, 2010; Yu et al. 2010). On the other hand, contrary views are proposed in other studies that articulated the importance of thermocline variations for CP ENSO events (Ren and Jin 2013; Wen et al. 2014). Our present work implies that the thermocline depth variation in the meridional direction might be of great importance in the change of feedback processes in the CP. However, this issue needs to be further investigated.

In addition, the mean zonal current change during the pre-1998 and post-1999 period was examined. The significant zonal current difference between the two epochs flows toward east in the equatorial central Pacific, featuring an interdecadal increase after 1998. The mean state change in the zonal current is incongruous with the SST zonal gradient, since a La Niña–like pattern is dominant after 1998 (McPhaden et al. 2011; Xiang et al. 2013). The oceanic current variation at the upper surface concurs with the change in sea surface height, but does not necessarily depend on the SST gradient change. The contribution of the mean zonal current change to the SST anomaly is relatively small, about one order of magnitude smaller than the three feedback processes we discussed here. Hence, it has a relatively minor influence on the interdacadal change in the rainfall anomaly pattern around 1998.

b. Could the main mode shift be attributed to a frequent occurrence of CP El Niño?

Many previous studies have focused on the interdecadal increase in the occurrence of CP El Niño (Yu et al. 2012; Xiang et al. 2013; Wang et al. 2013; Yeh et al. 2014, 2015). They pointed out that the CP El Niño events occurred more frequently after 1990 than before 1990 (e.g., Ashok et al. 2007; Kao and Yu 2009; Kug et al. 2009; Yeh et al. 2009; Lee and McPhaden 2010; Yeh et al. 2011; Kim et al. 2012; Yu et al. 2012; Yeh et al. 2015). The observational results herein show a very close relationship between the SST variability and rainfall anomaly pattern over the CP, which leads us to postulate that the frequent CP El Niño events might cause the westward shift in the precipitation anomaly pattern. Hence, to examine whether the interdecadal change in the spring precipitation mode is controlled by the frequency of CP El Niño occurrence, we applied the EOF analysis to the covariance matrix of precipitation anomalies for 1980–90 and 1991–2013, respectively (figure not shown), since many previous studies have suggested that the shift time of occurrence of the CP El Niño is around 1990 (e.g., Ashok et al. 2007; Kao and Yu 2009; Kug et al. 2009; Yeh et al. 2009; Lee and McPhaden 2010; Yeh et al. 2011; Kim et al. 2012; Yu et al. 2012; Yeh et al. 2015). During 1980–90, the EOF1 resembles the pattern shown in Fig. 2a, namely an EP-type pattern while the EOF2 resembles a CP-type pattern. During 1991–2013, the first mode is not significantly distinct from the other modes based on the method proposed by North et al. (1982). Furthermore, the EOF1 after 1990 is also similar to an EP-type pattern and the correlation coefficient between the EOF1 before 1990 is about 0.70, being significant at the 99% confidence level. The EOF analysis was also performed excluding the super EP El Niño events (1982/83 and 1997/98). No significant interdecadal change is identified in the leading mode of rainfall anomalies around 1990. Hence, the EOF results suggest a sophisticated relationship between CP El Niño and anomalous precipitation over the tropical Pacific, which suggests the need to further look into this issue in the future.

5. Why is the interdecadal change in the rainfall anomaly pattern observed only in boreal spring?

In this section, emphasis is placed on explaining the fact that the interdecadal shift from an EP-type to a CP-type pattern around 1998 in the leading mode of precipitation anomalies over tropical Pacific occurred only in boreal spring, which is clearly depicted herein (Fig. 1). The remaining question is what causes such a unique feature of the interannual variability of tropical precipitation anomaly in spring during different epochs.

It is first hypothesized that the life cycle of the ENSO-like SST anomaly distribution associated with the CP- or EP-type rainfall anomaly pattern might be very different, which leads to the unique feature of the rainfall pattern shift in spring. To shed light on this presumption, the composite analysis is used based on the following criterion. When PC1 of the precipitation anomaly in spring before (after) 1998 (Figs. 2c and 3c) is larger than 1.0 standard deviation the year is selected to represent the case of enhanced precipitation over the EP (CP). According to the criteria, three strong EP rainfall years (1983/1992/1998) are identified for the period 1980–98, while another three strong CP rainfall years (2003/2005/2010) are identified for 1999–2013. The results are shown in Figs. 11 and 12. Year 0 represents the strong rainfall year in spring and −1 the year before.

