Estimating the convective supply of nitrate and implied variability in export production over the North Atlantic

The maintenance and interannual variability of export production is partly controlled by the convective and Ekman supply of nutrients to the euphotic zone. This dynamical supply of nitrate to the euphotic zone is estimated over the North Atlantic from 1968 to 1993. First, a matrix of one-dimensional mixed-layer models are integrated over the North Atlantic forced by surface and Ekman fluxes of heat and freshwater. Second, nitrate fluxes to the euphotic zone are estimated by combining the mixed-layer thickness cycle and wind stress data with climatological nitrate profiles. The model suggests that the convective supply of nitrate ranges from 0.1 mol N m(-2)yr(-1) in the subtropics to 1.4 mol N m(-2)yr(-1) over the subpolar gyre. The interannual variability in convective supply reaches +/-0.2 mol N m(-2)yr(-1) This variability in nitrate supply is significantly correlated with the North Atlantic Oscillation index over parts of the central and western Atlantic but not over the eastern Atlantic. This convectively induced variability should modulate the levels of export production wherever the nitrate supply is limiting. The Ekman transfer is relatively small, reaching 0.1 mol N m(-2)yr(-1) over the subpolar gyre, but is important in maintaining nitrate concentrations within the seasonal boundary layer.