El Niño–Southern Oscillation (ENSO) atmospheric anomalies over the Indo–western Pacific — most prominently the anomalous anticyclone over the tropical Northwest Pacific (WNPAC) — are produced jointly by a seasonally modulated atmospheric response to equatorial Pacific sea surface temperature (the ENSO 'combination mode', which triggers the abrupt WNPAC onset around the El Niño peak) and by regional coupled ocean–atmosphere feedbacks in the Indo–western Pacific (the 'ocean capacitor', which sustains the WNPAC into the following summer). Xie and Zhou (2017) established this two-stage conceptual model, but left its quantitative validation open. The open problem is to convert this qualitative mechanism into a quantitatively partitioned, model- and event-type-robust attribution: how much of the observed variance and lifecycle of ENSO-related atmospheric anomalies each mechanism explains, whether the partition holds across eastern- versus central-Pacific events, fast- versus slow-decaying events, decadal regimes, and across current climate models whose WNPAC is biased; whether the statistical combination-tone signal is detectable in the finite observational record; and whether the seasonally resolved combination-mode perspective measurably improves operational seasonal forecasts. Later literature has delivered partial progress on each thread — a three-way partition with decadal non-stationarity, event-type-dependent mechanisms, traced CMIP5 bias origins, a proof-of-concept prediction gain for the Indian Ocean Dipole, and improved spectral significance in long model control runs — but no study closes the quantitative, general attribution or demonstrates the forecast benefit, so the problem remains open.

Background

The El Niño–Southern Oscillation (ENSO) is the dominant year-to-year climate fluctuation of the tropical Pacific, conventionally indexed by the Niño3.4 sea surface temperature (SST) anomaly (averaged over 5°S–5°N, 170°W–120°W). ENSO peaks in boreal winter, yet its influence on Asian and Indo–western Pacific climate persists into the following summer, chiefly through an anomalous anticyclonic circulation at low levels over the tropical Northwest Pacific (the WNPAC, also called the AAC), which steers moisture and shapes summer rainfall from eastern China to Japan. Two families of mechanism had been proposed for this lifecycle. The first is the 'combination mode' (C-mode): because the tropical Pacific climatological annual cycle and ENSO interact multiplicatively, an interannual ENSO signal multiplied by the seasonal cycle generates deterministic near-annual 'combination tone' frequencies (roughly 10- and 15-month periods). Stuecker and colleagues showed that this C-mode explains the abrupt transition of the Northwest Pacific circulation from a summer anomalous cyclone to a winter anomalous anticyclone while ENSO SST evolves smoothly, and that its spectral signature appears in tropical surface winds (Stuecker et al. 2013; Stuecker et al. 2015). That spectral evidence was contested by Li, Wang and Wang (2016), who argued the peaks could reflect ENSO's broad frequency spectrum rather than a distinct mode; Stuecker, Jin, Timmermann and McGregor (2016) replied, and the physical reality of seasonally modulated ENSO impacts is now broadly accepted. The second family is the regional 'Indo-western Pacific ocean capacitor' (IPOC): during El Niño, the tropical Indian Ocean warms and the Northwest Pacific cools; these SST anomalies persist after equatorial Pacific anomalies decay and, through atmospheric Kelvin-wave responses and the wind–evaporation–SST (WES) feedback — whereby surface wind anomalies modify evaporation and hence SST, which in turn reinforces the wind anomalies — maintain the WNPAC into the post-El Niño summer (Xie et al. 2009; Wu, Li and Zhou 2010). Xie and Zhou (2017) unified the two pictures. Using a seasonally stratified EOF (S-EOF) analysis — empirical orthogonal functions computed separately for each month of the ENSO year (June of the developing year to May of the decay year) — they showed algebraically that the combination-tone form is embedded in the linear seasonally stratified decomposition of the observed atmospheric response. In atmospheric model experiments with the Community Atmosphere Model version 5.3 (CAM5.3), they imposed either a time-invariant El Niño SST pattern in the equatorial Pacific alone (isolating seasonal modulation of the mean atmospheric state) or the full observed evolving composite globally versus over the Pacific only (POGA, Pacific-Ocean-Global-Atmosphere, versus GOGA, Global-Ocean-Global-Atmosphere experiments). The results: seasonal modulation of a fixed SST anomaly suffices for the abrupt WNPAC onset (C-mode), while SST anomalies outside the equatorial Pacific are essential for sustaining the WNPAC into the following summer (capacitor). Their forcing composite, however, used only three strong eastern-Pacific events (1972/73, 1982/83, 1997/98); central-Pacific (Modoki) events — El Niño events whose maximum warming lies over the central rather than the eastern equatorial Pacific — other amplitudes and decay speeds, and other models were not tested; the simulated anticyclone in the CAM5 GOGA run was biased weak and displaced northward; the quantitative variance partition between the two mechanisms was not computed; and the implications for seasonal prediction were left unexplored. Whether the combination-tone signal can be detected robustly in the finite observational record, given ENSO's broadband spectrum, also remained open. Subsequent work has filled parts of this agenda: Zhang et al. (2016) showed the combination mode dominates El Niño's impact over East Asia; Ren et al. (2016) evaluated the C-mode in CMIP5 (the Coupled Model Intercomparison Project, phase 5) models; Tao et al. (2018) traced CMIP5's weak-anticyclone bias to excessive cold-tongue and spurious western-Pacific SST biases; Feng et al. (2020) found that for central-Pacific events the WNPAC is reintensified by tropical North Atlantic warming; Jiang et al. (2020) documented decadal change of the C-mode under an ENSO regime shift; He, Zhou and Li (2019) found the WNPAC weakens in a warmer climate, dominated by the weakened atmospheric impact of the tropical Indian Ocean; Zhang et al. (2022) partitioned decaying-summer WNPAC maintenance among WNP cooling, Indian Ocean warming, and the C-mode and found the Indian Ocean influence is decadal in character; Zhao, Jin and Stuecker (2019) showed seasonally modulated ENSO forcing improves Indian Ocean Dipole (IOD, the east–west Indian Ocean SST seesaw) prediction beyond operational systems; and Stuecker (2023) reviewed the combination mode as a building block of tropical climate variability. None of these individually, or jointly, supplies the general quantitative attribution the source called for.

Problem Statement

Establish, quantitatively and generally, the two-stage mechanism by which ENSO drives seasonal atmospheric variability over the Indo–western Pacific — abrupt onset of the Northwest Pacific anomalous anticyclone (WNPAC) at the El Niño peak through the seasonally modulated atmospheric response to equatorial Pacific SST (the combination mode), and its persistence into the following summer through regional Indo–western Pacific coupled feedbacks (the ocean capacitor) — as proposed qualitatively by Xie and Zhou (2017). Specifically: (1) partition the variance and lifecycle of observed ENSO-related atmospheric anomalies over the Indo–western Pacific between the equatorial-Pacific-forced, seasonally modulated component and the regional SST-feedback component, for each stage of the ENSO year, with quantified uncertainty; (2) test whether this partition is robust across ENSO event types (eastern-Pacific versus central-Pacific events, differing decay speeds), across decadal regimes, and across current climate models, including diagnosis of the model biases (such as the weak, northward-displaced simulated anticyclone in CAM5-type experiments) that distort the partition; (3) quantify the statistical detectability of the combination-tone signal in the finite observational record, given ENSO's broadband spectrum; and (4) determine whether the seasonally resolved combination-mode perspective measurably improves operational seasonal forecasts of Indo–western Pacific circulation and its climate impacts relative to approaches based on ENSO indices and its known regional modes alone. The completion boundary is a quantitative attribution whose partition, robustness, and detectability are established across events and models, together with a demonstrated (or refuted) prediction benefit — answering, with evidence, how much each mechanism contributes and whether the framework carries operational predictive value.

