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The Southern Ocean upper-layer freshwater balance exerts a global climatic influence by modulating density stratification and biological productivity, and hence the exchange of heat and carbon between the atmosphere and the ocean interior. It is thus important to understand and quantify the time-varying freshwater inputs, which is challenging from measurements of salinity alone. Here we use seawater oxygen isotopes from samples collected between 2016 and 2021 along a transect spanning the Scotia and northern Weddell Seas to separate the freshwater contributions from sea ice and meteoric sources. The unprecedented retreat of sea ice in 2016 is evidenced as a strong increase in sea ice melt across the northern Weddell Sea, with surface values increasing approximately two percentage points between 2016 and 2018 and column inventories increasing approximately 1 to 2 m. Surface meteoric water concentrations exceeded 4% in early 2021 close to South Georgia due to meltwater from the A68 megaberg; smaller icebergs may influence meteoric water at other times also. Both these inputs highlight the importance of a changing cryosphere for upper-ocean freshening; potential future sea ice retreats and increases in iceberg calving would enhance the impacts of these freshwater sources on the ocean and climate. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]

The 5-year Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA) programme and its 1-year extension ENCORE (ENCORE is the National Capability ORCHESTRA Extension) was an approximately 11-million-pound programme involving seven UK research centres that finished in March 2022. The project sought to radically improve our ability to measure, understand and predict the exchange, storage and export of heat and carbon by the Southern Ocean. It achieved this through a series of milestone observational campaigns in combination with model development and analysis. Twelve cruises in the Weddell Sea and South Atlantic were undertaken, along with mooring, glider and profiler deployments and aircraft missions, all contributing to measurements of internal ocean and air–sea heat and carbon fluxes. Numerous forward and adjoint numerical experiments were developed and supported by the analysis of coupled climate models. The programme has resulted in over 100 peer-reviewed publications to date as well as significant impacts on climate assessments and policy and science coordination groups. Here, we summarize the research highlights of the programme and assess the progress achieved by ORCHESTRA/ENCORE and the questions it raises for the future. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]

The South Atlantic Ocean plays a key role in the heat exchange of the climate system, as it hosts the returning flow of the Atlantic Meridional Overturning Circulation (AMOC). To gain insights on this role, using data from three hydrographic cruises conducted in the South Atlantic Subtropical gyre at 34.5°S, 24°S, and 10°W, we identify water masses and compute absolute geostrophic circulation using inverse modeling. In the upper layers, the currents describe the South Atlantic anticyclonic gyre with the northwest flowing Benguela Current (26.3 ± 2.0 Sv at 34.5°S, and 21.2 ± 1.8 Sv at 24°S) flowing above the Mid‐Atlantic Ridge (MAR) between 22.4°S and 28.4°S (−19.2 ± 1.4 Sv), and the southward flowing Brazil Current (−16.5 ± 1.3 Sv at 34.5°S, and −7.3 ± 0.9 Sv at 24°S); the deep layers feature the southward transports of Deep Western Boundary Current (−13.9 ± 3.0 Sv at 34.5°S, and −8.7 ± 3.8 Sv at 24°S) and Deep Eastern Boundary Current (−15.1 ± 3.5 Sv at 34.5°S, and −16.3 ± 4.7 Sv at 24°S), with the interbasin west‐to‐east flow close to 24°S (7.5 ± 4.4 Sv); the abyssal waters present northward mass transports through the Argentina Basin (5.6 ± 1.1 Sv at 34.5°S, and 5.8 ± 1.5 Sv at 24°S) and Cape Basin (8.6 ± 3.5 Sv at 34.5°S–3.0 ± 0.8 Sv at 24°S) before returning southward (−2.2 ± 0.7 Sv at 24°S to −7.9 ± 3.6 Sv at 34.5°S), without any interbasin exchange across the MAR. In addition, we compute the upper AMOC strength (14.8 ± 1.0 and 17.5 ± 0.9 Sv), the equatorward heat transport (0.30 ± 0.05 and 0.80 ± 0.05 PW), and the freshwater flux (0.18 ± 0.02 and −0.07 ± 0.02 Sv) at 34.5°S and 24°S, respectively. Plain Language Summary: The location of the South Atlantic subtropical gyre plays a critical role in the Atlantic Meridional Overturning Circulation, which controls the Earth's climate system. We have examined the South Atlantic subtropical gyre's circulation between 34.5°S and 24°S and above the Mid‐Atlantic Ridge using data from three hydrographic cruises (34.5°S, 24°S, and 10°W). From these cruises, we have identified the water masses present on this region and computed their transport. Thus, we describe the path of the anticyclonic gyre and the northeast route of the Benguela Current in the upper water masses (<1,440 m depth); the southward flowing western and eastern boundary currents and an eastward interbasin flow close to 24°S above the Mid‐Atlantic Ridge in the deep water masses (those on the depth range from 1,440 to 3,800 m depth); and the northward flow of the abyssal water masses (>3,800 m depth). We have also reported the characteristic heat transport flowing northward across the subtropical South Atlantic Ocean, where evaporation dominates over precipitation. Key Points: The upper layers present the course of the anticyclonic South Atlantic subtropical gyre and the northeast route of the Benguela CurrentThe deep layers present the southward flowing boundary currents and an eastward interbasin flow close to 24°S above the Mid‐Atlantic RidgeThe heat transport flows northward across the subtropical South Atlantic Ocean, where evaporation dominates over precipitation [ABSTRACT FROM AUTHOR]

