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

Abstract

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 (< 200 km) of the frontal zone which marks a sharp transition between subtropical and subantarctic waters, hugging the northern flanks of the Crozet and Kerguelen Plateaus. Adding to this topographic control, the confluence of the ACC and the Agulhas Return Current north of Crozet further intensifies downstream baroclinic shear strengths. Eastward transport associated with the Agulhas Return Current at the entrance of the Crozet Basin at 53°E is estimated as 35 Sv (1 Sv = 106 m3/s), most of which recirculating northward into the northern part of the basin before reaching Kerguelen‐Amsterdam passage. About 75% of the ACC transport through Drake Passage pass through Kerguelen‐Amsterdam passage, the quasi‐totality of which being concentrated in the frontal zone composed of two merged fronts, the Subtropical and Subantarctic Fronts; the rest of the current passes south of Kerguelen. The Polar Front in this Crozet Basin area is not associated with any noticeable baroclinic shear and represents only a minor portion (5–7 Sv) of the ACC, in great contrast to that observed in the other sectors of the Southern Ocean. The Crozet Basin provides an important source for the formation and modification of water masses. Crozet Basin Mode Water formed locally just north of the ACC is of subtropical origin but shows significant zonal anomalies in its characteristics, with lighter mode (γθ > 26.5 kg/m3) in the western half of the basin and heavier mode (γθ < 26.7 kg/m3) 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.