References

  1. Xie, Shang-Ping, and Zhen-Qiang Zhou (2017). Seasonal Modulations of El Niño–Related Atmospheric Variability: Indo–Western Pacific Ocean Feedback. Journal of Climate 30(9):3461–3472. DOI: 10.1175/jcli-d-16-0713.1. https://doi.org/10.1175/jcli-d-16-0713.1
  2. Stuecker, M. F., A. Timmermann, F.-F. Jin, S. McGregor, and H.-L. Ren (2013). A combination mode of the annual cycle and the El Niño/Southern Oscillation. Nature Geoscience 6:540–544. DOI: 10.1038/ngeo1826. https://doi.org/10.1038/ngeo1826
  3. Stuecker, M. F., F.-F. Jin, A. Timmermann, and S. McGregor (2015). Combination Mode Dynamics of the Anomalous Northwest Pacific Anticyclone. Journal of Climate 28:1093–1111. DOI: 10.1175/jcli-d-14-00225.1. https://doi.org/10.1175/jcli-d-14-00225.1
  4. Li, Tim, Bin Wang, and Lu Wang (2016). Comments on “Combination Mode Dynamics of the Anomalous Northwest Pacific Anticyclone”. Journal of Climate 29(12):4685–4693. DOI: 10.1175/jcli-d-15-0385.1. https://doi.org/10.1175/jcli-d-15-0385.1
  5. Stuecker, M. F., F.-F. Jin, A. Timmermann, and S. McGregor (2016). Reply to “Comments on ‘Combination Mode Dynamics of the Anomalous Northwest Pacific Anticyclone’”. Journal of Climate 29(12):4695–4706. DOI: 10.1175/jcli-d-15-0558.1. https://doi.org/10.1175/jcli-d-15-0558.1
  6. Xie, Shang-Ping, Kaiming Hu, Jan Hafner, Hiroki Tokinaga, Yan Du, Gang Huang, and Takeaki Sampe (2009). Indian Ocean Capacitor Effect on Indo–Western Pacific Climate during the Summer following El Niño. Journal of Climate 22(3):730–747. DOI: 10.1175/2008jcli2544.1. https://doi.org/10.1175/2008jcli2544.1
  7. Wu, Bo, Tim Li, and Tianjun Zhou (2010). Relative Contributions of the Indian Ocean and Local SST Anomalies to the Maintenance of the Western North Pacific Anomalous Anticyclone during the El Niño Decaying Summer. Journal of Climate 23(11):2974–2986. DOI: 10.1175/2010jcli3300.1. https://doi.org/10.1175/2010jcli3300.1
  8. Zhang, Wenjun, Haiyan Li, Malte F. Stuecker, Fei-Fei Jin, and Andrew G. Turner (2016). A New Understanding of El Niño’s Impact over East Asia: Dominance of the ENSO Combination Mode. Journal of Climate 29(12):4347–4359. DOI: 10.1175/jcli-d-15-0104.1. https://doi.org/10.1175/jcli-d-15-0104.1
  9. Ren, Hong-Li, Jinqing Zuo, Fei-Fei Jin, and Malte F. Stuecker (2016). ENSO and annual cycle interaction: the combination mode representation in CMIP5 models. Climate Dynamics 46:3753–3765. DOI: 10.1007/s00382-015-2802-z. https://doi.org/10.1007/s00382-015-2802-z
  10. Tao, Weichen, Gang Huang, Renguang Wu, Kaiming Hu, Pengfei Wang, and Hainan Gong (2018). Origins of Biases in CMIP5 Models Simulating Northwest Pacific Summertime Atmospheric Circulation Anomalies during the Decaying Phase of ENSO. Journal of Climate 31(14):5707–5729. DOI: 10.1175/JCLI-D-17-0289.1. https://doi.org/10.1175/JCLI-D-17-0289.1
  11. Zhou, Zhen-Qiang, Shang-Ping Xie, Guang J. Zhang, and Wenyu Zhou (2018). Evaluating AMIP Skill in Simulating Interannual Variability over the Indo–Western Pacific. Journal of Climate 31(6):2253–2265. DOI: 10.1175/JCLI-D-17-0123.1. https://doi.org/10.1175/JCLI-D-17-0123.1
