Abstract
Ocean reanalyses integrate models and observations to provide a continuous and consistent reconstruction of the past physical and biogeochemical ocean states and variability. We present a reanalysis of the Mediterranean Sea biogeochemistry at a 1/24° resolution developed within the Copernicus Marine Environment Monitoring Service (CMEMS) framework. The reanalysis is based on the Biogeochemical Flux Model (BFM) coupled with a variational data assimilation scheme (3DVarBio) and forced by the Nucleus for European Modeling of the Ocean (NEMO)–OceanVar physical reanalysis and European Centre for medium-range weather forecasts (ECMWF) reanalysis ERA5 atmospheric fields. Covering the 1999–2019 period with daily means of 12 published and validated biogeochemical state variables, the reanalysis assimilates surface 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 including the evaluation of 12 state variables and fluxes and several process-oriented metrics. The results indicate an overall good reanalysis skill in simulating basin-wide values and variability in the biogeochemical variables. The uncertainty in reproducing observations at the mesoscale and weekly temporal scale is satisfactory for chlorophyll, nutrient, oxygen, and carbonate system variables in the epipelagic layers, whereas the uncertainty increases for a few variables (i.e., oxygen and ammonium) in the mesopelagic layers. The vertical dynamics of phytoplankton and nitrate are positively evaluated with specific metrics using BGC-Argo data. As a consequence of the continuous increases in temperature and salinity documented in the Mediterranean Sea over the last 20 years and atmospheric CO2 invasion, we observe basin-wide biogeochemical signals indicating surface deoxygenation, increases in alkalinity, and dissolved inorganic carbon concentrations, and decreases 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.
Highlights
Optimal integration of physical-biogeochemical observations and models is becoming increasingly urgent to support both scientific and broader environmental communities
We describe a novel version of the Copernicus Marine Environment Monitoring Service (CMEMS) biogeochemical reanalysis product covering the 1999– 2019 period, with a spatial resolution as high as 1/24◦ horizontally and 125 vertical levels, based on the most advanced version of the Mediterranean Sea Biogeochemical Flux Model system (MedBFM) operationally employed in CMEMS for short-term prediction purposes (Salon et al, 2019)
Level 1 Validation To illustrate the applicability of Level 1 validation, we provide a quality assessment of surface chlorophyll, Net primary production (NPP), and alkalinity, whereas a global overview of the normalized skill indicators based on the mean annual spatial distribution is provided in section “Synthesis of the Reanalysis Validation” for 10 state variables
Summary
Optimal integration of physical-biogeochemical observations and models is becoming increasingly urgent to support both scientific and broader environmental communities. Given their basin-wide, regular, and seamless spatial and temporal coverage, multidecadal reanalyses can constitute a robust basis to compute meaningful and specific environmental indicators that describe the states and trends of key biogeochemical features (e.g., essential climate and ecological variables; Bojinski et al, 2014; Miloslavich et al, 2018), fluxes, and processes that cannot be completely and extensively observed (e.g., the flux of CO2, primary production, and carbon sequestration) These indicators may support environmental state assessment and monitoring of climate change, and its impact on marine ecosystems, following national or international directives [e.g., European Marine Strategy Framework Directive (MSFD), UNEP/MAP]. OMIs for the ocean biogeochemistry presently account for pH, chlorophyll, CO2, nitrate, and oxygen minimum zones, whereas in most regional seas, only the surface chlorophyll OMI extracted from reprocessed satellite ocean color data is available
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