The biogeochemical transport by the Gulf Stream.

The Gulf Stream is important for the climate system through its transport and air-sea exchange of heat. What is less well accepted is the role of the Gulf Stream in the carbon cycle. Here we examine how the Gulf Stream provides a "biogeochemical stream", a sub-surface horizontal flux carrying waters with high concentrations of nutrients and low concentrations of anthropogenic carbon. Model experiments reveal particles released in dense layers at the start of the Gulf Stream follow trajectories extending into the subpolar gyre, while particles released at the surface are confined to the subtropics. Following a pathway to the subpolar gyre, the biogeochemical stream carries older, nutrient-rich and anthropogenically carbon-depleted waters along density layers and, when those dense layers outcrop into the mixed layer, enhances the subpolar drawdown of atmospheric carbon. This connectivity is supported by model sensitivity experiments revealing the subpolar upper ocean carbon content and upstream dense waters in the Gulf Stream connecting on timescales of 4 to 8 years. The likely effect of climate change on the biogeochemical stream is a decrease in the delivery of these older waters, both high in concentrations of nutrients and depleted in anthropogenic carbon, to the subpolar mixed layer, so weakening future North Atlantic carbon uptake from the atmosphere.