Ventilation of the Deep Western Boundary Current and Abyssal Western North Atlantic: Estimates from Tritium and 3He Distributions

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  • 1 Advanced Study Program, National Center for Atmospheric Research, Boulder, Colorado
  • | 2 Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
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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.

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