The Gulf Stream control of the North Atlantic carbon sink

We are all aware that atmospheric CO2 has risen over our lifetimes leading to global warming. The ocean has played an important role in moderating that atmospheric rise by taking up and storing 25% of the emitted carbon. However, the extent to which the ocean will continue to act in this manner as the ocean warms and becomes more acidic is unclear, as is the response in the future as we reduce carbon emissions to net zero in the next 20-30 years. A key region in addressing this question is the North Atlantic, which is disproportionately important for ocean carbon uptake. This carbon sink involves both the uptake of natural carbon (due to surface cooling and biological uptake) and of anthropogenic carbon (due to the rise in atmospheric CO2). A prevailing view is that this carbon sink will weaken in the future as surface warming decreases solubility and increases stratification, which inhibits the supply of nutrients and carbon to the surface ocean. However, this viewpoint takes a local perspective and does not account for the effect of the circulation in redistributing nutrients and carbon over the global ocean.

We wish to propose and test the alternative viewpoint that the circulation plays a central role in determining the carbon sink, by setting the supply of nutrients and carbon to the surface waters of the North Atlantic. In particular, there is a phenomenon - the western boundary current or Gulf Stream - that is crucial for this problem. We know that the Gulf Stream is important for supplying heat to higher latitudes, leading to a warmer European climate. However, its role in driving carbon uptake remains little explored. Surface observations show that there are elevated rates of carbon uptake downstream of the Gulf Stream. This uptake occurs as older waters carried below the surface by the Gulf Stream are transferred downstream to the surface. These older waters are rich in nutrients and depleted in anthropogenic carbon. When these waters outcrop to the surface, they determine the surface nutrient and carbon concentrations, and so control the carbon uptake from the atmosphere. How much carbon uptake is driven by this nutrient and carbon 'stream' in the North Atlantic depends on multiple climate-sensitive processes, including the density range of the stream, the Gulf Stream transport, and a suite of physical and biogeochemical processes occurring along its path.

We will use observations and models to comprehensively understand this pivotal phenomenon, distinguishing between several different mechanisms that transform the fluxes of properties at the beginning of the Gulf Stream to those entering the North Atlantic. We will make new measurements of how the Gulf Stream supply of nutrients and carbon varies all the way from Florida Straits to a carbon uptake hotspot downstream, a distance of over 2000 miles. We will employ moorings in Florida Straits to determine the nutrient and carbon properties at the start of the Gulf Stream. We will deploy a fleet of BioArgo floats and gliders to reveal how nutrients and carbon are conveyed from low to high latitudes, documenting their downstream evolution through the effects of physical transport, mixing and biological cycling. Our work programme sits between two ongoing observing arrays of the Atlantic meridional overturning circulation, RAPID at 26N and OSNAP between Labrador and Scotland, and these arrays place our observations in a wider context. We will test our ideas using experiments in circulation models, including assessing the sensitivity of the North Atlantic carbon sink to physical processes. Finally, we will evaluate how the carbon sink varies in climate model projections and establish whether the models' responses occur for the right reasons.

Unravelling these controls of the ocean carbon sink is crucial if we are to understand and credibly predict the future evolution of the carbon sink, especially given the uncertain ocean response to net zero emissions.