Fig. 11.
Fig. 11.

SST anomaly tendency (contour in 0.15°C month−1) and SST anomaly (shading in °C) along the equatorial (2°S–2°N) for (a),(b),(c) the three strong rainfall years and (d) their composite result during the pre-1998 period. The zeros lines are not displayed.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

Fig. 12.
Fig. 12.

As in Fig. 11, but for the post-1999 period.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

During the pre-1998 period, positive SST anomalies appear in the EP and CP in July(−1) and start to decay first in the EP from December(−1), as seen as Fig. 11. Note that the SST anomaly in the EP even tends to rise in spring of year 0 (Fig. 11d). Although the SST tendency is negative in MAM(0), SST anomalies are still positive in both the CP and EP, which favors enhanced precipitation there. During the post-1999 period (Fig. 12), positive SST tendency is dominant in the CP in JJA(−1). More importantly, two strikingly different features are observed. First, the SST warming tendency for the post-1999 period is much weaker than the pre-1998 period during its developing stages (year −1). Next in boreal spring, the negative SST tendency is located in the EP after 1998, instead of positive tendency before 1998. It accounts for the fact that the SST anomalies already became negative in the EP in spring after 1998, leading to suppressed precipitation there.

To figure out which physical oceanic process contributes to the CP warming and EP cooling in spring, the same composite analysis is carried out based on the three positive feedbacks (Fig. 13). Previous studies have suggested that these three feedback processes, especially TH feedback and ZA feedback processes, play a relatively important role in the growth and phase transition of ENSO (An et al. 1999; Jin and An 1999; Kang et al. 2001). Hence, it is reasonable to discuss the SST change through these positive feedback processes during the decaying spring. During the pre-1998 period, positive TH feedback and ZA feedback anomalies appear in both the EP and CP during the summer and autumn of year 1 (Figs. 13a and 13b), which contributes to anomalous warming there. After D(−1)JF(0), the sign of both the ZA and TH feedback anomaly changes to negative in the CP and EP. Meanwhile, negative EK feedback anomalies appear between 110° and 90°W (Fig. 13c). These negative feedback anomalies are conductive to the cooling of SST in both the CP and EP during the winter and spring of year 0. The circumstance during the post-1999 period is quite different from that during the pre-1998 period. First, the positive TH feedback anomaly center is located in the CP from the summer of year −1 to the spring of year 0 (Fig. 13d). Second, the positive ZA feedback anomaly between 180° and 110°W persists from July(−1) to March(0) (Fig. 13e). The positive contribution of TH and ZA feedback anomalies, which was observed from July(−1) to March(0), facilitates the persistent warming in the CP. Last, the TH feedback anomalies are negative between 120° and 90°W from July(−1) to the spring of year 0. In addition, the negative ZA feedback anomalies appear near 90°W from October(−1) to the following spring (Fig. 13e). The EK feedback anomaly changes to negative after December(−1) near 90°W (Fig. 13f), which is opposite to the sign of EK feedback there before 1998 (Fig. 13c). These oceanic processes might be conductive to the relatively fast decaying of SST anomalies in the EP. Consequently, a more significant warming tendency in the CP in both the previous winter and spring, together with the negative contribution of the three feedback anomalies in the EP during winter and following spring, jointly lead to a persistent warming in the CP and a rapid cooling in the EP after 1998. When the focus is on the spring of year 0, the maximum center of positive SST anomalies is located in the CP during the second epoch, accompanied by negative SST anomalies in the EP (Fig. 12d), which aids the formation of the different rainfall anomaly pattern. In addition, many studies have noted that the mean state has experienced a pronounced decadal change with a La Niña–like pattern around 1998 (McPhaden et al. 2011; Dai 2013; Xiang et al. 2013). Actually, the La Niña–like pattern of the mean state after 1998 with SST colder than the climatology in the EP, makes the EP’s SST easier to cool during spring.

Fig. 13.
Fig. 13.

