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The influence of the ocean circulation state on ocean carbon storage and CO2 drawdown potential in an Earth system model
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0003-4855-7767
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0002-4414-6859
Stockholm University, Faculty of Science, Department of Meteorology . Barcelona Supercomputer Center, Spain.
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Number of Authors: 52018 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 15, no 5, p. 1367-1393Article in journal (Refereed) Published
Abstract [en]

During the four most recent glacial cycles, atmospheric CO2 during glacial maxima has been lowered by about 90-100 ppm with respect to interglacials. There is widespread consensus that most of this carbon was partitioned in the ocean. It is, however, still debated which processes were dominant in achieving this increased carbon storage. In this paper, we use an Earth system model of intermediate complexity to explore the sensitivity of ocean carbon storage to ocean circulation state. We carry out a set of simulations in which we run the model to pre-industrial equilibrium, but in which we achieve different states of ocean circulation by changing forcing parameters such as wind stress, ocean diffusivity and atmospheric heat diffusivity. As a consequence, the ensemble members also have different ocean carbon reservoirs, global ocean average temperatures, biological pump efficiencies and conditions for air-sea CO2 disequilibrium. We analyse changes in total ocean carbon storage and separate it into contributions by the solubility pump, the biological pump and the CO2 disequilibrium component. We also relate these contributions to differences in the strength of the ocean overturning circulation. Depending on which ocean forcing parameter is tuned, the origin of the change in carbon storage is different. When wind stress or ocean diapycnal diffusivity is changed, the response of the biological pump gives the most important effect on ocean carbon storage, whereas when atmospheric heat diffusivity or ocean isopycnal diffusivity is changed, the solubility pump and the disequilibrium component are also important and sometimes dominant. Despite this complexity, we obtain a negative linear relationship between total ocean carbon and the combined strength of the northern and southern overturning cells. This relationship is robust to different reservoirs dominating the response to different forcing mechanisms. Finally, we conduct a drawdown experiment in which we investigate the capacity for increased carbon storage by artificially maximising the efficiency of the biological pump in our ensemble members. We conclude that different initial states for an ocean model result in different capacities for ocean carbon storage due to differences in the ocean circulation state and the origin of the carbon in the initial ocean carbon reservoir. This could explain why it is difficult to achieve comparable responses of the ocean carbon pumps in model intercomparison studies in which the initial states vary between models. We show that this effect of the initial state is quantifiable. The drawdown experiment highlights the importance of the strength of the biological pump in the control state for model studies of increased biological efficiency.

Place, publisher, year, edition, pages
2018. Vol. 15, no 5, p. 1367-1393
National Category
Biological Sciences Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-154780DOI: 10.5194/bg-15-1367-2018ISI: 000426909200001OAI: oai:DiVA.org:su-154780DiVA, id: diva2:1198626
Available from: 2018-04-18 Created: 2018-04-18 Last updated: 2020-02-06Bibliographically approved
In thesis
1. Model analysis of ocean carbon storage and transport across climate states
Open this publication in new window or tab >>Model analysis of ocean carbon storage and transport across climate states
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The ocean carbon cycle plays a fundamental role in the Earth’s climate system, on decadal to multi-millennial timescales. Of the carbon held in the ocean, the atmosphere, and the terrestrial biosphere combined, more than 90% resides in the ocean. Carbon enters the surface ocean through air-sea gas exchange and from terrestrial sources. It is transported to the deep ocean with the ocean circulation and through the so-called biological pump, where carbon is taken up in the surface ocean by photosynthetic organisms that fall down and decompose at depth. This thesis contributes to the understanding of the processes involved in ocean carbon storage and transport. It examines how these processes respond to model perturbations, and how this response influences our attempts to simulate glacial-interglacial fluctuations in atmospheric carbon dioxide (CO2).

The thesis investigates the response of the simulated ocean carbon storage, and distribution of the isotopic tracer δ13C, to changes in physical and biological parameters. In the included studies, we use observational as well as proxy records of oceanic properties to evaluate our model simulations. In addition, we use a climate model to interpret proxy evidence of glacial-interglacial changes in ocean δ13C. By using a separation framework, we identify the origin of the carbon in the model ocean, and attribute observed changes to the processes involved.

The results indicate a strong link between ocean carbon storage and the strength of the global ocean overturning circulation. Stronger circulation leads to less carbon storage through a weakening of the biological pump, and through reduced solubility due to an increase in global ocean average temperature.

In simulations of glacial climate, we find that biological adaptability to the surrounding nutrient conditions, through a flexible carbon-to-phosphorus ratio (C/P) in ocean photosynthesis, increases the ocean carbon storage compared to simulations where fixed C/P is applied. The biological flexibility improves the model’s ability to reproduce glacial atmospheric CO2. In line with previous research, we find freshwater input to the North Atlantic to be an important factor for reproducing glacial proxy records. The ensemble of simulations that achieve a good representation of glacial-interglacial δ13C indicates a deglacial whole-ocean change in δ13C of 0.28 ± 0.06‰.

The thesis underlines the importance of the initial state, and the choice of model parameterisations, for the outcome of model ensemble, and intercomparison studies. Finally, it proposes a new method for estimation of ocean carbon transport, and attribution of this transport to different water masses and carbon system processes.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2019. p. 42
Keywords
Oceanography, Climate, Climate model, Carbon cycle, Paleoclimate
National Category
Climate Research Oceanography, Hydrology and Water Resources Geosciences, Multidisciplinary
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172894 (URN)978-91-7797-829-9 (ISBN)978-91-7797-830-5 (ISBN)
Public defence
2019-10-25, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2019-10-02 Created: 2019-09-11 Last updated: 2019-09-24Bibliographically approved

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