Space and time variability of the Southern Ocean carbon budget

The Southern Ocean plays an important role in the uptake, transport and storage of carbon by the global oceans. These properties are dominated by the response to the rise in anthropogenic CO2 in the atmosphere, but they are modulated by climate variability and climate change. Here we explore the effect of climate variability and climate change on ocean carbon uptake and storage in the Southern Ocean. We assess the extent to which climate change may be distinguishable from the anthropogenic CO2 signal and from the natural background variability. We use a combination of biogeochemical ocean modelling and observations from the GLODAPv2020 database to detect climate fingerprints in dissolved inorganic carbon (DIC).We conduct an ensemble of hindcast model simulations of the period 1920-2019, using a global ocean biogeochemical model which incorporates plankton ecosystem dynamics based on twelve plankton functional types. We use the model ensemble to isolate the changes in DIC due to rising anthropogenic CO2 alone and the changes due to climatic drivers (both climate variability and climate change), to determine their relative roles in the emerging total DIC trends and patterns. We analyse these DIC trends for a climate fingerprint over the past four decades, across spatial scales from the Southern Ocean, to basin level and down to regional ship transects. Highly sampled ship transects were extracted from GLODAPv2020 to obtain locations with the maximum spatiotemporal coverage, to reduce the inherent biases in patchy observational data. Model results were sampled to the ship transects to compare the climate fingerprints directly to the observational data.Model results show a substantial change in DIC over a 35-year period, with a range of more than +/- 30 &#181;mol/L. In the surface ocean, both anthropogenic CO2 and climatic drivers act to increase DIC concentration, with the influence of anthropogenic CO2 dominating at lower latitudes and the influence of climatic drivers dominating at higher latitudes. In the deep ocean, the anthropogenic CO2 generally acts to increase DIC except in the subsurface waters at lower latitudes, while climatic drivers act to decrease DIC concentration. The combined fingerprint of anthropogenic CO2 and climatic drivers on DIC concentration is for an increasing trend at the surface and decreasing trends in low latitude subsurface waters. Preliminary comparison of the model fingerprints to observational ship transects will also be presented.

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A global CO2 flux dataset (2015–2019) inferred from OCO-2 retrievals using the Tan-Tracker inversion system

10.5194/essd-2021-210 ◽

2021 ◽

Author(s):

Zhe Jin ◽

Xiangjun Tian ◽

Rui Han ◽

Yu Fu ◽

Xin Li ◽

...

Keyword(s):

Tibetan Plateau ◽

Carbon Cycle ◽

Carbon Flux ◽

Carbon Budget ◽

Terrestrial Ecosystem ◽

Carbon Fluxes ◽

Global Carbon Cycle ◽

Ocean Carbon ◽

Global Carbon ◽

Regional Fluxes

Abstract. Accurate assessment of the various sources and sinks of carbon dioxide (CO2), especially terrestrial ecosystem and ocean fluxes with high uncertainties, is important for understanding of the global carbon cycle, supporting the formulation of climate policies, and projecting future climate change. Satellite retrievals of the column-averaged dry air mole fractions of CO2 (XCO2) are being widely used to improve carbon flux estimation due to their broad spatial coverage. However, there is no consensus on the robust estimates of regional fluxes. In this study, we present a global and regional resolved terrestrial ecosystem carbon flux (NEE) and ocean carbon flux dataset for 2015–2019. The dataset was generated using the Tan-Tracker inversion system by assimilating Observing Carbon Observatory 2 (OCO-2) column CO2 retrievals. The posterior NEE and ocean carbon fluxes were comprehensively validated by comparing posterior simulated CO2 concentrations with OCO-2 independent retrievals and Total Carbon Column Observing Network (TCCON) measurements. The validation showed that posterior carbon fluxes significantly improved the modelling of atmospheric CO2 concentrations, with global mean biases of 0.33 ppm against OCO-2 retrievals and 0.12 ppm against TCCON measurements. We described the characteristics of the dataset at global, regional, and Tibetan Plateau scales in terms of the carbon budget, annual and seasonal variations, and spatial distribution. The posterior 5-year annual mean global atmospheric CO2 growth rate was 5.35 PgC yr−1, which was within the uncertainty of the Global Carbon Budget 2020 estimate (5.49 PgC yr−1). The posterior annual mean NEE and ocean carbon fluxes were −4.07 and −3.33 PgC yr−1, respectively. Regional fluxes were analysed based on TransCom partitioning. All 11 land regions acted as carbon sinks, except for Tropical South America, which was almost neutral. The strongest carbon sinks were located in Boreal Asia, followed by Temperate Asia and North Africa. The entire Tibetan Plateau ecosystem was estimated as a carbon sink, taking up −49.52 TgC yr−1 on average, with the strongest sink occurring in eastern alpine meadows. These results indicate that our dataset captures surface carbon fluxes well and provides insight into the global carbon cycle. The dataset can be accessed at https://doi.org/10.11888/Meteoro.tpdc.271317 (Jin et al., 2021).

