Liang et al. (2026) Why the statistical relationship between east asian summer monsoon rainfall and the concurrent ENSO index is weak and nonsignificant

Identification

• Journal: npj Climate and Atmospheric Science
• Year: 2026
• Date: 2026-04-04
• Authors: Y. T. Liang, Lei Fan, Hui-Huang Fu, Chun Li, Qinyu Liu
• DOI: 10.1038/s41612-026-01381-0

Research Groups

• State Key Laboratory of Physical Oceanography, Ocean University of China
• College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao, China

Short Summary

This study explains the weak and non-significant statistical relationship between East Asian summer monsoon (EASM) rainfall and the concurrent ENSO index by demonstrating that two opposing phase relationships between the Pacific-Japan (PJ) teleconnection pattern and ENSO statistically cancel each other out. It reveals that a positive PJ pattern can be triggered by two distinct Indo-Pacific sea surface temperature (SST) anomaly patterns associated with opposite ENSO phases, each inducing different EASM rainfall responses.

Objective

• To investigate why the statistical relationship between East Asian summer monsoon (EASM) rainfall and the concurrent ENSO index is weak and non-significant.
• To determine if the Pacific-Japan (PJ) teleconnection pattern links ENSO and tropical East Asian regions with uncorrelated rainfall.
• To ascertain if two distinct tropical SST forcing mechanisms, each associated with opposite ENSO phases, can produce PJ anomalies of the same sign.

Study Configuration

• Spatial Scale: East Asia, tropical Indo-Pacific (20°S–40°N, 30°E–60°W), equatorial central-eastern Pacific (CEP), North Indian Ocean (NIO), Maritime Continent (MC), South China Sea (SCS), Yangtze River Valley, Japan, North China.
• Temporal Scale: Boreal summer (June–July–August, JJA), interannual variability, 1979–2022 (observational data), multi-decadal (model experiments).

Methodology and Data

• Models used:
  • SPEEDY (Simplified Parametrizations, primitivE-Equation DYnamics) atmospheric General Circulation Model (GCM) with T30 horizontal resolution (approximately 3.75° × 3.75°) and 8 vertical levels.
  • Multi-model ensemble mean of CMIP6 AMIP experiments.
• Data sources:
  • Sea surface temperature (SST): NOAA's Extended Reconstructed SST v5 (ERSST-v5).
  • Outgoing longwave radiation (OLR): NOAA's interpolated product.
  • Precipitation: NOAA's Precipitation Reconstruction (NOAA-PREC).
  • Atmospheric variables: NCEP/NCAR Reanalysis 1.
  • PJ pattern index derived using Singular Value Decomposition (SVD) of JJA 850-hPa geopotential height over East Asia (10°N-70°N, 100°E-160°E) and concurrent OLR over the Philippine region (7.5°N-25°N, 110°E-150°E).
  • ENSO index: Niño 3.4 index (5°S–5°N, 170°W–120°W).

Main Results

• The weak and non-significant correlation between EASM rainfall and the concurrent ENSO index is attributed to two opposing phase relationships between the Pacific-Japan (PJ) teleconnection pattern and ENSO, which cancel out statistically.
• Regions of non-significant ENSO-rainfall correlations over the southern EASM spatially coincide with the influence zone of the PJ circulation pattern.
• A positive PJ event can be triggered by two distinct tropical Indo-Pacific SST anomaly patterns, each associated with opposite ENSO phases:
  1. Indo-Pacific Tripole (IPT) pattern: Associated with a La Niña state (cold SST anomalies in the tropical Indian Ocean, especially the North Indian Ocean (NIO), and the equatorial central-eastern Pacific (CEP)). This forces a PJ pattern with a stronger northern anticyclonic anomaly (over Japan) and a weaker, northward-shifted southern cyclonic anomaly (over the South China Sea), leading to enhanced rainfall over North China.
  2. Indo-Pacific Dipole (IPD) pattern: Associated with an El Niño state (cold SST anomalies in the Indo-Pacific Pool and warm SST anomalies in the CEP). This forces a PJ pattern with a stronger southern cyclonic anomaly (over the South China Sea) and a weaker, spatially confined northern anticyclonic anomaly (near Japan), resulting in pronounced negative rainfall anomalies over the Yangtze River Basin.
• Atmospheric General Circulation Model (AGCM) experiments confirm that both NIO and Maritime Continent (MC) SST anomalies can independently excite the PJ pattern. CEP SST anomalies play a crucial role in shaping the PJ morphology: in the IPT pattern, CEP synergistically intensifies the northern PJ circulation; in the IPD pattern, CEP synergistically strengthens the southern PJ circulation but cannot excite the northern part alone.
• The two summer SST patterns (IPT and IPD) tend to originate from preceding winter La Niña conditions, but with distinct wintertime anomalous cyclonic centers over the northwest Pacific and tropical Indian Ocean SST anomalies, offering potential for early prediction.

Contributions

• Provides a novel mechanistic understanding for the long-standing "ENSO correlation paradox of EASM rainfall" by elucidating the role of PJ teleconnection dynamics in mediating ENSO’s impacts.
• Demonstrates that PJ events during summer ENSO are not merely products of intrinsic atmospheric stochastic variability but are driven by well-defined ENSO-related tropical SST forcing mechanisms.
• Identifies and characterizes two distinct spatial morphologies of the PJ pattern under two specific tropical Indo-Pacific SST forcing patterns (IPT and IPD), clarifying their respective formation mechanisms.
• Offers a viable new pathway for improving ENSO-based predictive skill for East Asian summer climate anomalies by emphasizing the need to account for the co-variability of the entire tropical Indo-Pacific SST field, rather than relying solely on a single ENSO index.
• Consolidates the theoretical foundation for ENSO-based seasonal climate forecasting.

Funding

• National Natural Science Foundation of China (Grant No. 41975089)

Citation

@article{Liang2026Why,
  author = {Liang, Y. T. and Fan, Lei and Fu, Hui-Huang and Li, Chun and Liu, Qinyu},
  title = {Why the statistical relationship between east asian summer monsoon rainfall and the concurrent ENSO index is weak and nonsignificant},
  journal = {npj Climate and Atmospheric Science},
  year = {2026},
  doi = {10.1038/s41612-026-01381-0},
  url = {https://doi.org/10.1038/s41612-026-01381-0}
}