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 principally in the eastern equatorial Pacific. Previous studies reduced excess simulated nutrients by increasing the complexity of modeled biogeochemistry, i.e., by adding pools for nutrients (and carbon) either in dissolved organic form or as plankton. Conversely, our study suggests that deficiencies in modeled circulation fields from global coarse-resolution ocean models are mostly responsible. This new interpretation stems from our use of an ocean general circulation model with higher resolution, which offers a more realistic equatorial circulation. We used the same biogeochemical model Hamburg ocean carbon cycle model, version 3, as in some of the previous studies. Our model-predicted distribution of PO(4)(3-) in the equatorial Pacific agrees reasonably well with the observations both at the surface and in the subsurface. Subsurface PO(4)(3-) concentrations in our model's eastern equatorial Pacific exceed observations by, at most, 15%, unlike coarser-resolution models. Improvement is due to enhanced meridional resolution (0.5 degrees) near the equator, which allows the model to simulate a vigorous equatorial undercurrent that brings in low-nutrient water from the western basin. Furthermore, the model upwells no nutrient-rich abyssal water into the surface equatorial Pacific. Our results suggest that dissolved organic carbon plays a minor role in the carbon budget of the equatorial Pacific.