The flow of Antarctic bottom water to the southwest Indian Ocean estimated using CFCs

Abstract

Observations of chlorofluorocarbons (CFCs), carbon tetrachloride, temperature, and salinity from five sections following the outflow path of Antarctic Bottom Water (AABW) into the southwest Indian Ocean are reported. The transient tracer data clearly show the plume of recently ventilated water whose hydrographic properties are progressively altered by mixing with the overlying waters. We use the CFC measurements to estimate the mean speeds (or transit times) and mixing rates (or dilutions) of the abyssal flow at each section using simple kinematic circulation models. Given our assumptions, the CFC ventilation age equals the transit time. The results suggest a transit time of 23±5 years (outflow speed of 1.2±0.3 cm s-1) to the Crozet-Kerguelen Gap with a dilution of 8-15 from the surface waters of the Weddell Sea. The estimated horizontal diffusivity is 30-70 m2 s-1, and the vertical diffusivity is 3-7×10-4 m2 s-1 Combined with the estimate of R. R. Dickson (unpublished data, 1998) for the AABW transport at this point, we conclude that a volume flux of 0.8-1.6 Sv (106 m3 s-1) is leaving the continental shelves of the Weddell Sea to eventually enter the abyssal Indian Ocean past Crozet Island.