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Discussion papers | Copyright
https://doi.org/10.5194/gmd-2018-192
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model evaluation paper 10 Aug 2018

Model evaluation paper | 10 Aug 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

Sensitivity of deep ocean biases to horizontal resolution in prototype CMIP6 simulations with AWI-CM1.0

Thomas Rackow1, Dmitry Sein1, Tido Semmler1, Sergey Danilov1, Nikolay Koldunov1,2, Dmitry Sidorenko1, Qiang Wang1, and Thomas Jung1,3 Thomas Rackow et al.
  • 1Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • 2MARUM, University of Bremen, Bremen, Germany
  • 3Institute of Environmental Physics, University of Bremen, Bremen, Germany

Abstract. CMIP5 models show substantial biases in the deep ocean that are larger than the level of natural variability and the response to enhanced greenhouse gas concentrations. Here we analyse the influence of horizontal resolution in a hierarchy of five multi-resolution simulations with the AWI Climate Model (AWI-CM), which employs a sea ice-ocean model component formulated on unstructured meshes. The ocean grid sizes considered range from a nominal resolution of ∼1° (CMIP5-type) up to locally eddy-resolving. We show that increasing ocean resolution locally to resolve ocean eddies leads to a major reduction in deep ocean biases. A detailed diagnosis of the simulations allows to identify the origins of the biases. We find that two major sources at the surface are responsible for the deep bias in the Atlantic Ocean. Furthermore, the Southern Ocean density structure is equally improved with locally explicitly resolved eddies compared to parameterized eddies. Part of the bias reduction can be traced back towards improved surface biases over outcropping regions, which are in contact with deeper ocean layers along isopycnal surfaces. Our prototype simulations provide guidance for the optimal choice of ocean grids for AWI-CM to be used in the final runs for phase 6 of the 'Coupled Model Intercomparison Project' (CMIP6) and for the related flagship simulations in the 'High Resolution Model Intercomparison Project' (HighResMIP). Quite remarkably, retaining resolution only in areas of high eddy activity along with excellent scalability characteristics of the unstructured-mesh sea ice-ocean model enables us to perform the multi-centennial climate simulations needed in a CMIP context at (locally) eddy-resolving resolution with a throughput of 5–6 simulated years per day.

Thomas Rackow et al.
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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Thomas Rackow et al.
Model code and software

The AWI Climate Model (AWI-CM), version 1.0 T. Rackow, D. Sein, T. Semmler, S. Danilov, N. Koldunov, D. Sidorenko, Q. Wang, and T. Jung https://doi.org/10.5281/zenodo.1342014

Video supplement

Sensitivity of deep ocean biases to horizontal resolution in prototype CMIP6 simulations: video supplements T. Rackow, D. Sein, T. Semmler, S. Danilov, N. Koldunov, D. Sidorenko, Q. Wang, and T. Jung https://doi.org/10.5281/zenodo.1323334

Thomas Rackow et al.
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Short summary
Current climate models show errors in the deep ocean that are larger than the level of natural variability and the response to enhanced greenhouse gas concentrations. These errors are thus larger than the signals we aim to predict. Using the AWI Climate Model, we show that increasing resolution to resolve eddies leads to a major reduction in deep ocean errors. This gives a strong case to aim for (locally) eddy-resolving computational grids in AWI's next-generation (CMIP6) model configuration.
Current climate models show errors in the deep ocean that are larger than the level of natural...
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