The eastern equatorial Pacific, home to exceptionally strong air-sea exchange of heat, water and carbon is a key region determining global climate variability. Consequently, the region puts our understanding of the interactions between atmosphere, ocean, and pelagic biogeochemical cycling, as expressed in numerical models, to the test. The major and most notorious problems associated with the atmospheric and oceanic model components are a spurious double-split Intertropical Convergence Zone (ITCZ), and a deficient representation of relatively low sea surface temperatures within a zonal band stretching from the western coast of South America far westward into the basin, the so-called “cold-tongue” (Figure 1).
 Given the especially tight coupling between the atmosphere and the ocean in the region, it is straightforward to seek a common cause of these notorious problems. On the other hand, it is just that tight (and nonlinear) coupling in combination with high uncertainties in flux estimates (e.g., Figure 2) which makes it difficult to even go the first step and identify which of the model components is most deficient. To complicate things further, the region is one example of few where biotic feedbacks on climate dynamics are significant because the solar radiation absorbed by phytoplankton pigments drives considerable heating of the surface ocean thereby modulating the coupling between ocean and atmosphere [e.g., Loeptien et al., 2009].
 Various attempts to identify deficiencies in the atmospheric and oceanic modules are documented in the literature. Among them are studies focusing on (1) cloud parameterizations in the atmosphere, (2) the effect of coarse spacial resolution of the ocean which fails to resolve tropical instability waves and topographic effects on the equatorial current system, (3) numerical subtleness associated with ocean-atmosphere couplers, and (4) the neglected effect of phytoplankton on the heat budget. Even so, the problems (double-split ITCZ and sea surface temperature bias) are apparently still exigent because they persist in all the IPCC AR4 models that were not flux-corrected.
 To this end, it seems remarkable that, of all model deficiencies in the region, the problem of spuriously enhanced nutrient concentrations in the deep eastern equatorial Pacific was apparently solved in isolation. This problem of coupled biogeochemical ocean circulation models was dubbed “nutrient trapping” by Najjar et al.  and, supposedly, was solved by Aumont et al.  who related it to a sluggish representation of the equatorial undercurrent in coarse resolution models. In contrast to Aumont et al. , we argue in section 1.1 that “nutrient trapping” is also a persistent problem still exigent insofar as it retards simulations of dissolved oxygen and, in turn, of denitrification.
 The remainder of the paper explores the sensitivity of the “nutrient trapping” problem by examining a suite of coupled ocean circulation ecosystem models. The suite is, compared to previous studies, relatively exhaustive. It comprises, e.g., varying horizontal resolutions down to eddy-permitting, differing vertical resolutions, differing atmospheric forcing as well as a wide range of ecosystem models. The latter differ with respect to both their prognostic variables and their associated parameters. A description of the simulations is summarized in section 2 and elaborated on in the Appendix A. Section 2.2 explains how we measure “nutrient trapping” in our simulations. Model results are presented, discussed, and summarized in sections 3, 4, and 5, respectively.
1.1 “Nutrient Trapping”—A Persistent Problem
 Concerns about anthropogenic radiative forcing triggered the development of global coupled ocean-carbon models with a realistic ocean topography and a rough representation of biotic effects on carbon cycles [Najjar et al., 1992; Maier-Reimer, 1993]. These models were in reasonable agreement with observations of phosphate in general, yet they shared a common flaw in the eastern equatorial Pacific: a pronounced, up to 50%, high bias in subsurface phosphate concentrations. This model deficiency is generally referred to as “nutrient trapping,” a term coined by Najjar et al.  to explain (deficient) model dynamics in that zone, which feeds the equatorial upwelling. Note that (as pointed out by Najjar et al. [ 1992]) “nutrient trapping” cannot explain spuriously enhanced nutrient concentrations deeper in the thermocline. Insofar the general usage of the term is inexact. Here we comply with the majority in order to avoid to invent another terminology. More specifically we use “nutrient trapping” to name spuriously enhanced water column nutrient inventories below the equatorial undercurrent.
