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The Southern Ocean plays an important role in the global oceanic uptake of CO2. Estimates of the air-sea CO2 flux are made using the partial pressure of CO2 at the sea surface (), but winter observations of the region historically have been sparse, with almost no coverage in the Pacific or Indian ocean sectors south of the Polar front in the period 2004–2017. Here, we use summertime observations of relevant properties in this region to identify subsurface waters that were last in contact with the atmosphere in the preceding winter, and then reconstruct “pseudo observations” of the wintertime . These greatly improve wintertime coverage south of the Polar Front in all sectors, improving the robustness of flux estimates there. We add the pseudo observations to other available observations of and use a multiple linear regression to produce a gap-filled time-evolving estimate of from which we calculate the air-sea flux. The inclusion of the pseudo observations increases outgassing at the beginning of the period, but the effect reduces with time. We estimate a 2004–2017 long-term mean flux of −0.02 ± 0.02 Pg C yr−1 for the Southern Ocean south of the Polar Front, similar to comparable studies based on shipboard data. However, we diverge somewhat from an estimate which utilized autonomous float data for recent years: we find a small sink in 2017 of −0.08 ± 0.03 Pg C yr−1 where the float-based estimate suggested a small source. Plain Language Summary The Southern Ocean is an important region where carbon dioxide (CO2) gets absorbed into the ocean, however, the observations that allow us to calculate the flux are lacking. Estimates of the atmosphere-ocean flux of CO2 rely on observations of surface CO2 concentrations collected on board ships, which are especially sparse in the winter and in the most southerly parts of the Southern Ocean. In this study, we have used observations from below the surface taken in summertime to reconstruct estimates of the wintertime surface CO2

The strength of the Meridional Overturning Circulation (MOC) in the North Atlantic is dependent upon the formation of dense waters that occurs at high northern latitudes. Wintertime deep convection in the Labrador and Irminger Seas forms the intermediate water mass known as Labrador Sea Water (LSW). Changes in the rate of formation and subsequent export of LSW are thought to play a role in MOC variability, but formation rates are uncertain and the link between formation and export is complex. We present the first observation-based application of a recently developed Regional Thermohaline Inverse Method (RTHIM) to a region encompassing the Arctic and North Atlantic subpolar gyre for the years 2013, 2014 and 2015. RTHIM is a novel method which can diagnose the formation and export rates of water masses such as the LSW identified by their temperature and salinity, apportioning the formation rates into contributions from surface fluxes and interior mixing. We find LSW formation rates of up to 12 Sv during 2014-15, a period of strong wintertime convection, and around half that value during 2013 when convection was weak. We also show that the newly convected water is not exported directly, but instead is mixed isopycnally with warm, salty waters that have been advected into the region, before the products are then exported. RTHIM solutions for 2015 volume, heat and freshwater transports are compared with observations from a mooring array deployed for the Overturning in the Subpolar North Atlantic Program and show good agreement, lending validity to our results.

The strength of the Meridional Overturning Circulation (MOC) in the North Atlantic is dependent upon the formation of dense waters that occurs at high northern latitudes. Wintertime deep convection in the Labrador and Irminger Seas forms the intermediate water mass known as Labrador Sea Water (LSW). Changes in the rate of formation and subsequent export of LSW are thought to play a role in MOC variability, but formation rates are uncertain and the link between formation and export is complex. We present the first observation-based application of a recently developed Regional Thermohaline Inverse Method (RTHIM) to a region encompassing the Arctic and part of the North Atlantic subpolar gyre for the years 2013, 2014 and 2015. RTHIM is a novel method which can diagnose the formation and export rates of water masses such as the LSW identified by their temperature and salinity, apportioning the formation rates into contributions from surface fluxes and interior mixing. We find LSW formation rates of up to 12 Sv during 2014-15, a period of strong wintertime convection, and around half that value during 2013 when convection was weak. We also show that the newly convected water is not exported directly, but instead is mixed isopycnally with warm, salty waters that have been advected into the region, before the products are then exported. RTHIM solutions for 2015 volume, heat and freshwater transports are compared with observations from a mooring array deployed for the Overturning in the Subpolar North Atlantic Program (OSNAP) and show good agreement, lending validity to our results.

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 2615-2634, doi:10.1002/2017JC013536., In this work, we use realistic isopycnal velocities with a 3-D eddy diffusivity to advect and diffuse a tracer in the Antarctic Circumpolar Current, beginning in the Southeast Pacific and progressing through Drake Passage. We prescribe a diapycnal diffusivity which takes one value in the SE Pacific west of 678W and another value in Drake Passage east of that longitude, and optimize the diffusivities using a cost function to give a best fit to experimental data from the DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) tracer, released near the boundary between the Upper and Lower Circumpolar Deep Water. We find that diapycnal diffusivity is enhanced 20-fold in Drake Passage compared with the SE Pacific, consistent with previous estimates obtained using a simpler advection-diffusion model with constant, but different, zonal velocities east and west of 678W. Our result shows that diapycnal mixing in the ACC plays a significant role in transferring buoyancy within the Meridional Overturning Circulation., UK Natural Environment Research Council Grant Numbers: NE/F020252/1 , NE/N018028/1 , NE/J007757/1; US National Science Foundation Grant Numbers: OCE 0622825 , OCE 1232962, 2018-09-26

A Regional Thermohaline Inverse Method (RTHIM) is presented that estimates velocities through the section bounding an enclosed domain and transformation rates due to interior mixing within the domain, given inputs of surface boundary fluxes of heat and salt and interior distributions of salinity and temperature. The method works by invoking a volumetric balance in thermohaline coordinates between the transformation due to mixing, surface fluxes and advection, while constraining the mixing to be down tracer gradients. The method is validated using a 20-year mean of outputs from the NEMO model in an Arctic and subpolar North Atlantic domain, bound to the south by a section with a mean latitude of 66°N. RTHIM solutions agree well with the NEMO model ‘truth’ and are robust to a range of parameters; the MOC, heat and freshwater transports calculated from an ensemble of RTHIM solutions are within 12%, 10% and 19%, respectively, of the NEMO values. There is also bulk agreement between RTHIM solution transformation rates due to mixing and those diagnosed from NEMO. Localized differences in diagnosed mixing may be used to guide the development of mixing parameterizations in models such as NEMO, whose downgradient diffusive closures with prescribed diffusivity may be considered oversimplified and too restrictive.

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) was designed as a multi-pronged US and UK CLIVAR effort to measure and to better understand diapycnal mixing and along-isopycnal eddy transport in the Antarctic Circumpolar Current (ACC), because these processes together appear to play a key role in the Meridional Overturning Circulation (MOC) (Gille et al, 2007). The project represents an unusual effort to evaluate simultaneously the roles of diapycnal and isopycnal mixing, and the program has benefited from close collaboration between observationalists, theoreticians and modelers. Fieldwork for DIMES began in early 2009, and the initial phase of the field observations is now wrapping up. This article provides a brief preliminary summary of early DIMES findings.