Composite map of (left) the TH feedback anomalies, (middle) the ZA feedback anomalies, and (right) the EK feedback anomalies based on strong rainfall years of (upper) pre-1998 and (bottom) post-1999 period. The unit is °C month−1.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

To further detect the unique feature of the spring precipitation anomalies, another assumption is that the variability of some physical processes might be the largest in boreal spring, which possibly makes this interdecadal change more outstanding in spring through oceanic dynamic coupling processes. As mentioned in section 3, the ZA feedback process plays a crucial role in the interdecadal shift in the leading mode of spring precipitation anomalies. As a result, our attention is devoted to the annual change in the anomalous zonal current and mean zonal SST gradient. The time series of the anomalous zonal current for 1980–98 and 1999–2013 are shown in Figs. 14a and 14b, respectively. During both the pre-1998 and post-1999 period, the anomalous zonal current flowing toward the west is strongest from March to May in the strong rainfall years (red line in Figs. 14a and 14b). The strongest westward zonal current anomaly at the upper ocean in spring makes it more effective for the growth of CP warming pattern through the ZA feedback process, resulting in an enhanced rainfall anomaly over the CP. Furthermore, the mean zonal SST gradient during the two epochs is presented in Fig. 14c. Note that their difference, shown as a purple line in Fig. 14c, reaches its peak in boreal winter, approximately 0.5°C. In spring, it is about 0.3°C, which exerts confined impacts on the interdecadal change in the tropical Pacific precipitation anomaly in spring as mentioned in previous sections.

Fig. 14.
Fig. 14.

Annual variability of anomalous zonal current at upper ocean for (top) the pre-1998 and (middle) post-1999 period. The red lines represent the strong rainfall years and the gray lines the other years. The black thick line denotes the averaged anomalous zonal current for different epochs. (bottom) The annual variation of the zonal mean SST gradient. The scale for the purple line is given on the right y axis.

Citation: Journal of Climate 29, 16; 10.1175/JCLI-D-15-0462.1

From the view of the life cycle of the ENSO-like SST anomaly pattern, the major discrepancy between the two epochs is the SST anomaly pattern in spring (Figs. 11 and 12). The rapid decaying of SST anomalies in the EP after 1998 results from the weakness of the EK feedback. The annual variation of the SST anomaly and the La Niña–like mean state favors the fact that the leading mode shift of the tropical precipitation only occurs in boreal spring. From the perspective of variance in the related variables in the annual cycle, the anomalous zonal current is largest in spring. Accordingly, both these assumptions might be considered as possible reasons that cause the uniqueness of spring rainfall variability over the tropical Pacific.

6. Summary and discussion

The interdecadal change in the leading mode of the precipitation anomaly over the tropical Pacific during boreal spring has been investigated in the present study. During the pre-1998 period, the main mode of the spring precipitation anomaly is characterized by a zonal dipole pattern with a positive center over the EP and a negative center over the equatorial western Pacific, which is delineated by an EP-type pattern. During the post-1999 period, the leading mode of the spring precipitation anomaly features a zonal triple pattern with a positive center over the western Pacific and CP (namely a CP-type pattern) and a negative center over the Maritime Continent and the equatorial southeast Pacific, which is spatially correlated with the pattern of EOF2 during the pre-1998 period (R = 0.80). The results are suggestive of an interdecadal shift in the leading mode of the tropical Pacific precipitation anomaly during spring around the late 1990s. It was found that the leading mode of the precipitation anomaly during the two epochs is closely associated with a different SST anomaly pattern. The regression and SVD analysis add supports to the fact that the EP-type (CP-type) rainfall pattern is closely correlated with the maximum warming of SST anomaly in the EP (CP).

Then, the investigation has been undertaken in order to figure out which physical processes mainly contribute to the growth of the SST anomaly in the CP, namely, the formation of the CP-type rainfall pattern during the second epoch. Three positive feedback processes, including the zonal advection (ZA) feedback, Ekman pumping (EK) feedback, and thermocline (TH) feedback, were examined. Evidence shows a significant enhanced trend of the ZA feedback process in the CP, which may mainly determine the predominant warming there during the post-1999 period. In comparison to the contribution of the mean zonal SST gradient, the variability of the anomalous zonal current at the upper ocean is dominant in the ZA feedback change. Our present study also suggests the importance of the thermocline depth variation in the meridional direction since the anomalous zonal current is controlled by a geostrophic current, which is associated with the thermocline change.