 Sensitivity studies suggested that the problem was not related to a deficient parameterization of sinking particulate organic matter (POM). For example, Maier-Reimer  found that the scale of the problem was insensitive toward changes of the parameterization of the POM remineralization. In his model framework, “nutrient trapping” could merely be shifted in the vertical and distributed over a larger volume. He speculated that the problem is associated with flaws in his physical circulation model, “... perhaps as a consequence of the coarse vertical resolution of the models.”
 Najjar et al. , on the other hand, showed that a fundamentally different understanding of the biogeochemical cycling and associated export pathways of organic matter to depth resulted in a model, apparently consistent with observations. Their concept includes an additional explicit representation of dissolved organic matter (DOM) which does not, in contrast to particulate organic matter, sink. This decouples remineralization of organic material (and associated buildup of inorganic nutrients and oxygen deficit) at depth from local photosynthetic production above—if the time scale of DOM decay is long enough (Appendix B) to ensure horizontal export out of the equatorial region.
 After it became apparent that the Najjar et al.  fix to the problem must be accompanied with a simulated DOM pool, way higher than can be inferred from (admittedly sparse, compare Figure 3) observations, various studies presented evidence that the problem lies in the simulated ocean circulation [Matear and Holloway, 1995; Anderson and Sarmiento, 1995; Oschlies, 2000].
 To date, and given that an adequate numerical algorithm is applied [Oschlies, 2000], Aumont et al.  summarize the state-of-the-art. By increasing the meridional resolution to 0.5° they model a stronger, more realistic, equatorial undercurrent that feeds the upwelling in the eastern equatorial Pacific with low-nutrient water from the western basin. This, in turn, effects simulated subsurface nutrient concentrations which exceed observed values by less than 15%. Because their model does not include a DOM pool, this indeed is apparently the “the ocean circulation solution” to the “nutrient trapping” problem.
 However, two open questions remain. First, we argue in section 1.1.1 that the role of particulate organic matter (POM) is unclear in the “ocean circulation solution” of Aumont et al. . Their POM compartment could be unrealistically high and might have taken over the role of the unrealistically high DOM concentrations that are needed in other models. Second, we argue (in section 1.1.2) that even a bias as low as 10% in simulated phosphate would not be a real solution to the “nutrient trapping” problem, because the associated oxygen bias renders vast areas of the Pacific suboxic thereby retarding simulations of denitrification.
1.1.1 Decoupling Role of DOM and POM in Models
 The definition of DOM in global coupled biogeochemical ocean circulation models is ambiguous. The operational definition is based on size, or on the ability to pass a filter of certain mesh size. Most models do not care about size and the model-relevant trait is that DOM is considered to be dissolved, or at least “too small to sink.” The introduction of DOM influences model solutions in that it decouples local primary production from local export of organic material to depth because divergent horizontal surface currents tend to transport DOM away from its production site. After its decay, remineralized nutrients are prone to drive photosynthetic production and associated export elsewhere, i.e., downstream. Because primary production is highly correlated with diffusive or advective nutrient supply to the euphotic zone it is admissible to conclude that the functional role of DOM in a biogeochemical model, is to decouple local export production from the local supply of inorganic nutrients to the euphotic zone. It is hard to overrate the impact of DOM dynamics in models given that a biogeochemical module does, mechanistically, merely two things: first, it redistributes inorganic nutrients supplied to the surface vertically (mimicking sinking and decaying organic matter). Second, it leaves the remainder of the supplied nutrients at the surface (be it in inorganic or organic form) which, as explained above, decouples local nutrient supply from local export. Hence, it is not surprising why—although neither its production nor its decomposition are comprehensively understood—DOM is generally explicitly simulated, and why models show such a high sensitivity toward the exact formulation of DOM dynamics (as reported by, e.g., Kwon and Primeau [ 2006]). Counterintuitive, on the other hand, is that standing surface stocks of plankton and detritus—compartments generally attributed to POM rather than to DOM—are similar to DOM. Bluntly put, POM is identical to DOM because surface currents do not differentiate between sinking and non-sinking organic matter, i.e., both are flushed downstream. Hence, the extent of decoupling between local nutrient supply and local export is effected by the divergent horizontal transport of total nutrients in the euphotic zone, including all organic and inorganic forms.
