Ventilation of the Subtropical North Atlantic: Locations and Times of Last Ventilation Estimated Using Tracer Constraints From GEOTRACES Section GA03

The ventilation of the subtropical North Atlantic along GEOTRACES section GA03 is quantified in terms of where and how long ago water was last in the mixed layer. Measurements of T, S, PO4, CFC-11, CFC-12, SF6, and estimates of prebomb C-14 are deconvolved for the boundary propagator G using a maximum-entropy approach. From G, we calculate the fractions of water last ventilated in specified surface regions (w). We estimate that (5613)% of the water deeper than 1,000 m was ventilated in northern high latitudes, (155)% in the Mediterranean, and (2712)% in the Southern Ocean. Below the thermocline and outside the deep western boundary current, mean ages of (w)-ventilated water exceed a century. Consequently, memory of where last ventilation occurred tends to get lost and the deep mean-age patterns of (w)-ventilated water are broadly similar for all (w). The mean ventilation ages, averaged over the section with (w)-fraction weights, are roughly 200 years for all deep water masses except for water last ventilated south of the Antarctic divergence, which is about twice as old. The uncertainties in the section-mean profiles of the (w) fractions and their mean ages are approximate to 50% and approximate to 20%, respectively. The (w) fractions have vertically diffuse overlapping patterns suggesting significant diapycnal mixing, consistent with century-scale mean ages. We quantify the seasonal cycle of ventilation and find that in both hemispheres peak ventilation occurs during late winter and early spring, but Northern Hemisphere ventilated deep waters have a more pronounced seasonal cycle with nearly zero summertime ventilation.