Quantifying nutrient supply to the Southern Ocean

Using nutrient concentrations on neutral density surfaces in the Southern Ocean and climatological winds, both of which are fairly well constrained, we have estimated the long‐term average of net export from the upper ocean (i.e., the net supply of nutrients in Circumpolar Deep Water and Antarctic Intermediate Water to the surface layer) over the whole of the Southern Ocean south of the wind stress maximum at an average latitude of 50°S. We calculate net new production (equivalent to export production) values of 51 ± 3 Tmol Si yr −1 and 14 ± 3 Tmol N yr −1 . The latter is equivalent to 1.1 ± 0.2 Pg C yr −1 when scaled with the Redfield ratio of C to N. These values are in good agreement with recent observational and modeling estimates and are reasonably in line with inverse model calculations. Interpolating the high‐quality data from meridional World Ocean Circulation Experiment sections onto the neutral density surfaces revealed remarkable constancy of nutrient concentrations at all longitudes and depths below about 340 ± 100 m, which we call the surface‐influenced depth. This indicates that isopycnic stirring by eddies in the Southern Ocean is efficient at homogenizing nutrient concentrations and removing any signature of remineralization. The large depth of the surface‐influenced depth, over twice the winter mixed layer depth, also indicates that eddies must be responsible for transferring nutrient deficits resulting from drawdown in the surface layer across the pycnocline to several hundred meters deep.