On the source of Gulf Stream nutrients

Along density surfaces, nutrient concentrations in the Gulf Stream are elevated relative to concentrations to either side of the current. We assess the source of these elevated nutrient concentrations in the western boundary current using historical hydrographic data. The analysis is extended to the separated Gulf Stream with four hydrographic sections recently occupied as part of the Climate Variability and Predictability Program (CLIVAR) Mode Water Dynamics Experiment. The results of this analysis suggest that imported, extrasubtropical waters are the primary source of the elevated nutrient concentrations. Because the high nutrient signature is likely imported, diapycnal mixing need not be invoked to explain the Gulf Stream's high nutrient concentrations, as had been proposed in the past. Moreover, nutrients do not increase along the length of the stream, further suggesting that the stream's high nutrient signature is imported rather than manufactured by processes within the current. The imported nutrients are likely advected into the North Atlantic within the low-salinity water masses that contribute to the shallow limb of the meridional overturning circulation. Thus the availability of nutrients in the North Atlantic may be linked to upstream processes in the tropics and possibly the Southern Hemisphere as well as to variability in the volume of imported water and its distribution in density space.