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Ocean Science An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/os-2020-6
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2020-6
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  24 Feb 2020

24 Feb 2020

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A revised version of this preprint was accepted for the journal OS and is expected to appear here in due course.

Assimilation of chlorophyll data into a stochastic ensemble simulation for the North Atlantic ocean

Yeray Santana-Falcón1, Pierre Brasseur1, Jean Michel Brankart1, and Florent Garnier2 Yeray Santana-Falcón et al.
  • 1Université Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, Grenoble, France
  • 2LEGOS, University of Toulouse, CNRS, IRD, CNES, UPS, Toulouse, France

Abstract. Satellite-derived surface chlorophyll data are daily assimilated into a three-dimensional 24 member ensemble configuration of an online-coupled NEMO-PISCES model for the North Atlantic ocean. A one-year multivariate assimilation experiment is performed to evaluate the impacts on analyses and forecast ensembles. Our results demonstrate that the integration of data improves surface analysis and forecast chlorophyll representation in a major part of the model domain, where the assimilated simulation outperforms the probabilistic skills of a non-assimilated analogous simulation. However, improvements are dependent on the reliability of the prior free ensemble. A regional diagnosis shows that surface chlorophyll is overestimated in the northern limit of the subtropical North Atlantic, where the prior ensemble spread does not cover the observation's variability. There, the system cannot deal with corrections that alter the equilibrium between the observed and unobserved state variables producing instabilities that propagate into the forecast. To alleviate these inconsistencies, a one-month sensitivity experiment in which the assimilation process is only applied to model fluctuations is performed. Results suggest the use of this methodology may decrease the effect of corrections on the correlations between state vectors. Overall, the experiments presented here evidence the need of refining the description of model's uncertainties according to the biogeochemical characteristics of each oceanic region.

Yeray Santana-Falcón et al.

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Yeray Santana-Falcón et al.

Yeray Santana-Falcón et al.

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Latest update: 19 Oct 2020
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Short summary
Data assimilation is the most comprehensive strategy to estimate the biogeochemical state of the ocean. Here, surface Chl-a data are daily assimilated into a 24 member NEMO-PISCES ensemble configuration to implement a complete 4D assimilation system. Results show the assimilation increases the skills of the ensemble, though a regional diagnosis suggests that the description of model and observations uncertainties needs to be refined according to the biogeochemical characteristics of each region.
Data assimilation is the most comprehensive strategy to estimate the biogeochemical state of the...
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