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Carbon-dioxide Ocean-Atmosphere exchange

The oceans and atmosphere interact and exchange large amounts of soluble gases including carbon dioxide (CO2), which is taken up by microscopic marine plants known as phytoplankton. 

As phytoplankton accumulate the chlorophyll they contain changes the colour of the ocean surface.Satellites are able to detect this change, enabling scientists to map the global distribution and concentration of this chlorophyll over time which reflects the natural cycle of CO2 absorption and release back into the atmosphere

Phytoplankton as seen from Space

Caption: Phytoplankton in the Baltic sea as seen by the Copernicus Sentinel-2 satellite.

These ‘Ocean Colour’ observations have been used by the ESA Climate Change Initiative Climate Modelling User Group (CMUG) to understand this important carbon sink and improve models used to predict the evolution of the Earth’s climate system.

The CMUG focussed on a robust, 19 year long, global satellite timeserie generated by the CCI Ocean Colour project team, spanning that meets the requirements of Global Observing Climate System.

They assimilated this observational timeseries into FOAM-HADOCC, the UK Met Office’s global coupled ocean-biogeochemical model, which is used to generate short-range, seasonal and decadal predictions of the Earth System and climate. 

Assimilation: positive impact of ocean colour

The impact was positive: assimilation of these data consistently reduced the chlorophyll bias seen in the model, with the model results better matching independent in-situ observations than before.

Caption: The availability of carbon dioxide dissolved in the ocean is a driver of phytoplankton growth, which can change the colour of the top layer of the oceans. CMUG calculated the exchange of this greenhouse gas by assimilating ocean colour data into an ocean climate model which simulates the biochemical and physical properties of the oceans.

CMUG note that exploitation of satellite ocean colour data is still in its infancy but the improved spatial coverage of the Ocean Colour data, particularly in regions important for biological and fisheries production, looks very promising for future data assimilation.

About the authors
Shubha Sathyendranath is Head of Remote Sensing and Marine Optics at Plymouth Marine Laboratory and is the Assistant Director for the Partnership for the Observation of the Global Oceans. Shubha leads the OC CCI ECV team. Her research interests include ocean colour modelling, spectral characteristics of light penetration underwater, bio-optical properties of phytoplankton, modelling primary production, bio-geochemical cycles in the sea, climate change, biological-physical interactions in the marine system, ecological provinces in the sea, ecological indicators and phytoplantkon functional types.
David Ford has worked on ocean modelling and data assimilation research at the Met Office since 2008. His main focus is on developing and validating the pre-operational global coupled physical-biogeochemical ocean modelling system. His work covers time scales from near-real-time forecasts to decadal reanalyses, with both operational and climate research applications. David has also worked on shelf seas biogeochemical modelling and validation, and developing error covariances for the data assimilation component of the Met Office’s operational physical ocean forecasting system.