Measured and modeled CFC distribution of lower North Atlantic Deep Water in the Guiana Basin

Abstract

The circulation of the Overflow Lower North Atlantic Deep Water (OLNADW) in the Guiana Basin is studied by the measured and modeled distribution of the chlorofluorocarbon component CFC-11. The OLNADW is formed in the northern Atlantic and transported south to the equatorial region as the densest part of the Deep Western Boundary Current (DWBC), characterized by CFC and oxygen maxima. Some former studies proposed a northwestward recirculation of the boundary current in the Guiana Basin, because the measured southeastward DWBC transport was larger than the calculated net transport across the entire width of the basin. Despite the variability of the DWBC reported recently, this discrepancy seems to hold. An advective-diffusive model is presented, which includes recirculation in the northern Guiana Basin as well as horizontal diffusion, and the results are compared with a CFC survey in March 1994. Because of high CFC concentration in the boundary current one would expect to find a high CFC signal in the recirculated water mass near the Mid-Atlantic Ridge as well. However, this was not observed. Modeled CFC distributions and observations only agree if the presumed recirculation is located in the center of the northern Guiana Basin or if no westward recirculation exists at all. It was found that the strength of the recirculation is about 3±1.5 Sv, which is smaller than previously supposed. Copyright 1998 by the American Geophysical Union.