Branching of Antarctic Bottom Water Flow in the Middle Part of the Vema Channel Revealed by Direct Velocity Measurements

Abstract

The Antarctic Bottom Water flow in the Vema Channel of the Southwest Atlantic is one of the strongest abyssal flows in the World Ocean. The spatial structure of abyssal waters in the channel has been previously studied by different authors only at two locations: at the Vema Sill in the southern part of the channel and at the Vema Extension in its northern part. Here we report new measurements that reveal an additional, shallower branch of the current in its middle part. These new observations show that two individual current branches are observed in the middle, previously unexplored part of the Vema Channel at approximately 28°S. Repeated measurements have also revealed strong simultaneous variations in thermohaline properties and bottom current direction in the shallower branch, indicating that abyssal waters transported to the south differ from those transported to the north due to differences in mixing between water masses in the channel. The new temperature observations show almost the same mean warming rates in different regions of the Vema Channel varying from 1.8 to 2.4 mK/year during the last two decades. New observations show that both studied branches transport the Antarctic Bottom Water between the Argentine and Brazil basins; moreover, the additional shallow branch changes its direction and the transport here can be directed both northward and southward.