Articles | Volume 21, issue 3
https://doi.org/10.5194/os-21-1167-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-21-1167-2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Research article
| 
01 Jul 2025
Research article |
| 01 Jul 2025
| 01 Jul 2025
Dual-tracer constraints on the inverse Gaussian transit time distribution improve the estimation of water mass ages and their temporal trends in the tropical thermocline
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Wolfgang Koeve
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Andreas Oschlies
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Kiel University, Kiel, Germany
Yan-Chun He
Nansen Environmental and Remote Sensing Center, Bjerknes Centre for Climate Research, Bergen, Norway
Tronje Peer Kemena
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Lennart Gerke
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Iris Kriest
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
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Lennart Gerke, Toste Tanhua, William A. Nesbitt, Samuel W. Stevens, and Douglas W. R. Wallace
Ocean Sci., 22, 1391–1407, https://doi.org/10.5194/os-22-1391-2026, https://doi.org/10.5194/os-22-1391-2026, 2026
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Transient tracer data, measured for the first time in 2022 in the Gulf of St. Lawrence, reveal older deep waters in the east than the west, contrary to expected estuarine circulation, indicating increased influence of older, warmer, less oxygenated North Atlantic Central Water over younger, oxygen-rich Labrador Current Water. While consistent with previous reports of increasing NACW contribution, our results contradict claims of a complete shift to NACW by 2021, showing that LCW still persists.
Lina Garcia-Suarez, Katja Fennel, Neha Mehendale, Tronje Kemena, and David P. Keller
Ocean Sci., 22, 1183–1193, https://doi.org/10.5194/os-22-1183-2026, https://doi.org/10.5194/os-22-1183-2026, 2026
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This study shows that regional ocean warming can make the Gulf Stream appear to shift north more rapidly than it actually does. Temperature-based proxies, like the Gulf Stream North Wall, overestimate changes in its position. Methods based on sea surface height provide a more accurate view. These results help improve how we track changes in ocean currents and avoid misinterpreting signs of climate-related shifts.
Chiara Ciscato, Neha Mehendale, Tronje P. Kemena, Sandy Avrutin, and David P. Keller
EGUsphere, https://doi.org/10.5194/egusphere-2026-926, https://doi.org/10.5194/egusphere-2026-926, 2026
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While reducing emissions, technologies are being developed to sequester carbon dioxide (CO2) from the air. Among the proposed options is ocean alkalinity enhancement (OAE), which works to lower the CO2 concentration in seawater and accelerate its uptake from the atmosphere as a compensatory effect. As seasonality is important for the annual air-sea CO2 exchange, this research investigates OAE impacts on the seasonal carbon cycle analysing outputs from a numerical earth system model.
Jakob Dörr, Carlo Mans, Marius Årthun, Kristofer Döös, Dafydd Gwyn Evans, and Yanchun He
Ocean Sci., 22, 565–585, https://doi.org/10.5194/os-22-565-2026, https://doi.org/10.5194/os-22-565-2026, 2026
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The Arctic Ocean plays a key role in the global ocean circulation by producing dense waters that feed the lower limb of the Atlantic meridional overturning circulation (AMOC). We use a high-resolution ocean simulation to investigate the pathways and mechanisms through which these dense waters are formed in the Arctic. Our results show that surface cooling in the Barents Sea dominates the dense water production, but that internal mixing plays a role at high densities.
Wolfgang Koeve and Ivy Frenger
EGUsphere, https://doi.org/10.5194/egusphere-2026-304, https://doi.org/10.5194/egusphere-2026-304, 2026
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Apparent oxygen utilization (AOU) is widely used to diagnose oceanic oxygen loss and marine carbon cycle changes because it can be derived from observations and climate models. However, it assumes perfect oxygen equilibration at the ocean surface, an assumption that introduces uncertainty. Using an Earth System model of intermediate complexity, we reassess the validity of the AOU approximation for steady-state and future climate scenarios.
