Transfer von Nordatlantischem Tiefenwasser durch den Südatlantik mit Tracer-Verteilungen und Transitzeit-Verteilungen

Abstract

We examine the spreading of North Atlantic Deep Water (NADW) in the tropical Atlantic and subtropical South Atlantic by means of transit time distributions (TTDs) which are fitted to observations of the transient tracers CFC-11, CFC-12, CCl4 and tritium. The TTDs are derived under the assumption of steady-state flow, and the flow being formally represented by one-dimensional advection and mixing. The latter includes all non-advective processes of the transport, such as eddies, recirculations and temporal variability of the transport field. In comparison with conventional tracer dating methods, which use the approach of a purely advective transport, this method represents the next higher approximation.With the approach chosen here, the TTDs depend on a limited number of parameters only: the mean age t, and the Péclet number Pe as the ratio of advection and mixing. Additionally an explicitly tracer-free dilution V is used to account for mixing with waters that are old enough to contain little or no tracer. It has turned out that the use of different tracers and of repeated observations usually does not provide enough independent information to determine the parameters of the TTDs consistently. Therefore we assume t and V to vary linearly with the distance and that Pe is constant. This allows us to use all relevant tracer observations at different locations and times to determine the free parameters of the TTDs. Conventionally a constant apparent tracer saturation in the source waters is used as a tracer boundary condition. To obtain a more realistic boundary condition a simple mixing model for the source water masses has been devised. This approach ensures a realistic approximation of the time evolution of the tracer concentration ratios in the source waters.Using these TTDs we examine the transport of upper and lower NADW in the South Atlantic in the deep western boundary current, along the equator, and along a zonal section at 11°S.