Publications

In climate studies, it is crucial to distinguish between changes caused by natural variability and those resulting from external forcing. Here we use a suite of numerical experiments based on the ECCO-Darwin ocean biogeochemistry model to separate the impact of the atmospheric carbon dioxide (CO₂) growth rate and climate on the ocean carbon sink - with a goal of disentangling the space-time variability of the dominant drivers. When globally integrated, the variable atmospheric growth rate and climate exhibit similar magnitude impacts on ocean carbon uptake. At local scales, interannual variability in air-sea CO₂ flux is dominated by climate. The implications of our study for real-world ocean observing systems are clear: in order to detect future changes in the ocean sink due to slowing atmospheric CO₂ growth rates, better observing systems and constraints on climate-driven ocean variability are required.

Nitrous oxide (N₂O) is a long-lived potent greenhouse gas and stratospheric ozone-depleting substance that has been accumulating in the atmosphere since the preindustrial period. The mole fraction of atmospheric N₂O has increased by nearly 25 % from 270 ppb (parts per billion) in 1750 to 336 ppb in 2022, with the fastest annual growth rate since 1980 of more than 1.3 ppb yr^-1 in both 2020 and 2021. According to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR6), the relative contribution of N₂O to the total enhanced effective radiative forcing of greenhouse gases was 6.4 % for 1750-2022. As a core component of our global greenhouse gas assessments coordinated by the Global Carbon Project (GCP), our global N₂O budget incorporates both natural and anthropogenic sources and sinks and accounts for the interactions between nitrogen additions and the biogeochemical processes that control N₂O emissions. We use bottom-up (BU: inventory, statistical extrapolation of flux measurements, and process-based land and ocean modeling) and top-down (TD: atmospheric measurement-based inversion) approaches. We provide a comprehensive quantification of global N₂O sources and sinks in 21 natural and anthropogenic categories in 18 regions between 1980 and 2020. We estimate that total annual anthropogenic N₂O emissions have increased 40% (or 1.9 Tg N yr^-1) in the past 4 decades (1980-2020). Direct agricultural emissions in 2020 (3.9 Tg N yr^-1, best estimate) represent the large majority of anthropogenic emissions, followed by other direct anthropogenic sources, including fossil fuel and industry, waste and wastewater, and biomass burning (2.1 Tg N yr), and indirect anthropogenic sources (1.3 Tg N yr^-1) . For the year 2020, our best estimate of total BU emissions for natural and anthropogenic sources was 18.5 (lower-upper bounds: 10.6-27.0) Tg N yr^-1, close to our TD estimate of 17.0 (16.6-17.4) Tg N yr^-1. For the 2010-2019 period, the annual BU decadal-average emissions for both natural and anthropogenic sources were 18.2 (10.6-25.9) Tg N yr^-1 and TD emissions were 17.4 (15.8-19.20) Tg N yr^-1. The once top emitter Europe has reduced its emissions by 31 % since the 1980s, while those of emerging economies have grown, making China the top emitter since the 2010s. The observed atmospheric N₂O concentrations in recent years have exceeded projected levels under all scenarios in the Coupled Model Intercomparison Project Phase 6 (CMIP6), underscoring the importance of reducing anthropogenic N₂O emissions. To evaluate mitigation efforts and contribute to the Global Stocktake of the United Nations Framework Convention on Climate Change, we propose the establishment of a global network for monitoring and modeling N₂O from the surface through to the stratosphere. The data presented in this work can be downloaded from https://doi.org/10.18160/RQ8P-2Z4R (Tian et al., 2023).

