A Biogeochemical Southern Ocean State Estimate (B‐SOSE) is introduced that includes carbon and oxygen fields as well as nutrient cycles. The state estimate is constrained with observations while maintaining closed budgets and obeying dynamical and thermodynamic balances. Observations from profiling floats, shipboard data, underway measurements, and satellites are used for assimilation. The years 2008–2012 are chosen due to the relative abundance of oxygen observations from Argo floats during this time. The skill of the state estimate at fitting the data is assessed. The agreement is best for fields that are constrained with the most observations, such as surface pCO2 in Drake Passage (44% of the variance captured) and oxygen profiles (over 60% of the variance captured at 200 and 1000 m). The validity of adjoint method optimization for coupled physical‐biogeochemical state estimation is demonstrated with a series of gradient check experiments. The method is shown to be mature and ready to synthesize in situ biogeochemical observations as they become more available. Documenting the B‐SOSE configuration and diagnosing the strengths and weaknesses of the solution informs usage of this product as both a climate baseline and as a way to test hypotheses. Transport of Intermediate Waters across 32°S supplies significant amounts of nitrate to the Atlantic Ocean (5.57 ± 2.94 Tmol yr−1) and Indian Ocean (5.09 ± 3.06 Tmol yr−1), but much less nitrate reaches the Pacific Ocean (1.78 ± 1.91 Tmol yr−1). Estimates of air‐sea carbon dioxide fluxes south of 50°S suggest a mean uptake of 0.18 Pg C/yr for the time period analyzed.

The Weddell Gyre (WG) is one of the main oceanographic features of the Southern Ocean south of the Antarctic Circumpolar Current which plays an influential role in global ocean circulation as well as gas exchange with the atmosphere. We review the state‐of‐the art knowledge concerning the WG from an interdisciplinary perspective, uncovering critical aspects needed to understand this system's role in shaping the future evolution of oceanic heat and carbon uptake over the next decades. The main limitations in our knowledge are related to the conditions in this extreme and remote environment, where the polar night, very low air temperatures, and presence of sea ice year‐round hamper field and remotely sensed measurements. We highlight the importance of winter and under‐ice conditions in the southern WG, the role that new technology will play to overcome present‐day sampling limitations, the importance of the WG connectivity to the low‐latitude oceans and atmosphere, and the expected intensification of the WG circulation as the westerly winds intensify. Greater international cooperation is needed to define key sampling locations that can be visited by any research vessel in the region. Existing transects sampled since the 1980s along the Prime Meridian and along an East‐West section at ~62°S should be maintained with regularity to provide answers to the relevant questions. This approach will provide long‐term data to determine trends and will improve representation of processes for regional, Antarctic‐wide, and global modeling efforts—thereby enhancing predictions of the WG in global ocean circulation and climate.

: The competitive ability of phytoplankton cells is simulated in a model chemostat in which cell size is considered to be an adaptive trait. Parameters describing nutrient uptake kinetics are constrained by empirically derived allometric rules. With a steady input of a single nutrient, the evolutionarily stable cell size is selected through competition among phytoplankton. We find that large cells may be favored when (1) phytoplankton growth is limited by the rate at which internally stored inorganic nutrients can be converted into biomass, and (2) maximum quotas increase with size faster than minimum quotas. Increased internal quotas then accelerate the rate of biomass production in large cells, despite their enhanced requirements for resources. The evolutionarily stable strategy is set by the allometric relationships for nutrient uptake kinetics and by metabolism. Images: Anne W. Thompson (left); Jim Ehrman, Zoe Finkel, Mount Allison Digital Microscopy Lab (right)

We analyze the variability of air‐sea fluxes of carbon dioxide and oxygen in the Southern Ocean during the period 1993–2003 in a biogeochemical and physical simulation of the global ocean. Our results suggest that the nonseasonal variability is primarily driven by changes in entrainment of carbon‐rich, oxygen‐poor waters into the mixed layer during winter convection episodes. The Southern Annular Mode (SAM), known to impact the variability of air‐sea fluxes of carbon dioxide, is also found to affect oxygen fluxes. We find that El Niño–Southern Oscillation (ENSO) also plays an important role in generating interannual variability in air‐sea fluxes of carbon and oxygen. Anomalies driven by SAM and ENSO constitute a significant fraction of the simulated variability; the two climate indices are associated with surface heat fluxes, which control the modeled mixed layer depth variability. We adopt a Lagrangian view of tracers advected along the Antarctic Circumpolar Current (ACC) to highlight the importance of convective mixing in inducing anomalous air‐sea fluxes of carbon dioxide and oxygen. The idealized Lagrangian model captures the principal features of the variability simulated by the more complex model, suggesting that knowledge of entrainment, temperature, and mean geostrophic flow in the mixed layer is sufficient to obtain a first‐order description of the large‐scale variability in air‐sea transfer of soluble gases. Distinct spatial and temporal patterns arise from the different equilibration timescales of the two gases.

The spatial and temporal distributions of sea surface temperature (SST) anomalies in the Antarctic Circumpolar Current (ACC) are investigated, using monthly data from the NCEP-NCAR reanalysis for the period 1980-2004. Patterns of atmospheric forcing are identified in observations of sea level pressure and air-sea heat fluxes. It is found that a significant fraction of SST variability in the ACC can be understood as a linear response to surface forcing by the Southern Annular Mode (SAM) and remote forcing by ENSO. The physical mechanisms rely on the interplay between atmospheric variability and mean advection by the ACC. SAM and ENSO drive a low-level anomalous circulation pattern localized over the South Pacific Ocean, inducing surface heat fluxes and Ekman heat advection anomalies. A simple model of SST propagating in the ACC, forced with heat fluxes estimated from the reanalysis, suggests that surface heat fluxes and Ekman heat advection are equally important in driving the observed SST variability. Further diagnostics indicate that SST anomalies, generated mainly upstream of Drake Passage, are subsequently advected by the ACC and damped after a couple of years. It is suggested that SST variability along the path of the ACC is largely a passive response of the oceanic mixed layer to atmospheric forcing.