The formation of Antarctic Bottom Water (AABW) is a key process in the global ocean circulation, but modeling the formation and downslope flow of AABW represents an ongoing challenge for ocean and climate models due to the high horizontal resolution required. Here, we assess the formation and export of AABW to the abyss and its sensitivity to horizontal model resolution in a circumpolar ocean‐sea ice model available at horizontal resolutions of 1/10°, 1/20°, and 1/40°. The formation of Dense Shelf Water (DSW), the precursor of AABW, reduces with increasing model resolution in most formation regions due to shelf freshening. Increased eddy activity with higher model resolution flattens the isopycnals in the open Southern Ocean and enables access of lighter, fresher waters onto the continental shelf. Despite the decrease in DSW formation, the total offshore AABW transport increases with increasing model resolution, especially across the 2,500 m isobath, due to less diapycnal mixing during the downslope flow. This resolution dependency is strongest in the Ross and Weddell Seas, the two most important regions of AABW formation. We conclude that a horizontal resolution of 1/10° is sufficient to simulate AABW export from East Antarctica, in agreement with theory of the downslope flow of dense plumes, but finer resolutions of up to 1/40° increase the offshore transport and may be required to resolve the AABW export in the Weddell and Ross Seas.

Dense water formation on the Antarctic continental shelf is the main process by which Antarctic Bottom Waters form and is fundamental to the abyssal overturning circulation. However, most ocean models fail to simulate Antarctic dense water formation on the continental shelf and flow down the continental slope (i.e., overflow) due to resolution constraints. While the impact of horizontal and vertical resolution on the overflows has been previously studied, the effect of surface vertical resolution on dense water formation remains unexplored. To address this gap, we vary the surface ocean grid cell of two dense water‐forming models from 1.1 to 5.1 m thickness. We used two ocean and sea ice models, each employing a different boundary layer parameterization scheme. In one model, thickening the surface cell to 5.1 m reduced dense water formation by 64% and led to the complete cessation of the overflow after 10 years of simulation. In the other, the same thickening decreased dense water formation by 32% and overflow by 67% over the same period. The dense water formation reduction in the experiments with thicker surface grid cells is explained by a southward shift in the surface Ekman transport, which brings light offshore waters to the coast and limits dense water formation at the continental shelf. Although dense water formation responds to surface layer thickening in both models, differences in sea ice production contribute to greater sensitivity in one case, where a weaker sea ice formation in the 5.1 m configuration further decreases dense water production. These results highlight that a high vertical resolution at the ocean surface is required to form Antarctic dense waters.

Abstract Ice sheet melting into the Southern Ocean can change the formation and properties of the Antarctic Bottom Water (AABW). Ocean models often mimic ice sheet melting by adding freshwater fluxes in the Southern Ocean under diverse spatial distributions. We use a global ocean and sea‐ice model to explore whether the spatial distribution and magnitude of meltwater fluxes can alter AABW properties and formation. We find that a realistic spatially varying meltwater flux sustains AABW with higher salinities compared to simulations with uniform meltwater fluxes. Finally, we show that increases in ice sheet melting above 12% since 1958 can trigger AABW freshening rates similar to those observed in the Southern Ocean since 1990, suggesting that the increasing Antarctic meltwater discharge can drive the observed AABW freshening.

In a warming climate, the Global Meridional Overturning Circulation (GMOC) is expected to change significantly with a risk of disrupting the global redistribution of ocean properties that sustains marine ecosystems, carbon cycle, and others. Here we make a novel attempt to utilize a diagnostic ocean & sea-ice model to estimate the GMOC and its interdecadal changes since the mid-1950s that are consistent with historical hydrographic observations. We find that significant changes in the GMOC have already occurred, most notably in the upper and lower overturning cells in the Southern Ocean. The former has expanded poleward and into denser water and strengthened by 3–4 Sv since the mid-1970s, while the latter has contracted and weakened by a similar rate during the same period. These changes are driven by the increasing Southern Hemisphere (SH) Ferrel cell and associated increases in the westerlies and the surface buoyancy loss over its sinking branch, and the increasing Antarctic meltwater discharge, in response to ozone depletion in the SH stratosphere and increasing atmospheric CO 2 . A large-scale readjustment of the GMOC seems to be underway in the South Atlantic and Indo-Pacific Oceans since the mid-2000s in response to the Southern Ocean changes.

