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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-152
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-152
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 15 May 2020

Submitted as: research article | 15 May 2020

Review status
This preprint is currently under review for the journal BG.

Assimilating synthetic Biogeochemical-Argo and ocean colour observations into a global ocean model to inform observing system design

David Ford David Ford
  • Met Office, FitzRoy Road, Exeter, EX1 3PB, UK

Abstract. A set of observing system simulation experiments has been performed to explore the impact on global ocean biogeochemical reanalyses of assimilating chlorophyll from remotely sensed ocean colour, and assess the potential impact of assimilating in situ observations of chlorophyll, nitrate, oxygen, and pH from a proposed array of Biogeochemical-Argo (BGC-Argo) floats. Two different potential BGC-Argo array distributions were tested: one where biogeochemical sensors are placed on all current Argo floats, and one where biogeochemical sensors are placed on a quarter of current Argo floats. This latter approximately corresponds to the proposed BGC-Argo array of 1000 floats. Different strategies for updating model variables when assimilating ocean colour were assessed. All similarly improved the assimilated variable surface chlorophyll, but had a mixed impact on the wider ecosystem and carbon cycle, degrading some key variables of interest. Assimilating BGC-Argo data gave no added benefit over ocean colour in terms of simulating surface chlorophyll, but for most other variables, including sub-surface chlorophyll, adding BGC-Argo greatly improved results throughout the water column. This included surface partial pressure of carbon dioxide (pCO2), which was not assimilated but is an important output of reanalyses. Both BGC-Argo array distributions gave benefits, with greater improvements seen with more observations. From the point of view of ocean reanalysis, it is recommended to proceed with development of BGC-Argo as a priority. The proposed array of 1000 floats will lead to clear improvements in reanalyses, with a larger array likely to bring further benefits. The ocean colour satellite observing system should also be maintained, as ocean colour and BGC-Argo will provide complementary benefits. There is also much potential to improve the use of existing observations, particularly in terms of multivariate balancing, through improving assimilation methodologies.

David Ford

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Status: open (until 26 Jun 2020)
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Short summary
Biogeochemical-Argo floats are starting to routinely measure ocean chlorophyll, nutrients, oxygen and pH. This study generated synthetic observations representing two potential Biogeochemical-Argo observing system designs, and created a data assimilation scheme to combine them with an ocean model. The proposed system of 1000 floats brought clear benefits to model results, with additional floats giving further benefit. Existing satellite ocean colour observations gave complementary information.
Biogeochemical-Argo floats are starting to routinely measure ocean chlorophyll, nutrients,...
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