Laurence A. Anderson

Application of biogeochemical models to the study of marine ecosystems is pervasive, yet objective quantification of these models' performance is rare. Here, 12 lower trophic level models of varying complexity are objectively assessed in two distinct regions (equatorial Pacific and Arabian Sea). Each model was run within an identical one‐dimensional physical framework. A consistent variational adjoint implementation assimilating chlorophyll‐a, nitrate, export, and primary productivity was applied and the same metrics were used to assess model skill. Experiments were performed in which data were assimilated from each site individually and from both sites simultaneously. A cross‐validation experiment was also conducted whereby data were assimilated from one site and the resulting optimal parameters were used to generate a simulation for the second site. When a single pelagic regime is considered, the simplest models fit the data as well as those with multiple phytoplankton functional groups. However, those with multiple phytoplankton functional groups produced lower misfits when the models are required to simulate both regimes using identical parameter values. The cross‐validation experiments revealed that as long as only a few key biogeochemical parameters were optimized, the models with greater phytoplankton complexity were generally more portable. Furthermore, models with multiple zooplankton compartments did not necessarily outperform models with single zooplankton compartments, even when zooplankton biomass data are assimilated. Finally, even when different models produced similar least squares model‐data misfits, they often did so via very different element flow pathways, highlighting the need for more comprehensive data sets that uniquely constrain these pathways.

We examine the role of dissolved organic matter (DOM) and the stoichiometric ratios of organic matter remineralization in determining the magnitude and distribution of remineralization of organic matter in the oceanic water column, and the impact of this remineralization on tracer distributions. Our aim is to improve the parameterization of relevant processes in ocean general circulation models by bringing the models into closer agreement with new observational constraints that suggest substantial differences from previous work (lower DOM levels and higher −O₂/P ratios). We used phosphate and apparent oxygen utilization (AOU) to analyze the effect of the remineralization profile on water column tracer distributions. The primary impact of DOM cycling is to modify the distribution of remineralization over that obtained from a model with particle cycling only. Changing the oxygen‐to‐phosphorus stoichiometric ratio modifies the magnitude of oxygen utilization, but preserves its basic distribution. We find that even a small amount of DOM is sufficient to prevent the problem of nutrient trapping. Improved phosphate and AOU simulations are obtained when the amount of DOM is reduced and the deep ocean −O₂/P ratio is increased in accord with recent observations of these properties.

A new formulation of the general problem of the large‐scale energetics for the global oceans is presented and analyzed. Using a variety of ocean surface observations, some of the terms in the energy balance equations, such as the time rates of change, the generation rates of available gravitational potential energy G(P) and kinetic energy G(K), and the conversion rate from available gravitational potential energy into kinetic energy C(P, K), are estimated for annual and seasonal mean conditions. An attempt is also made to measure the uncertainty of these terms in order to assess the reliability of the results. The interseasonal variability is analyzed showing that the winter hemisphere represents the most active region of the globe. Using the data and some reasonable assumptions, pictures of the energy cycle in the global oceans for yearly and seasonal conditions are constructed. The hemispheric and global dissipation rates of available gravitational potential energy D(P) and of kinetic energy D(K) are estimated as residuals, assuming that the contribution from the internal energy C(I, K) is only a minor factor. After analyzing the general consistency of the picture of the energy cycle for the oceans, the conclusion is reached that the G(P) and G(K) terms are about equally important terms needed to describe and understand the structure and dynamics of the global ocean circulation.