This data contains the unprocessed output from the five equilibrium simulations used in the paper "Antarctic Bottom Water sensitivity to spatio-temporal variations in Antarctic meltwater fluxes". Simulations were run using the ocean and sea-ice components of CESM1, and the atmosphere was data-driven, and based on ERA5 reanalysis from 1958 until 1980. All data is in netcdf, and includes the description of flag values, and units. The five simulations differ in the freshwater flux scheme as described below:Simulation UNIF - files: ga1.f09_g16.308Forced with freshwater fluxes from the Antarctic Ice Sheet (AIS), uniformly distributed around the Antarctic coast. Total freshwater flux from AIS: 2075 Gt/yrSimulation BM - files: ga1.f09_g16.208Forced with asymmetric zonal freshwater fluxes from the AIS.Total freshwater flux from AIS: 2075 Gt/yrSimulation VARI - files: ga1.f09_g16.108Forced with asymmetric zonal and meridional freshwater fluxes from the AIS. Total freshwater flux from AIS: 2075 Gt/yrSimulation CV - files: ga1.f09_g16.408Forced with asymmetric meridional freshwater fluxes from the AIS, to mimic iceberg melting. Total freshwater flux from AIS: 934 Gt/yrSimulation VARI120% - files: ga1.f09_g16.x18Same as VARI, but the total freshwater fluxes were increased by 20%. Total freshwater flux from AIS: 2490 Gt/yr[30S 90S] , [180W 180E] Variables: Ocean Temperature [Celius], Salinity [PSU], Sea ice Fraction [fraction], Salt flux from sea ice [kg/m2/s], and ocean overturning [SV] For more information, check:https://doi.org/10.1002/essoar.10512610.1

The northern hemisphere experienced an abrupt cold event ~ 8200 years ago (the 8.2 ka event) that was triggered by the release of meltwater into the Labrador Sea, and resulting in a weakening of the poleward oceanic heat transport. Although this event has been considered a possible analogue for future ocean circulation changes due to the projected Greenland Ice Sheet (GIS) melting, large uncertainties in the amount and rate of freshwater released during the 8.2 ka event make such a comparison difficult. In this study, we compare sea surface temperatures and oxygen isotope ratios from 28 isotope-enabled model simulations with 35 paleoproxy records to constrain the meltwater released during the 8.2 ka event. Our results suggest that a combination of 5.3 m of meltwater in sea level rise equivalent (SLR) released over a thousand years, with a short intensification over ~ 130 years (an additional 2.2 m of equivalent SLR) due to routing of the Canadian river discharge, best reproduces the proxy anomalies. Our estimate is of the same order of magnitude as projected future GIS melting rates under the high emission scenario RCP8.5.

Climate anomalies due to Lake Agassiz outbursts and Hudson Bay ice dome melting are commonly considered triggers of North American atmospheric cooling. However, in the Southern Hemisphere, these freshwater fluxes are mostly associated with increased precipitation and a possible intensification of the South American Monsoon System (SAMS). Here, we tested how the SAMS responded to early-Holocene meltwater events. Based on both proxy data and simulations, we find that sea surface temperatures (SSTs) and precipitation indicate a freshwater-driven strengthening of the SAMS due to a weakening of the South Atlantic subtropical dipole. Simulated SAMS strengthening accounts for up to 50% of the variance in early-Holocene precipitation in South America. In turn, changes in the South Atlantic Subtropical Dipole accounts for up to 31% of the variance in South Atlantic SSTs. Additionally, we propose that the stronger SAMS in the early Holocene might have been due to a freshwater-driven weakening of the southeasterly trade winds. Slower trade winds weaken the zonal and meridional surface water transport, concentrating warm waters in the northeastern South Atlantic.

