Elaine L. McDonagh

The ocean's critical role in absorbing anthropogenic carbon dioxide (CO ₂ ) and the resulting changes in the ocean carbon cycle are monitored by global‐scale observing networks, including the Global Ocean Ship‐based Investigations Program (GO‐SHIP), Surface Ocean CO ₂ Observing Network (SOCONET), and Biogeochemical‐Argo (BGC‐Argo). Given the scale of the challenge to characterize the evolving ocean carbon cycle, no single network is sufficient; a coordinated, collaborative, cross‐network approach is essential. This manuscript describes the overarching research questions driving these three networks and details their inherent synergies and dependencies, such as GO‐SHIP and SOCONET delivering reference‐quality measurements for validation of other in situ and satellite‐based data, GO‐SHIP providing opportunities for float deployment, and BGC‐Argo filling gaps with high‐frequency autonomous data. We provide recommendations for optimizing these relationships and an intra‐network strategy moving forward. As representatives of the science committees for these networks, we are committed to sustaining and enhancing the collaborative structure necessary to fill critical knowledge gaps in ocean carbon cycle research, improve climate prediction, and enable future ocean ecosystem forecasting.

Here, we review scientific advances based on the second decade of Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP) repeat hydrographic observations, collected between 2014 and 2023, building on an earlier review of GO-SHIP's first decade and its predecessor programs, Climate and Ocean Variability, Predictability, and Change (CLIVAR) and the World Ocean Circulation Experiment (WOCE). During the second decade of GO-SHIP, 50 reference sections were occupied by 10 nations. Scientific advances include continued monitoring of the oceanic uptake of deep heat and anthropogenic carbon associated with climate change, as well as concomitant salinity, oxygen, and nutrient changes. In addition, we present results from new methods to analyze GO-SHIP observations that provide new insights into the patterns of anthropogenic warming in the ocean and water mass inversions that inform full-depth ocean ventilation. Finally, in this decade, GO-SHIP has expanded its scope to incorporate regional seas that represent hotspots of climate-driven change (the Arctic Ocean and Mediterranean Sea) and built a nascent biological component (Bio-GO-SHIP), and we review key results from these efforts.

Global climate change is concurrently and profoundly altering the ocean’s physical and biogeochemical environment. The intermediate water in the South Atlantic and Indian Oceans, which together constitute a critical component of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), lies at the heart of these changes. The intermediate layer of the South Atlantic is ventilated by two primary sources: relatively fresher and younger waters (characterised by low Apparent Oxygen Utilization, AOU) originating from the Pacific Ocean, and saltier and older waters advected from the Indian Ocean via the Agulhas System. However, the extent to which the intrusion of saline, older Indian Ocean waters via the Agulhas System modulates the stability of the AMOC’s upper limb remains poorly understood. Specifically, the temporal variability and long-term contribution of the Indian Ocean waters to the South Atlantic intermediate layer remains a knowledge gap.Here, we focus on observational evidence of and investigate the underlying mechanisms driving the influence of Indian Ocean intermediate waters on the Atlantic Ocean in a warming climate. We examine AOU-salinity covariability across decadal to multi-decadal time scales within South Atlantic intermediate water. This analysis integrates high-quality observational databases of temperature, salinity, dissolved oxygen, and water age, as well as repeat hydrographic sections, allowing us to link their observed variability to changes in circulation and mixing, while considering oxygen disequilibrium effects and the influence of the biological carbon pump in changing AOU.We find an increasing influence of Indian Ocean water in the South Atlantic at the intermediate layer over the past 30 years. The most strongly impacted regions are identified. In addition, we quantify the impact of Indian Ocean influence on the South Atlantic and show that this signal has become progressively detectable over the past 30 years, but has not yet exceeded the level of internal variability, indicating an ongoing ‘Indianization’ of the South Atlantic intermediate layer. We identify the underlying mechanisms related to the increasingly positive phase of the Southern Annular Mode (SAM) and an associated multidecadal increase in Agulhas leakage. Finally, we will discuss the potential implications of this phenomenon for the long-term stability of the AMOC.

