The subpolar North Atlantic (SPNA) is one of the few regions where the deep ocean is in direct contact with the atmosphere, making it a key location for interior ocean ventilation through gas exchange. We use observational data to analyze large-scale patterns of mean annual air-sea flux, biological production and consumption, and physical transport of oxygen for the subpolar North Atlantic Ocean (45N-65N), finding a net annual flux of 48.1±14.6 Tmol (1Tmol = 10^12 mol) of oxygen from the atmosphere into the ocean, largely balanced by a removal of oxygen through physical transport. Wintertime increases in oxygen content in isopycnal layers match the location and magnitude of net oxygen uptake from the atmosphere, supporting the connection between air-sea gas exchange at the surface and ventilation of deeper layers. Integrated over the whole SPNA, 90% of the net oxygen influx and 80% of the seasonal oxygen content increase occur at densities of σ0 < 27.6 kg m-3, in the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). The subpolar gyre (SPG) is ventilated with oxygen largely at these lower densities, accumulating oxygen along its cyclonic pathway from the North Atlantic Current towards the Labrador Sea. Our results thus suggest that the subpolar gyre is oxygenated cumulatively throughout the SPNA, as mode waters formed each winter become progressively denser and more oxygenated along the SPG’s path, culminating in the oxygen-rich Labrador Sea Water which is ultimately exported to the rest of the ocean in the lower branch of the AMOC.

Oceanic macroturbulence is efficient at stirring and transporting tracers. The dynamical properties of this stirring can be characterized by statistically quantifying tracer structures. Here, we characterize the macroscale (1-100 km) tracer structures observed by two Seagliders downstream of the Southwest Indian Ridge (SWIR) in the Antarctic Circumpolar Current (ACC). These are some of the first glider observations in an energetic standing meander of the ACC, regions associated with enhanced ventilation. The small-scale density variance in the mixed layer (ML) was relatively enhanced near the surface and base of the ML, while being muted in the middle, suggesting the formation mechanism to be associated to ML instabilities and eddies. In addition, ML density fronts were formed by comparable contributions from temperature and salinity gradients, suggesting the dominant role of stirring, over air-sea interactions, in their formation and sustainability. In the interior, along-isopycnal spectra and structure functions of spice indicated that there is relatively lower variance at smaller scales than would be expected based on non-local stirring, suggesting that flows smaller than the deformation radius play a role in the cascade of tracers to small scales. These interior spice anomalies spanned across isopycnals, and were found to be about 3-5 times flatter than the aspect ratio that would be expected for O(1) Burger number flows like interior QG dynamics, suggesting the ratio of vertical shear to horizontal strain is greater than $N/f$. This further supports that small-scale flows, with high-mode vertical structures, stir tracers and impact tracer distributions.

Despite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high-latitude Southern Ocean. This is due in part to an historical paucity of observations in this remote, turbulent, and seasonally ice-covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different frontal zones of the Antarctic Circumpolar Current (ACC)—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon-rich waters at the base of the mixed layer supplies DIC which is then, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern ACC, which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind-driven transport of DIC from the SIZ. These results stem from the first observation-based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region.

Global estimates of mesoscale vertical velocity remain poorly constrained due to a historical lack of adequate observations on the spatial and temporal scales needed to measure these small magnitude velocities. However, with the wide-spread and frequent observations collected by the Argo array of autonomous profiling floats, we can now better quantify mesoscale vertical velocities throughout the global ocean. We use the underutilized trajectory data from the Argo array to estimate the time evolution of isotherm displacement around a float as it drifts at 1000 dbar, allowing us to quantify vertical velocity averaged over approximately 4.5 days for that pressure level. The resulting estimates have a non-normal, high-peak, and heavy-tail distribution. The vertical velocity distribution has a mean value of (1.9±0.02)×10-6 m s-1 and a median value of (1.3± 0.2)×10-7 m s-1, but the high-magnitude events can be up to the order of 10−4 m s-1 , We find that vertical velocity is highly spatially variable and is largely associated with a combination of topographic features and horizontal flow. These are some of the first observational estimates of mesoscale vertical velocity to be taken across such large swaths of the ocean without assumptions of uniformity or reliance on horizontal divergence.