Quoted from: Palmer, J. R., and I. J. Totterdell. "Production and export in a global ocean ecosystem model." Deep Sea Research Part I: Oceanographic Research Papers 48, no. 5 (2001): 1169-1198.  https://doi.org/10.1016/S0967-0637(00)00080-7 

        The Hadley Centre Ocean Carbon Cycle (HadOCC) model has been developed for use in global ocean carbon cycle modelling. Our knowledge of both the natural marine carbon cycle and its response to perturbation is still far from complete, but it is widely accepted that the oceans are taking up around 2 Gigatonnes (Gt) of the anthropogenic carbon; see Schimel et al. (1995) for an overview. It is necessary to use models in order to better understand the processes which transport the carbon into the deep ocean and to assess the possible changes in the natural cycle due to global warming. The earliest models were box models (e.g. Siegenthaler and Joos, 1992) but with increasing computing power and better parametrisations of physical and biogeochemical processes a number of studies were made using ocean general circulation models (e.g. Sarmiento et al., 1992; Maier-Reimer, 1993). The first such models had at best a simple parametrisation of the biologically mediated processes: basic estimations of the uptake of anthropogenic CO2 could be made but it was not possible to accurately assess the strengths of any carbon cycle feedbacks which involve climate change and the ocean biology. Sarmiento and Le Quéré (1996) found in a simulation of the global carbon cycle for the next 100 years that changes in the amount and geographical pattern of marine primary production, the amount of carbon fixed by phytoplankton during photosynthetic growth, could have significant effects. Six and Maier-Reimer (1996) presented the first ocean carbon cycle model that featured an explicit representation of the marine ecosystem. This paper describes a new model which has been developed for both ocean-only and coupled ocean–atmosphere–vegetation carbon cycle studies.

        The HadOCC model is based on the Cox (1984) type ocean general circulation model used at the UK Meteorological Office (including the Hadley Centre). The model simulates the essential aspects of carbonate chemistry and biological production and export. Several extra tracers are carried to model the carbon cycle. These are dissolved inorganic carbon (DIC), total alkalinity, a nitrogenous nutrient, phytoplankton, zooplankton and sinking detritus. The model can be divided conceptually into separate inorganic and biological parts. Inorganic carbonate chemistry and partial pressure physics are well understood and can be reproduced with fair accuracy even in a simple carbon cycle model. The export of biologically generated soft tissue (organic matter) and hard tissue (carbonate) to the deep ocean, collectively known as the biological pump, is much more difficult to model. There are many direct measurements of total primary productivity made using 14C incubation methods, as well as indirect estimates based on remotely sensed surface chlorophyll concentrations, to which model predictions can be compared. But it is not total primary production that is the key biological driving field for an ocean carbon cycle model. Nutrients can be cycled through the food web many times before being lost to the deep ocean, and it is the final export of carbon and nutrients to deeper waters, largely as a sinking particle flux, that determines the circulation of carbon and nutrients on timescales of decades to centuries and that must be modelled.

        The requirement for the HadOCC model to run century-scale carbon cycle simulations, sometimes coupled to an atmospheric model, means that it has to be extremely simple in order not to use excessive computer resources. A simple model can also be more easily analysed, which is a useful feature if the model shows unexpected behaviour as part of a coupled system. We also believe that a simple model, properly constructed, is less likely to respond in an unreasonable way to changes in climatological forcings (for example). We made the deliberate choice during the development of the model to include only certain major processes, as described in the following sections and in the appendix. We have not included processes such as nitrogen fixation since, having only an incomplete understanding of the controlling factors, we cannot guarantee that any parametrisation we use will not lead to an unreasonable response of the model to climate change in the scenario runs. The model is not intended to reproduce the patterns of primary production in particular regions in particular years, as it seems unlikely that the long-term operation of the global ocean carbon cycle is sensitive to processes at such time and space scales. However, in a future study we intend to force the model with climatological fields featuring realistic interannual variability and examine the resulting variability in the primary productivity field.