This study characterized ocean biological carbon pump metrics in the second iteration of the REgional Carbon Cycle Assessment and Processes (RECCAP2) project. The analysis here focused on comparisons of global and biome-scale regional patterns in particulate organic carbon (POC) production and sinking flux from the RECCAP2 ocean biogeochemical model ensemble against observational products derived from satellite remote sensing, sediment traps, and geochemical methods. There was generally good model-data agreement in mean large-scale spatial patterns, but with substantial spread across the model ensemble and observational products. The global-integrated, model ensemble-mean export production, taken as the sinking POC flux at 100 m (6.08 ± 1.17 PgC yr⁻¹), and export ratio defined as sinking flux divided by net primary production (0.154 ± 0.026) both fell at the lower end of observational estimates. Comparison with observational constraints also suggested that the model ensemble may have underestimated regional biological CO₂ drawdown and air-sea CO₂ flux in high productivity regions. Reasonable model-data agreement was found for global-integrated, ensemble-mean sinking POC flux into the deep ocean at 1,000 m (0.65 ± 0.24 PgC yr⁻¹) and the transfer efficiency defined as flux at 1,000 m divided by flux at 100 m (0.122 ± 0.041), with both variables exhibiting considerable regional variability. The RECCAP2 analysis presents standard ocean biological carbon pump metrics for assessing biogeochemical model skill, metrics that are crucial for further modeling efforts to resolve remaining uncertainties involving system-level interactions between ocean physics and biogeochemistry.

Nitrous oxide (N₂O) is a greenhouse gas and stratospheric ozone-depleting substance with large and growing anthropogenic emissions. Previous studies identified the influx of N₂O-depleted air from the stratosphere to partly cause the seasonality in tropospheric N₂O (aN₂O), but other contributions remain unclear. Here, we combine surface fluxes from eight land and four ocean models from phase 2 of the Nitrogen/N₂O Model Intercomparison Project with tropospheric transport modeling to simulate aN₂O at eight remote air sampling sites for modern and pre-industrial periods. Models show general agreement on the seasonal phasing of zonal-average N₂O fluxes for most sites, but seasonal peak-to-peak amplitudes differ several-fold across models. The modeled seasonal amplitude of surface aN₂O ranges from 0.25 to 0.80 ppb (interquartile ranges 21%-52% of median) for land, 0.14-0.25 ppb (17%-68%) for ocean, and 0.28-0.77 ppb (23%-52%) for combined flux contributions. The observed seasonal amplitude ranges from 0.34 to 1.08 ppb for these sites. The stratospheric contributions to aN₂O, inferred by the difference between the surface-troposphere model and observations, show 16%-126% larger amplitudes and minima delayed by ∼1 month compared to Northern Hemisphere site observations. Land fluxes and their seasonal amplitude have increased since the pre-industrial era and are projected to grow further under anthropogenic activities. Our results demonstrate the increasing importance of land fluxes for aN₂O seasonality. Considering the large model spread, in situ aN₂O observations and atmospheric transport-chemistry models will provide opportunities for constraining terrestrial and oceanic biosphere models, critical for projecting carbon-nitrogen cycles under ongoing global warming.

Abstract. The ocean carbon sink plays a critical role in climate, absorbing anthropogenic carbon from the atmosphere and mitigating climate change. The sink shows significant variability on decadal timescales, but estimates from models and observations disagree with one another, raising uncertainty over the magnitude of the sink, its variability, and its driving mechanisms. There is a need to reconcile observation-based estimates of air-sea CO₂ fluxes with those of the changing ocean carbon inventory in order to improve our understanding of the sink, and doing so requires knowledge of how carbon is transported within the interior by the ocean circulation. Here we employ a recently developed optimal transformation method (OTM) that uses water-mass theory to relate interior changes in tracer distributions to transports and mixing and boundary forcings, and we extend its application to include carbon using synthetic data. We validate the method using model outputs from a biogeochemical state estimate, and we test its ability to recover boundary carbon fluxes and interior transports consistent with changes in heat, salt, and carbon. Our results show that the OTM effectively reconciles boundary carbon fluxes with interior carbon distributions when given a range of prior fluxes. The OTM shows considerable skill in its reconstructions, reducing root-mean-squared errors from biased priors between model "truth" and reconstructed boundary carbon fluxes by up to 71 %, with the bias of the reconstructions consistently ≤0.06 molCm^-2yr^-1 globally. Inter-basin transports of carbon also compare well with the model truth, with residuals <0.25 Pg C yr⁻¹ for reconstructions produced using a range of priors. The OTM has significant potential for application to reconcile observational estimates of air-sea CO₂ fluxes with the interior accumulation of anthropogenic carbon.

