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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model evaluation paper 22 Feb 2019

Model evaluation paper | 22 Feb 2019

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

On the discretization of the ice thickness distribution in the NEMO3.6-LIM3 global ocean–sea ice model

François Massonnet1, Antoine Barthélemy1, Koffi Worou1, Thierry Fichefet1, Martin Vancoppenolle2, Clément Rousset2, and Eduardo Moreno-Chamarro3 François Massonnet et al.
  • 1Georges Lemaître Centre for Earth and Climate Research, Earth and Life Institute, Université catholique de Louvain, Louvain-la-Neuve, Belgium
  • 2Sorbonne Universités (UPMC Paris 6), LOCEAN-IPSL, CNRS/ IRD/MNHN, Paris, France
  • 3Barcelona Supercomputing Center (Centro Nacional de Supercomputación), Nexus II-Planta 1 C/ Jordi Girona, 29, 08034 Barcelona, Spain

Abstract. The Ice Thickness Distribution (ITD) is one of the core constituents of modern sea ice models. The ITD accounts for the unresolved spatial variability of sea ice thickness within each model grid cell. While there is a general consensus on the added physical realism brought by the ITD, how to implement it remains an open question. Here, we use the ocean--sea ice general circulation model NEMO3.6-LIM3 forced by atmospheric reanalyses to test how the ITD discretization (number of ice thickness categories, positions of the category boundaries) impacts the simulated mean Arctic and Antarctic sea ice states. We find that winter ice volumes in both hemispheres increase with the number of categories, and attribute that increase to a net enhancement of basal ice growth rates. The range of simulated mean winter volumes in the various experiments amounts to ~ 30 % and ~ 10 % of the reference values (run with 5 categories) in the Arctic and Antarctic, respectively. This suggests that the way the ITD is discretized has a significant influence on the model mean state, all other things being equal. We also find that the existence of a thick category with lower bounds at ~ 4 m and ~ 2 m for the Arctic and Antarctic, respectively, is a prerequisite for allowing the storage of deformed ice, and therefore for fostering thermodynamic growth in thinner categories. Our analysis finally suggests that increasing the resolution of the ITD without changing the lower limit of the upper category results in small but not negligible variations of ice volume and extent. Our study proposes for the first time a bi-polar process-based explanation of the origin of mean state changes when the ITD discretization is modified. The sensitivity experiments conducted in this study, based on one model, emphasize that the choice of category positions, especially of thickest categories, has a primary influence on the simulated mean sea ice states while the number of categories and resolution have only a secondary influence.

François Massonnet et al.
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François Massonnet et al.
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Model source code, scripts and link to the data F. Massonnet, A. Barthélemy, K. Worou, T. Fichefet, M. Vancoppenolle, C. Rousset, and E. Moreno-Chamarro

François Massonnet et al.
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Publications Copernicus
Short summary
Sea ice thickness varies considerably on spatial scales of several meters. However, contemporary climate models cannot resolve such scales yet. This is why sea ice models used in climate models include an ice thickness distribution (ITD) to account for this unresolved variability. Here, we explore with the ocean-sea ice model NEMO3.6-LIM3 the sensitivity of simulated mean Arctic and Antarctic sea ice states to the way the ITD is discretized.
Sea ice thickness varies considerably on spatial scales of several meters. However, contemporary...