In this study we evaluate the benthic fluxes of silicic acid along the West Antarctic Peninsula (WAP). Silicic acid (DSi) is one of the macronutrients essential in fuelling biological hot spots of diatom‐dominated primary production along the WAP. Here we measure the concentration and stable silicon isotopic composition of DSi in porewater profiles, biogenic silica content (BSi), and diatom abundance from sediment cores collected along the WAP. We couple these measurements with reaction‐transport modeling, to assess the DSi flux and the processes that release this key nutrient from the sediment into the overlying waters. Our results show that the benthic DSi flux is dominated by the diffusive flux, which is estimated to be equivalent to 26.7 ± 2.7 Gmol yr−1 for the WAP continental shelf. The DSi isotope profiles reveal the important impact of sedimentary processes on porewater DSi and suggest that biogenic silica dissolution is the main source of DSi in porewaters and consequently of the benthic fluxes. Our integrated data‐model assessment highlights the impact of surface productivity on sedimentary processes and the dynamic environment of core‐top sediments where dissolution and reverse weathering reactions control DSi exchanges. Plain Language Summary: The West Antarctic Peninsula (WAP) is a highly productive region dominated by siliceous algae, diatoms. Biomass distribution is patchy along the WAP, and these hot spots are potentially enhanced by essential nutrients, that is, dissolved silicon, released from shallow marine sediments. We use observations of porewater concentrations and isotopes to show that the dissolved silicon flux from the sediments before the major diatom bloom period is low compared to the open Southern Ocean. However, both our observations and modeling reveal a strong link between surface water uptake and sedimentary release of dissolved silicon, such that benthic fluxes will increase during the summer season. We predict a significant variability in benthic silicon flux, tied to diatom productivity, which is highly sensitive to climatically driven changes in sea ice dynamics. Key Points: Dissolved silicon fluxes from marine sediment along the West Antarctic Peninsula continental shelf are reported for the first timeThe silicon isotopic composition of porewater is used to assess diagenetic processes and the silicon budget at the sediment‐water interfaceWe find no clear evidence of a significant source of silicon from marine sediment along the West Antarctic Peninsula continental shelf [ABSTRACT FROM AUTHOR]

Uncertainty exists in the time-mean total transport of the Antarctic Circumpolar Current (ACC), the world's strongest ocean current. The two most recent observational programs in Drake Passage, DRAKE and cDrake, yielded transports of 141 and 173.3 Sv, respectively. In this paper, we use a realistic 1/12° global ocean simulation to interpret these observational estimates and reconcile their differences. We first show that the modeled ACC transport in the upper 1,000 m is in excellent agreement with repeat shipboard acoustic Doppler current profiler (SADCP) transects and that the exponentially decaying transport profile in the model is consistent with the profile derived from repeat hydrographic data. By further comparing the model results to the cDrake and DRAKE observations, we argue that the modeled 157.3 Sv transport, that is, approximately the average of the cDrake and DRAKE estimates, is actually representative of the time-mean ACC transport through the Drake Passage. The cDrake experiment overestimated the barotropic contribution in part because the array undersampled the deep recirculation southwest of the Shackleton Fracture Zone, whereas the surface geostrophic currents used in the DRAKE estimate yielded a weaker near-surface transport than implied by the SADCP data. We also find that the modeled baroclinic and barotropic transports are not correlated; thus, monitoring either baroclinic or barotropic transport alone may be insufficient to assess the temporal variability of the total ACC transport. [ABSTRACT FROM AUTHOR]

Variability in the heat delivery by Circumpolar Deep Water (CDW) is responsible for modulating the basal melting of the Amundsen Sea ice shelves. However, the mechanisms controlling the CDW inflow to the region's continental shelf remain little understood. Here, a high-resolution regional model is used to assess the processes governing heat delivery to the Amundsen Sea. The key mechanisms are identified by decomposing CDW temperature variability into two components associated with 1) changes in the depth of isopycnals [heave (HVE)], and 2) changes in the temperature of isopycnals [water mass property changes (WMP)]. In the Dotson--Getz trough, CDW temperature variability is primarily associated with WMP. The deeper thermocline and shallower shelf break hinder CDW access to that trough, and CDW inflow is regulated by the uplift of isopycnals at the shelf break--which is itself controlled by wind-driven variations in the speed of an undercurrent flowing eastward along the continental slope. In contrast, CDW temperature variability in the Pine Island--Thwaites trough is mainly linked to HVE. The shallower thermocline and deeper shelf break there permit CDW to persistently access the continental shelf. CDW temperature in the area responds to wind-driven modulation of the water mass on-shelf volume by changes in the rate of inflow across the shelf break and in Ekman pumping-induced vertical displacement of isopycnals within the shelf. The western and eastern Amundsen Sea thus represent distinct regimes, in which wind forcing governs CDW-mediated heat delivery via different dynamics. [ABSTRACT FROM AUTHOR]

