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- Author or Editor: William J. Jenkins x
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Abstract
Isopycnal maps of 3H–3He age (τ) with about 100 km resolution have been obtained for a 1000-km scale area in the eastern subtropical North Atlantic. The midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m−2 s−1. Analysis of the distributions within the context of advection-diffusion equations reveals that the larger scale 3H–3He age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of Ekman downwelling. Thus, isopycnal velocities can be estimated from 1/∇τ, but record only the velocity component normal to the outcrop. The velocities thus obtained agree well with geostrophic estimates, although the comparison is flawed by the fundamental mismatch in timescales between the two techniques. Backward extrapolation of the maps to zero-age outcrop positions indicate that the contribution of unventilated, older, recirculated water to the gyre flow above 600 m depth is small in this area (no more than 20%). Ventilation of these density horizons thus is two to three times greater than inferred from Ekman pumping alone. A simple argument concerning the interaction of large scale flow with the topography of the depth of winter mixing shows that it is consistent with this process being a primary ventilation/subduction process for the subtropical gyre thermocline.
Advection–diffusion analysis of the AOU distributions on these surfaces indicates that oxygen utilization rates may be estimated using δAOU/δτ to an accuracy of order 10%. Such estimates appear consistent with previous work, and curvature in the AOU versus τ relationship may be a reflection of a southeastward increase in surface primary productivity.
Abstract
Isopycnal maps of 3H–3He age (τ) with about 100 km resolution have been obtained for a 1000-km scale area in the eastern subtropical North Atlantic. The midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m−2 s−1. Analysis of the distributions within the context of advection-diffusion equations reveals that the larger scale 3H–3He age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of Ekman downwelling. Thus, isopycnal velocities can be estimated from 1/∇τ, but record only the velocity component normal to the outcrop. The velocities thus obtained agree well with geostrophic estimates, although the comparison is flawed by the fundamental mismatch in timescales between the two techniques. Backward extrapolation of the maps to zero-age outcrop positions indicate that the contribution of unventilated, older, recirculated water to the gyre flow above 600 m depth is small in this area (no more than 20%). Ventilation of these density horizons thus is two to three times greater than inferred from Ekman pumping alone. A simple argument concerning the interaction of large scale flow with the topography of the depth of winter mixing shows that it is consistent with this process being a primary ventilation/subduction process for the subtropical gyre thermocline.
Advection–diffusion analysis of the AOU distributions on these surfaces indicates that oxygen utilization rates may be estimated using δAOU/δτ to an accuracy of order 10%. Such estimates appear consistent with previous work, and curvature in the AOU versus τ relationship may be a reflection of a southeastward increase in surface primary productivity.
Abstract
The tritium and excess 3He data from the 1981 TTO/NAS program are used to study the time scales for the ventilation of the deep western basin by recently formed North Atlantic Deep Water (NADW). The large-scale distributions of tritium and 3He in the deep North Atlantic are presented, and tracer inventories are computed for individual deep water basins. The bulk of the bomb tritium (and thus new NADW) resided in 1981 in the deep Labrador Sea and western subpolar gyre, with a slightly smaller amount in the deep western subtropical gyre. The maximum excess 3He values were located south of the overflows in the Labrador Sea the result of competition between ventilation and in situ tritium decay. The subpolar gyre was also the site of the strongest increase in decay-corrected tritium (∼120%) and excess 3He (∼100%) between the 1972 GEOSECS survey and the 1981 TTO/NAS program. The observed deep water tritium inventory is in reasonable agreement with model tracer inputs computed for the combined overflows from the Greenland/Norwegian Seas.
Elevated tritium and anomalous 3He values are found in the deep western boundary current (DWBC) along the entire North American coast. The cross-stream and alongstream structure of the transient tracer distributions in the DWBC is examined using high-resolution, midlatitude sections and a composite boundary current section from the overflows to the tropics. The observed evolution of tritium and excess 3He along the DWBC are used, along with the large-scale tracer distributions, to constrain a conceptual ventilation model for the deep western basin. The model results highlight the important role of turbulent mixing and/or recirculation between the DWBC and the interior and suggest that on average the water in the boundary current is exchanged with the interior every 2500–3500 km. The net effect of the large recirculation between the boundary current and the interior is twofold: rapid O(10–15 years) ventilation of the deep Labrador Sea and western subpolar gyre by newly formed NADW and reduction in the southward spreading rate of NADW to about 0.75–1.5 cm s−1, a factor of 5–10 smaller than observed DWBC velocities. The results have important implications for understanding the response of the deep North Atlantic to climatic variability on decadal time scales and for the invasion of anthropogenic pollutants (e.g., CO2) into the deep ocean.
Abstract
The tritium and excess 3He data from the 1981 TTO/NAS program are used to study the time scales for the ventilation of the deep western basin by recently formed North Atlantic Deep Water (NADW). The large-scale distributions of tritium and 3He in the deep North Atlantic are presented, and tracer inventories are computed for individual deep water basins. The bulk of the bomb tritium (and thus new NADW) resided in 1981 in the deep Labrador Sea and western subpolar gyre, with a slightly smaller amount in the deep western subtropical gyre. The maximum excess 3He values were located south of the overflows in the Labrador Sea the result of competition between ventilation and in situ tritium decay. The subpolar gyre was also the site of the strongest increase in decay-corrected tritium (∼120%) and excess 3He (∼100%) between the 1972 GEOSECS survey and the 1981 TTO/NAS program. The observed deep water tritium inventory is in reasonable agreement with model tracer inputs computed for the combined overflows from the Greenland/Norwegian Seas.
Elevated tritium and anomalous 3He values are found in the deep western boundary current (DWBC) along the entire North American coast. The cross-stream and alongstream structure of the transient tracer distributions in the DWBC is examined using high-resolution, midlatitude sections and a composite boundary current section from the overflows to the tropics. The observed evolution of tritium and excess 3He along the DWBC are used, along with the large-scale tracer distributions, to constrain a conceptual ventilation model for the deep western basin. The model results highlight the important role of turbulent mixing and/or recirculation between the DWBC and the interior and suggest that on average the water in the boundary current is exchanged with the interior every 2500–3500 km. The net effect of the large recirculation between the boundary current and the interior is twofold: rapid O(10–15 years) ventilation of the deep Labrador Sea and western subpolar gyre by newly formed NADW and reduction in the southward spreading rate of NADW to about 0.75–1.5 cm s−1, a factor of 5–10 smaller than observed DWBC velocities. The results have important implications for understanding the response of the deep North Atlantic to climatic variability on decadal time scales and for the invasion of anthropogenic pollutants (e.g., CO2) into the deep ocean.
Abstract
An analysis of the physical mechanisms contributing to the ventilation of the lower subtropical thermocline (26.5 < σ θ < 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 θ = 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.
Abstract
An analysis of the physical mechanisms contributing to the ventilation of the lower subtropical thermocline (26.5 < σ θ < 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 θ = 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.