William A. Nesbitt, Alfonso O. Mucci, Toste Tanhua, Yves Gélinas, Jean-Éric Tremblay, Gwénaëlle Chaillou, Ludovic Pascal, Caroline Fradette, Lennart Gerke, Samuel W. Stevens, Mathilde Jutras, Marjolaine Blais, Martine Lizotte, Michel Starr, and Douglas W. R. Wallace
Earth Syst. Sci. Data Discuss., https://doi.org/10.5194/essd-2026-7, https://doi.org/10.5194/essd-2026-7, 2026
Revised manuscript under review for ESSD
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Over the past few decades, the St. Lawrence Estuary and Gulf have shown clear trends in oxygen depletion and acidification. This data description paper brings together twenty years of measurements from the St. Lawrence Estuary, Gulf of St. Lawrence, and Saguenay Fjord, carefully performing quality control procedures on data, and makes them publicly available. The resulting dataset supports future research, monitoring, and environmental management in this sensitive marine system.
Chia-Te Chien, Markus Pahlow, Christopher J. Somes, Markus Schartau, and Andreas Oschlies
EGUsphere, https://doi.org/10.5194/egusphere-2025-4907, https://doi.org/10.5194/egusphere-2025-4907, 2025
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We aim to investigate the variability and drivers of atmospheric pCO2 drawdown during the Last Glacial Maximum using an ensemble of model simulations. Our ensemble explores the effects of parameter uncertainty and different forcing scenarios on marine biogeochemistry and pCO2. We show that changes in ocean circulation, nutrient utilization, and iron availability regulate the efficiency of the biological carbon pump, accounting for a substantial portion of the glacial–interglacial pCO2 change.
Lise Seland Graff, Jerry Tjiputra, Ada Gjermundsen, Andreas Born, Jens Boldingh Debernard, Heiko Goelzer, Yan-Chun He, Petra Margaretha Langebroek, Aleksi Nummelin, Dirk Olivié, Øyvind Seland, Trude Storelvmo, Mats Bentsen, Chuncheng Guo, Andrea Rosendahl, Dandan Tao, Thomas Toniazzo, Camille Li, Stephen Outten, and Michael Schulz
Earth Syst. Dynam., 16, 1671–1698, https://doi.org/10.5194/esd-16-1671-2025, https://doi.org/10.5194/esd-16-1671-2025, 2025
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The magnitude of future Arctic amplification is highly uncertain. Using the Norwegian Earth System Model, we explore the effect of improving the representation of clouds, ocean eddies, the Greenland ice sheet, sea ice, and ozone on the projected Arctic winter warming in a coordinated experiment set. These improvements all lead to enhanced projected Arctic warming, with the largest changes found in the sea ice retreat regions and the largest uncertainty found on the Atlantic side.
Timothée Bourgeois, Giang T. Tran, Aurich Jeltsch-Thömmes, Jörg Schwinger, Friederike Fröb, Thomas L. Frölicher, Thorsten Blenckner, Olivier Torres, Jean Negrel, David P. Keller, Andreas Oschlies, Laurent Bopp, and Fortunat Joos
Biogeosciences, 22, 5435–5462, https://doi.org/10.5194/bg-22-5435-2025, https://doi.org/10.5194/bg-22-5435-2025, 2025
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Anthropogenic greenhouse gas emissions significantly impact ocean ecosystems through climate change and acidification, leading to either progressive or abrupt changes. This study maps the crossing of physical and ecological limits for various ocean impact metrics under three emission scenarios. Using Earth system models, we identify when these limits are exceeded, highlighting the urgent need for ambitious climate action to safeguard the world's oceans and ecosystems.
William A. Nesbitt, Samuel W. Stevens, Alfonso O. Mucci, Lennart Gerke, Toste Tanhua, Gwénaëlle Chaillou, and Douglas W. R. Wallace
Ocean Sci., 21, 2179–2195, https://doi.org/10.5194/os-21-2179-2025, https://doi.org/10.5194/os-21-2179-2025, 2025
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We combine two decades of oxygen data with new carbon observations and a tracer-informed model to quantify oxygen loss and carbon buildup in the deep waters of the Gulf and Lower St. Lawrence Estuary. We then test a novel idea: reoxygenating these waters with the oxygen produced as a by-product from green-hydrogen production. Our results suggest this could significantly reduce hypoxia, though full recovery would require larger inputs.
