Decadal Ventilation and Mixing of Indian Ocean Waters

Chlorofluorocarbon (CFC) and hydrographic data from the World Ocean Circulation Experiment (WOCE) Indian Ocean expedition are used to evaluate contributions to decadal ventilation of water masses. At a given density, CFC-derived ages increase and concentrations decrease from the south to north, with lowest concentrations and oldest ages in Bay of Bengal. Average ages for thermocline water are zero to 40 years, and for intermediate water they are less than 10 years to more than 40 years. As compared with the Marginal Seas or throughflow, the most significant source of CFCs for the Indian Ocean south of 12oN is the Southern Hemisphere. A simple calculation is used to show this is the case even at intermediate levels due to differences in gas solubilities and mixing of Antarctic Intermediate Water and Red Sea Water.Bottom Water in the Australia-Antarctic Basin is higher in CFC concentrations than to the west in the Enderby Basin, due to the shorter distance of this water to the Adelie Land coast and Ross Sea sources. However, by 40oS, CFC concentrations in the Bottom Water of the Crozet Basin originating from the Weddell Sea are similar to those in the South Australia Basin. Independent observations, which show that Bottom Water undergoes elevated mixing between the Australia-Antarctic Basin and before entering the subtropics, are consistent with high CFC dilutions (3to 14-fold) and a substantial concentration decrease (factor of 5) south to north of the Southeast Indian Ridge. CFCbearing Bottom Water with ages of 30 years or more is transported into the subtropical South Indian Ocean by three western boundary currents, and highest concentrations are observed in the westernmost current. During WOCE, CFC-bearing Bottom Water reaches to about 30oS in the Perth Basin, and to 20oS in the Mascarene Basin. Comparing