Spreading velocities and dilution of North Atlantic Deep Water in the tropical Atlantic based on CFC time series

[1] Repeated observations of CFC-11 and CFC-12 in the western tropical Atlantic over the period from 1990 to 2002 are used to fit parameters of the age spectra for several North Atlantic Deep Water components: the upper Labrador Sea water (ULSW), the Labrador Sea water (LSW), and the lower North Atlantic Deep Water (LNADW). The most robust results are the mean spreading velocities, u, from the formation region in the Labrador Sea and the Denmark Strait, of 1.5 +/- 0.3 cm/s for the two LSW water masses and 2.0 +/- 0.2 cm/s for LNADW, respectively. These values are equivalent to an age of about 15 - 22 years for all three components at 16degreesN and 21 - 31 years at the equator at 35degreesW. The results are compatible with simpler tracer methods, as CFC ratios or effective'' ages. The fractions, f, of the young'' component (i.e., the fraction of the water mass ventilated in the last 50 years), turned out to be quite small, ranging from 25% to less than 5%. The downstream decrease of f indicates mixing of young NADW with CFC-poor older NADW vintages and/or mixing with other CFC-free water masses from the Southern Hemisphere. The lowest fractions were found in the LNADW, with a sharp downstream decrease from the equator at 35 degreesW to 10 degreesS, probably caused by the bifurcation into a zonal flow along 2 degreesS and a southward flow along the Brazilian coast. Unlike the small uncertainties estimated for the inferred velocities, isopycnal diffusivities, k, range from 100 to 10000 m(2)/s. The temporal evolution of f in the LSW shows a maximum, propagating from 7 degreesN in 1996, downstream to 5degreesS in the Deep Western Boundary Current, and along 2degreesS to 23degreesW. We interpret this signal as the arrival of the intensely ventilated LSW formed in 1972 - 1976. Decreasing fractions of the ULSW and LSW south of the equator between 2000 and 2002 are presumably caused by local dynamics.