On the transport and modification of Antarctic Bottom Water in the Vema Channel

The Vema Channel is a deep passage across the Rio Grande Rise in the South Atlantic through which Antarctic Bottom Water (AABW) must flow on its way northward from the Argentine Basin to the Brazil Basin and eventually into the North Atlantic. Both dynamic computation and direct current measurement based on recently acquired data indicate that the volume transport of AABW is about 4 x 10(6) m(3)/sec northward with a standard deviation of about 1.2 x 10(6) m(3)/sec. There are no known exits for AABW below 1 degrees C out of the Brazil Basin and it is estimated by heat flux balance that if AABW leaves this basin across isopycnals, a diffusion rate of 3-4 cm(2)/sec so directed is required. There is a sharp water mass transition between the two basins across the Rio Grande Rise with AABW in the Argentine Basin being distinctively fresher and colder in the potential temperature range from 0.2 degrees C to 2.0 degrees C at the same density. Cold tongues of fresh water advected into the Vema Channel may be thoroughly mixed by lateral eddy diffusion at a rate estimated to be 4 x 10(6) cm(2)/sec. This process demands a supply of Brazil Basin AABW from the north consistent with an observed weak southward flow to the east of the much more intense northward jet. Isopycnals show a reversal in slope with depth within the channel (but not outside) such that the coldest water is in the west at shallow AABW depths but in the east near the. bottom. East of the channel axis there are thick bottom boundary layers which are nearly homogeneous in the vertical but horizontally stratified. We suggest a dynamical, nonmixing, mechanism for producing these features. Dissolved silicate measurements reveal a filament of low concentration, presumably North Atlantic Deep Water, which is located over the channel axis at 2500 m depth. This is some 1000 m above the Rio Grande Rise and 2000 m above the Channel floor.