Greenland's marine-terminating glaciers connect the ice sheet to the ocean and provide a critical boundary where heat, freshwater, and nutrient exchanges take place. Buoyant freshwater runoff from inland ice sheet melt is discharged at the base of marine-terminating glaciers, forming vigorous upwelling plumes. It is understood that subglacial plumes modify waters near glacier fronts and increase submarine glacier melt by entraining warm ambient waters at depth. However, ocean observations along Greenland's coastal margins remain biased toward summer months which limits accurate estimation of ocean forcing on glacier retreat and acceleration. Here, we fill a key observational gap in northwest Greenland by describing seasonal hydrographic variation at glacier fronts in Melville Bay using in situ observations from moorings deployed year-round, CTDs, and profiling floats. We evaluated local and remote forcing using remote sensing and reanalysis data products alongside a high-resolution ocean model. Analysis of the year-round hydrographic data revealed consistent above-sill seasonality in temperature and salinity. The warmest, saltiest waters occurred in spring (April-May) and primed glaciers for enhanced submarine melt in summer when meltwater plumes entrain deep waters. Waters were coldest and freshest in early winter (November-December) after summer melt from sea ice, glacier ice, and icebergs provided cold freshwater along the shelf. Ocean variability was greatest in the summer and fall, coincident with increased freshwater runoff and large wind events before winter sea ice formation. Results increase our mechanistic understanding of Greenland ice-ocean interactions and enable improvements in ocean model parameterization.

The Pine Island and Thwaites Ice Shelves (PIIS/TIS) in the Amundsen Sea are melting rapidly and impacting global sea levels. The thermocline depth (TD) variability, the interface between cold Winter Water and warm modified Circumpolar Deep Water (mCDW), at the PIIS/TIS front strongly correlates with basal melt rates, but the drivers of its interannual variability remain uncertain. Here, using an ocean model, we propose that the strength of the eastern Amundsen Sea on-shelf circulation primarily controls TD variability and consequent PIIS/TIS melt rates. The TD variability occurs because the on-shelf circulation meanders following the submarine glacial trough, creating vertical velocity through bottom Ekman dynamics. We suggest that a strong or weak ocean circulation, possibly linked to remote winds in the Bellingshausen Sea, generates corresponding changes in bottom Ekman convergence, which modulates mCDW upwelling and TD variability. We show that interannual variability of off-shelf zonal winds has a minor effect on ocean heat intrusion into PIIS/TIS cavities, contrary to the widely accepted concept.

We investigate the use of atmospheric oxygen (O₂) and carbon dioxide (CO₂) measurements for the estimation of the fossil fuel component of atmospheric CO₂ in the UK. Atmospheric potential oxygen (APO) - a tracer that combines O₂ and CO₂, minimizing the influence of terrestrial biosphere fluxes - is simulated at three sites in the UK, two of which make APO measurements. We present a set of model experiments that estimate the sensitivity of APO simulations to key inputs: fluxes from the ocean, fossil fuel flux magnitude and distribution, the APO baseline, and the exchange ratio of O₂ to CO₂ fluxes from fossil fuel combustion and the terrestrial biosphere. To estimate the influence of uncertainties in ocean fluxes, we compare three ocean O₂ flux estimates from the NEMO-ERSEM, the ECCO-Darwin ocean model, and the Jena CarboScope (JC) APO inversion. The sensitivity of APO to fossil fuel emission magnitudes and to terrestrial biosphere and fossil fuel exchange ratios is investigated through Monte Carlo sampling within literature uncertainty ranges and by comparing different inventory estimates. We focus our model-data analysis on the year 2015 as ocean fluxes are not available for later years. As APO measurements are only available for one UK site at this time, our analysis focuses on the Weybourne station. Model-data comparisons for two additional UK sites (Heathfield and Ridge Hill) in 2021, using ocean flux climatologies, are presented in the Supplement. Of the factors that could potentially compromise simulated APO-derived fossil fuel CO₂ (ffCO₂) estimates, we find that the ocean O₂ flux estimate has the largest overall influence at the three sites in the UK. At times, this influence is comparable in magnitude to the contribution of simulated fossil fuel CO₂ to simulated APO. We find that simulations using different ocean fluxes differ from each other substantially. No single model estimate, or a model estimate that assumed zero ocean flux, provided a significantly closer fit than any other. Furthermore, the uncertainty in the ocean contribution to APO could lead to uncertainty in defining an appropriate regional background from the data. Our findings suggest that the contribution of non-terrestrial sources needs to be better accounted for in model simulations of APO in the UK to reduce the potential influence on inferred fossil fuel CO₂ using APO.

