Comparison of dynamic models to predict the concentration of a photochemical tracer in the upper ocean as a function of depth and time

Three one-dimensional upper-ocean boundary-dynamics models are used to predict the concentration of a short-lived natural photochemical tracer (CO) as a function of time and depth. The models are coupled with parameterizations of the production and destruction rates of the tracer using chemical rate constants from published experimental studies. The model predictions of the upper-ocean [CO] do not agree closely with the measured concentrations obtained during one expedition, probably mostly because of errors in the rate constants used in the chemical model (published values of the in situ rate constants show much variability). An optimization scheme is described to find the values of the production and destruction rate constants that minimize the difference betweenthe modelled and measured upper-ocean [CO], and the uncertainty is assessed by Monte Carlo simulation. Using the optimized estimates of the chemical production and destruction rate constants, the simulated upper-ocean [CO] shows close agreement among the three models. It is still difficult to quantitatively determine which of the models is the best description for observed tracer concentrations because of weaknesses in the existing in situ data sets.