River runoff into the continental shelf affects the coastal environment in many ways: from sediment input and marine sediments resuspension to meso-scale eddies formation in case of higher river discharges. Therefore, for ocean modeling is crucial to understand the seasonal and interannual river runoff variability. Brazil stands out when it comes to riverine discharge, since 6 out of the 50 largest rivers flows within the country limits. Here, a compilation of all river discharge data provided by the Brazilian National Water Agency resulted in the identification of 97 exorheic rivers (with time series longer than 15 years), including 10 unmonitored ones which had climatologies estimated by the proposed methodology. This compilation took a considerable effort, as streamflow stations were often far from the discharge location. It was also necessary to overcome the challenge associated with the Brazilian climate heterogeneity in order to apply the regionalization method. The riverine input into the ocean was estimated at the mouth of the rivers, with the overall discharge analysis taking into consideration a division of the Brazilian Continental Shelf in five sectors. These results analyzed here are also made available for public use. Overall, the Brazilian northern continental shelf receives about 95% of the total annual river runoff (212,148 m3 s−1), followed by the northeastern continental shelf with 1.5% (3345 m3 s−1), the eastern continental shelf with 1.4% (3097 m3 s−1) and the southern continental shelf with about 1.3% (2997 m3 s−1). The southeastern continental shelf receives only 0.8% of the total discharge (1909 m3 s−1). Throughout the year, the major river discharges were observed from summer to winter months, following the different climate and precipitation patterns along the country. The interannual variability of the discharges was assessed, also looking at similarities regarding both El Niño Southern Oscillation and Pacific Decadal Oscillation.

Open ocean deep convection is a common source of error in the representation of Antarctic Bottom Water (AABW) formation in ocean general circulation models. Although those events are well described in non-assimilatory ocean simulations, the recent appearance of a massive open ocean polynya in the Estimating the Circulation and Climate of the Ocean Phase II reanalysis product (ECCO2) raises questions on which mechanisms are responsible for those spurious events and whether they are also present in other state-of-the-art assimilatory reanalysis products. To investigate this issue, we evaluate how three recently released high-resolution ocean reanalysis products form AABW in their simulations. We found that two of the products create AABW by open ocean deep convection events in the Weddell Sea that are triggered by the interaction of sea ice with the Warm Deep Water, which shows that the assimilation of sea ice is not enough to avoid the appearance of open ocean polynyas. The third reanalysis, My Ocean University Reading UR025.4, creates AABW using a rather dynamically accurate mechanism. The UR025.4 product depicts both continental shelf convection and the export of Dense Shelf Water to the open ocean. Although the accuracy of the AABW formation in this reanalysis product represents an advancement in the representation of the Southern Ocean dynamics, the differences between the real and simulated processes suggest that substantial improvements in the ocean reanalysis products are still needed to accurately represent AABW formation.

Surface sediments collected in the intertidal zone of Paraguaçu estuary in July, 2013, were analyzed for organic matter, nitrogen, phosphorus, grain size fractions and partial concentrations of 16 metals. The USEPA 3051A method and ICP-OES and CV-AAS techniques were chosen to metal analysis. Pollution indices (EF, Igeo and PIN) and a comparison with sediment quality guidelines (UET, ERL, ERM, TEL and PEL of NOAA) were conducted in order to evaluate the potential metal impacts over the area. Principal Component Analysis (PCA) and Pearson correlation results showed the importance of organic matter content and the fine-grained fraction of sediments on the control of the bioavailable metals distribution. The Paraguaçu estuary already has anthropogenic enrichment relative to the background level, especially for Mn, whose values exceeded almost 30 times the background at one site (Mn: 1197.30 mg kg(-1)). However, metal levels are still below the reference values with the exception of Hg at one site (Hg: 0.25 mg kg(-1), exceeded TEL and ERL).