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. 5°S, 10°S, and 28°S. 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 0.1×106 m3 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 6.7×106 m3 s−1 westward at Lats. 5°S and 10°S, respectively, and 1.8×106 m3 s−1 eastward at Lat. 28°S. As in the southward flow through Denmark Strait, density stratification explains the splitting of westward currents at 5°S and 10°S 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 28°S is less certain, but might be related to the particular geometry of the West Australian Basin.