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1. Ocean Ventilation Controls the Contrasting Anthropogenic CO ₂ Uptake Rates Between the Western and Eastern South Atlantic Ocean Basins

   https://doi.org/10.1029/2021GB007265  

   Gao, Hui; Cai, Wei‐Jun; Jin, Meibing; Dong, Changming; Timmerman, Amanda H. V. (June 2022, Global Biogeochemical Cycles)

   Abstract The South Atlantic Ocean is an important region for anthropogenic CO₂(C_anth) uptake and storage in the world ocean, yet is less studied. Here, after an extensive sensitivity test and method comparison, we applied an extended multiple linear regression method with six characteristic water masses to estimate C_anthchange or increase (ΔC_anth) between 1980s and 2010s in the South Atlantic Ocean using two meridional transects (A16S and A13.5) and one zonal transect (A10). Over a period of about 25 years, the basin‐wide ΔC_anthwas 3.86 ± 0.14 Pg C decade⁻¹. The two basins flanking the Mid‐Atlantic Ridge had different meridional patterns of ΔC_anth, yielding an average depth‐integrated ΔC_anthin the top 2000 m of 0.91 ± 0.25 mol m⁻² yr⁻¹along A16S on the west and 0.57 ± 0.22 mol m⁻² yr⁻¹along A13.5 on the east. The west‐east basin ΔC_anthcontrasts were most prominent in the tropical region (0–20°S) in the Surface Water (SW), approximately from equator to 35°S in the Subantarctic Mode Water (Subantarctic Mode Water (SAMW)), and all latitudes in the Antarctic Intermediate Water (AAIW). Less ΔC_anthin the eastern basin than the western basin was caused by weaker ventilation driven by SAMW and AAIW formation and subduction and stronger Antarctic Bottom Water (AABW) formation in the former than the latter. In addition to the spatial heterogeneity, C_anthincrease rates accelerated from the 1990s to the 2000s, consistent with the overall increase in air‐sea CO₂exchange in the South Atlantic Ocean. 

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2. The Influence of Surface Fluxes on Export of Southern Ocean Intermediate and Mode Water in Coupled Climate Models

   https://doi.org/10.1029/2024JC021841  

   Almeida, Lucas; Mazloff, Matthew_R; Mata, Mauricio_M (November 2024, Journal of Geophysical Research: Oceans)

   Abstract The Southern Ocean (SO) plays a crucial role in the process of sequestering heat and carbon dioxide from the atmosphere and transferring them to the deep ocean. This process is intricately linked to the formation of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW), which are pivotal components of the Meridional Overturning Circulation (MOC) and have a substantial impact on the global climate balance. AAIW and SAMW take shape in specific regions of the Southern Ocean due to the influence of strong winds, buoyancy fluxes, and their effects, such as convection, the development of thick mixed layers, and wind‐driven subduction. These water masses subsequently flow northward, contributing to the ventilation of the intermediate layers within the subtropical gyres. In this study, our focus lies on investigating the regional aspects of AAIW and SAMW transformation in CMIP6 models. We accomplish this by analyzing the relationship between the meridional transport of these water masses and air‐sea fluxes, particularly Ekman pumping, freshwater fluxes, and heat fluxes. Our findings reveal that the highest transformation rates occur in the Indian sector of the Southern Ocean, with notable values also observed in the southeast Pacific and south of Africa. Additionally, we assess the potential changes in these formation regions under future scenarios projected for the end of the 21st century. Although the patterns of formation regions remain consistent, there is a significant decrease in the transformation process. 

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3. The Impact of Southern Ocean Ekman Pumping, Heat and Freshwater Flux Variability on Intermediate and Mode Water Export in CMIP Models: Present and Future Scenarios

   https://doi.org/10.1029/2021JC017173  

   Almeida, Lucas; Mazloff, Matthew R.; Mata, Mauricio M. (May 2021, Journal of Geophysical Research: Oceans)

