Revisiting nutrient trapping in global coupled biogeochemical ocean circulation models

We analyze an extensive set of global coupled biogeochemical ocean circulation models. The focus is on the equatorial Pacific. In all simulations, which are consistent with observed standing stocks of relevant biogeochemical species at the surface, we find spuriously enhanced (reduced) macronutrient (oxygen) concentrations in the deep eastern equatorial Pacific. This modeling problem, apparently endemic to global coupled biogeochemical ocean circulation models, was coined nutrient trapping by Najjar et al. (1992). In contrast to Aumont et al. (1999), we argue that nutrient trapping is still a persistent problem, even in eddy-permitting models and, further, that the scale of the problem retards model projections of nitrogen cycling. In line with previous work, our results indicate that a deficient circulation is at the core of the problem rather than an admittedly poor quantitative understanding of biogeochemical cycles. More specifically, we present indications that nutrient trapping in models is a result of a spuriously damped Equatorial Intermediate (zonal) Current System and Equatorial Deep Jets-phenomenon which await a comprehensive understanding and have, to date, not been successfully simulated.