The ice shelves in the Bellingshausen Sea are melting and thinning rapidly due to modified Circumpolar Deep Water (mCDW) intrusions carrying heat toward ice-shelf cavities. Observations are, however, sparse in time and space, and extensive model-data comparisons have never been possible. Here, using a circulation model of the region and ship-based observations, we show that the simulated water mass distributions in several troughs traversing mCDW inflows are in good agreement with observations, implying that our model has the skills to simulate hydrographic structures as well as on-shelf ocean circulations. It takes 7.9 and 11.7 months for mCDW to travel to the George VI Ice Shelf cavities through the Belgica and Marguerite troughs, respectively. Ice-shelf melting is mainly caused by mCDW intrusions along the Belgica and Marguerite troughs, with the heat transport through the former being ∼2.8 times larger than that through the latter. The mCDW intrusions toward the George VI Ice Shelf show little seasonal variability, while those toward the Venable Ice Shelf show seasonal variability, with higher velocities in summer likely caused by coastal trapped waves. We also conduct particle experiments tracking glacial meltwater. After 2 years of model integration, ∼33% of the released particles are located in the Amundsen Sea, supporting a linkage between Bellingshausen Sea ice-shelf meltwater and Amundsen Sea upper ocean hydrography.

In recent decades, the Greenland ice sheet has been losing mass through glacier retreat and ice flow acceleration. This mass loss is linked with variations in submarine melt, yet existing ocean models are either coarse global simulations focused on decadal-scale variability or fine-scale simulations for process-based investigations. Here, we unite these scales with a framework to downscale from a global state estimate (15 km) into a regional model (3 km) that resolves circulation on the continental shelf. We further downscale into a fjord-scale model (500 m) that resolves circulation inside fjords and quantifies melt. We demonstrate this approach in Scoresby Sund, East Greenland, and find that interannual variations in submarine melt at Daugaard-Jensen glacier induced by ocean temperature variability are consistent with the decadal changes in glacier ice dynamics. This study provides a framework by which coarse-resolution models can be refined to quantify glacier submarine melt for future ice sheet projections.

In the boreal spring of 2023, an extreme coastal El Niño struck the coastal regions of Peru and Ecuador, causing devastating rainfalls, flooding, and record dengue outbreaks. Observations and ocean model experiments reveal that northerly alongshore winds and westerly wind anomalies in the eastern equatorial Pacific, initially associated with a record-strong Madden-Julian Oscillation and cyclonic disturbance off Peru in March, drove the coastal warming through suppressed coastal upwelling and downwelling Kelvin waves. Atmospheric model simulations indicate that the coastal warming in turn favors the observed wind anomalies over the far eastern tropical Pacific by triggering atmospheric deep convection. This implies a positive feedback between the coastal warming and the winds, which further amplifies the coastal warming. In May, the seasonal background cooling precludes deep convection and the coastal Bjerknes feedback, leading to the weakening of the coastal El Niño. This coastal El Niño is rare but predictable at 1 month lead, which is useful to protect lives and properties.