   Abstract Subduction in the Antarctic circumpolar region of the Southern Ocean (SO) results in the formation of Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW). Theoretical understanding predicts that subduction rates of these waters masses is driven by wind stress curl and buoyancy fluxes. The objective of this work is to evaluate the extent to which AAIW and SAMW variability are correlated to SO air‐sea fluxes and how potential changes to those forcings would impact the future water mass export rates. We correlate the water mass volume transport at 30°S with Ekman pumping, freshwater and heat fluxes in the Coupled Model Intercomparison Project. The export of these water masses varies across models, with most overestimating the total transport. Correlation coefficients between the air‐sea fluxes and exports are consistent with theoretical expectations. In the picontrol/historical scenarios, the highest correlations with AAIW export variability are heat flux, while Ekman pumping best explains SAMW. However, multivariate regressions show that both AAIW and SAMW export variability are better explained using the combination of all three fluxes. In future scenario simulations air‐sea fluxes trend significantly in the catastrophic scenario (RCP8.5 and SSP8.5). Both AAIW and SAMW are still highly correlated to the fluxes, but with different correlation coefficients. The dominant forcing components even change from the present simulations to the future scenario runs. Thus, correlations between AAIW and SAMW transports and air‐sea fluxes are not stationary in time, limiting the predictive skill of statistical models and highlighting the importance of using complex climate models. 

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4. Iceberg Alley and Subantarctic Ice and Ocean Dynamics

   https://doi.org/10.14379/iodp.proc.382.2021  

   Weber, M.E.; Raymo, M.E.; Peck, V.L.; Williams, T. (May 2021, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   International Ocean Discovery Program Expedition 382, Iceberg Alley and Subantarctic Ice and Ocean Dynamics, investigated the long-term climate history of Antarctica, seeking to understand how polar ice sheets responded to changes in insolation and atmospheric CO2 in the past and how ice sheet evolution influenced global sea level and vice versa. Five sites (U1534–U1538) were drilled east of the Drake Passage: two sites at 53.2°S at the northern edge of the Scotia Sea and three sites at 57.4°–59.4°S in the southern Scotia Sea. We recovered continuously deposited late Neogene sediments to reconstruct the past history and variability in Antarctic Ice Sheet (AIS) mass loss and associated changes in oceanic and atmospheric circulation. The sites from the southern Scotia Sea (Sites U1536–U1538) will be used to study the Neogene flux of icebergs through “Iceberg Alley,” the main pathway along which icebergs calved from the margin of the AIS travel as they move equatorward into the warmer waters of the Antarctic Circumpolar Current (ACC). In particular, sediments from this area will allow us to assess the magnitude of iceberg flux during key times of AIS evolution, including the following: • The middle Miocene glacial intensification of the East Antarctic Ice Sheet, • The mid-Pliocene warm period, • The late Pliocene glacial expansion of the West Antarctic Ice Sheet, • The mid-Pleistocene transition (MPT), and • The “warm interglacials” and glacial terminations of the last 800 ky. We will use the geochemical provenance of iceberg-rafted detritus and other glacially eroded material to determine regional sources of AIS mass loss. We will also address interhemispheric phasing of ice sheet growth and decay, study the distribution and history of land-based versus marine-based ice sheets around the continent over time, and explore the links between AIS variability and global sea level. By comparing north–south variations across the Scotia Sea between the Pirie Basin (Site U1538) and the Dove Basin (Sites U1536 and U1537), Expedition 382 will also deliver critical information on how climate changes in the Southern Ocean affect ocean circulation through the Drake Passage, meridional overturning in the region, water mass production, ocean–atmosphere CO2 transfer by wind-induced upwelling, sea ice variability, bottom water outflow from the Weddell Sea, Antarctic weathering inputs, and changes in oceanic and atmospheric fronts in the vicinity of the ACC. Comparing changes in dust proxy records between the Scotia Sea and Antarctic ice cores will also provide a detailed reconstruction of changes in the Southern Hemisphere westerlies on millennial and orbital timescales for the last 800 ky. Extending the ocean dust record beyond the last 800 ky will help to evaluate dust-climate couplings since the Pliocene, the potential role of dust in iron fertilization and atmospheric CO2 drawdown during glacials, and whether dust input to Antarctica played a role in the MPT. The principal scientific objective of Subantarctic Front Sites U1534 and U1535 at the northern limit of the Scotia Sea is to reconstruct and understand how intermediate water formation in the southwest Atlantic responds to changes in connectivity between the Atlantic and Pacific basins, the “cold water route.” The Subantarctic Front contourite drift, deposited between 400 and 2000 m water depth on the northern flank of an east–west trending trough off the Chilean continental shelf, is ideally situated to monitor millennial- to orbital-scale variability in the export of Antarctic Intermediate Water beneath the Subantarctic Front. During Expedition 382, we recovered continuously deposited sediments from this drift spanning the late Pleistocene (from ~0.78 Ma to recent) and from the late Pliocene (~3.1–2.6 Ma). These sites are expected to yield a wide array of paleoceanographic records that can be used to interpret past changes in the density structure of the Atlantic sector of the Southern Ocean, track migrations of the Subantarctic Front, and give insights into the role and evolution of the cold water route over significant climate episodes, including the following: • The most recent warm interglacials of the late Pleistocene and • The intensification of Northern Hemisphere glaciation. 