Furthermore, observational results show that the interdecadal change in the leading mode of the precipitation anomaly occurred only in boreal spring. To examine the unique feature of the spring precipitation anomaly, two possible assumptions were put forward here. From the view of the life cycle of the ENSO-like SST pattern, the relatively rapid decaying of the SST anomalies before spring in the EP after 1998 favors the formation of the CP-type rainfall pattern. The La Niña–like mean state makes it easier to decay because of a colder condition in the EP during the second epoch. In addition, the variance of anomalous zonal current, which is a major factor in the ZA feedback, is largest in spring. It was indicative of a possibility that the remarkable feature of the tropical precipitation anomaly pattern change was observed only in spring.

It is worthwhile mentioning that the atmospheric circulation associated with the different rainfall pattern shows striking discrepancies before and after 1998. Based on the regression analysis, it was found that the associated sea level pressure after 1998 exhibits a circumglobal wavelike pattern in the subtropical Southern Hemisphere, which was not observed before 1998 (figure not shown). Hence, it is necessary to investigate the impact of the different leading modes of the tropical precipitation anomaly on the surrounding atmosphere, which could also be demonstrated by a simple numerical model. In addition, it is noted that the rainfall anomaly pattern over the Indian Ocean has also experienced an interdecadal change around the late 1990s. The relationship between the interdecadal change in the rainfall anomaly pattern over the tropical Pacific and that over the Indian Ocean is unclear. These related scientific questions need to be further analyzed in the future.

Acknowledgments

The comments of anonymous reviewers have greatly improved this manuscript. This research was jointly supported by the National Natural Science Foundation of China (Grants 41175076 and 41530530) and the National Key Basic Research and Development Projects of China (2014CB953901). RW acknowledges the support of the National Natural Science Foundation of China (Grants 41275081 and 41475081). YG acknowledges the support of the high-performance grid computing platform of Sun Yat-sen University.

REFERENCES

  • Adler, R. F., and Coauthors, 2003: The version-2 Global Precipitation Climatology Project (GPCP) Monthly Precipitation Analysis (1979–present). J. Hydrometeor., 4, 11471167, doi:10.1175/1525-7541(2003)004<1147:TVGPCP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • An, S.-I., F. Jin, and I.-S. Kang, 1999: The role of zonal advection feedback in phase transition and growth of ENSO in the Cane-Zebiak model. J. Meteor. Soc. Japan, 77, 11511160.

    • Search Google Scholar
    • Export Citation
  • 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, doi:10.1029/2006JC003798.

    • Search Google Scholar
    • Export Citation
  • Behringer, D., and Y. Xue, 2004: Evaluation of the global ocean data assimilation system at NCEP: The Pacific Ocean. Eighth Symp. on Integrated Observing and Assimilation Systems for Atmosphere, Oceans, and Land Surface, Seattle, WA, Amer. Meteor. Soc., 2.3. [Available online at https://ams.confex.com/ams/pdfpapers/70720.pdf.]

  • Bratcher, A. J., and B. S. Giese, 2002: Tropical Pacific decadal variability and global warming. Geophys. Res. Lett., 29, 1918, doi:10.1029/2002GL015191.

    • Search Google Scholar
    • Export Citation
  • Cai, W. J., and T. Cowan, 2009: La Niño Modoki impacts Australia autumn rainfall variability. Geophys. Res. Lett., 36, L12805, doi:10.1029/2009GL037885.

    • Search Google Scholar
    • Export Citation
  • Carton, J. A., and B. S. Giese, 2008: A reanalysis of ocean climate using Simple Ocean Data Assimilation (SODA). Mon. Wea. Rev., 136, 29993017, doi:10.1175/2007MWR1978.1.

    • Search Google Scholar
    • Export Citation
  • Chang, P., and S. G. Philander, 1994: A coupled ocean–atmosphere instability of relevance to the seasonal cycle. J. Atmos. Sci., 51, 36273648, doi:10.1175/1520-0469(1994)051<3627:ACOIOR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Chen, Z., Z. Wen, R. Wu, P. Zhao, and J. Cao, 2014: Influence of two types of El Niños on the East Asian climate during boreal summer: A numerical study. Climate Dyn., 43, 469481, doi:10.1007/s00382-013-1943-1.

