Temporal evolution of tritium-³He age in the North Atlantic : implications for thermocline ventilation

Abstract

This thesis is a study of the physical mechanisms that ventilate the subtropical thermocline of the eastern North Atlantic. The starting point is an analysis of the existent historical database of natural and anthropogenic tracers, with special emphasis on 3He and tritium, that can be used to infer rates of ventilation. If the flow is predominantly advective, the temporal evolution of coupled transient tracers can be used to define a tracer age which measures the elapsed time since a water parcel was resident in the surface mixed layer. A principle finding is that the observed tracer age shows a large and systematic change over time. Tritium-3He age in the eastern Atlantic thermocline is seen to increase over time; the magnitude of the change is greatest for the deeper, more slowly ventilated layers of the thermocline. The first hypothesis examined is that the observed shift in the tracer age field is the manifestation of a slackening of the physical ventilation. A time series of the meridional geostrophic velocity shear in the eastern Atlantic shows no indication of a change in the strength of the large-scale circulation. Uncertainty of the geostrophic calculation due to data sparsity and mesoscale eddy contamination prevents conclusive rejection of the hypothesis of a changing circulation. There are other tracers which offer useful clues: comparison of the tritium-3He age field with dissolved oxygen reveals a temporal trend in the property-property correlation. The spatial structure of the oxygen field, however, shows no long-term evolution over time. From this line of evidence it is concluded that the physical ventilation of the thermocline has not altered over time and, therefore, the temporal change in the tritium-3He age field must be the signal of the tritium invasion itself. A second hypothesis, which analysis shows is more consistent with the observations, is that the changing tracer age is a consequence of mixing effects in the ventilation of 3He and tritium. 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. A one-dimensional model is constructed based on the assumption that the totality of the fluid in the thermocline derives from subduction out of the surface mixed layer. The temporal behavior of the tracer age field is found to be dependent on the radiotracer Peclet number, which measures the ratio of the diffusive and advective time scales. In a model with steady circulation, the observed temporal behavior of the tracer age field can be reproduced only when the effects of lateral mixing play a significant role in the process of ventilation. The vertical structure and magnitude of the implied lateral diffusivity are, however, inconsistent with other observations. The numerical simulations are next extended to two-dimensions to allow for the presence of a pool of unventilated, re-circulated water within the anti-cyclonic, subtropical gyre. Comparison of the model with the observed transient tracer field in the lower thermocline shows consistency with conventional estimates of lateral mixing rates only when the diffusively ventilated "pool" region extends across the entire zonal domain of the gyre. In contrast, the transient tracer fields in the upper portion of the thermocline are best reproduced when the isopycnal surfaces are ventilated by advection directly from the surface mixed layer. The results obtained here are consistent with numerical simulations which reveal a prominent role for mesoscale eddies in the ventilation of the thermocline.