Gonzalez et al. (2025) We Need to Simulate More Northern ITCZs and Less Southern ITCZs Over the East Pacific Ocean in Coupled Climate Models

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Identification

• Journal: Journal of Geophysical Research Atmospheres
• Year: 2025
• Date: 2025-12-17
• Authors: Alex O. Gonzalez, Fouzia Fahrin, Guðrún Magnúsdóttir, Alex Kinsella, Indrani Ganguly
• DOI: 10.1029/2025jd043650

Research Groups

Not available in the provided text.

Short Summary

This study developed an algorithm to classify daily Intertropical Convergence Zone (ITCZ) states and found that Coupled Model Intercomparison Project 6 (CMIP6) models significantly underestimate northern hemisphere ITCZs and overestimate southern hemisphere ITCZs over the east Pacific, challenging the traditional "double ITCZ bias" interpretation. Reanalyses also exhibit distinct ITCZ state biases and errors in interannual variability.

Objective

• To explore daily characteristics of the Intertropical Convergence Zone (ITCZ) in observations, reanalyses, and 25 Coupled Model Intercomparison Project 6 (CMIP6) models over the east Pacific Ocean to understand persistent tropical precipitation biases.

Study Configuration

• Spatial Scale: East Pacific Ocean.
• Temporal Scale: Daily characteristics, focusing on January through May, and interannual variability.

Methodology and Data

• Models used: 25 Coupled Model Intercomparison Project 6 (CMIP6) models.
• Data sources: Observations, reanalyses, and CMIP6 model outputs. An algorithm was devised and applied to determine a region's dominant daily ITCZ configuration (ITCZ state) based on the daily mean precipitation field. The five defined ITCZ states are: northern hemisphere (nITCZ), southern hemisphere (sITCZ), double (dITCZ), equatorial (eITCZ), and absent (aITCZ).

Main Results

• Nearly all CMIP6 models gravely underestimate northern hemisphere ITCZs (nITCZs) and overestimate southern hemisphere ITCZs (sITCZs) during January through May.
• All reanalyses also underestimate nITCZs and overestimate equatorial ITCZs (eITCZs).
• Errors in ITCZ state interannual variability are consistent with mean errors in reanalyses.
• Southern hemisphere ITCZ (sITCZ) interannual variability is far too low relative to the mean in most CMIP6 models.
• All reanalyses and CMIP6 models overestimate precipitation rates in the southern hemisphere ITCZ band for double ITCZs (dITCZs) and sITCZs, suggesting errors of atmospheric origin.

Contributions

• Devised and applied a novel algorithm to classify daily ITCZ configurations into five distinct states (nITCZ, sITCZ, dITCZ, eITCZ, aITCZ).
• Provided a refined understanding of tropical precipitation biases, demonstrating that the "double ITCZ bias" colloquialism may be misleading by revealing specific daily ITCZ state biases (e.g., nITCZ underestimation, sITCZ overestimation) in CMIP6 models.
• Quantified ITCZ state biases and errors in interannual variability for CMIP6 models and reanalyses over the east Pacific, highlighting systematic model deficiencies.

Funding

Not available in the provided text.

Citation

@article{Gonzalez2025We,
  author = {Gonzalez, Alex O. and Fahrin, Fouzia and Magnúsdóttir, Guðrún and Kinsella, Alex and Ganguly, Indrani},
  title = {We Need to Simulate More Northern ITCZs and Less Southern ITCZs Over the East Pacific Ocean in Coupled Climate Models},
  journal = {Journal of Geophysical Research Atmospheres},
  year = {2025},
  doi = {10.1029/2025jd043650},
  url = {https://doi.org/10.1029/2025jd043650}
}