No abstract

An improved method has been developed for the separation of the natural and bomb components of the radiocarbon in the ocean. The improvement involves the use of a very strong correlation between natural radiocarbon and dissolved silica. This method is applied to radiocarbon measurements made on samples collected during the Geochemical Ocean Sections Study (GEOSECS), Transient Tracers in the Ocean (TTO) and South Atlantic Ventilation Experiment (SAVE) expeditions. On the basis of this new separation we provide not only an estimate of the global inventory of bomb 14C at the time of the GEOSECS survey but also the distribution of bomb radiocarbon along four thermocline isopycnals in each ocean. We also document the evolution of the bomb 14C inventory and penetration along thermocline isopycnals in the North Atlantic Ocean between the times of the GEOSECS (1972–1973) and TTO (1980–1982) surveys and in the South Atlantic Ocean between the times of the GEOSECS (1973) and SAVE (1987–1989) surveys. In addition, we show that the bomb tritium to bomb 14C ratio (expressed in the tritium unit (TU) 81 units/100‰) for waters entering the thermocline of the northern hemisphere is about 9 times higher than for those entering the southern hemisphere thermocline. This contrast offers long‐term potential as an indicator of inter‐hemispheric transport of upper ocean waters.

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Data collected in 1988–1989, as part of the South Atlantic Ventilation Experiment, have been combined with the historical database to study the circulation and water mass variability of the abyssal water in the Argentine Basin. A map of potential temperature at 4000 m used as an indication of geostrophic shear defines a south and western intensified crescent‐shaped abyssal recirculation. Within this recirculation, and its northward extension to the Brazil Basin, Antarctic Bottom Water (AABW) properties have undergone two modifications during the 1980s: (1) The water mass cooled (0.05°C) and freshened (0.008 in salinity ratio) on surfaces of constant density. (2) The densest layer of AABW was altered to less dense water through mixing or advection out of the study area. This water mass change does not appear to have affected the flow pattern. Data collected in 1983 and 1988 to the north in the Brazil Basin show penetration of the freshwater mass in the deep western boundary current to between 18°S and 10°S, indicating very rapid propagation of the anomaly from the Argentine Basin into the Brazil Basin as a deep western boundary current. It is suggested that open ocean convective events within the Weddell Sea contributed to the change in AABW documented here.

Deep ocean inventories of dissolved chlorofluorocarbon‐11 (CFC‐11) along representative sections off Antarctica provide the first estimate of the overall strength of all dense water sources in the Southern Ocean. Their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the Thermohaline Circulation (THC). The bottom layer is supplied via sinking of Antarctic Bottom Water (AABW) produced at a few continental shelves. The middle layer receives the offshore injection of ventilated Modified Circumpolar Deep Water (MCDW) produced along much of the lengthy Antarctic Slope Front. The top layer is ventilated by northward export of Antarctic Surface Water into the Upper Circumpolar Deep Water of the Antarctic Circumpolar Current. Average Southern Ocean inputs to the upper two layers of the deep ocean for the 1970–1990 period are derived on the basis of the CFC‐11 distributions along meridional sections, the mean CFC‐11 saturations of all water mass ingredients, and the inferred mixing leading to production of dense source waters. About 5.4 ± 1.7 Sv (1 Sv = 106 m3s−1) of near‐freezing Shelf Water ventilate the bottom layer, and 4.7 ± 1.7 Sv and 3.6 ± 1.3 Sv of cold Antarctic Surface Water ventilate the middle and top layers. Therefore the total contribution of ventilated Southern Ocean waters to the lower limb of the global THC is about 14 Sv. This is close to the about 17 Sv estimated for North Atlantic near‐surface sources from CFC‐11 inventories. Their entrainment of 7.4 ± 2.4 Sv of CFC‐poor subsurface Lower Circumpolar Deep Water during the formation and sinking of AABW and MCDW raises the total Southern Ocean input to the deep ocean to about 21 Sv.

