The Ekman transfer of nutrients and maintenance of new production over the North Atlantic

Abstract

The maintenance of new production requires a supply of nutrients to the euphotic zone to offset the loss through biological export. The dynamical supply of nutrients is usually discussed in terms of the vertical transfer from nutrient-rich, deep waters. However, the horizontal transfer is important in regions of downwelling over subtropical gyres, where nutrients may be transported across the intergyre boundaries by the surface Ekman drift or geostrophic eddies from the neighbouring nutrient-rich, upwelling regions. The Ekman transfer of nitrate to the euphotic layer is diagnosed from climatology over the North Atlantic. The vertical Ekman supply of nitrate is found to be significant over the subpolar gyre, the tropics and eastern boundary, whereas the horizontal transfer is found to be dominant at the intergyre boundaries. On the northern flank of the subtropical gyre, the Ekman transfer provides a source of nitrate from 0.03 to 0.06 mol N m -2 yr -1, corresponding to a contribution to new production of between 0.4 and 0.8 mol C m -2 yr -1. This estimate represents a significant fraction of the total new production of typically 1 mol C m -2 yr -1 suggested by both remote chlorophyll and sediment trap observations. A simplified nitrogen cycle model is used to assess the role of the Ekman supply over the North Atlantic. In the model the Ekman supply of nitrate leads to a plume of nitrate and enhanced productivity extending up to 1000 km into the subtropical gyre from the intergyre boundaries. This lateral scale is controlled by the seasonal cycle of the mixed layer and the remineralisation of the particulate organic fallout.