The flux and mixing rates of Antarctic bottom water within the North Atlantic

Abstract

Calculation was made of the northward flow of Antarctic Bottom Water into the western North Atlantic Basin through a passageway approximately 300 km wide between the Ceara Rise and the Mid‐Atlantic Ridge at about 4°N. Two moorings carrying four current meters each were placed in the region where currents were expected to be the greatest and were left for 360 days. The current was not steady, but exhibited large surges with approximately a 60‐day time scale whose cause is unknown. Current meters in the fastest flowing region all exhibited a net northward flow. It is estimated that approximately 0.8×10 6 m 3 s −1 of water colder than 1.9°C (potential temperature) flowed on the average into the North Atlantic, with this number possibly being too small by 0.3×10 6 m 3 s −1 or too big by 0.1×10 6 m 3 s −1 depending upon estimates of the width of the region, which the current meters sample. However, these numbers differ from geostrophic estimates of 1.98×10 6 m 3 s −1 , the disagreement coming almost entirely from the fact that the geostrophic estimates give a sizeable flux of water colder than 1.2°C while the current meters do not. Based upon the two different flux rates, residence times and mixing coefficients are calculated for Antarctic Bottom Water for each 0.1°C potential temperature interval from 1.0°C to 1.9°C. Finally, some comments on the dynamics of the northward flow are made.