A conductivity-temperature-depth and tracer chemistry section in the southeast South Atlantic in December 1989 and January 1990 presents strong evidence that there is a significant interocean exchange of thermocline and intermediate water between the South Atlantic and Indian oceans. Eastward flowing water at 10øW composed of South Atlantic Central (thermocline) Water is too enriched with chlorofluoromethanes 11 and 12 and oxygen to be the sole source of similar O-S water within the northward flowing Benguela Current. About two thirds of the Benguela Current thermocline transport is drawn from the Indian Ocean; the rest is South Atlantic water that has folded into the Benguela Current in association with the Agulhas eddy-shedding process. South Atlantic Central water passes in the Indian Ocean by a route to the south of the Agulhas Return Current. The South Atlantic water loops back to the Atlantic within the Indian Ocean, perhaps mostly within the Agulhas recirculation cell of the southwest Indian Ocean. Linkage of Atlantic and Indian Ocean water diminishes with increasing depth; it extends through the lower thermocline into the Antarctic Intermediate Water (AAIW) (about 50% is derived from the Indian Ocean) but not into the deep water. While much of the interocean exchange remains on an approximate horizontal "isopycnal" plane, as much as 10 x i0 6 m 3 s -! ofindian Ocean water within the 25 x 10 6 m 3 s -I Bengueia Current, mostly derived from the lower thermocline and AAIW, may balance deeper Atlantic export of North Atlantic Deep Water (NADW). The addition of salt water from the evaporative Indian Ocean into the South Atlantic Ocean thermocline and AAIW levels may precondition the Atlantic for NADW formation. While AAIW seems to be the chief feed for NADW, the bulk of it enters the subtropical South Atlantic, spiked with Indian Ocean salt, within the Benguela Current rather than along the western boundary of the South Atlantic. Paper number 92JC00485. 0148-0227/92/92J C-00485 $05.00 overall heat and salinity budgets of the South Atlantic and may play a role in the global thermohaline circulation [Gordon, 1985, 1986, 1988]. In effect, the Atlantic's salinity is increased by drawing salty water from the evaporative Indian Ocean (which north of 30øS loses fresh water to the atmosphere at a rate of 5.1 x 105 m 3 s -1 only slightly less than the Atlantic Ocean [Baumgartner and Reichel, 1975]). Estimates of the leakage of Indian Ocean water into the South Atlantic by eddies and plumes range from 3 to 20 Sv (1 Sv = 106 m 3 s-i). It is not an easy matter of calculating this value, since the Indian Ocean Central Water and the South Atlantic Central Water (SACW) have very similar potential temperature-salinity (0-S) structure [Gordon, 1985]. Gordon et al. [1987] calculate 10 Sv of IOCW and Antarctic Intermediate Water (AAIW) entering the Atlantic in late 1983' Whirworth and Nowlin [1987] show inflow of nearly 20 Sv in early 1984. Bennett's [1988] evaluation of the 1983 and 1984 data arrives at lower values, 6.3 and 9....

[1] The Arctic Ocean constitutes a large body of water that is still relatively poorly surveyed because of logistical difficulties, although the importance of the Arctic Ocean for global circulation and climate is widely recognized. For instance, the concentration and inventory of anthropogenic CO 2 (C ant ) in the Arctic Ocean are not properly known despite its relatively large volume of well-ventilated waters. In this work, we have synthesized available transient tracer measurements (e.g., CFCs and SF 6 ) made during more than two decades by the authors. The tracer data are used to estimate the ventilation of the Arctic Ocean, to infer deep-water pathways, and to estimate the Arctic Ocean inventory of C ant . For these calculations, we used the transit time distribution (TTD) concept that makes tracer measurements collected over several decades comparable with each other. The bottom water in the Arctic Ocean has CFC values close to the detection limit, with somewhat higher values in the Eurasian Basin. The ventilation time for the intermediate water column is shorter in the Eurasian Basin ($200 years) than in the Canadian Basin ($300 years). We calculate the Arctic Ocean C ant inventory range to be 2.5 to 3.3 Pg-C, normalized to 2005, i.e., $2% of the global ocean C ant inventory despite being composed of only $1% of the global ocean volume. In a similar fashion, we use the TTD field to calculate the Arctic Ocean inventory of CFC-11 to be 26.2 ± 2.6 Â 10 6 moles for year 1994, which is $5% of the global ocean CFC-11 inventory.

In 1997, a unique hydrographic and chlorofluorocarbon (CFC: component CFC-11) dataset was obtained in the subpolar North Atlantic. To estimate the synopticity of the 1997 data, the recent temporal evolution of the CFC and Labrador Sea Water (LSW) thickness fields are examined. In the western Atlantic north of 50ЊN, the LSW thickness decreased considerably from 1994-97, while the mean CFC concentrations did not change much. South of 50ЊN and in the eastern Atlantic, the CFC concentration increased with little or no change in the LSW thickness. On shorter timescales, local anomalies due to the presence of eddies are observed, but for space scales larger than the eddies the dataset can be treated as being synoptic over the 1997 observation period.The spreading of LSW in the subpolar North Atlantic is described in detail using gridded CFC and LSW thickness fields combined with Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float trajectories. The gridded fields are also used to calculate the CFC-11 inventory in the LSW from 40Њ to 65ЊN, and from 10Њ to 60ЊW. In total, 2300 Ϯ 250 tons of CFC-11 (equivalent to 16.6 million moles) were brought into the LSW by deep convection. In 1997, 28% of the inventory was still found in the Labrador Sea west of 45ЊW and 31% of the inventory was located in the eastern Atlantic.The CFC inventory in the LSW was used to estimate the lower limits of LSW formation rates. At a constant formation rate, a value of 4.4-5.6 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ) is obtained. If the denser modes of LSW are ventilated only in periods with intense convection, the minimum formation rate of LSW in 1988-94 is 8.1-10.8 Sv, and 1. 8-2.4 Sv in 1995-97.