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5. Expedition 382 Scientific Prospectus: Iceberg Alley and South Falkland Slope Ice and Ocean Dynamics

   https://doi.org/10.14379/iodp.sp.382.2018  

   Weber, M.E.; Raymo, M.E.; Peck, V.L.; Williams, T. (May 2018, Scientific prospectus)

   null (Ed.)

   International Ocean Discovery Program Expedition 382, Iceberg Alley and South Falkland Slope Ice and Ocean Dynamics, will investigate the long-term climate history of Antarctica, seeking to understand how polar ice sheets responded to changes in atmospheric CO2 in the past and how ice sheet evolution influenced global sea level. We will drill six sites in the Scotia Sea, east of the Antarctic Peninsula, providing the first deep drilling in this region of the Southern Ocean. We expect to recover >600 m of late Neogene sediment that will be used to reconstruct the past history and variability in Antarctic Ice Sheet (AIS) mass loss and associated changes in oceanic and atmospheric circulation. Expedition 382 expects to deliver the first spatially and temporally integrated record of iceberg flux from “Iceberg Alley,” the main pathway by which icebergs are calved from the margin of the AIS and travel equatorward into warmer waters of the Antarctic Circumpolar Current (ACC). In particular, we will characterize the magnitude of iceberg flux during key times of AIS evolution: • The middle Miocene glacial intensification of the East Antarctic Ice Sheet, • The mid-Pliocene warm interval, • The late Pliocene glacial expansion of the West Antarctic Ice Sheet, • The mid-Pleistocene transition, and • The “warm interglacials” and glacial terminations of the last 800 ky. We will use the geochemical provenance of iceberg-rafted detritus and other glacially eroded material to determine regional sources of AIS mass loss in this region, address interhemispheric phasing of ice sheet growth and decay, study the distribution and history of land-based versus marine-based ice sheets around the continent over time, and explore the links between AIS variability and global sea level. By comparing north–south variations across the Scotia Sea, Expedition 382 will also deliver critical information on how climate changes in the Southern Ocean affect ocean circulation through the Drake Passage, meridional overturning in the region, water-mass production, CO2 transfer by wind-induced upwelling, sea ice variability, bottom water outflow from the Weddell Sea, Antarctic weathering inputs, and changes in oceanic and atmospheric fronts in the vicinity of the ACC. Comparing changes in dust proxy records between the Scotia Sea and Antarctic ice cores will also provide a detailed reconstruction of changes in the Southern Hemisphere westerlies on millennial and orbital timescales for the last 800 ky. Extending the ocean dust record beyond the last 800 ky will help to evaluate climate-dust couplings since the Pliocene, the potential role of dust in iron fertilization and atmospheric CO2 drawdown during glacials, and whether dust input to Antarctica played a role in the mid-Pleistocene transition. The principal scientific objective of the South Falkland Slope sites to the north is to reconstruct and understand how ocean circulation and intermediate water formation responds to changes in climate with a special focus on the connectivity between the Atlantic and Pacific basins. The South Falkland Slope Drift, a contourite drift on the Falkland margin deposited between 400 and 2000 m water depth, is ideally situated to monitor millennial- to orbital-scale variability in the export of Antarctic Intermediate Water beneath the Subantarctic Front over at least the last 2 My. We anticipate that these sites will yield a wide array of paleoceanographic records that can be used to interpret past changes in the density structure of the Atlantic sector of the Southern Ocean and track the migration of the Subantarctic Front. We expect the cored sediments to capture the following significant climate episodes: • The most recent warm interglacials of the late Pleistocene; • The mid-Pleistocene transition, when δ18O records shifted from dominantly 41 to 100 ky periodicity; and possibly • Mid-Pliocene warm intervals, often invoked as the best analog for possible future climate change. 

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