The Gulf Stream is important for the climate system through its transport and air-sea exchange of heat. What is less well accepted is the role of the Gulf Stream in the carbon cycle. Here we examine how the Gulf Stream provides a “biogeochemical stream”, a sub-surface horizontal flux carrying waters with high concentrations of nutrients and low concentrations of anthropogenic carbon. Model experiments reveal particles released in dense layers at the start of the Gulf Stream follow trajectories extending into the subpolar gyre, while particles released at the surface are confined to the subtropics. Following a pathway to the subpolar gyre, the biogeochemical stream carries older, nutrient-rich and anthropogenically carbon-depleted waters along density layers and, when those dense layers outcrop into the mixed layer, enhances the subpolar drawdown of atmospheric carbon. This connectivity is supported by model sensitivity experiments revealing the subpolar upper ocean carbon content and upstream dense waters in the Gulf Stream connecting on timescales of 4 to 8 years. The likely effect of climate change on the biogeochemical stream is a decrease in the delivery of these older waters, both high in concentrations of nutrients and depleted in anthropogenic carbon, to the subpolar mixed layer, so weakening future North Atlantic carbon uptake from the atmosphere.

Oxygen is fundamental to ocean biogeochemical processes, with deoxygenation potentially reducing biodiversity, and disrupting biogeochemical cycles. In recent decades, the global ocean oxygen concentration has been decreasing, but this decrease is underestimated in numerical ocean models by as much as 50 %. Mechanisms responsible for this deoxygenation include (i) solubility-driven deoxygenation due to ocean warming, and (ii) changes in the remineralised signal due to either a change in the supply of biological material to depth or a change in circulation leading to change in the residence time of water, and hence the accumulation of the remineralised oxygen deficit, or a combination of both. The magnitude of oxygen change due to each process is currently unclear. Here, we describe and implement a new method to decompose oxygen change into its constituent parts by linking each process to concomitant changes in temperature and dissolved inorganic carbon. Using observations on a repeated section of the North Atlantic at 24.5° N, we show that the consistent oxygen decrease observed since 1992 in the upper 2000 m has been dominated by an increase in remineralisation-related oxygen-consumption. While warming-driven solubility changes have a much smaller impact on the upper ocean in comparison, their impact has trebled in the past twenty years, suggesting they will become an increasingly significant driver of deoxygenation with future warming. Remineralisation-related oxygen consumption peaks at a depth of approximately 600 m, where it is responsible for up to 70 % of the total deoxygenation. While this study does not determine the exact cause of the remineralisation-driven change, little change in primary productivity has been observed in the region, suggesting that a change in ocean circulation is indirectly driving the majority of deoxygenation in the Subtropical North Atlantic, via a non-local change in remineralisation.

Oxygen is fundamental to ocean biogeochemical processes, with deoxygenation potentially reducing biodiversity, and disrupting biogeochemical cycles. In recent decades, the global ocean oxygen concentration has been decreasing, but this decrease is underestimated in numerical ocean models by as much as 50 %. Mechanisms responsible for this deoxygenation include solubility-driven deoxygenation driven by ocean warming, and changes in the amount, rates and spatial patterns of remineralisation. However, the magnitude of change in oxygen due to each process is currently unclear. Here, we use a new method to decompose oxygen change into its constituent parts by linking each process to concomitant changes in temperature and dissolved inorganic carbon. Using observations across a repeated section of the North Atlantic at 24.5° N, we show that the consistent oxygen decrease observed since 1992 in the upper 2000 m has been dominated by an increase in remineralisation-related oxygen-consumption. While warming-driven solubility changes have a much smaller impact on the upper ocean in comparison, the impact has trebled in the past twenty years, suggesting they will become an increasingly significant driver of deoxygenation with future warming. Remineralisation-related oxygen consumption peaks at a depth of approximately 600 m, where it is responsible for up to 70 % of the total deoxygenation. This remineralisation-driven change may be caused by a change in the supply of biological material to depth, a change in circulation leading to change in the residence time of water in the North Atlantic and hence the accumulation of the remineralised oxygen deficit, or a combination of both. While this study does not determine the exact cause, previously little change in productivity in has been observed in the region, suggesting ocean circulation is indirectly driving the majority of deoxygenation in the Subtropical North Atlantic, via a non-local change in remineralisation.

