# The biogeochemical transport by the Gulf Stream

## TL;DR

The Gulf Stream carries nutrient-rich, low-anthropogenic carbon water to the subpolar North Atlantic, enhancing carbon uptake from the atmosphere.

## Contribution

The paper introduces the concept of a 'biogeochemical stream' within the Gulf Stream and its role in the carbon cycle.

## Key findings

- Particles in dense layers of the Gulf Stream reach the subpolar gyre, enhancing carbon drawdown.
- Model experiments show a 4 to 8-year timescale for connecting upstream dense waters to the subpolar region.
- Climate change may reduce the delivery of nutrient-rich, carbon-depleted waters to the subpolar mixed layer.

## Abstract

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.

The Gulf Stream provides a sub-surface horizontal flux carrying high concentrations of nutrients and low concentrations of anthropogenic carbon affecting the subpolar North Atlantic carbon, according to data analyses, model and adjoint simulations.