On the Role of the Gulf Stream in the Changing Atlantic Nutrient Circulation During the 21st Century

The Gulf Stream is part of the upper limb of the Atlantic meridional overturning circulation (AMOC) and the western boundary current of the North Atlantic sub­ tropical gyre. The Gulf Stream is also a nutrient stream. It transports macronutrients (nitrate, phosphate, silicate) necessary for marine phytoplankton growth along the eastern continental margin of the United States from the Straits of Florida to Cape Hatteras at globally significant rates. At Cape Hatteras, the Gulf Stream separates from the coast and carries its nutrients to the northeast off the continental slope and into deep water. There, waters of recent tropical, subtropical and subpolar origins con­ verge and both the volume and nutrient transport increase in a great junction of the global ocean circulation. Further east, near the Grand Banks of Newfoundland, the waters and nutrients of the Gulf Stream diverge again into the subtropical and subpolar gyres. As nutrients and water move northward, they rise along sloping isopycnals and are eventually advected into the surface mixed layer or entrained by seasonal mixed‐layer deepening. This upwards nutrient flux is compensated for by downwards nutrient fluxes associated with physical and biogeochemical processes. A significant fraction of the inorganic nutrient entering the mixed layer is trans­ formed directly to denser North Atlantic Deep Water and sinks as the incoming water loses heat to the atmosphere. However, another significant fraction of the inor­ ganic nutrient entering the mixed layer is converted to organic form by phytoplankton. A fraction of this organic nutrient sinks via particles to denser water, where it is remineralized, and the other fraction of the organic nutrient (i.e., the nonsinking dissolved part) is transformed On the Role of the Gulf Stream in the Changing Atlantic Nutrient Circulation During the 21st Century