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

David Ford

doi:10.5194/bg-18-509-2021

Abstract

Abstract. A set of observing system simulation experiments was performed. This assessed the impact on global ocean biogeochemical reanalyses of assimilating chlorophyll from remotely sensed ocean colour and in situ observations of chlorophyll, nitrate, oxygen, and pH from a proposed array of Biogeochemical-Argo (BGC-Argo) floats. Two potential BGC-Argo array distributions were tested: one for which biogeochemical sensors are placed on all current Argo floats and one for which biogeochemical sensors are placed on a quarter of current Argo floats. Assimilating BGC-Argo data greatly improved model results throughout the water column. This included surface partial pressure of carbon dioxide (pCO2), which is an important output of reanalyses. In terms of surface chlorophyll, assimilating ocean colour effectively constrained the model, with BGC-Argo providing no added benefit at the global scale. The vertical distribution of chlorophyll was improved by assimilating BGC-Argo data. 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.

Highlights

Throughout the ocean, physical and chemical processes interact with a teeming ecosystem to affect all life on Earth
This paper describes the development of a scheme to assimilate profiles of Chl a, NO3, O2, and pH into an updated version of the Met Office’s global physical–biogeochemical ocean reanalysis system
This paper describes the updated model, newly developed assimilation scheme, and set-up of the observing system simulation experiments (OSSEs)

Summary

Introduction

Throughout the ocean, physical and chemical processes interact with a teeming ecosystem to affect all life on Earth. The upwelling of nutrient-rich waters fuels the growth of phytoplankton, which form the base of the marine food web and contribute half the planet’s primary production (Field et al, 1998). Oxygen is required for marine and terrestrial life, and its availability depends on ocean circulation, solubility, and biological activity. Carbon is taken up at the sea surface, at a rate contingent on physics and biology, and transported throughout the ocean. Some is quickly released back to the atmosphere. All these phenomena are regulated by an array of processes which display variability on a range of scales from milliseconds to millennia and from nanometres to ocean basins

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