North Atlantic Subtropical Mode Water (NASTMW) serves as a major conduit for dissolved carbon to penetrate into the ocean interior by its wintertime outcropping events. Prior research on NASTMW has concentrated on its physical formation and destruction, as well as Lagrangian pathways and timescales of water into and out of NASTMW. In this study, we examine how dissolved inorganic carbon (DIC) concentrations are modified along Lagrangian pathways of NASTMW on subannual timescales. We introduce Lagrangian parcels into a physical-biogeochemical model and release these parcels annually over two decades. For different pathways into, out of, and within NASTMW, we calculate changes in DIC concentrations along the path (ΔDIC), distinguishing contributions from vertical mixing and biogeochemical processes. While the mean ΔDIC for parcels that persist within NASTMW in one year is relatively small at +6 µmol/L, this masks underlying dynamics: individual parcels undergo interspersed DIC depletion and enrichment, spanning several timescales and magnitudes. The strongest ΔDIC is during subduction of water parcels (+101 µmol/L  in one year), followed by transport out of NASTMW due to increases in density in water parcels (+10 µmol/L). Most DIC enrichment and depletion regimes span timescales of weeks, related to phytoplankton blooms. However, mixing and biogeochemical processes often oppose one another at short timescales, so the largest net DIC changes occur at timescales of more than 30 days. Our new Lagrangian approach complements bulk Eulerian approaches, which average out this underlying complexity, and is relevant to other biogeochemical studies, for example on marine carbon dioxide removal.

The Southern Ocean plays a critical role in the uptake, transport and storage of carbon by the global oceans. It is the ocean’s largest sink of CO2, yet it is also among the regions with the lowest storage of anthropogenic carbon. This behaviour results from the unique combination of high winds driving the upwelling of deep waters and the subduction and northwards transport of surface carbon. Here we identify the indirect effect of climate-related changes in ocean conditions relative to the direct effect of anthropogenic changes in atmospheric CO2 on the reorganisation of carbon in the Southern Ocean using a combination of modelling and observations. We show that the effect of climate variability and climate change on the storage of carbon in the Southern Ocean is nearly as large as the effect of anthropogenic CO2 during the period 1998-2018 compared with a climatology around the year 1995. We identify a distinct climate fingerprint in dissolved inorganic carbon (DIC), with elevated DIC concentration in the surface ocean that reinforces the anthropogenic CO2 signal, and reduced DIC concentration in the subsurface ocean that offsets the anthropogenic CO2 signal. The fingerprint is strongest at lower latitudes (30°S-55°S). This fingerprint could serve to monitor the highly uncertain evolution of carbon within this critical ocean basin, and better identify its drivers.

The oceans are a major carbon sink. Sea surface temperature (SST) is a crucial variable in the calculation of the air-sea carbon dioxide (CO2;) flux from surface observations. Any bias in the SST or any upper ocean vertical temperature gradient (e.g., the cool skin effect) potentially generates a bias in the CO2 flux estimates. A recent study suggested a substantial increase (~50% or ~0.9 Pg C yr-1) in the global ocean CO2 uptake due to this temperature effect. Here, we use a gold standard buoy SST dataset as the reference to assess the accuracy of in-situ SST used for flux calculation. A physical model is then used to estimate the cool skin effect, which varies with latitude. The bias-corrected SST (assessed by buoy SST) coupled with the physics-based cool skin correction increases the average ocean CO2 uptake by ~35% (0.6 Pg C yr-1) for 1982 to 2020, which is significantly smaller than the previous correction. After these temperature considerations, we estimate an average net ocean CO2 uptake of 2.2 +- 0.4 Pg C yr-1 for 1994 to 2007 based on an ensemble of surface observation-based flux estimates, in line with the independent interior ocean carbon storage estimate corrected for the river induced natural outgassing flux (2.1 +- 0.4 Pg C yr-1).