Antarctic Bottom Water in the Vema Fracture Zone

A section of 46 CTD/Lowered Acoustic Doppler Current Profiler stations along the Vema Fracture Zone (VFZ) was made for the first time with a repeat of the transverse section on the main sill (at 41 degrees W) for the sixth time. Potential temperature of Antarctic Bottom Water when it flows into the fracture increases from 1.3 degrees C to 1.6 degrees C. This temperature increase is much smaller than in the Romanche Fracture Zone because in the VFZ there are not many transverse sills that force the flow to become turbulent. A zone with minimal velocities of the bottom flows exists in the western part of the VFZ up to the main sill. A current with velocities of 0.25 m/s flows above the low velocity zone at a depth of about 4,000 m. Underwater spillways with hydraulically controlled flow of bottom water down the slope from 4,500 m to 5,000 m at speeds up to 0.40 m/s were found east of the sill (in the central and southern channels). The bottom current accelerates as it flows down, but then its kinetic energy decreases. The bottom current slows down and mixes with surrounding waters. A thin (similar to 30 m) bottom flow descends further to the deep depression. In the southern channel of the VFZ, bottom water flows into a deep (5,400 m) basin. The coldest and densest water reaches this depression during rare inflows. Plain Language Summary Antarctic Bottom Water formed on Antarctic slopes slowly flows northward. The flow passes many deep basins and abyssal channels between them until it reaches the Vema Fracture Zone (VFZ). The VFZ is the main conduit in the Mid-Atlantic Ridge for the bottom water flow to the Northeast Atlantic; this flow fills the abyssal basins of the Northeast Atlantic. Unlike the Romanche and Chain fracture zones near the equator, Antarctic Bottom Water only slightly warms here while flowing through the fracture because there are not many transverse sills that can prevent the bottom flow and induce mixing with North Atlantic Deep Water occupying the overlying water. The fastest current flows over a low velocity zone with low friction. After overflowing the main sill, the water descends from 4,500 m to 5,000 m as a spillway, in which the flow partly mixes with the surrounding waters.