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dc.contributor.authorRobbins, Paul E.  Concept link
dc.coverage.spatialEastern North Atlantic
dc.date.accessioned2013-01-11T19:08:03Z
dc.date.available2013-01-11T19:08:03Z
dc.date.issued1997-09
dc.identifier.urihttps://hdl.handle.net/1912/5712
dc.descriptionSubmitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1997en_US
dc.description.abstractThis 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.en_US
dc.description.sponsorshipThis research was supported by Office of Naval Research AASERT contract number N00014-95-l-0824.en_US
dc.format.mimetypeapplication/pdf
dc.language.isoen_USen_US
dc.publisherMassachusetts Institute of Technology and Woods Hole Oceanographic Institutionen_US
dc.relation.ispartofseriesWHOI Thesesen_US
dc.subjectThermoclinesen_US
dc.subjectTritium datingen_US
dc.subjectOceanus (Ship : 1975-) Cruise OC52en_US
dc.subjectOceanus (Ship : 1975-) Cruise OC79en_US
dc.subjectOceanus (Ship : 1975-) Cruise OC202en_US
dc.subjectAtlantis II (Ship : 1963-) Cruise AII107en_US
dc.subjectAtlantis II (Ship : 1963-) Cruise AII109en_US
dc.subjectMeteor (Ship) Cruise M56en_US
dc.subjectMeteor (Ship) Cruise M64en_US
dc.subjectMeteor (Ship) Cruise M69en_US
dc.subjectEndeavor (Ship: 1976-) Cruise EN143en_US
dc.titleTemporal evolution of tritium-³He age in the North Atlantic : implications for thermocline ventilationen_US
dc.typeThesisen_US
dc.identifier.doi10.1575/1912/5712


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