Miriam Tivig, David P. Keller, and Andreas Oschlies
Biogeosciences, 21, 4469–4493, https://doi.org/10.5194/bg-21-4469-2024, https://doi.org/10.5194/bg-21-4469-2024, 2024
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Marine biological production is highly dependent on the availability of nitrogen and phosphorus. Rivers are the main source of phosphorus to the oceans but poorly represented in global model oceans. We include dissolved nitrogen and phosphorus from river export in a global model ocean and find that the addition of riverine phosphorus affects marine biology on millennial timescales more than riverine nitrogen alone. Globally, riverine phosphorus input increases primary production rates.
Na Li, Christopher J. Somes, Angela Landolfi, Chia-Te Chien, Markus Pahlow, and Andreas Oschlies
Biogeosciences, 21, 4361–4380, https://doi.org/10.5194/bg-21-4361-2024, https://doi.org/10.5194/bg-21-4361-2024, 2024
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N is a crucial nutrient that limits phytoplankton growth in large ocean areas. The amount of oceanic N is governed by the balance of N2 fixation and denitrification. Here we incorporate benthic denitrification into an Earth system model with variable particulate stoichiometry. Our model compares better to the observed surface nutrient distributions, marine N2 fixation, and primary production. Benthic denitrification plays an important role in marine N and C cycling and hence the global climate.
Tianfei Xue, Jens Terhaar, A. E. Friederike Prowe, Thomas L. Frölicher, Andreas Oschlies, and Ivy Frenger
Biogeosciences, 21, 2473–2491, https://doi.org/10.5194/bg-21-2473-2024, https://doi.org/10.5194/bg-21-2473-2024, 2024
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Phytoplankton play a crucial role in marine ecosystems. However, climate change's impact on phytoplankton biomass remains uncertain, particularly in the Southern Ocean. In this region, phytoplankton biomass within the water column is likely to remain stable in response to climate change, as supported by models. This stability arises from a shallower mixed layer, favoring phytoplankton growth but also increasing zooplankton grazing due to phytoplankton concentration near the surface.
Andreas Oschlies, Lennart T. Bach, Rosalind E. M. Rickaby, Terre Satterfield, Romany Webb, and Jean-Pierre Gattuso
State Planet, 2-oae2023, 1, https://doi.org/10.5194/sp-2-oae2023-1-2023, https://doi.org/10.5194/sp-2-oae2023-1-2023, 2023
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Reaching promised climate targets will require the deployment of carbon dioxide removal (CDR). Marine CDR options receive more and more interest. Based on idealized theoretical studies, ocean alkalinity enhancement (OAE) appears as a promising marine CDR method. We provide an overview on the current situation of developing OAE as a marine CDR method and describe the history that has led to the creation of the OAE research best practice guide.
Katja Fennel, Matthew C. Long, Christopher Algar, Brendan Carter, David Keller, Arnaud Laurent, Jann Paul Mattern, Ruth Musgrave, Andreas Oschlies, Josiane Ostiguy, Jaime B. Palter, and Daniel B. Whitt
State Planet, 2-oae2023, 9, https://doi.org/10.5194/sp-2-oae2023-9-2023, https://doi.org/10.5194/sp-2-oae2023-9-2023, 2023
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This paper describes biogeochemical models and modelling techniques for applications related to ocean alkalinity enhancement (OAE) research. Many of the most pressing OAE-related research questions cannot be addressed by observation alone but will require a combination of skilful models and observations. We present illustrative examples with references to further information; describe limitations, caveats, and future research needs; and provide practical recommendations.
Iris Kriest, Julia Getzlaff, Angela Landolfi, Volkmar Sauerland, Markus Schartau, and Andreas Oschlies
Biogeosciences, 20, 2645–2669, https://doi.org/10.5194/bg-20-2645-2023, https://doi.org/10.5194/bg-20-2645-2023, 2023
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Global biogeochemical ocean models are often subjectively assessed and tuned against observations. We applied different strategies to calibrate a global model against observations. Although the calibrated models show similar tracer distributions at the surface, they differ in global biogeochemical fluxes, especially in global particle flux. Simulated global volume of oxygen minimum zones varies strongly with calibration strategy and over time, rendering its temporal extrapolation difficult.
