3H and 3He in the Beta Triangle: Observations of Gyre ventilation and oxygen utilization rates

Isopycnal maps of 3H--2He age (tau) 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/deltau, 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 tau 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 processing being a primary ventilation/subduction process for the subtropical gyre thermocline