Oxygen is fundamental to ocean biogeochemical processes, with deoxygenation potentially reducing biodiversity, and disrupting biogeochemical cycles. In recent decades, the global ocean oxygen concentration has been decreasing, but this decrease is underestimated in numerical ocean models by as much as 50 %. Mechanisms responsible for this deoxygenation include solubility-driven deoxygenation driven by ocean warming, and changes in the amount, rates and spatial patterns of remineralisation. However, the magnitude of change in oxygen due to each process is currently unclear. Here, we use a new method to decompose oxygen change into its constituent parts by linking each process to concomitant changes in temperature and dissolved inorganic carbon. Using observations across a repeated section of the North Atlantic at 24.5° N, we show that the consistent oxygen decrease observed since 1992 in the upper 2000 m has been dominated by an increase in remineralisation-related oxygen-consumption. While warming-driven solubility changes have a much smaller impact on the upper ocean in comparison, the impact has trebled in the past twenty years, suggesting they will become an increasingly significant driver of deoxygenation with future warming. Remineralisation-related oxygen consumption peaks at a depth of approximately 600 m, where it is responsible for up to 70 % of the total deoxygenation. This remineralisation-driven change may be caused by a change in the supply of biological material to depth, a change in circulation leading to change in the residence time of water in the North Atlantic and hence the accumulation of the remineralised oxygen deficit, or a combination of both. While this study does not determine the exact cause, previously little change in productivity in has been observed in the region, suggesting ocean circulation is indirectly driving the majority of deoxygenation in the Subtropical North Atlantic, via a non-local change in remineralisation.

Oxygen is fundamental to ocean biogeochemical processes, with deoxygenation potentially reducing biodiversity, and disrupting biogeochemical cycles. In recent decades, the global ocean oxygen concentration has been decreasing, but this decrease is underestimated in numerical ocean models by as much as 50 %. Mechanisms responsible for this deoxygenation include solubility-driven deoxygenation driven by ocean warming, and changes in the amount, rates and spatial patterns of remineralisation. However, the magnitude of change in oxygen due to each process is currently unclear. Here, we use a new method to decompose oxygen change into its constituent parts by linking each process to concomitant changes in temperature and dissolved inorganic carbon. Using observations across a repeated section of the North Atlantic at 24.5° N, we show that the consistent oxygen decrease observed since 1992 in the upper 2000 m has been dominated by an increase in remineralisation-related oxygen-consumption. While warming-driven solubility changes have a much smaller impact on the upper ocean in comparison, the impact has trebled in the past twenty years, suggesting they will become an increasingly significant driver of deoxygenation with future warming. Remineralisation-related oxygen consumption peaks at a depth of approximately 600 m, where it is responsible for up to 70 % of the total deoxygenation. This remineralisation-driven change may be caused by a change in the supply of biological material to depth, a change in circulation leading to change in the residence time of water in the North Atlantic and hence the accumulation of the remineralised oxygen deficit, or a combination of both. While this study does not determine the exact cause, previously little change in productivity in has been observed in the region, suggesting ocean circulation is indirectly driving the majority of deoxygenation in the Subtropical North Atlantic, via a non-local change in remineralisation.

Changes in the hydrological cycle can affect ocean circulation and ventilation. Freshwater enters the ocean as meteoric water (MW; precipitation, river runoff, and glacial discharge) and sea ice meltwater (SIM). These inputs are traced using seawater salinity and stable oxygen isotopes in seawater, . We apply a self‐organizing map, a machine learning technique, to water mass properties to estimate the global distribution of the isotopic signature of MW by characterizing distinct salinity‐ relationships from two comprehensive data sets. The inferred is then used in a three‐endmember mixing model to provide a globally coherent MW and SIM contributions to the extratropical ocean freshwater budget. Through the use of , our results show the role of MW and SIM in dense water formation and the resulting interhemispheric asymmetry in the freshwater sources that fill the interior ocean freshwater budget. Trends drawn in ‐S space show a significant decrease in sea ice formation driving the freshening of Antarctic bottom water for the 1980–2023 period, whereas SIM is significantly increasing in parts of the Arctic halocline. The different roles of sea ice in dense water formation has implications for future ocean circulation under climate change, where machine learning techniques applied to have been proven to have utility in detecting such changes.