Melting of ice shelves can energize a wide range of ocean currents, from three-dimensional turbulence to relatively large-scale boundary currents. Here, we conduct high-resolution simulations of the western Amundsen Sea to show that submesoscale eddies are prevalent inside ice shelf cavities. The simulations indicate energetic submesoscale eddies at the top and bottom ocean boundary layers, regions with sharp topographic slopes and strong lateral buoyancy gradients. These eddies play a substantial role in the vertical and lateral (along-isopycnal) heat advection toward the ice shelf base, enhancing the basal melting in all simulated cavities. In turn, the meltwater provides strong buoyancy gradients that energize the submesoscale variability, forming a positive loop that could affect the overall efficiency of heat exchange between the ocean and the ice shelf cavity. Our study implies that submesoscale-induced enhancement of basal melting may be a ubiquitous process that needs to be parameterized in coarse-resolution climate models.

The Paris Agreement calls for emissions reductions to limit climate change, but how will the carbon cycle change if it is successful? The land and oceans currently absorb roughly half of anthropogenic emissions, but this fraction will decline in the future. The amount of carbon that can be released before climate is mitigated depends on the amount of carbon the ocean and terrestrial ecosystems can absorb. Policy is based on model projections, but observations and theory suggest that climate effects emerging in today's climate will increase and carbon cycle tipping points may be crossed. Warming temperatures, drought, and a slowing growth rate of CO₂ itself will reduce land and ocean sinks and create new sources, making carbon sequestration in forests, soils, and other land and aquatic vegetation more difficult. Observations, data-assimilative models, and prediction systems are needed for managing ongoing long-term changes to land and ocean systems after achieving net-zero emissions.

• International agreements call for stabilizing climate at 1.5° above preindustrial, while the world is already seeing damaging extremes below that.
• If climate is stabilized near the 1.5° target, the driving force for most sinks will slow, while feedbacks from the warmer climate will continue to cause sources.
• Once emissions are reduced to net zero, carbon cycle-climate feedbacks will require observations to support ongoing active management to maintain storage.

Based on latest estimates (e.g., https://sealevel.nasa.gov), global mean sea level has risen nearly 100 mm since 1993. However, the rate of rise has not been constant in space or time and recent observations (since ∼2008) reveal pronounced regional acceleration in the Gulf of Mexico (GoM). Here we use model solutions and observational data to identify the physical mechanisms responsible for enhanced rates of coastal sea-level rise in this region. We quantify the effect of offshore subsurface ocean warming on coastal sea-level rise and its relationship to regional hypsometry, the distribution of ocean area with depth. Using an Estimating the Circulation and Climate of the Ocean (ECCO) state estimate, we establish that coastal sea-level changes at the 10-year timescale are largely the result of changes in regional ocean mass, reflected in ocean bottom pressure. These coastal bottom pressure changes reflect both net mass flux into the Gulf, as well as internal mass redistribution within the Gulf, which can be understood as an isostatic ocean response to subsurface warming. We test the relationships among coastal sea-level, bottom pressure, and subsurface warming identified in ECCO using observations from satellite gravimetry, altimetry, tide gauges, and Argo floats. Estimates of mass redistribution explain a significant fraction of coastal sea-level trends observed by tide gauges. For instance, at St. Petersburg, Florida, this mass redistribution mechanism accounts for >50% of the coastal sea-level trend observed between 2008 and 2017. This study thus elucidates a physical mechanism whereby coastal sea-level responds to open-ocean subsurface density change.

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO₂) and releasing nitrous oxide (N₂O) and methane (CH₄). In this second phase of the Regional Carbon Cycle Assessment and Processes (RECCAP2), we quantify global coastal ocean fluxes of CO₂, N₂O and CH₄ using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO₂ in both observational products and models, but the magnitude of the median net global coastal uptake is ∼60% larger in models (−0.72 vs. −0.44 PgC year⁻¹, 1998-2018, coastal ocean extending to 300 km offshore or 1,000 m isobath with area of 77 million km²). We attribute most of this model-product difference to the seasonality in sea surface CO₂ partial pressure at mid- and high-latitudes, where models simulate stronger winter CO₂ uptake. The coastal ocean CO₂ sink has increased in the past decades but the available time-resolving observation-based products and models show large discrepancies in the magnitude of this increase. The global coastal ocean is a major source of N₂O (+0.70 PgCO₂-e year⁻¹ in observational product and +0.54 PgCO₂ -e year⁻¹ in model median) and CH₄ (+0.21 PgCO₂-e year⁻¹ in observational product), which offsets a substantial proportion of the coastal CO₂ uptake in the net radiative balance (30%-60% in CO₂-equivalents), highlighting the importance of considering the three greenhouse gases when examining the influence of the coastal ocean on climate.

