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: This thesis studies the physical mechanisms which ventilate the subtropical thermocline of the eastern North Atlantic. The coupled transient tracers tritium and 3He define a tracer age diagnostic which estimates the elapsed time since a water parcel was resident at the ocean surface. Analysis of the historical database of 3He and tritium measurements reveals large, systematic changes in the observed tracer age. The first hypothesis examined is that the observed shift in tracer age results from a weakening of the physical ventilation. Examination of a time-series of the meridional geostrophic velocity and the large-scale oxygen distribution, both based on historical hydrography, reveal no evidence for such a change. Therefore, the temporal trend in observed tritium-3He age cannot be explained by an alteration of the physical ventilation of the thermocline. A second hypothesis is that the temporal change in the tritium-3He age field reflects a signal of the tritium invasion itself. Numerical simulations of the thermocline ventilation of 3H and 3He are performed to examine the steadiness of the tracer age field under different advective-diffusive regimes. The temporal behavior of the tracer age field is dependent on the relative importance of the advection and diffusion of the flow field. The observed temporal behavior of the tracer age field is evidence of a significant role for lateral mixing in the ventilation of the thermocline.

Temporal evolution of tritium-�He age in the North Atlantic : implications for thermocline ventilation