High-Resolution Reanalysis of the Mediterranean Sea Biogeochemistry

Ocean reanalyses integrate models and observations to provide a continuous and consistent reconstruction of the past physical and biogeochemical ocean state and variability. We present a reanalysis of the Mediterranean Sea biogeochemistry at a 1/24^o resolution developed within the Copernicus Marine Service framework. The reanalysis is based on the Biogeochemical Flux Model (BFM) coupled with a variational data assimilation scheme (3DVarBio) and forced by the NEMO–OceanVar Mediterranean reanalysis and the ERA5 atmospheric reanalysis. Covering the 1999–2021 period, the reanalysis assimilates ESA-CCI satellite chlorophyll data and integrates EMODnet data as initial conditions, in addition to considering World Ocean Atlas data at the Atlantic boundary, CO2 atmospheric observations, and yearly estimates of riverine nutrient inputs.

With the use of multiple observation sources (remote, in situ, and BGC-Argo), the quality of the biogeochemical reanalysis is qualitatively and quantitatively assessed at three validation levels. Results of the first validation-level indicate an overall pretty good reanalysis skill in simulating basin-wide values and variability in the biogeochemical variables, such as phytoplankton biomass, net primary production and CO2 air-sea flux. Then, chlorophyll, nutrients, oxygen, and carbonate system variables show also satisfactory uncertainty in reproducing in situ observations at the mesoscale and weekly temporal scale. The uncertainty increases for a few variables (i.e., oxygen and ammonium) in the mesopelagic layers. Finally, using specific and process-oriented skill metrics based on BGC-Argo data, the vertical dynamics of phytoplankton and nitrate are positively assessed.

As a consequence of the continuous increases in temperature, salinity and atmospheric CO2 in the Mediterranean Sea over the last 20 years, the reanalysis results indicate basin-wide biogeochemical signals of surface deoxygenation, increase in alkalinity and dissolved inorganic carbon concentrations, and decrease in pH at the surface. The new, high-resolution reanalysis, open and freely available from the Copernicus Marine Service, allows users from different communities to investigate the spatial and temporal variability in 12 biogeochemical variables and fluxes at different scales (from the mesoscale to the basin-wide scale and from daily to multiyear scales) and the interaction between physical and biogeochemical processes shaping Mediterranean marine ecosystem functioning.