  12. He, Chao, Tianjun Zhou, and Tim Li (2019). Weakened Anomalous Western North Pacific Anticyclone during an El Niño–Decaying Summer under a Warmer Climate: Dominant Role of the Weakened Impact of the Tropical Indian Ocean on the Atmosphere. Journal of Climate 32(1):213–230. DOI: 10.1175/JCLI-D-18-0033.1. https://doi.org/10.1175/JCLI-D-18-0033.1
  13. Zhao, Sen, Fei-Fei Jin, and Malte F. Stuecker (2019). Improved Predictability of the Indian Ocean Dipole Using Seasonally Modulated ENSO Forcing Forecasts. Geophysical Research Letters 46(16):9980–9990. DOI: 10.1029/2019GL084196. https://doi.org/10.1029/2019GL084196
  14. Feng, Juan, Wen Chen, and Xiaocong Wang (2020). Reintensification of the Anomalous Western North Pacific Anticyclone during the El Niño Modoki Decaying Summer: Relative Importance of Tropical Atlantic and Pacific SST Anomalies. Journal of Climate 33(8):3271–3288. DOI: 10.1175/jcli-d-19-0154.1. https://doi.org/10.1175/jcli-d-19-0154.1
  15. Jiang, Feng, Wenjun Zhang, Malte F. Stuecker, and Fei-Fei Jin (2020). Decadal Change of Combination Mode Spatiotemporal Characteristics due to an ENSO Regime Shift. Journal of Climate 33(12):5239–5251. DOI: 10.1175/jcli-d-19-0822.1. https://doi.org/10.1175/jcli-d-19-0822.1
  16. Song, Jinjie, Philip J. Klotzbach, Yihong Duan, and Xiaoyong Zhuge (2021). Modulation of Tropical Cyclone Formation Over the Western North Pacific by the ENSO Combination Mode. Geophysical Research Letters 48:e2020GL091606. DOI: 10.1029/2020gl091606. https://doi.org/10.1029/2020gl091606
  17. Fukuda, Yoshihiro, Masahiro Watanabe, and Fei-Fei Jin (2021). Mode of Precipitation Variability Generated by Coupling of ENSO With Seasonal Cycle in the Tropical Pacific. Geophysical Research Letters 48:e2021GL095204. DOI: 10.1029/2021gl095204. https://doi.org/10.1029/2021gl095204
  18. Zhang, Wenjun, Zuoli Yu, Feng Jiang, Xin Geng, and Renhe Zhang (2022). Important role of the ENSO combination mode in the maintenance of the anomalous anticyclone over the western North Pacific in boreal summer. Science China Earth Sciences 65:1379–1387. DOI: 10.1007/s11430-021-9908-5. https://doi.org/10.1007/s11430-021-9908-5
  19. Darshana, P., Jasti S. Chowdary, Anant Parekh, and C. Gnanaseelan (2022). Relationship between the Indo-western Pacific Ocean capacitor mode and Indian summer monsoon rainfall in CMIP6 models. Climate Dynamics 59:393–415. DOI: 10.1007/s00382-021-06133-9. https://doi.org/10.1007/s00382-021-06133-9
  20. Du, Yan, Zesheng Chen, Shang-Ping Xie, Lianyi Zhang, Ying Zhang, and Yinan Cai (2022). Drivers and characteristics of the Indo-western Pacific Ocean capacitor. Frontiers in Climate 4:1014138. DOI: 10.3389/fclim.2022.1014138. https://doi.org/10.3389/fclim.2022.1014138
  21. Stuecker, Malte F. (2023). The climate variability trio: stochastic fluctuations, El Niño, and the seasonal cycle. Geoscience Letters 10:51. DOI: 10.1186/s40562-023-00305-7. https://doi.org/10.1186/s40562-023-00305-7
  22. Zhu, Zhiwei, Ying Yang, and Boqi Liu (2025). Physics-Based Predictions of the Month-by-Month Summer Western North Pacific Anomalous Anticyclone. Journal of Climate 38:2187–2203. DOI: 10.1175/jcli-d-24-0340.1. https://doi.org/10.1175/jcli-d-24-0340.1