Melt rates of West Antarctic ice shelves in the Amundsen Sea track large decadal variations in the volume of warm water at their outlets. This variability is generally attributed to wind-driven variations in warm water transport toward ice shelves. Inspired by conceptual representations of the global overturning circulation, we introduce a simple model for the evolution of the thermocline, which caps the warm water layer at the ice-shelf front. This model demonstrates that interannual variations in coastal polynya buoyancy forcing can generate large decadal-scale thermocline depth variations, even when the supply of warm water from the shelf-break is fixed. The modeled variability involves transitions between bistable high and low melt regimes, enabled by feedbacks between basal melt rates and ice front stratification strength. Our simple model captures observed variations in near-coast thermocline depth and stratification strength, and poses an alternative mechanism for warm water volume changes to wind-driven theories.

As a contribution to the Regional Carbon Cycle Assessment and Processes phase 2 (RECCAP2) project, we present synthesized estimates of Arctic Ocean sea-air CO₂ fluxes and their uncertainties from surface ocean pCO₂ -observation products, ocean biogeochemical hindcast and data assimilation models, and atmospheric inversions. For the period of 1985-2018, the Arctic Ocean was a net sink of CO₂ of 116 ± 4 TgC yr⁻¹ in the pCO₂ products, 92 ± 30 TgC yr⁻¹ in the models, and 91 ± 21 TgC yr⁻¹ in the atmospheric inversions. The CO₂ uptake peaks in late summer and early autumn, and is low in winter when sea ice inhibits sea-air fluxes. The long-term mean CO₂ uptake in the Arctic Ocean is primarily caused by steady-state fluxes of natural carbon (70% ± 15%), and enhanced by the atmospheric CO₂ increase (19% ± 5%) and climate change (11% ± 18%). The annual mean CO₂ uptake increased from 1985 to 2018 at a rate of 31 ± 13 TgC yr⁻¹  dec⁻¹ in the pCO₂ products, 10 ± 4 TgC yr⁻¹  dec ⁻¹ in the models, and 32 ± 16 TgC yr⁻¹  dec⁻¹ in the atmospheric inversions. Moreover, 77% ± 38% of the trend in the net CO₂ uptake over time is caused by climate change, primarily due to rapid sea ice loss in recent years. Furthermore, true uncertainties may be larger than the given ensemble standard deviations due to common structural biases across all individual estimates.

This contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985-2018, using a combination of models and observation-based products. The mean sea-air CO₂ flux from 1985 to 2018 is −1.6 ± 0.2 PgC yr⁻¹ based on an ensemble of reconstructions of the history of sea surface pCO₂ (pCO₂ products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO₂ , which is estimated at −2.1 ± 0.3 PgC yr⁻¹ by an ensemble of ocean biogeochemical models, and −2.4 ± 0.1 PgC yr⁻¹ by two ocean circulation inverse models. The ocean also degasses about 0.65 ± 0.3 PgC yr⁻¹ of terrestrially derived CO₂ , but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO₂ products reconstruct a trend in the ocean carbon sink of −0.61 ± 0.12 PgC yr⁻¹  decade⁻¹ , while biogeochemical models and inverse models diagnose an anthropogenic CO₂ -driven trend of −0.34 ± 0.06 and −0.41 ± 0.03 PgC yr⁻¹  decade⁻¹, respectively. This implies a climate-forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate-driven variability exceeding the CO₂-forced variability by 2-3 times. These results suggest that anthropogenic CO₂ dominates the ocean CO₂ sink, while climate-driven variability is potentially large but highly uncertain and not consistently captured across different methods.

