Ages and age spectra of Eastern Mediterranean Deep Water

Observations of transient tracers such as tritium, helium and CFCs can be used to obtain information on the ages of water masses. The ocean general circulation model Modular Ocean Model-Array (MOMA) is used to compute the distribution of anthropogenic tracers as well as the ‘ideal’ age and age spectra for the Eastern Mediterranean. Substantial differences between tracer concentration age, tracer ratio age and the ideal age are revealed. The reason for these discrepancies is the fact that a water parcel in the ocean interior has to be regarded as a composition of fluid elements with different ages and different tracer concentrations. Therefore the concept of age spectra (distribution of transit times), describing the composition of the water parcel, is more adequate than using a single age. The computed age spectra are analysed in how far they are reflecting the circulation of Eastern Mediterranean Deep Water, especially its spreading and upwelling velocities. The methods used for the computation of the age spectra assume a steady ocean circulation. Short- and long-term variability of the model lead to a falsification of the calculated age spectra. The comparison between tracer concentrations and ideal age derived from the age spectra with the values computed directly in the model as passive tracers allows an estimation of the accuracy of the computed age spectra. The distribution of transit times for the deep water range, which are characterized by one pronounced maximum, suggests an approximation of the deep water masses consisting of only two parts, a young and an old component. Based on this assumption, it is possible to obtain a good estimation of the ideal age derived from measured tracer data.