Jiajun Wu, David P. Keller, and Andreas Oschlies
Earth Syst. Dynam., 14, 185–221, https://doi.org/10.5194/esd-14-185-2023, https://doi.org/10.5194/esd-14-185-2023, 2023
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In this study we investigate an ocean-based carbon dioxide removal method: macroalgae open-ocean mariculture and sinking (MOS), which aims to cultivate seaweed in the open-ocean surface and to sink matured biomass quickly to the deep seafloor. Our results suggest that MOS has considerable potential as an ocean-based CDR method. However, MOS has inherent side effects on marine ecosystems and biogeochemistry, which will require careful evaluation beyond this first idealized modeling study.
Chia-Te Chien, Jonathan V. Durgadoo, Dana Ehlert, Ivy Frenger, David P. Keller, Wolfgang Koeve, Iris Kriest, Angela Landolfi, Lavinia Patara, Sebastian Wahl, and Andreas Oschlies
Geosci. Model Dev., 15, 5987–6024, https://doi.org/10.5194/gmd-15-5987-2022, https://doi.org/10.5194/gmd-15-5987-2022, 2022
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We present the implementation and evaluation of a marine biogeochemical model, Model of Oceanic Pelagic Stoichiometry (MOPS) in the Flexible Ocean and Climate Infrastructure (FOCI) climate model. FOCI-MOPS enables the simulation of marine biological processes, the marine carbon, nitrogen and oxygen cycles, and air–sea gas exchange of CO2 and O2. As shown by our evaluation, FOCI-MOPS shows an overall adequate performance that makes it an appropriate tool for Earth climate system simulations.
Sophy Oliver, Coralia Cartis, Iris Kriest, Simon F. B Tett, and Samar Khatiwala
Geosci. Model Dev., 15, 3537–3554, https://doi.org/10.5194/gmd-15-3537-2022, https://doi.org/10.5194/gmd-15-3537-2022, 2022
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Global ocean biogeochemical models are used within Earth system models which are used to predict future climate change. However, these are very computationally expensive to run and therefore are rarely routinely improved or calibrated to real oceanic observations. Here we apply a new, fast optimisation algorithm to one such model and show that it can calibrate the model much faster than previously managed, therefore encouraging further ocean biogeochemical model improvements.
Tianfei Xue, Ivy Frenger, A. E. Friederike Prowe, Yonss Saranga José, and Andreas Oschlies
Biogeosciences, 19, 455–475, https://doi.org/10.5194/bg-19-455-2022, https://doi.org/10.5194/bg-19-455-2022, 2022
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The Peruvian system supports 10 % of the world's fishing yield. In the Peruvian system, wind and earth’s rotation bring cold, nutrient-rich water to the surface and allow phytoplankton to grow. But observations show that it grows worse at high upwelling. Using a model, we find that high upwelling happens when air mixes the water the most. Then phytoplankton is diluted and grows slowly due to low light and cool upwelled water. This study helps to estimate how it might change in a warming climate.
Karin Kvale, David P. Keller, Wolfgang Koeve, Katrin J. Meissner, Christopher J. Somes, Wanxuan Yao, and Andreas Oschlies
Geosci. Model Dev., 14, 7255–7285, https://doi.org/10.5194/gmd-14-7255-2021, https://doi.org/10.5194/gmd-14-7255-2021, 2021
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We present a new model of biological marine silicate cycling for the University of Victoria Earth System Climate Model (UVic ESCM). This new model adds diatoms, which are a key aspect of the biological carbon pump, to an existing ecosystem model. Our modifications change how the model responds to warming, with net primary production declining more strongly than in previous versions. Diatoms in particular are simulated to decline with climate warming due to their high nutrient requirements.
