Transit Time Distribution based on the ECCO-JPL Ocean Data Assimilation

Oceanic water mass is a mixture of waters with varying ages, and the Transit Time Distribution (TTD) measures its age spectrum. We construct a model-based TTD using the data-constrained circulation fields from the ECCO-JPL Ocean Data Assimilation, and test it against the observed and directly simulated distribution of pCFC-11 from the Pacific and Atlantic basins. The ECCO-JPL circulation provides overall reliable estimates of the upper ocean ventilation rates suitable for biogeochemical studies. Observed distributions of pCFC-11 in the upper ocean thermocline are well reproduced by the convolution integral of the model-based TTD (mean bias < 6%, spatial correlation > 0.87) but there are significant regional biases in particular near the base of the thermocline and in the deep water formation regions. The model underestimates the deep pCFC-11 (> 2000 m) in the North Atlantic and in the Southern Ocean. The ratio between the mean and the spread of the age spectrum (Γ/Δ) is close to unity (mean = 1.04, median = 0.99) in the ventilated thermocline of the Pacific basin but there are significant regional variations of the ratio.