    • Search Google Scholar
    • Export Citation
  • Chung, P.-H., and T. Li, 2013: Interdecadal relationship between the mean state and El Niño types. J. Climate, 26, 361379, doi:10.1175/JCLI-D-12-00106.1.

    • Search Google Scholar
    • Export Citation
  • Dai, A., 2013: The influence of the inter-decadal Pacific oscillation on U.S. precipitation during 1923–2010. Climate Dyn., 41, 633646, doi:10.1007/s00382-012-1446-5.

    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597, doi:10.1002/qj.828.

    • Search Google Scholar
    • Export Citation
  • Ding, Q., E. J. Steig, D. S. Battisti, and M. Kuttel, 2011: Winter warming in West Antarctica caused by central tropical Pacific warming. Nat. Geosci., 4, 398403, doi:10.1038/ngeo1129.

    • Search Google Scholar
    • Export Citation
  • Feng, J., and J. Li, 2011: Influence of El Niño Modoki on spring rainfall over south China. J. Geophys. Res., 116, D13102, doi:10.1029/2010JD015160.

    • Search Google Scholar
    • Export Citation
  • Feng, J., and J. Li, 2013: Contrasting impacts of two types of ENSO on the boreal spring Hadley circulation. J. Climate, 26, 47734789, doi:10.1175/JCLI-D-12-00298.1.

    • Search Google Scholar
    • Export Citation
  • Feng, J., L. Wang, W. Chen, S. K. Fong, and K. C. Leong, 2010: Different impacts of two types of Pacific Ocean warming on Southeast Asian rainfall during boreal winter. J. Geophys. Res., 115, D24122, doi:10.1029/2010JD014761.

    • 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, doi:10.1002/joc.2217.

    • Search Google Scholar
    • Export Citation
  • Graf, H.-F., and D. Zanchettin, 2012: Central Pacific El Niño, the “subtropical bridge,” and Eurasian climate. J. Geophys. Res., 117, D01102, doi:10.1029/2011JD016493.

    • Search Google Scholar
    • Export Citation
  • Guo, Y., Z. Wen, R. Wu, R. Lu, and Z. Chen, 2015: Impact of tropical Pacific precipitation anomaly on the East Asian upper-tropospheric westerly jet during the boreal winter. J. Climate, 28, 64576474, doi:10.1175/JCLI-D-14-00674.1.

    • Search Google Scholar
    • Export Citation
  • Jin, F., and S.-I. An, 1999: Thermocline and zonal advective feedback within the equatorial ocean recharge oscillator model for ENSO. Geophys. Res. Lett., 26, 29892992, doi:10.1029/1999GL002297.

    • Search Google Scholar
    • Export Citation
  • Jin, F., S. T. Kim, and L. Bejarano, 2006: Acoupled-stability index for ENSO. Geophys. Res. Lett., 33, L23708, doi:10.1029/2006GL027221.

    • Search Google Scholar
    • Export Citation
  • Kang, I.-S., S.-I. An, and F. Jin, 2001: A systematic approximation of the SST anomaly equation for ENSO. J. Meteor. Soc. Japan, 79, 110, doi:10.2151/jmsj.79.1.

    • Search Google Scholar
    • Export Citation
  • Kao, H.-Y., and J.-Y. Yu, 2009: Contrasting eastern Pacific and central Pacific types of ENSO. J. Climate, 22, 615632, doi:10.1175/2008JCLI2309.1.

    • Search Google Scholar
    • Export Citation
  • Kim, H.-M., P. J. Webster, and J. A. Curry, 2009: Impact of shifting patterns of Pacific Ocean warming on North Atlantic tropical cyclones. Science, 325, 7780, doi:10.1126/science.1174062.

    • Search Google Scholar
    • Export Citation
  • Kim, J.-S., K.-Y. Kim, and S.-W. Yeh, 2012: Statistical evidence for the natural variation of the central Pacific El Niño. J. Geophys. Res., 117, C06014, doi:10.1029/2012JC008003.

    • Search Google Scholar
    • Export Citation
  • Kim, S. T., and J.-Y. Yu, 2012: The two types of ENSO in CMIP5 models. Geophys. Res. Lett., 39, L11704, doi:10.1029/2012GL052006.

