Seminar Ozean, Eis, Atmosphäre

One important application of transient tracer data is to estimate water ages, i.e., transit times of a water parcel from its source area in the mixed layer to the location of observation in the interior. Conventional dating concepts like tracer concentration age or tracer ratio age model the transport as purely advectiv pipe flow, but in reality the transport follows an ensemble of trajectories, so that the water parcel sampled is a composite of contributions of different age.
For a better approximation we model ages using an age distribution formally corresponding to one-dimensional advection and mixing. We assume a Gaussian distribution function with free parameters mean age t and age variance s²; we additionally allow for a dilution by water old enough to be tracer free (dilution factor d). Parameters are obtained by fitting predicted tracer concentrations (cobs = folding the distribution function n(t,s²,d) with realistic, time dependent tracer concentrations c₀ in the mixed layer) to observations for different and even non-synoptic tracers (CFC11, CFC12, CCl4), 8 degree N - 30 degree S.
Typical mean ages and dilution factors in upper North Atlantic Deep Water close to the western boundary are 30 years and 10, respectively, with significant increases southward and eastward. The fitted age distributions allow the construction of realistic tracer concentration time histories all through the tracer period, which can serve as internal tracer boundary conditions in ocean models.