BIORYS4: A New Global Ocean Biogeochemical Reanalysis within Copernicus Marine Service

We present BIORYS4, a new Global Ocean Biogeochemical Reanalysis developed at Mercator Ocean International. within the Copernicus Marine Service framework. BIORYS4 provides 3D biogeochemical (BGC) fields at a quarter degree horizontal resolution and 75 vertical levels, spanning from 1993 to the present. The BGC fields are simulated using the PISCES-v2 (Pelagic Interactions Scheme for Carbon and Ecosystem Studies, version 2) model (Aumont et al. 2015), forced by the Global Ocean Physics Reanalysis, also developed at Mercator Ocean International within the Copernicus Marine Service. The BGC simulation is constrained through two approaches: climatological relaxation and data assimilation using a Singular Evolutive Extended Kalman Filter (SEEK) implemented in the Mercator Assimilation System. Dissolved inorganic nitrate, phosphate, silicate and iron, dissolved organic carbonand dissolved oxygen are relaxed toward monthly climatologies, and total alkalinity is relaxed toward a annual climatology using a 1-year relaxation timescale to preserve the model's internal interannual variability. Dissolved inorganic carbon is additionally relaxed toward interannual fields to account for anthropogenic emissions. To further constrain the model, the chlorophyll concentration derived from Ocean Color data is assimilated, updating the chlorophyll, nitrate, and silicate representations over the mixed layer. We constrain carbonate system variables through the assimilation of dissolved inorganic carbon and total alkalinity derived from a neural network product based on the Surface Ocean CO2 Atlas (SOCAT). Comparisons with a wide range of observational datasets demonstrate that BIORYS4 provides a robust representation of global biogeochemical processes. In particular, the assimilation of SOCAT-based carbonate variables significantly improves the simulated surface partial pressure of CO2 and air-sea CO2 fluxes, highlighting that direct assimilation of these fluxes could further enhance model accuracy and better resolve regional and temporal dynamics. The BIORYS4 global ocean biogeochemical reanalysis will be available freely through the Copernicus Marine Service, serving diverse scientific and operational user communities.