Estimating climatological age from a model-derived oxygen-age relationshipin the Mediterranean

Climatological oxygen observations suggest that the deep water of the weste rn Mediterranean is better ventilated than that of the eastern Mediterranea n. In this work, the oxygen distribution is modelled using an off-lint trac er model driven by velocity fields from an eddy-permitting general circulat ion model. The model includes an air-sea transfer of oxygen, entrainment, a nd an interior oxygen consumption which varies in each basin according to e stimates of the background new production. The modelled oxygen distribution shows higher values in the western basin when compared with the eastern ba sin, despite higher oxygen utilisation in the west, although the model unde restimates the west-east contrast. In addition, an off-line ideal age integ ration is performed that shows ages reaching 100 yr in the eastern basin an d 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 ox ygen utilisation (AOU) and age for each basin. A climatological age distrib ution is then inferred from the observed AOU distribution using the model-d erived relationships. The predicted age reaches 80-120 yr and 20-40 yr in t he bottom waters of the eastern and western basins respectively. The errors in this approach are likely to be significant, bring +/-20 yr in the east and +/-40 yr in the west. However, the results are broadly in accord with i ndependent estimates of overturning timescales-70 to 120 yr and 40 yr for t he eastern and western basins, respectively. (C) 1998 Elsevier Science B.V. All rights reserved.