Model-derived estimates of new production: New results point towards lower values

Model-derived estimates of marine new production are found to display systematic covariations with the underlying model architecture. Almost regardless of the formulation of biogeochemical processes, model-derived estimates of new production have more than doubled from about 0.15 mol N m−2 yr−1 to values around 0.4 mol N m−2 yr−1 when turning from early box models to more recent investigations using coarse-resolution general circulation models. Because none of these models resolves eddies, which have been shown to enhance biological production, a further increase in simulated new production with increasing model resolution might be expected. This study presents results from an eddy-permitting coupled biological–physical model that suggest a basin-scale new production of less than 0.3 mol N m−2 yr−1 for the North Atlantic, i.e. substantially less than values typical for coarse-resolution models. Sensitivity experiments reveal that the amount of diapycnal mixing, described either explicitly or implicitly in the numerical discretization schemes, has a considerable effect on the simulated input of nutrients into the euphotic zone. Implications for coarse-resolution models used until now are that unrealistically high levels of explicit and implicit diapycnal diffusion may have been responsible for unrealistically high estimates of new production.