Climate change is affecting a wide range of global systems, with polar ecosystems experiencing the most rapid change. Although climate impacts affect lower-trophic-level and short-lived species most directly, it is less clear how long-lived and mobile species will respond to rapid polar warming because they may have the short-term ability to accommodate ecological disruptions while adapting to new conditions. We found that the population dynamics of an iconic and highly mobile polar-associated species are tightly coupled to Arctic prey availability and access to feeding areas. When low prey biomass coincided with high ice cover, gray whales experienced major mortality events, each reducing the population by 15 to 25%. This suggests that even mobile, long-lived species are sensitive to dynamic and changing conditions as the Arctic warms.

Iceberg A-68 separated from the Larsen C Ice Shelf in July 2017 and the impact of this event on the local ocean circulation has yet to be assessed. Here, we conduct numerical simulations of ocean dynamics near and below the ice shelf pre- and post-calving. Results agree with in situ and remote observations of the area as they indicate that basal melt is primarily controlled by wintertime sea-ice formation, which in turn produces High Salinity Shelf Water (HSSW). After the calving event, we simulate a 50% increase in HSSW intrusion under the ice shelf, enhancing ocean heat delivery by 30%. This results in doubling of the melt rate under Gipps Ice Rise, suggesting a positive feedback for further retreat that could destabilize the Larsen C Ice Shelf. Assessing the impact of ice-front retreat on the heat delivery under the ice is crucial to better understand ice-shelf dynamics in a warming environment.

Ice shelf basal melting is the primary mechanism driving mass loss from the Antarctic Ice Sheet, yet it is unknown how the localized melt enhancement from subglacial discharge will affect future Antarctic glacial retreat. We develop a parameterization of ice shelf basal melt that accounts for both ocean and subglacial discharge forcing and apply it in future projections of Denman and Scott Glaciers, East Antarctica, through 2300. In forward simulations, subglacial discharge accelerates the onset of retreat of these systems into the deepest continental trench on Earth by 25 years. During this retreat, Denman Glacier alone contributes 0.33 millimeters per year to global sea level rise, comparable to half of the contemporary sea level contribution of the entire Antarctic Ice Sheet. Our results stress the importance of resolving complex interactions between the ice, ocean, and subglacial environments in future Antarctic Ice Sheet projections.

We assess the Southern Ocean CO₂ uptake (1985-2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO₂ with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr⁻¹ ) and pCO₂ -observation-based products (0.73 ± 0.07 PgC yr⁻¹ ). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present-day net uptake is to first order a response to rising atmospheric CO₂ , driving large amounts of anthropogenic CO₂ (C_ant ) into the ocean, thereby overcompensating the loss of natural CO₂ to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO₂ -products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr⁻¹  decade⁻¹ , respectively). This is despite nearly identical pCO₂ trends in GOBMs and pCO₂ -products when both products are compared only at the locations where pCO₂ was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non-thermal processes of biology and mixing/circulation. Ocean interior accumulation of C_ant points to an underestimate of C_ant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle.

Meltwater content and pathways determine the impact of Antarctica's melting ice shelves on ocean circulation and climate. Using ocean glider observations, we quantify meltwater distribution and transport within the Bellingshausen Sea's Belgica Trough. Meltwater is present at different densities and with different turbidities: both are indicative of a layer's ice shelf of origin. To investigate how ice-shelf origin separates meltwater into different export pathways, we compare these observations with high-resolution tracer-release model simulations. Meltwater filaments branch off the Antarctic Coastal Current into the southwestern trough. Meltwater also enters the Belgica Trough in the northwest via an extended western pathway, hence the greater observed southward (0.50 mSv) than northward (0.17 mSv) meltwater transport. Together, the observations and simulations reveal meltwater retention within a cyclonic in-trough gyre, which has the potential to promote climactically important feedbacks on circulation and future melting.

Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up, and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: 1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation and 2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies is an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations.