Using satellite-derived backscattering coefficients in addition to chlorophyll data to constrain a simple marine biogeochemical model

Abstract. Biogeochemical models of the ocean carbon cycle are frequently validated by, or tuned to, satellite chlorophyll data. However, ocean carbon cycle models are required to accurately model the movement of carbon, not chlorophyll, and due to the high variability of the carbon to chlorophyll ratio in phytoplankton, chlorophyll is not a robust proxy for carbon. Using inherent optical property (IOP) inversion algorithms it is now possible to also derive the amount of light backscattered by the upper ocean (b_b) which is related to the amount of particulate organic carbon (POC) present. Using empirical relationships between POC and b_b, a 1-D marine biogeochemical model is used to simulate b_b at 490 nm thereby allowing the model to be compared with both remotely-sensed chlorophyll or b_b data. Here I investigate the possibility of using b_b in conjunction with chlorophyll data to help constrain the parameters in a simple 1-D NPZD model. The parameters of the biogeochemical model are tuned with a genetic algorithm, so that the model is fitted to either chlorophyll data or to both chlorophyll and b_b data at three sites in the Atlantic with very different characteristics. Several inherent optical property (IOP) algorithms are available for estimating b_b, three of which are used here. The effect of the different b_b datasets on the behaviour of the tuned model is examined to ascertain whether the uncertainty in b_b is significant. The results show that the addition of b_b data does not consistently alter the same model parameters at each site and in fact can lead to some parameters becoming less well constrained, implying there is still much work to be done on the mechanisms relating chlorophyll to POC and b_b within the model. However, this study does indicate that including b_b data has the potential to significantly effect the modelled mixed layer detritus and that uncertainties in b_b due to the different IOP algorithms are not particularly significant.