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

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.