Age tracers in an ocean GCM

Observations of transient tracers such as tritium and helium-3 (He) are frequently combined to construct `age-likea quantities generally interpreted to represent time elapsed since a #uid parcel was last at the surface. In a turbulent (`di!usivea) environment such as the ocean, we must regard the #uid parcel as being composed of material #uid elements that have spent di!erent lengths of time since their last contact with the surface. Hence, they are characterized by an age spectrum or distribution of transit times. In this study we explore the concepts of tracer-derived `agesa and the transit-time probability density function (PDF) with the aim of improving our understanding of their interpretation. Using an ocean general circulation model, we illustrate the e!ect of mixing on tracer-derived `agesa within the Atlantic Ocean. The mixing biases such ages towards younger values with respect to the ideal or mean age of a water parcel. In the North Atlantic, this bias is particularly pronounced in the thermocline because of large vertical gradients in tracer concentration, and in the deep ocean, where the penetration of recently ventilated water creates large gradients along the isopycnal surfaces. In contrast, the e!ect of mixing appears to be relatively small in the subtropical subduction region. Calculations of the transit-time PDF in the ocean model show, however, that the mean age can potentially be very large because of contributions from long transit-time pathways, in spite of the fact that such pathway