Estimating climatological age from a model-derived oxygen–age relationship in the Mediterranean

Abstract

Climatological oxygen observations suggest that the deep water of the western Mediterranean is better ventilated than that of the eastern Mediterranean. In this work, the oxygen distribution is modelled using an off-line tracer model driven by velocity fields from an eddy-permitting general circulation model. The model includes an air–sea transfer of oxygen, entrainment, and an interior oxygen consumption which varies in each basin according to estimates of the background new production. The modelled oxygen distribution shows higher values in the western basin when compared with the eastern basin, despite higher oxygen utilisation in the west, although the model underestimates the west–east contrast. In addition, an off-line ideal age integration is performed that shows ages reaching 100 yr in the eastern basin and 50 yr in the western basin, although the latter value should be viewed as an over-estimate as the modelled deep convection is too weak. These model integrations are combined to produce a relationship between the apparent oxygen utilisation (AOU) and age for each basin. A climatological age distribution is then inferred from the observed AOU distribution using the model-derived relationships. The predicted age reaches 80–120 yr and 20–40 yr in the bottom waters of the eastern and western basins respectively. The errors in this approach are likely to be significant, being ±20 yr in the east and ±40 yr in the west. However, the results are broadly in accord with independent estimates of overturning timescales—70 to 120 yr and 40 yr for the eastern and western basins, respectively.