 Aumont et al.  claim to reduce the “nutrient trapping” without introducing DOM. At the same time they included a spurious POM compartment. Spurious—because in their framework POM is not remineralized under low oxygen concentrations, i.e., it accumulates. It cannot be ruled out that their POM accumulates up to unrealistically high levels thereby causing an unrealistically high (organic) nutrient transport out of the “nutrient trapping” region, or in other words, the POM compartment of Aumont et al. , which was not assessed against observations, might decouple nutrient supply to the euphotic zone from local remineralization of POM at depth, just as an unrealistically high DOM compartment would.
 There is a side aspect to the similarity between POM and DOM as regards their capacity to decouple nutrient supply to the surface from local remineralization at depth. That is, in models without an explicit representation of POM, its decoupling role described above has to be taken over, or mimicked by DOM. Hence, we are left uncertain on the question of how much of the supposedly excessive DOM pool must be understood as a proxy for POM in model frameworks similar to the one of Najjar et al.  and Maier-Reimer .
1.1.2 The Associated Oxygen Problem
 Aumont et al.  presented a model solution where the bias between simulated phosphate concentrations and observations is below 15%. This is impressive compared to the bias of up to 50% reported by Najjar et al.  and Maier-Reimer . However, we argue that even a supposedly small bias of 10% in simulated subsurface phosphate concentrations transfers into a huge error of the area hosting suboxic conditions below the surface (assuming that most of the bias is effected by increased accumulated remineralization, in contrast to be effected by spurious preformed values). This, in turn, effects an increase of that fraction of the global export production that is denitrified. The following gedankenexperiment highlights the scale of the problem.
 Assuming that denitrification occurs in suboxic regions defined by a 4.5 mmol O2 m − 3 threshold (note that this is similar to what is used in state-of-the-art models) [e.g., Keller et al., 2012], the WOA oxygen climatology [Garcia et al., 2010a] suggests that denitrification in the Pacific is restricted to an area of ≈ 6 × 105 km2 close to the American coast, with a vertical extent of less than 500m (the barely visible, colored regions in Figure 4a). A comparison with photosynthetic rates derived from satellite-based chlorophyll concentration (Figure 4c) reveals no evidence that suboxia is correlated with unusually high export production and associated rates of oxygen utilization. Hence, in the real ocean, sites of pelagic denitrification are set by a combination of rather low (although significant) export production and a sluggish circulation and unusually low ventilation [e.g., Karstensen et al., 2008].
 In contrast, a model which features a realistic ocean circulation and surface nutrient distribution, but which is biased high by 10% in phosphate at depth, would have an oxygen distribution , given by
is the oxygen climatology reduced by an oxygen consumption corresponding to a 10% increase (hence the factor 0.1 in equation (1)) of remineralized phosphate at depth. is the Redfield ratio ( − 160 mol O2 (mol P) − 1). Figure 4b shows the vertical extent of suboxia diagnosed from . A comparison with Figure 4a reveals the relation between a spurious increase of remineralization at depth and spuriously increased areas hosting favorable conditions for pelagic denitrification: the 10% bias in phosphate translates to an increase of suboxic areas by two orders of magnitude. Maybe more important, they now also cover areas with especially high photosynthetic rates and associated export production. This increased export reaching suboxic zones feeds an unrealistically high denitrification. In addition, the spurious suboxia may well retard the sensitivity of denitrification toward circulation changes (as effected by, e.g., a changing climate) since the suboxia hosting the lion's share of the denitrification is set by extraordinary high rates of export production rather than by a sluggish circulation as is the case in the real ocean.