Over the past century, tide gauges in Hawaii have recorded interdecadal sea level variations that are coherent along the island chain. The generation of this signal and its relationship to other interdecadal variability are investigated, with a focus on the last decade. Hawaii sea level is correlated with sea surface height (SSH) over a significant portion of the North Pacific Ocean, and with the Pacific–North America (PNA) index, which represents teleconnections between tropical and midlatitude atmospheric variations. Similar variations extend well below the thermocline in World Ocean Atlas temperature. Comparison with NCEP reanalysis wind and pressure shows that high (low) sea level phases around Hawaii are associated with an increase (decrease) in the strength of the Aleutian low. The associated wind stress curl pattern is dynamically consistent with observed sea level anomalies, suggesting that sea level at Hawaii represents large-scale changes that are directly wind-forced in concert with the PNA. Atmospheric modulation, as opposed to Rossby wave propagation, may explain the linkage of Hawaii sea level to North American sea level and ENSO events. A wind-forced, baroclinic Rossby wave model replicates some aspects of the interdecadal SSH variations and their spatial structure but fails to predict them in detail near Hawaii. The accuracy of wind products in this region and over this time period may be a limiting factor. Variations in mixed layer temperature due to surface heat flux anomalies may also contribute to the interdecadal sea level signal at Hawaii. [ABSTRACT FROM AUTHOR]

The Southern Ocean upper-layer freshwater balance exerts a global climatic influence by modulating density stratification and biological productivity, and hence the exchange of heat and carbon between the atmosphere and the ocean interior. It is thus important to understand and quantify the time-varying freshwater inputs, which is challenging from measurements of salinity alone. Here we use seawater oxygen isotopes from samples collected between 2016 and 2021 along a transect spanning the Scotia and northern Weddell Seas to separate the freshwater contributions from sea ice and meteoric sources. The unprecedented retreat of sea ice in 2016 is evidenced as a strong increase in sea ice melt across the northern Weddell Sea, with surface values increasing approximately two percentage points between 2016 and 2018 and column inventories increasing approximately 1 to 2 m. Surface meteoric water concentrations exceeded 4% in early 2021 close to South Georgia due to meltwater from the A68 megaberg; smaller icebergs may influence meteoric water at other times also. Both these inputs highlight the importance of a changing cryosphere for upper-ocean freshening; potential future sea ice retreats and increases in iceberg calving would enhance the impacts of these freshwater sources on the ocean and climate. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

The 5-year Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA) programme and its 1-year extension ENCORE (ENCORE is the National Capability ORCHESTRA Extension) was an approximately 11-million-pound programme involving seven UK research centres that finished in March 2022. The project sought to radically improve our ability to measure, understand and predict the exchange, storage and export of heat and carbon by the Southern Ocean. It achieved this through a series of milestone observational campaigns in combination with model development and analysis. Twelve cruises in the Weddell Sea and South Atlantic were undertaken, along with mooring, glider and profiler deployments and aircraft missions, all contributing to measurements of internal ocean and air-sea heat and carbon fluxes. Numerous forward and adjoint numerical experiments were developed and supported by the analysis of coupled climate models. The programme has resulted in over 100 peer-reviewed publications to date as well as significant impacts on climate assessments and policy and science coordination groups. Here, we summarize the research highlights of the programme and assess the progress achieved by ORCHESTRA/ENCORE and the questions it raises for the future. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

The West Antarctic Peninsula shelf is a region of high seasonal primary production which supports a large and productive food web, where macronutrients and inorganic carbon are sourced primarily from intrusions of warm saline Circumpolar Deep Water. We examined the cross-shelf modification of this water mass during mid-summer 2015 to understand the supply of nutrients and carbon to the productive surface ocean, and their subsequent uptake and cycling. We show that nitrate, phosphate, silicic acid and inorganic carbon are progressively enriched in subsurface waters across the shelf, contrary to cross-shelf reductions in heat, salinity and density. We use nutrient stoichiometric and isotopic approaches to invoke remineralization of organic matter, including nitrification below the euphotic surface layer, and dissolution of biogenic silica in deeper waters and potentially shelf sediment porewaters, as the primary drivers of cross-shelf enrichments. Regenerated nitrate and phosphate account for a significant proportion of the total pools of these nutrients in the upper ocean, with implications for the seasonal carbon sink. Understanding nutrient and carbon dynamics in this region now will inform predictions of future biogeochemical changes in the context of substantial variability and ongoing changes in the physical environment.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'., (© 2018 The Authors.)