Geoscientific Model Development www.geosci-model-dev.net 1991-959X 1991-9603 3 1 2010 10.5194/gmd-3-143-2010 http://www.geosci-model-dev.net/3/143/2010/ http://www.geosci-model-dev.net/3/143/2010/gmd-3-143-2010.html http://www.geosci-model-dev.net/3/143/2010/gmd-3-143-2010.pdf 143 167 2010-02-16 An isopycnic ocean carbon cycle model K. M. Assmann karen.assmann@bjerknes.uib.no M. Bentsen J. Segschneider C. Heinze Bjerknes Centre for Climate Research, Bergen, Norway Nansen Remote Sensing and Environmental Research Centre, Bergen, Norway Max-Planck Institute for Meteorology, Hamburg, Germany Geophysical Institute, University of Bergen, Bergen, Norway The carbon cycle is a major forcing component in the global climate system. Modelling studies, aiming to explain recent and past climatic changes and to project future ones, increasingly include the interaction between the physical and biogeochemical systems. Their ocean components are generally z-coordinate models that are conceptually easy to use but that employ a vertical coordinate that is alien to the real ocean structure. Here, we present first results from a newly-developed isopycnic carbon cycle model and demonstrate the viability of using an isopycnic physical component for this purpose. As expected, the model represents well the interior ocean transport of biogeochemical tracers and produces realistic tracer distributions. Difficulties in employing a purely isopycnic coordinate lie mainly in the treatment of the surface boundary layer which is often represented by a bulk mixed layer. The most significant adjustments of the ocean biogeochemistry model HAMOCC, for use with an isopycnic coordinate, were in the representation of upper ocean biological production. We present a series of sensitivity studies exploring the effect of changes in biogeochemical and physical processes on export production and nutrient distribution. 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