Frontal structure, water masses, and circulation in the Crozet Basin

Due to topographic steering by the Crozet and Kerguelen Plateaus, the Antarctic Circumpolar Current (ACC) core in the Crozet Basin area is shifted to its northernmost position in the Southern Ocean, along the southern limb of the South Indian subtropical gyre. Here the jet‐like current is embedded within a narrow band ( 26.5 kg/m 3 ) in the western half of the basin and heavier mode (γ θ < 26.7 kg/m 3 ) in its eastern half. These different varieties of Crozet Basin Mode Water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone. The area north of the Kerguelen Plateau appears to be an important source of water mass modification. Antarctic Intermediate Water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher, oxygen‐richer, colder subantarctic water. Energetic eddy activities observed along the frontal zone likely enhance such cross‐frontal exchange of water masses. Equatorward spreading of the modified thermocline water masses (Mode Water, Antarctic Intermediate Water) occurs as part of the anticyclonic circulation in the South Indian subtropical gyre which is centered within the basin. Strong injection of North Indian Deep Water into the ACC south of Amsterdam Island is clearly evidenced for the first time. This deep oxygen minimum water centered at about 3000 m can be traced to the east of Madagascar, from where it spreads southward through the eastern part of the Crozet Basin. Due to the influence of this water mass, North Atlantic Deep Water characteristics of the Circumpolar Deep Water are significantly altered in Kerguelen‐Amsterdam passage. Our results indicate a cyclonic deep‐level circulation pattern in the Crozet Basin, with a northward flow (9 Sv) of Circumpolar Deep Water / Antarctic Bottom Water as a deep western boundary current and a partially compensating southward flow (3 Sv) of North Indian Deep Water through the eastern part of the basin. It is concluded that the Crozet Basin constitutes an important passage for the exchange of water masses between the Southern Ocean and the Indian Ocean.