Maria-Theresia Verwega, Christopher J. Somes, Markus Schartau, Robyn Elizabeth Tuerena, Anne Lorrain, Andreas Oschlies, and Thomas Slawig
Earth Syst. Sci. Data, 13, 4861–4880, https://doi.org/10.5194/essd-13-4861-2021, https://doi.org/10.5194/essd-13-4861-2021, 2021
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This work describes a ready-to-use collection of particulate organic carbon stable isotope ratio data sets. It covers the 1960s–2010s and all main oceans, providing meta-information and gridded data. The best coverage exists in Atlantic, Indian and Southern Ocean surface waters during the 1990s. It indicates no major difference between methods and shows decreasing values towards high latitudes, with the lowest in the Southern Ocean, and a long-term decline in all regions but the Southern Ocean.
Miriam Tivig, David P. Keller, and Andreas Oschlies
Biogeosciences, 18, 5327–5350, https://doi.org/10.5194/bg-18-5327-2021, https://doi.org/10.5194/bg-18-5327-2021, 2021
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Nitrogen is one of the most important elements for life in the ocean. A major source is the riverine discharge of dissolved nitrogen. While global models often omit rivers as a nutrient source, we included nitrogen from rivers in our Earth system model and found that additional nitrogen affected marine biology not only locally but also in regions far off the coast. Depending on regional conditions, primary production was enhanced or even decreased due to internal feedbacks in the nitrogen cycle.
Henrike Schmidt, Julia Getzlaff, Ulrike Löptien, and Andreas Oschlies
Ocean Sci., 17, 1303–1320, https://doi.org/10.5194/os-17-1303-2021, https://doi.org/10.5194/os-17-1303-2021, 2021
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Oxygen-poor regions in the open ocean restrict marine habitats. Global climate simulations show large uncertainties regarding the prediction of these areas. We analyse the representation of the simulated oxygen minimum zones in the Arabian Sea using 10 climate models. We give an overview of the main deficiencies that cause the model–data misfit in oxygen concentrations. This detailed process analysis shall foster future model improvements regarding the oxygen minimum zone in the Arabian Sea.
Jaard Hauschildt, Soeren Thomsen, Vincent Echevin, Andreas Oschlies, Yonss Saranga José, Gerd Krahmann, Laura A. Bristow, and Gaute Lavik
Biogeosciences, 18, 3605–3629, https://doi.org/10.5194/bg-18-3605-2021, https://doi.org/10.5194/bg-18-3605-2021, 2021
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In this paper we quantify the subduction of upwelled nitrate due to physical processes on the order of several kilometers in the coastal upwelling off Peru and its effect on primary production. We also compare the prepresentation of these processes in a high-resolution simulation (~2.5 km) with a more coarsely resolved simulation (~12 km). To do this, we combine high-resolution shipboard observations of physical and biogeochemical parameters with a complex biogeochemical model configuration.
Mariana Hill Cruz, Iris Kriest, Yonss Saranga José, Rainer Kiko, Helena Hauss, and Andreas Oschlies
Biogeosciences, 18, 2891–2916, https://doi.org/10.5194/bg-18-2891-2021, https://doi.org/10.5194/bg-18-2891-2021, 2021
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In this study we use a regional biogeochemical model of the eastern tropical South Pacific Ocean to implicitly simulate the effect that fluctuations in populations of small pelagic fish, such as anchovy and sardine, may have on the biogeochemistry of the northern Humboldt Current System. To do so, we vary the zooplankton mortality in the model, under the assumption that these fishes eat zooplankton. We also evaluate the model for the first time against mesozooplankton observations.
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Short summary
We evaluated the effectiveness of the inverse Gaussian transit time distribution (IG-TTD) with respect to estimating the mean state and temporal changes of seawater age, defined as the duration since water last had contact with the atmosphere, within the tropical thermocline. Results suggest that the IG-TTD underestimates seawater age. Moreover, the IG-TTD constrained by a single tracer gives spurious trends in water age. Incorporating an additional tracer improves IG-TTD's accuracy for estimating temporal change of seawater age.
We evaluated the effectiveness of the inverse Gaussian transit time distribution (IG-TTD) with...
