The effect of marine aggregate parameterisations on nutrients and oxygen minimum zones in a global biogeochemical model

Abstract

Abstract. Particle aggregation determines the particle flux length\nscale and affects the marine oxygen concentration and thus the volume of\noxygen minimum zones (OMZs) that are of special relevance for ocean nutrient\ncycles and marine ecosystems and that have been found to expand faster than\ncan be explained by current state-of-the-art models. To investigate the\nimpact of particle aggregation on global model performance, we carried out a\nsensitivity study with different parameterisations of marine aggregates and\ntwo different model resolutions. Model performance was investigated with\nrespect to global nutrient and oxygen concentrations, as well as extent and\nlocation of OMZs. Results show that including an aggregation model improves\nthe representation of OMZs. Moreover, we found that besides a fine spatial\nresolution of the model grid, the consideration of porous particles, an\nintermediate-to-high particle sinking speed and a moderate-to-high\nstickiness improve the model fit to both global distributions of dissolved\ninorganic tracers and regional patterns of OMZs, compared to a model without\naggregation. Our model results therefore suggest that improvements not only\nin the model physics but also in the description of particle aggregation\nprocesses can play a substantial role in improving the representation of\ndissolved inorganic tracers and OMZs on a global scale. However, dissolved\ninorganic tracers are apparently not sufficient for a global model\ncalibration, which could necessitate global model calibration against a\nglobal observational dataset of marine organic particles.