On the sources of Weddell Gyre Antarctic Bottom Water

In March-April 1995, as part of the World Ocean Circulation Experiment sect ion A23, we completed 49 hydrographic stations across the Weddell Gyre and southern Antarctic Circumpolar Current, from the Antarctic continental shel f (72.5 degrees S, 16.5 degrees W) to South Georgia (55 degrees S, 34.5 deg rees W). Chlorofluorocarbon (CFC-11, CFC-12, and CFC-113) data collected at these stations reveal that distinct sources renew the Antarctic Bottom Wat er (defined as waters with potential temperatures less than 0 degrees C) of the Weddell Gyre. Weddell Sea Bottom Water (defined as waters with potenti al temperatures less than -0.7 degrees C) formed in the western Weddell Sea has CFC concentrations about 5 to 6 times higher in the eastward flowing n orthern Weddell Gyre than in the westward flowing southern limb. Our CFC me asurements suggest that distinct sources of Weddell Sea Bottom Water exist: in,the western Weddell Sea, in agreement with previous descriptions based o n potential temperature and salinity signals. In the northern Weddell Gyre, high CFC concentrations in Weddell Sea Deep Water, potential temperatures between 0 degrees C and -0.7 degrees C, confirm the long-recognized sources for this water mass in the western and southwestern Weddell Sea. In the so uthern Weddell Gyre at about 20 degrees W and along the Antarctic continent al slope, Weddell Sea Deep Water with potential temperatures around -0.45 d egrees C shows a deep CFC maximum about 1000 m above the seafloor. CFC conc entrations in this deep southern core are about 80% of those of new Weddell Sea Deep Water in the northern Weddell Gyre near 30 degrees W. The A23 CFC and hydrographic data are not consistent with the hypothesis that Weddell Sea Deep Waters are derived from a single source in the western Weddell Sea , Instead, these tracers suggest that an important portion of the Weddell S ea Deep Water in the southern Weddell Gyre originates outside the western W eddell Sea, probably near the Amery Basin and environs, around 75 degrees E . These features of the circulation and renewal of the deep Weddell Gyre sh ould be carefully considered in simulations dealing with fluxes; pathways, and formation rates of Antarctic Bottom Water.