Abstract:
Global biogeochemical ocean models are able to reproduce the mean annual flux of CO2 in the Southern Ocean, while they lack the ability to reproduce the seasonal and intra-seasonal modes correctly. It is likely that the role of biological uptake in mediating the exchange flux has been overlooked with respect to the supposedly larger amount of carbon involved in the processes of the solubility pump. Current estimates of phytoplankton growth cycle in the Southern Ocean are generally biased and it is becoming more evident that the timing and intensity of the bloom in the Sub-Antarctic Zone (SAZ) and frontal region is remarkably diverse. This work presents results from the TRAIN-SOPP project (Southern Ocean Primary Production in the Earth System), which explored the regional features of the seasonal cycle in satellite data and in Earth System model output from the last round of the Climate Model lntercomparison Project (CMIPS). The analysis of weekly climatological ocean colour data confirmed that the basin is characterised by the existence of regions that are not purely zonal or meridional in the spatial domain. A supervised local analysis revealed that models generally overestimate phytoplankton biomass in the SAZ and are characterised by a shift in the bloom timing of about 2-3 months. However, a regional unsupervised clustering analysis indicated that this bias is due to most models simulating features that are more similar to the sub-tropical zone rather than indicative of the SAZ. These discrepancies are mostly driven by inadequacies in the simulation of the major physical features, but also the parameterization of phytoplankton acclimation mechanism that is included in the models may play a role.
