Production and export in a global ocean ecosystem model

The Hadley Centre Ocean Carbon Cycle (HadOCC) model is a coupled physical–biogeochemical model of the ocean carbon cycle. It features an explicit representation of the marine ecosystem, which is assumed to be limited by nitrogen availability. The biogeochemical compartments are dissolved nutrient, total CO2, total alkalinity, phytoplankton, zooplankton and detritus. The results of the standard simulation are presented. The annual primary production predicted by the model (47.7 Gt C yr−1) compares well to the estimates made by Longhurst et al. (1995, J. Plankton Res., 17, 1245) and Antoine et al. (1996, Global Biogeochem. Cycles, 10, 57). The HadOCC model finds high production in the sub-polar North Pacific and North Atlantic Oceans, and around the Antarctic convergence, and low production in the sub-tropical gyres. However in disagreement with the observations of Longhurst et al. and Antoine et al., the model predicts very high production in the eastern equatorial Pacific Ocean. The export flux of carbon in the model agrees well with data from deep-water sediment traps. In order to examine the factors controlling production in the ocean, additional simulations have been run. A nutrient-restoring simulation confirms that the areas with the highest primary production are those with the greatest nutrient supply. A reduced wind-stress experiment demonstrates that the high production found in the equatorial Pacific is driven by excessive upwelling of nutrient-rich water. Three further simulations show that nutrient supply at high latitudes, and hence production there, is sensitive to the parameters and climatological forcings of the mixed layer sub-model.