Nutrient trapping in the equatorial Pacific: The ocean circulation solution

Nutrient trapping is a chronic problem found in global carbon cycle models with particle-only remineralization schemes. It is defined as the excess of subsurface nutrient concentrations relative to observations and occurs pri ncipally in the eastern equatorial Pacific. Previous studies reduced excess simulated nutrients by increasing the complexity of modeled biogeochemistr y, i.e., by adding pools for nutrients (and carbon) either in dissolved org anic form or as plankton. Conversely, our study suggests that deficiencies in modeled circulation fields from global coarse-resolution ocean models ar e mostly responsible. This new interpretation stems from our use of an ocea n general circulation model with higher resolution, which offers a more rea listic equatorial circulation. We used the same biogeochemical model Hambur g ocean carbon cycle model, version 3, as in some of the previous studies. Our model-predicted distribution of PO43- in the equatorial Pacific agrees reasonably well with the observations both at the surface and in the subsur face. Subsurface PO43- concentrations in our model's eastern equatorial Pac ific exceed observations by, at most, 15%, unlike coarser-resolution models . Improvement is due to enhanced meridional resolution (0.5 degrees) near t he equator, which allows the model to simulate a vigorous equatorial underc urrent that brings in low-nutrient water from the western basin. Furthermor e, the model upwells no nutrient-rich abyssal water into the surface equato rial Pacific. Our results suggest that dissolved organic carbon plays a min or role in the carbon budget of the equatorial Pacific.