A study of North Atlantic ventilation using transient tracers

Abstract

The oceanic distributions of tritium 3H), 3He, and the chlorofluorocarbons (CFCs) can be used to constrain the time-scales of the major ventilation pathways for an ocean basin such as the North Atlantic. I present a new global model function, developed from a factor analysis of the WMO/IAEA data set, for predicting the spatial and temporal variability of bomb-tritium in precipitation. Model estimates for the atmospheric 3H delivery to the North Atlantic are recomputed and combined with advective 3H input estimates in a budget for the North Atlantic Basin. Key features of the model budget include refined estimates of the 3H vapor flux and southward advection of 3H in the low salinity, surface flow from the Arctic. Arctic tritium sources contribute about half of the observed increase (40%) in the decay corrected tritium inventory from the 1972 GEOSECS program and the 1981 TTO/NAS program. The 3H concentration in the intermediate and deep waters for the sub-polar North Atlantic increased substantially between 1972 and 1981. A time dependent model for the 3H and 3He inflow to the abyssal Atlantic from the Nordic Seas is developed. The 3H and 3He distributions in the abyssal North Atlantic and Deep Western Boundary Current (DWBC) are also presented. A simple model of abyssal circulation is constructed using the model Nordic Seas overflow curves, the observed tracer gradients in the DWBC, and the GEOSECS and TTO tracer inventories for the deep basins. Although the tracer concentrations in the boundary current are rather insensitive to the velocity of the boundary current, they do place bounds on the magnitude of recirculation between the boundary current and the interior. On average, a volume equal to the boundary current transport is entrained/detrained over a length scale of about 5000 km. About half of the overflow water entering the western basin of North Atlantic since the mid-1960's has been mixed into the deep Labrador Sea and subpolar gyre. The effects of tracer surface boundary conditions on thermocline ventilation and oxygen utilization rate estimates are discussed. Tracers that equilibrate rapidly with the atmosphere, such as 3He and the CFCs, have lower apparent ventilation time scales than t racers, such as tritium and radiocarbon, t hat are reset slowly in the surface layer. The results of a simple box-mixing model are compared with tritium and 3He data from a 1979 survey of the eastern subtropical North Atlantic. On shallow density surfaces, the computed tritium ventilation rates are two to three times slower than those for 3He; deeper in the thermocline, the two tracer ventilation rates converge. This trend may be related to the decreasing effectiveness of 3He gas exchange in equilibrating the deeper winter mixed layers of the more northerly isopycnal outcrops. Box models using limited surface exchange tracers (e.g. tritium and 14C) can under predict oxygen utilization rates (OUR) by up to 3 times due to differences between tracer and oxygen boundary conditions while 3He may overestimate OUR by 10- 20%. I present and discuss the distributions of two chlorofluorocarbons (CFCs) in the eastern North Atlantic measured on a 1988 hydrographic cruise between Iceland and the equator. CFC tagged seawater fills the entire sub-polar water column and subtropical thermocline. Measurable CFC levels are found at the ocean bottom as far south as 35°N; the CFC penetration depth shoals to about 750 meters in the tropics. The CFC data are used to illustrate the ventilation time-scales for the water masses in the eas tern basin and to calculate OUR values in the subtropical thermocline. The CFC data in the tropical oxygen minimum off of Africa are significantly lower than the values on similar density surfaces in the subtropics, providing support for the idea that the tropical oxygen minima are controlled primarily by physical rather than biological mechanisms. The evolution of the tropical and subtropical CFC distributions between the 1972-73 TTO/TAS program and the 1988 cruise are also examined. Other features of the CFC data include a clear signal of Labrador Sea Water mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, a northward flowing deep boundary current along the eastern margin of the basin, and a mid-depth equatorial plume of upper North Atlantic Deep Water.