The overflows across the Ninetyeast Ridge

Water deeper than, say, 3800 m in the Central Indian Basin comes from the West Australian Basin across deep saddles on the Ninetyeast Ridge near Lats. 5degreesS, 10degreesS. and 28degreesS. The WOCE Hydrographic Program in the Indian Ocean (1994-1996) provided opportunities for more detailed measurements of water properties in these overflows than any made earlier. With zero-velocity surfaces inferred from contrasts in oxygen and silicic-acid concentrations and from distributions of specific-volume anomaly, geostrophic estimates of these overflow rates are, respectively, 1.0, 1.1, and 0.1 x 10(6) m(3) s(-1) (westward). These constructions also imply strong zonal flows of mid-depth water (between 2000 db and the zero-velocity surfaces) through the passages: 2.2 and 6.7 x 10(6) m(3) s(-1) westward at Lats. 5degreesS and 10degreesS, respectively, and 1.8 x 10(6) m(3) s(-1) eastward at Lat. 28degreesS. As in the southward flow through Denmark Strait, density stratification explains the splitting of westward currents at 5degreesS and 10degreesS into mid-depth and deep flows as they pass over their saddles (but not why such vertically extended flows should exist in the first place). The cause of opposed eastward flow at 28degreesS is less certain, but might be related to the particular geometry of the West Australian Basin. (C) 2002 Elsevier Science Ltd. All rights reserved.