Intra-annual variability of carbon signature and transport in the North Atlantic Ocean

The ocean is the largest carbon reservoir on Earth, and a major sink for the excess of CO₂ (anthropogenic carbon) emitted to the atmosphere by human activities. Having removed about a quater of these emissions since the beginning of the industrial era, ocean’s key role in climate is particularty outstanding in the North Atlantic (NA). A combination of physical and biological processes makes the NA a key-role region for the natural and anthropogenic carbon uptake and storage, and hence for the global carbon cycle. Traditionally, the seasonal carbon cycle has been assumed to respond to natural variability, unnafected by the ongoing anthropogenic increase of atmospheric CO₂. Recent model projections, however, point otherwise, yet observational evidence to verify these predictions is still missing. Here we examine seasonal cycle in dissolved inorganic carbon (DIC) and its (surface-2000 dbar) transport, estimated using in-situ data and neural networks, across the OVIDE (GO-SHIP A25) section, from 1993 to 2021 at a monthly resolution. Our results highlight that changes in temperature, dissolved oxygen and ocean circulation are key components driving the seasonal DIC variability. DIC concentrations are higher in years with strong winter mixing regimes (which bring more nutrient-rich waters to the surface, favouring photosynthesis, and more (remineralized) carbon back to the surface). Seasonal DIC transport fluctuations are found significant compared to the mean (e.g. +/- 25% in the upper branch of the meridional overturning circulation), putting into relevance that caution is needed if assuming that single-cruise occupations are representative of the annual state. We also observe a yearly variant seasonal imbalance, with a significant reduction over the past two decades in the upper branch of the meridional overturning circulation. These results underscore the importance of considering intra-annual variability in the North Atlantic's carbon cycle when addressing climate change.