Preprints
https://doi.org/10.5194/egusphere-2025-4425
https://doi.org/10.5194/egusphere-2025-4425
09 Oct 2025
 | 09 Oct 2025
Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Seasonal to long-term variability of natural and anthropogenic carbon concentrations and transports in the subpolar North Atlantic Ocean

Raphaël Bajon, Lidia I. Carracedo, Herlé Mercier, Rémy Asselot, and Fiz F. Pérez

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is integral to the climate system, transporting heat and anthropogenic carbon across the North Atlantic (NA) from subtropical to subpolar latitudes. This physical mechanism promotes the uptake and sequestration of atmospheric CO2 through surface cooling as warm water advances northward and consequently sinks through deep winter convection. Using ship-based observations, ocean reanalyses, neural networks, and a back-calculation approach, we present a 30-year monthly time series of contemporary carbon (natural, Cnat and anthropogenic, Cant) concentrations and transports at the A25-OVIDE hydrographic section in the subpolar NA Ocean, and assess their variability from seasonal to long-term scales. We divided the section into essential layers, including the upper branch of the AMOC (uMOC) and the mixed layer (ML). Our findings indicate that the full-section-averaged Cnat concentration shows no significant trend over the 30-year period. In contrast, the full-section-averaged Cant concentration increased by more than one third over the 30-year period, attributed to anthropogenic influences and atmospheric CO2 increase. Seasonal and interannual variability is more pronounced in the uMOC and in the ML, where deep convection and biological activity impact their concentration. The seasonal deepening of the ML in winter contributes two thirds and one half of its ML concentration for Cnat and Cant, respectively, the rest being attributed to biology and solubility. The Cant and Cnat transports are predominantly determined by the variability of volume transport, except for the decadal trend in Cant transport which is primarily influenced by changes in Cant concentration. The variability in tracer transport is the largest in the uMOC, which exhibits a seasonal peak-to-peak amplitude of approximately 25 % of the annual mean tracer transport. These results offer new insights to refine model representations and improve our understanding of the subpolar NA carbon dynamics.

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Raphaël Bajon, Lidia I. Carracedo, Herlé Mercier, Rémy Asselot, and Fiz F. Pérez

Status: open (until 20 Nov 2025)

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Raphaël Bajon, Lidia I. Carracedo, Herlé Mercier, Rémy Asselot, and Fiz F. Pérez
Raphaël Bajon, Lidia I. Carracedo, Herlé Mercier, Rémy Asselot, and Fiz F. Pérez
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Short summary
We analyzed three decades of carbon concentration and transport in the subpolar gyre of the North Atlantic Ocean, a key region in ocean carbon cycle. Natural carbon remained stable, but human-derived carbon rose by over 30 %. Upper layers exhibit strong seasonal to interannual variability driven by physical and biological processes. Carbon transport was tightly linked to circulation patterns. In the longterm, transport of human-derived carbon is proportional to the increase in its concentration.
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