The Importance of Lateral Diffusion for the Ventilation of the Lower Thermocline in the Subtropical North Atlantic

Abstract

An analysis of the physical mechanisms contributing to the ventilation of the lower subtropical thermocline (26.5 < σ? < 27.3) of the North Atlantic is presented. Examination of the surface forcing suggests that this density range in the Atlantic should be strongly ventilated by flow from the surface winter mixed layer. In contrast to this expectation, the isopycnic distribution of tracers within the shielded thermocline fails to show evidence 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 isopycnal surface outcrops. Tracer properties of recently ventilated waters enter the gyre by diffusive exchange across the Azores Front. Evidence of this diffusive ventilation based on both steady-state and transient tracers is presented. Mean basin-scale property distributions on σ? = 27.0 are diagnosed from an expanded high quality hydrographic database. The Montgomery streamfunction revels no evidence of pathways for direct geostrophic ventilation on this density horizon; low values of potential vorticity are confined to the region of formation north of the Azores Current. To complement the examination of the steady-state tracer distribution, an interpretation of the temporal evolution of the tritium?3He age in the eastern Atlantic is considered. The penetration of the coupled tritium and 3He tracers provide a sensitive diagnostic of the effects of mixing. Lateral mixing creates robust and predictable changes in measured Eulerian tritium?3He age in response to the oceanic input of anthropogenic tritium. Simple kinematic models of the ventilation of tritium and 3He are compared with the observed temporal character of the tracer age field. Circulation scenarios characterized by net export of fluid from the surface mixed layer into the lower subtropical thermocline require excessively large magnitudes of lateral diffusivity (≥4000 m2 s?1) to accurately simulate the transient tracer observations. On the other hand, the observations can be reconciled with canonical magnitudes of lateral diffusion (1000?1500 m2 s?1) if the ventilation of properties is mediated by diffusive transmission across the Azores Current accompanied by only negligible net transport of mass.