  • Kug, J.-S., F.-F. Jin, and S.-I. An, 2009: Two types of El Niño events: Cold tongue El Niño and warm pool El Niño. J. Climate, 22, 14991515, doi:10.1175/2008JCLI2624.1.

    • Search Google Scholar
    • Export Citation
  • Kug, J.-S., J. Choi, S.-I. An, F.-F. Jin, and A. T. Wittenberg, 2010: Warm pool and cold tongue El Niño events as simulated by the GFDL 2.1 coupled GCM. J. Climate, 23, 12261239, doi:10.1175/2009JCLI3293.1.

    • Search Google Scholar
    • Export Citation
  • Lee, S.-K., C. Wang, and D. B. Enfield, 2010: On the impact of central Pacific warming events on Atlantic tropical storm activity. Geophys. Res. Lett., 37, L17702, doi:10.1029/2010GL044459.

    • 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, doi:10.1029/2010GL044007.

    • Search Google Scholar
    • Export Citation
  • Li, H. L., H. J. Wang, and Y. Z. Yin, 2012: Interdecadal variation of the West African summer monsoon during 1979-2010 and associated variability. Climate Dyn., 39, 28832894, doi:10.1007/s00382-012-1426-9.

    • Search Google Scholar
    • Export Citation
  • Li, T., 1997: Phase transition of the El Niño–Southern Oscillation: A stationary SST mode. J. Atmos. Sci., 54, 28722887, doi:10.1175/1520-0469(1997)054<2872:PTOTEN>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • McPhaden, M. J., T. Lee, and D. McClurg, 2011: El Niño and its relationship to changing background conditions in the tropical Pacific Ocean. Geophys. Res. Lett., 38, L15709, doi:10.1029/2011GL048275.

    • Search Google Scholar
    • Export Citation
  • North, G. R., T. L. Bell, R. F. Cahalan, and F. J. Moeng, 1982: Sampling errors in the estimation of empirical orthogonal functions. Mon. Wea. Rev., 110, 699706, doi:10.1175/1520-0493(1982)110<0699:SEITEO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Praveen Kumar, B., J. Vialard, M. Lengaigne, V. S. N. Murty, M. J. McPhaden, M. F. Cronin, F. Pinsard, and K. Gopala Reddy, 2013: TropFlux wind stresses over the tropical oceans: Evaluation and comparison with other products. Climate Dyn., 40, 20492071, doi:10.1007/s00382-012-1455-4.

    • 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, doi:10.1029/2002JD002670.

    • Search Google Scholar
    • Export Citation
  • Ren, H.-L., and F.-F. Jin, 2013: Recharge oscillator mechanisms in two types of ENSO. J. Climate, 26, 65066523, doi:10.1175/JCLI-D-12-00601.1.

    • Search Google Scholar
    • Export Citation
  • Simpson, I. R., R. Seager, M. Ting, and T. A. Shaw, 2016: Causes of change in Northern Hemisphere winter meridional winds and regional hydroclimate. Nat. Climate Change, 6, 6570, doi:10.1038/nclimate2783.

    • Search Google Scholar
    • Export Citation
  • Su, J., R. Zhang, T. Li, X. Rong, J.-S. Kug, and C.-C. Hong, 2010: Causes of the El Niño and La Niña amplitude asymmetry in the equatorial eastern Pacific. J. Climate, 23, 605617, doi:10.1175/2009JCLI2894.1.

    • Search Google Scholar
    • Export Citation
  • Taschetto, A. S., and M. H. England, 2009: El Niño Modoki impacts on Australian rainfall. J. Climate, 22, 31673174, doi:10.1175/2008JCLI2589.1.

    • Search Google Scholar
    • Export Citation
  • Wang, C., and X. Wang, 2013: Classifying El Niño Modoki I and II by different impacts on rainfall in southern China and typhoon tracks. J. Climate, 26, 13221338, doi:10.1175/JCLI-D-12-00107.1.

    • Search Google Scholar
    • Export Citation
  • Wang, S.-Y., M. L’Heureux, and J.-H. Yoon, 2013: Are greenhouse gases changing ENSO precursors in the western North Pacific? J. Climate, 26, 63096322, doi:10.1175/JCLI-D-12-00360.1.

