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 0-40 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 12 degrees N 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 that 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 40 degrees S, 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 (3-14-fold) and a substantial concentration decrease (factor of 5) south to north of the Southeast Indian Ridge. CFC-bearing bottom waters with ages 30 years or more are 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 30 degrees S in the Perth Basin, and to 20 degrees S in the Mascarene Basin. Comparing subtropical bottom water-CFC concentrations with those of the South Pacific and Atlantic oceans, at comparable latitudes, Indian Ocean bottom water-CFC concentrations are lower, consistent with its high dissipation rates from tidal mixing and current fluctuations as shown elsewhere. Thus, the generally high dilutions and low CFC concentrations in bottom water of the Indian Ocean are due to distance to the water mass source regions and the relative effectiveness of mixing. While it is not surprising that at thermocline, intermediate, and bottom levels, the significant ventilation sources on decadal time scales are all from the south, the CFCs show how local sources and mixing within the ocean affect the ventilation. (c) 2007 Elsevier Ltd. All rights reserved.