The Maintenance of High Primary Production in the Absence of Ekman Upwelling: The Supply of Nutrients to the Intergyre North Atlantic

Ekman suction and pumping are often invoked to explain the observed difference in primary production and chlorophyll a between the North Atlantic subpolar and subtropical gyres. Between the gyres, the intergyre region can be loosely defined by its lack of a strong Ekman suction or pumping of nutrients. Despite the lack of a strong Ekman supply of nutrients, the mean seasonal cycle in chlorophyll a in the intergyre is remarkably similar to that seen in the subpolar gyre. In this thesis, we present research on mechanisms for nutrient supply to the intergyre that might support its high production. Using biogeochemical and physical reanalysis ocean data products, a nutrient budget is constructed for a region in the eastern North Atlantic, within the intergyre. Analysis of this budget shows that the seasonal entrainment flux, resultant from the movement of the mixed layer across vertical nutrient gradients, is responsible for the majority of the nutrient supply to the region. Hydrographic ocean data and particle trajectories run in a numerical model are then used to show that the waters seasonally entrained into the intergyre mixed layer likely originate in the Gulf Stream, linking nutrient supply in the North Atlantic intergyre to high downstream nutrient fluxes observed in the Gulf Stream.