Tuning and assessment of the HYCOM-NORWECOM V2.1 biogeochemical modeling system for the North Atlantic and Arctic oceans

Abstract. The HYCOM-NORWECOM (HYbrid Coordinate Ocean Model–NORWegian ECOlogical Model) modeling system is used both for basic research and as a part of the forecasting system for the Arctic Marine Forecasting Centre through the MyOcean project. Here we present a revised version of this model. The present model, as well as the sensitivity simulations leading up to this version, have been compared to a data set of in situ measurements of nutrient and chlorophyll from the Norwegian Sea and the Atlantic sector of the Arctic Ocean. The model revisions having the most impact included adding diatoms to the diet of microzooplankton, increasing microzooplankton grazing rate and decreasing the silicate-to-nitrate ratio in diatoms. Model runs are performed both with a coarse- (~ 50 km) and higher-resolution (~ 15 km) model configuration, both covering the North Atlantic and Arctic oceans. While the new model formulation improves the results in both the coarse- and high-resolution model, the nutrient bias is smaller in the high-resolution model, probably as a result of the better resolution of the main processes and improved circulation. The final revised version delivers satisfactory results for all three nutrients as well as improved results for chlorophyll in terms of the annual cycle amplitude. However, for chlorophyll the correlation with in situ data remains relatively low. Besides the large uncertainties associated with observational data this is possibly caused by the fact that constant C:N- and Chl:N ratios are implemented in the model.