Geoscientific Model Development Discussions www.geosci-model-dev-discuss.net 1991-9611 1991-962X 2 2 2009 10.5194/gmdd-2-1023-2009 http://www.geosci-model-dev-discuss.net/2/1023/2009/ http://www.geosci-model-dev-discuss.net/2/1023/2009/gmdd-2-1023-2009.html http://www.geosci-model-dev-discuss.net/2/1023/2009/gmdd-2-1023-2009.pdf 1023 1079 2009-07-27 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 Geophysical Institute, University of Bergen, Bergen, Norway Nansen Remote Sensing and Environmental Research Centre, Bergen, Norway Max-Planck Institute for Meteorology, Hamburg, Germany 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 thus increasingly include the interaction between the physical and biogeochemical systems. Their ocean components are generally <i>z</i>-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 interior ocean transport of biogeochemical tracers well 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 biogeochemical code for use with an isopycnic coordinate are 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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