The Use of Anthropogenic Tritium and Helium-3 to Study Subtropical Gyre Ventilation and Circulation

With tritium and helium-3 (³He) data from the Transient Tracers in the Ocean (TTO) expedition and from two other contemporaneous cruises, a synoptic picture of the ventilation and circulation of the subtropical North Atlantic is built. We will see clear evidence of gyre circulation in the tritium-³He age distributions on the shallower isopycnals, permitting estimates of the rates of circulation averaged over timescales from months to decades. The entry points of fluid into the main thermocline and the pathways of exchange with the upper ocean on seasonal to decade timescales appear clearly. It is the time-averaged transport processes on those timescales that are important to the uptake of carbon dioxide by the ocean. The overall relation between tritium and ³He within the subtropics exhibits a systematic hook-like pattern that is consistent with `strong' ventilation of the gyre thermocline; a fluid parcel entering the gyre thermocline making only about one circuit around the gyre before being ventilated. Finally, we present a time-series of <sup>3</sup>He measurements made over a period of two years near Bermuda. The mixed layer is demonstrably supersaturated in this isotope throughout a large part of the year, requiring a gas-exchange flux of this isotope to the atmosphere. Model results are presented that permit the calculation of the in situ solubility isotope ratio anomaly for helium (as affected by bubble injection and gas exchange), and that can be used to estimate the upward flux of this isotope. Because only a small fraction of this flux can be produced in the mixed layer, this helium must be `mined' from the main thermocline. The computed flux is consistent with the long-term evolution of the inventories of tritium and ³He within the main thermocline. This flux has implications regarding the vertical transport of material within and from the permanent thermocline. A single observation of what may be one of the processes responsible for this upward flux is discussed. The large-scale distributions of tritium and tritium-³He age on a few representative density horizons within the North Atlantic have been presented, and begin to indicate the nature and pathways of ventilation. These distributions are consistent with what we know about gyre circulation, but present us with evidence of very strong ventilation occurring in the northwestern segment of the subtropical gyre, as well as the previously supposed entry point of fluid in the northeastern section. We can see the westward march of the outcrop entry point from the Bay of Biscay for the shallowest surface, to the Labrador Sea, and then northward to the Greenland Sea for the deepest horizon. Interior gradients of the tritium-³He age appear to be an important diagnostic feature of ventilation processes in the North Atlantic basin.