Skill assessment of the PELAGOS global ocean biogeochemistry model over the period 1980-2000

Global Ocean Biogeochemistry General Circula- tion Models are useful tools to study biogeochemical pro- cesses at global and large scales under current climate and future scenario conditions. The credibility of future esti- mates is however dependent on the model skill in captur- ing the observed multi-annual variability of firstly the mean bulk biogeochemical properties, and secondly the rates at which organic matter is processed within the food web. For this double purpose, the results of a multi-annual simula- tion of the global ocean biogeochemical model PELAGOS have been objectively compared with multi-variate observa- tions from the last 20 years of the 20th century, both con- sidering bulk variables and carbon production/consumption rates. Simulated net primary production (NPP) is compa- rable with satellite-derived estimates at the global scale and when compared with an independent data-set of in situ ob- servations in the equatorial Pacific. The usage of objective skill indicators allowed us to demonstrate the importance of comparing like with like when considering carbon transfor- mation processes. NPP scores improve substantially when in situ data are compared with modeled NPP which takes into account the excretion of freshly-produced dissolved or- ganic carbon (DOC). It is thus recommended that DOC mea- surements be performed during in situ NPP measurements to quantify the actual production of organic carbon in the sur- face ocean. The chlorophyll bias in the Southern Ocean that affects this model as well as several others is linked to the inadequate representation of the mixed layer seasonal cycle in the region. A sensitivity experiment confirms that the ar- tificial increase of mixed layer depths towards the observed values substantially reduces the bias. Our assessment results qualify the model for studies of carbon transformation in the surface ocean and metabolic balances. Within the limits of the model assumption and known biases, PELAGOS indi- cates a net heterotrophic balance especially in the more olig- otrophic regions of the Atlantic during the boreal winter pe- riod. However, at the annual time scale and over the global ocean, the model suggests that the surface ocean is close to a weakly positive autotrophic balance in accordance with re- cent experimental findings and geochemical considerations.