Intrabasin comparison of surface radiocarbon levels in the Indian Ocean between coral records and three-dimensional global ocean models

Oceanic uptake and transport of bomb-produced radiocarbon is used as a diagnostic in global ocean models to test parameterizations of mixing and air-sea gas exchange between the ocean and atmosphere. A model's ability to simulate bomb-produced C-14 is also a good indicator of its ability to predict uptake of anthropogenic CO2. We have conducted a model-data comparison of surface radiocarbon time series from coral records from the coasts of Kenya and Sumatra and a suite of dynamical three-dimensional ocean models that were included in the second phase of the Ocean Carbon-Cycle Model Intercomparison Project. The coral records comprise the first intrabasin record of surface water ΔC-14 variability in the equatorial Indian Ocean and provide an independent evaluation of model performance. Differing treatments of lateral subgrid scale mixing in different models appear to be less important than other, unknown factors in explaining differences in results between the models. Those models that include a dynamic vertical mixing scheme appear to be more capable of matching observed coral radiocarbon time series. However, among models with the same parameterization of lateral subgrid scale mixing, there is a large degree of variation, suggesting that at both sites, factors such as resolution, topography, physical forcing and horizontal advection are more important than mixing parameterization in explaining intermodel differences. None of the models reproduce the time lag in the rate of bomb C-14 response between the Kenya and Sumatra coral sites. Future efforts are needed to improve model simulation of radiocarbon in surface waters in the equatorial Indian Ocean.