¹⁴C-age tracers in global ocean circulation models

Abstract. The natural abundance of ¹⁴C in total CO₂ dissolved in seawater (DIC) is a property applied to evaluate the water age structure and circulation in the ocean and in ocean models. In this study we use three different representations of the global ocean circulation augmented with a suite of idealised tracers to study the potential and limitations of using natural ¹⁴C to determine water age, which is the time elapsed since a body of water has been in contact with the atmosphere. We find that, globally, bulk ¹⁴C-age is dominated by two equally important components, one associated with ageing, i.e. the time component of circulation, and one associated with a "preformed ¹⁴C-age". The latter quantity exists because of the slow and incomplete atmosphere–ocean equilibration of ¹⁴C particularly in high latitudes where many water masses form. In the ocean's interior, preformed ¹⁴C-age behaves like a passive tracer. The relative contribution of the preformed component to bulk ¹⁴C-age varies regionally within a given model, but also between models. Regional variability in the Atlantic Ocean is associated with the mixing of waters with very different end members of preformed ¹⁴C-age. Here, variations in the preformed component over space and time mask the circulation component to an extent that its patterns are not detectable from bulk ¹⁴C-age. Between models, the variability of preformed ¹⁴C-age can also be considerable (factor of 2), related to the combination of physical model parameters, which influence circulation dynamics or gas exchange. The preformed component was found to be very sensitive to gas exchange and moderately sensitive to ice cover. In our model evaluation, the choice of the gas-exchange constant from within the currently accepted range of uncertainty had such a strong influence on preformed and bulk ¹⁴C-age that if model evaluation would be based on bulk ¹⁴C-age, it could easily impair the evaluation and tuning of a model's circulation on global and regional scales. Based on the results of this study, we propose that considering preformed ¹⁴C-age is critical for a correct assessment of circulation in ocean models.