VENTILATION OF THE DEEP WESTERN BOUNDARY CURRENT AND ABYSSAL WESTERN NORTH-ATLANTIC - ESTIMATES FROM TRITIUM AND HE-3 DISTRIBUTIONS

The tritium and excess He-3 data from the 1981 TTO/NAS program are use d to study the time scales for the ventilation of the deep western bas in by recently formed North Atlantic Deep Water (NADW). The large-scal e distributions of tritium and He-3 in the deep North Atlantic are pre sented, and tracer inventories are computed for individual deep water basins. The bulk of the bomb tritium (and thus new NADW) resided in 19 81 in the deep Labrador Sea and western subpolar gyre, with a slightly smaller amount in the deep western subtropical gyre. The maximum exce ss He-3 values were located south of the overflows in the Labrador Sea , the result of competition between ventilation and in situ tritium de cay. The subpolar gyre was also the site of the strongest increases in decay-corrected tritium (approximately 120%) and excess He-3(approxim ately 100%) between the 1972 GEOSECS survey and the 1981 TTO/NAS progr am. The observed deep water tritium inventory is in reasonable agreeme nt with model tracer inputs computed for the combined overflows from t he Greenland/Norwegian Seas. Elevated tritium and anomalous He-3 value s are found in the deep western boundary current (DWBC) along the enti re 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 sec tion from the overflows to the tropics. The observed evolution of trit ium and excess He-3 along the DWBC are used, along with the large-scal e tracer distributions, to constrain a conceptual ventilation model fo r the deep western basin. The model results highlight the important ro le of turbulent mixing and/or recirculation between the DWBC and the i nterior and suggest that on average the water in the boundary current is exchanged with the interior every 2500-3500 km. The net effect of t he large recirculation between the boundary current and the interior i s twofold: rapid O(10-15 years) ventilation of the deep Labrador Sea a nd western subpolar gyre by newly formed NADW and reduction in the sou thward 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 i mplications 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.