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Volume 11, issue 9 | Copyright
Geosci. Model Dev., 11, 3795-3805, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model evaluation paper 24 Sep 2018

Model evaluation paper | 24 Sep 2018

Verification of the mixed layer depth in the OceanMAPS operational forecast model for Austral autumn

Daniel Boettger1, Robin Robertson2, and Gary B. Brassington3 Daniel Boettger et al.
  • 1School of Mathematics and Statistics, University of New South Wales, Sydney, 2052, Australia
  • 2Xiamen University Malaysia, Selangor Darul Ehsan, 43900 Sepang, Malaysia
  • 3Bureau of Meteorology, Sydney, 2000, Australia

Abstract. The ocean mixed layer depth is an important parameter describing the exchange of fluxes between the atmosphere and ocean. In ocean modelling a key factor in the accurate representation of the mixed layer is the parameterization of vertical mixing. An ideal opportunity to investigate the impact of different mixing schemes was provided when the Australian Bureau of Meteorology upgraded its operational ocean forecasting model, OceanMAPS to version 3.0. In terms of the mixed layer, the main difference between the old and new model versions was a change of vertical mixing scheme from that of Chen et al. (1994) to the General Ocean Turbulence Model.

The model estimates of the mixed layer depth were compared with those derived from Argo observations. Both versions of the model exhibited a deep bias in tropical latitudes and a shallow bias in the Southern Ocean, consistent with previous studies. The bias, however, was greatly reduced in version 3.0, and variance between model runs decreased. Additionally, model skill against climatology also improved significantly. Further analysis discounted changes to model resolution outside of the Australian region having a significant impact on these results, leaving the change in vertical mixing scheme as the main factor in the assessed improvements to mixed layer depth representation.

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Short summary
This study focuses on the impact of the model vertical mixing parameterisation on the representation of the mixed layer depth (MLD) in ocean forecast models. We compare data from two recent versions of the OceanMAPS forecast system, and find that while there were large improvements in the later version of the model, the skill of each parameterisation varies with spatial location.
This study focuses on the impact of the model vertical mixing parameterisation on the...