Zhang et al. (2025) Simulation of the South China Sea summer monsoon effect on the Indian Ocean dipole in community integrated earth system model

Identification

• Journal: Climate Dynamics
• Year: 2025
• Date: 2025-12-19
• Authors: Yazhou Zhang, Jianping Li, Yuejiang Shi, Jianping Li, Bin Zuo, Yang Zhao, Yina Diao, Ting Liu
• DOI: 10.1007/s00382-025-08001-2

Research Groups

• State Key Laboratory of Physical Oceanography (POL)/Frontiers Science Center for Deep Ocean Multispheres and Earth System (FDOMES)/Key Laboratory of Physical Oceanography/Academy of the Future Ocean/College of Oceanic and Atmospheric Science/Center for Ocean Carbon Neutrality, Ocean University of China, Qingdao, China
• Ministry of Education Key Laboratory for Earth System Modeling, Department of Earth System Science, Tsinghua University, Beijing, China
• Laboratory for Ocean Dynamics and Climate, Qingdao Marine Science and Technology Center, Qingdao, China
• PLA Dalian Naval Academy, Dalian, China
• State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China
• Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

Short Summary

This study evaluates the Community Integrated Earth System Model's (CIESM) ability to simulate the South China Sea Summer Monsoon (SCSSM)–Indian Ocean Dipole (IOD) connection and its mechanisms. It finds that while CIESM reproduces the SCSSM-IOD linkage, it significantly overestimates its strength due to unrealistic SCSSM–ENSO interactions and associated biases in atmospheric bridges and surface zonal winds.

Objective

• To evaluate the performance of the Community Integrated Earth System Model (CIESM) in simulating the connection between the South China Sea Summer Monsoon (SCSSM) and the Indian Ocean Dipole (IOD), and to understand the underlying physical mechanisms and model biases.

Study Configuration

• Spatial Scale:
  • Atmospheric variables: 2.5° × 2.5° horizontal grid.
  • Sea Surface Temperature (SST): 2° × 2° grid.
  • Sea Surface Height (SSH): 0.5° × 0.5° resolution.
  • SCSSM domain: 100°–125° E, 0°–25° N.
  • Tropical Indian Ocean (IOD): 40°–120° E, 20° S–20° N.
  • CIESM atmospheric component: approximately 1° horizontal resolution, 30 vertical hybrid layers.
• Temporal Scale:
  • Analysis period: 1948–2014 (SSH: 1948–2010).
  • Climatological mean period: 1979–2005.
  • Seasonal focus: Boreal summer (June–July–August, JJA) and autumn (September–October–November, SON).

Methodology and Data

• Models used:
  • Community Integrated Earth System Model (CIESM) (based on NCAR CESM1, with Community Atmospheric Model version 5 (CAM5) and Parallel Ocean Program version 2 (POP2)).
  • Multi-model ensemble (MME) mean from 23 CMIP6 models (first realization, r1i1p1f1, historical simulations).
• Data sources:
  • Observation/Reanalysis:
    • NCEP–NCAR Reanalysis I (NCEP1) for monthly mean atmospheric variables (winds, latent heat flux, precipitation).
    • Extended Reconstructed Sea Surface Temperature version 5 (ERSSTv5) from NOAA for SST.
    • Simple Ocean Data Assimilation reanalysis (SODA v2.2.4) for SSH.
  • Indices:
    • SCSSM index (SCSSMI) defined by Li and Zeng (2002, 2003) using 925 hPa wind.
    • IOD mode index (DMI) as the difference in SST anomalies between the tropical western Indian Ocean (50°–70° E, 10° S–10° N) and eastern Indian Ocean (90°–110° E, 10° S–0°).
    • ENSO measured by Niño3.4 index (area-averaged SST anomalies over 170° E–120° W, 5° S–5° N).
  • Methods: Empirical Orthogonal Function (EOF) analysis, pattern correlation, regression analysis, and two-tailed Student’s t-test.

Main Results

• The multi-model ensemble (MME) from 23 CMIP6 models effectively reproduces the observed significant correlation coefficients (approximately 0.6) between the SCSSM and IOD during boreal summer and autumn.
• CIESM simulates a significantly higher SCSSM–IOD correlation (approximately 0.8) than observed and the MME, indicating an overestimation of their association.
• CIESM overestimates climatological mean lower-level winds, particularly cross-equator flows over the tropical western Indian Ocean, and underestimates precipitation in the SCSSM region.
• The simulated IOD amplitude in CIESM is markedly stronger than observations and the MME, with a pattern correlation coefficient of 0.58 (the lowest among all models), linked to large cold SST anomalies extending westward from the eastern tropical Indian Ocean.
• CIESM substantially weakens the atmospheric bridges (precipitation dipole and regional Hadley circulation) over the Western North Pacific (WNP) and Southern Maritime Continent (SMC) compared to observations, primarily due to biases in precipitation anomaly magnitude and spatial distribution over the WNP.
• In CIESM, SCSSM-induced latent heat flux anomalies over the eastern Indian Ocean exhibit the opposite sign to observations, suggesting IOD growth is dominated by oceanic processes rather than coupled air-sea feedbacks.
• The exaggerated SCSSM impact on the IOD in CIESM is primarily attributed to unrealistic SCSSM–ENSO interactions, which lead to amplified ENSO-induced vertical motions over the Maritime Continent and strengthened surface zonal winds over the tropical eastern Indian Ocean.
• Models, especially CIESM, consistently reproduce significantly positive SST anomalies over the tropical central-eastern Pacific from the preceding December–January–February (DJF) to JJA, contrasting with observed negative/weakening/reversing anomalies. This leads to a stronger Walker circulation and amplified surface zonal wind anomalies.
• Cold tongue biases over the tropical eastern Indian Ocean in CIESM (reduced in MME) are significantly correlated with IOD amplitude (correlation of -0.82) and the SCSSM–IOD connection (correlation of -0.58), further contributing to the overestimated IOD intensity.

Contributions

• Provides a systematic evaluation of the Community Integrated Earth System Model (CIESM) and CMIP6 models in simulating the SCSSM–IOD connection and its underlying atmospheric and oceanic mechanisms.
• Identifies specific biases in CIESM, particularly the overestimation of the SCSSM–IOD linkage, and attributes these biases to unrealistic SCSSM–ENSO interactions and cold tongue biases.
• Highlights the critical importance of accurate representation of SCSSM–ENSO relationships and reduction of cold tongue biases for improving IOD simulations and predictions in climate models.
• Offers comprehensive insights for future model development by comparing CIESM performance against observations and a multi-model ensemble of CMIP6 models.

Funding

• National Natural Science Foundation of China (NSFC) Project (42130607, 42305187)
• Laoshan Laboratory (No. LSKJ202202600)
• Shandong Natural Science Foundation Project (ZR2024MD055)
• Open fund of Ministry of Education Key Laboratory for Earth System Modeling, Department of Earth System Science, Tsinghua University

Citation

@article{Zhang2025Simulation,
  author = {Zhang, Yazhou and Li, Jianping and Shi, Yuejiang and Li, Jianping and Zuo, Bin and Zhao, Yang and Diao, Yina and Liu, Ting},
  title = {Simulation of the South China Sea summer monsoon effect on the Indian Ocean dipole in community integrated earth system model},
  journal = {Climate Dynamics},
  year = {2025},
  doi = {10.1007/s00382-025-08001-2},
  url = {https://doi.org/10.1007/s00382-025-08001-2}
}