The importance of lateral diffusion for the ventilation of the lower thermocline in the subtropical North Atlantic

An analysis of the physical mechanisms contributing to the ventilation of t he lower subtropical thermocline (26.5 < sigma(theta) < 27.3) of the North Atlantic is presented. Examination of the surface forcing suggests that thi s density range in the Atlantic should be strongly ventilated by how from t he surface winter mixed layer. In contrast to this expectation, the isopycn ic distribution of tracers within the shielded thermocline fails to show ev idence of net advective penetration of recently ventilated waters into the eastern North Atlantic. Instead, the presence of the Azores Current appears to block the net southward invasion of mass from the region of the isopycn al surface outcrops. Tracer properties of recently ventilated waters enter the gyre by diffusive exchange across the Azores Front. Evidence of this di ffusive ventilation based on both steady-stare and transient tracers is pre sented. Mean basin-scale property distributions on sigma(theta) = 27.0 are diagnosed from an expanded high quality hydrographic database. The Montgome ry streamfunction revels no evidence of pathways for direct geostrophic ven tilation on this density horizon; low values of potential vorticity are con fined to the region of formation north of the Azores Current. To complement the examination of the steady-state tracer distribution; an i nterpretation of the temporal evolution of the tritium-He-3 age in the east ern Atlantic is considered. The penetration of the coupled tritium and He-3 tracers provide a sensitive diagnostic of the effects of mixing. Lateral m ixing creates robust and predictable changes in measured Eulerian tritium-H e-3 age in response to the oceanic input of anthropogenic tritium, Simple k inematic models of the ventilation of tritium and He-3 are compared with th e observed temporal character of the tracer age field. Circulation scenario s characterized by net export of fluid from the surface mixed layer into th e lower subtropical thermocline require excessively large magnitudes of lat eral diffusivity (greater than or equal to 4000 m(2) s(-1)) to accurately s imulate the transient tracer observations. On the other hand, the observati ons can be reconciled with canonical magnitudes of lateral diffusion (1000- 1500 m(2) s(-1)) if the ventilation of properties is mediated by diffusive transmission across the Azores Current accompanied by only negligible net t ransport of mass.