Huneke et al. (2025) The ACCESS-CM2 climate model with a higher resolution ocean-sea ice component (1/4°)

Identification

• Journal: Geoscientific model development
• Year: 2025
• Date: 2025-12-15
• Authors: Wilma G. C. Huneke, Andrew McC. Hogg, Martin Dix, Daohua Bi, Arnold Sullivan, Shayne McGregor, Chiara Holgate, Siobhan O’Farrell, Micael J. T. Oliveira
• DOI: 10.5194/gmd-18-9991-2025

Research Groups

• Research School of Earth Sciences, Australian National University, Canberra, Australia
• Australian Research Council Centre of Excellence for Climate Extremes, Canberra, Australia
• Australian Research Council Centre of Excellence for the Weather of the 21st Century, Canberra, Australia
• Australia's Climate Simulator (ACCESS-NRI), Australian National University, Canberra, Australia
• CSIRO Environment, Aspendale, Australia
• School of Earth Atmosphere and Environment, Monash University Melbourne, Victoria, Australia

Short Summary

This paper introduces and evaluates ACCESS-CM2-025, a new configuration of the Australian Community Climate and Earth System Simulator coupled model with a higher resolution (0.25°) ocean-sea ice component. The study finds that while the higher resolution improves the representation of ocean mesoscale variability, ENSO, and North Atlantic deep convection, many biases from the lower-resolution version (ACCESS-CM2-1) persist, particularly those linked to the atmospheric model component.

Objective

• To assess the impact of a higher resolution (0.25°) ocean-sea ice component in the ACCESS-CM2 climate model on the simulated climate, focusing on intrinsic ocean variability, mean state, and large-scale decadal climate variability, compared to a 1° resolution version and forced ocean-sea ice models.

Study Configuration

• Spatial Scale: Global, with a focus on specific regions like western boundary currents, Southern Ocean, North Atlantic, and tropical Pacific. Ocean-sea ice component resolution: 0.25° (ACCESS-CM2-025) and 1° (ACCESS-CM2-1). Atmospheric component resolution: N96 grid (1.25° latitude by 1.875° longitude).
• Temporal Scale: 500-year present-day simulations, with analysis focusing on the last 100 years (model years 400–499) for time-mean analyses.

Methodology and Data

• Models used:
  • Coupled models: ACCESS-CM2-025 (new, 0.25° ocean-sea ice, N96 atmosphere), ACCESS-CM2-1 (existing, 1° ocean-sea ice, N96 atmosphere).
  • Ocean-sea ice only models: ACCESS-OM2-025 (0.25° ocean-sea ice), ACCESS-OM2-1 (1° ocean-sea ice).
  • Atmospheric submodel: Met Office Unified Model (UM) version 10.6 GA7.1.
  • Land surface submodel: CABLE version 2.5.
  • Ocean submodel: MOM version 5.1.
  • Sea ice submodel: CICE version 5.1.2.
  • Coupler: OASIS-MCT.
• Data sources:
  • Observations for comparison: World Ocean Atlas 2013 (WOA13) for temperature and salinity, AVISO SSALTO/DUACS altimetry for sea level anomaly, National Snow and Ice Data Centre (NSIDC) for sea ice extent, IFREMER v4.1 for latent heat flux (evaporation), Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST) for ENSO.
  • Reanalysis for forced models: JRA55-do v1.3 atmospheric reanalysis product.
  • Bathymetry: GEBCO_2014 v20150318 30 arcsec topography (for CM2-025), modified GFDL-CM2.5 bathymetry (for OM2-025/OM2-1), legacy dataset (for CM2-1).

Main Results

• Improved Ocean Dynamics: ACCESS-CM2-025 shows a faster spin-up of deep ocean temperatures and is more energetic in high eddy-active regions (western boundary currents, ACC), better capturing the spatial pattern and amplitude of mesoscale variability compared to ACCESS-CM2-1 and forced models.
• ENSO Representation: ACCESS-CM2-025 features a more realistic ENSO life cycle and seasonality, with a significantly reduced biennality compared to the lower-resolution ACCESS-CM2-1.
• North Atlantic Improvements: ACCESS-CM2-025 better captures the observed maximum mixed layer depth (MLD) in the North Atlantic, being largely sea ice-free and able to convect in the Labrador Sea, unlike ACCESS-CM2-1. It also shows reduced deep convection south of Iceland.
• Persistent Biases: Both ACCESS-CM2 models share many biases, particularly near the sea surface and affecting sea ice coverage, primarily attributed to the atmospheric model component (e.g., warm Southern Ocean SST bias due to insufficient cloud cover, resulting in lower summer sea ice extent in CM2-025).
• Overturning Circulation: All ACCESS configurations underestimate the observed Atlantic Meridional Overturning Circulation (AMOC) strength at 26° N. ACCESS-CM2-025 forms noticeably denser Antarctic Bottom Water (AABW) than observed, with colder and saltier waters south of 55° S.
• Multidecadal Variability: ACCESS-CM2-025 exhibits spurious multidecadal variability in the North Atlantic, evident in SST, AMOC transport, sea ice coverage, and MLD, originating from intermittent deep convection in the Labrador Sea. This variability is not present in the lower-resolution version.

Contributions

• Introduction and comprehensive evaluation of a new, higher-resolution (0.25°) ocean-sea ice configuration of the ACCESS-CM2 coupled climate model (ACCESS-CM2-025).
• Detailed assessment of the impact of increased ocean resolution on key climate parameters, including ocean mesoscale variability, ENSO, mixed layer depth, overturning circulation, and sea ice, in both coupled and forced model contexts.
• Identification of specific improvements (e.g., mesoscale activity, ENSO life cycle, North Atlantic deep convection) and persistent biases (e.g., Southern Ocean SST, sea ice extent, tropical Pacific SSS) linked to atmospheric or remaining oceanic model deficiencies.
• Highlighting the emergence of spurious multidecadal variability in the North Atlantic in the higher-resolution coupled model, providing a critical insight for future climate modeling studies.

Funding

• Australian Research Council Centre of Excellence for Climate Extremes (CE170100023)
• Australian Research Council Centre of Excellence for the Weather of the 21st Century (CE230100012)
• National Computational Infrastructure (NCI) in Canberra, Australia (supported by the Australian Commonwealth Government)
• ACCESS-NRI's model ACCESS-CM2 infrastructure (enabled by the Australian Government's National Collaborative Research Infrastructure Strategy - NCRIS)
• CSIRO
• Earth Systems and Climate Change Hub (NESP2)
• Climate Systems Hub (NESP2)

Citation

@article{Huneke2025ACCESSCM2,
  author = {Huneke, Wilma G. C. and Hogg, Andrew McC. and Dix, Martin and Bi, Daohua and Sullivan, Arnold and McGregor, Shayne and Holgate, Chiara and O’Farrell, Siobhan and Oliveira, Micael J. T.},
  title = {The ACCESS-CM2 climate model with a higher resolution ocean-sea ice component (1/4°)},
  journal = {Geoscientific model development},
  year = {2025},
  doi = {10.5194/gmd-18-9991-2025},
  url = {https://doi.org/10.5194/gmd-18-9991-2025}
}