Stefano Ciavatta, Plymouth marine Laboratory/National centre for Earth Observation

Decadal reanalysis of biogeochemical indicators and fluxes in the North West European shelf: Towards operational ecology

de Mora Lee, Kay Susan, Brewin R, David Ford


In this work we performed the first decadal reanalysis of the biogeochemistry of the North West European shelf, by assimilating the last generation of remotely sensed chlorophyll data (ESA’s Climate Change Initiative – Ocean Colour) into a state-of-the-art physical-ecosystem model (POLCOMS-ERSEM), by using the Ensemble Kalman filter (EnKF). The EnKF was applied in a localized form with variable radius, and it used the log-transformation of the observations and of the fourth four analysed model states. Not-assimilated in situ observations of chlorophyll, nutrients, dissolved oxygen, temperature and salinity were used to evaluate skill metrics of the assimilation performance, i.e. root mean square error, bias, Pearson correlation. The reanalysis improved markedly the simulation and forecast of the satellite chlorophyll data. The assimilative simulation had also some skill in reproducing the spatial-temporal patterns of oxygen, nitrate and silicate. The analysis of the simulated biogeochemical processes showed that the North West European shelf is a net sink for atmospheric CO2. This holds for the largest part of the region, and takes account of the inter-annual variability of the CO2 fluxes, as well as of the errors associated to the model estimates. Biological processes provide a major contribution to the carbon in-gassing into this shelf-sea ecosystem. The use of the reanalysis dataset to support operational biological oceanography and for novel applications such as size-class chlorophyll assimilation was explored, by analysing the spatial-temporal structure of the correlations among states in the ensemble output. We found that the identified structure has the potential to characterize the model error covariance matrix in a variational, state-of-the art operational system for the North West European shelf-sea ecosystem.