Flow of Bottom and Deep Water in the Amirante Passage and Mascarene Basin

Abstract. In the Indian Ocean the Amirante Passage is the sill through which rel-atively cold, fresh, oxygen-rich, and nutrient-poor bottom water spreads northward into the Somali Basin from the Mascarene Basin. The passage is also a conduit through which relatively warm, salty, oxygen-poor, and nutrient-rich deep water spreads south. Previous estimates for northward transport of bottom water in the passage have been made from station pairs and sections without benefit of tracer measurements. Previous estimates of southward transport of deep water are scarce. Three hydrographic sections were made across the passage in 1995 and 1996 as part of the World Ocean Circulation Experiment (WOCE). Two WOCE sections were also made perpendicular to the western boundary in the Mascarene Basin, just south of the passage. The geostrophic shear field is used with the salinity, dissolved oxygen, and silica distributions to select a range of zero-velocity surfaces (ZVSs) on potential isotherms from 1.0 ø to 1.1øC (hence a range of geostrophic transports) for which the flow direction is consistent with the tracer distributions. Objective mapping is used to obtain flux estimates below the deepest common level of station pairs. Estimates in the Mascarene Basin result in a bottom water volume transport from 2.5 to 3.8 x 10 • m 3 s-1 northward toward the passage below the ZVSs and a deep-water transport between the ZVSs and 2.5øC from 11.6 to 6.4 x 10 • m a s- • southward. Estimates within the passage result in transports from 1.0 to 1.7 x 10 • m a s- • northward for the bottom water and from 8.6 to 3.8 x 10 • m 3 s- • southward for the deep water. 1