    • Search Google Scholar
    • Export Citation
  • Wang, X., and C. Wang, 2014: Different impacts of various El Niño events on the Indian Ocean Dipole. Climate Dyn., 42, 9911005, doi:10.1007/s00382-013-1711-2.

    • Search Google Scholar
    • Export Citation
  • Wen, C., A. Kumar, Y. Xue, and M. J. McPhaden, 2014: Changes in tropical Pacific thermocline depth and their relationship to ENSO after 1999. J. Climate, 27, 72307249, doi:10.1175/JCLI-D-13-00518.1.

    • Search Google Scholar
    • Export Citation
  • Weng, H., S. K. Behera, and T. Yamagata, 2009: Anomalous winter climate conditions in the Pacific Rim during recent El Niño Modoki and El Niño events. Climate Dyn., 32, 663674, doi:10.1007/s00382-008-0394-6.

    • Search Google Scholar
    • Export Citation
  • Wu, B., T. Zhou, and T. Li, 2012: Two distinct modes of tropical Indian Ocean precipitation in boreal winter and their impacts on equatorial western Pacific. J. Climate, 25, 921938, doi:10.1175/JCLI-D-11-00065.1.

    • Search Google Scholar
    • Export Citation
  • Wu, R., B. P. Kirtman, and V. Krishnamurthy, 2008: An asymmetric mode of tropical Indian Ocean rainfall variability in boreal spring. J. Geophys. Res., 113, D05104, doi:10.1029/2007JD008931.

    • Search Google Scholar
    • Export Citation
  • Wu, R., Z. Wen, S. Yang, and Y. Li, 2010: An interdecadal change in southern China summer rainfall around 1992/93. J. Climate, 23, 23892403, doi:10.1175/2009JCLI3336.1.

    • Search Google Scholar
    • Export Citation
  • Xiang, B., B. Wang, and T. Li, 2013: A new paradigm for the predominance of standing central Pacific warming after the late 1990s. Climate Dyn., 41, 327340, doi:10.1007/s00382-012-1427-8.

    • Search Google Scholar
    • Export Citation
  • Xie, Z., Y. Du, and S. Yang, 2015: Zonal extension and retraction of the subtropical westerly jet stream and evolution of precipitation over East Asia and the western Pacific. J. Climate, 28, 67836798, doi:10.1175/JCLI-D-14-00649.1.

    • 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, doi:10.1038/nature08316.

    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., B. P. Kirtman, J.-S. Kug, W. Park, and M. Latif, 2011: Natural variability of the central Pacific El Niño event on multi-centennial timescales. Geophys. Res. Lett., 38, L02704, doi:10.1029/2010GL045886.

    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., J.-S. Kug, and S.-I. An, 2014: Recent progresses on two types of El Niño: Observations, dynamics, and future changes. Asia-Pac. J. Atmos. Sci., 50, 6981, doi:10.1007/s13143-014-0028-3.

    • Search Google Scholar
    • Export Citation
  • Yeh, S.-W., X. Wang, C. Wang, and B. Dewitte, 2015: On the relationship between the North Pacific climate variability and the central Pacific El Niño. J. Climate, 28, 663677, doi:10.1175/JCLI-D-14-00137.1.

    • Search Google Scholar
    • Export Citation
  • Yu, J.-Y., H.-Y. Kao, and T. Lee, 2010: Subtropics-related interannual sea surface temperature variability in the central equatorial Pacific. J. Climate, 23, 28692884, doi:10.1175/2010JCLI3171.1.

    • Search Google Scholar
    • Export Citation
  • Yu, J.-Y., M.-M. Lu, and S.-T. Kim, 2012: A change in the relationship between tropical central Pacific SST variability and the extratropical atmosphere around 1990. Environ. Res. Lett., 7, 034025, doi:10.1088/1748-9326/7/3/034025.

    • Search Google Scholar
    • Export Citation
  • Zhu, Y. L., H. J. Wang, W. Zhou, and J. H. Ma, 2011: Recent changes in the summer precipitation pattern in East China and the background circulation. Climate Dyn., 36, 14631473, doi:10.1007/s00382-010-0852-9.

    • Search Google Scholar
    • Export Citation
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