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Diagnosing CO2 emission-induced feedbacks between the Southern Ocean carbon cycle and the climate system: A multiple Earth System Model analysis using a water mass tracking approach

Journal of Climate ◽

10.1175/jcli-d-20-0889.1 ◽

2021 ◽

pp. 1-62

Author(s):

Tilla Roy ◽

Jean Baptiste Sallée ◽

Laurent Bopp ◽

Nicolas Metzl

Keyword(s):

Climate Change ◽

Southern Ocean ◽

Water Mass ◽

Buffering Capacity ◽

Climate Impacts ◽

System Model ◽

Earth System ◽

Local Climate ◽

Mode Waters ◽

Ocean Carbon

AbstractAnthropogenic CO2 emission-induced feedbacks between the carbon cycle and the climate system perturb the efficiency of atmospheric CO2 uptake by land and ocean carbon reservoirs. The Southern Ocean is a region where these feedbacks can be largest and differ most among Earth System Model projections of 21st century climate change. To improve our mechanistic understanding of these feedbacks, we develop an automated procedure that tracks changes in the positions of Southern Ocean water masses and their carbon uptake. In an idealised ensemble of climate change projections, we diagnose two carbon–concentration feedbacks driven by atmospheric CO2 (due to increasing air-sea CO2 partial pressure difference, dpCO2, and reducing carbonate buffering capacity) and two carbon–climate feedbacks driven by climate change (due to changes in the water mass surface outcrop areas and local climate impacts). Collectively these feedbacks increase the CO2 uptake by the Southern Ocean and account for one-fifth of the global uptake of CO2 emissions. The increase in CO2 uptake is primarily dpCO2-driven, with Antarctic intermediate waters making the largest contribution; the remaining three feedbacks partially offset this increase (by ~25%), with maximum reductions in Subantarctic mode waters. The process dominating the decrease in CO2 uptake is water mass-dependent: reduction in carbonate buffering capacity in Subtropical and Subantarctic mode waters, local climate impacts in Antarctic intermediate waters, and reduction in outcrop areas in circumpolar deep waters and Antarctic bottom waters. Intermodel variability in the feedbacks is predominately dpCO2–driven and should be a focus of efforts to constrain projection uncertainty.

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The Southern Ocean carbon sink 1985-2018: first results of the RECCAP2 project

10.5194/egusphere-egu21-1834 ◽

2021 ◽

Author(s):

Judith Hauck ◽

Luke Gregor ◽

Cara Nissen ◽

Eric Mortenson ◽

Seth Bushinsky ◽

...

Keyword(s):

Southern Ocean ◽

Seasonal Cycle ◽

Carbon Sink ◽

Global Ocean ◽

Biogeochemical Models ◽

Wide Range ◽

Data Products ◽

First Results ◽

Ocean Carbon ◽

Indian Sector

The Southern Ocean is the main gateway for anthropogenic CO2 into the ocean owing to the upwelling of old water masses with low anthropogenic CO2 concentration, and the transport of the newly equilibrated surface waters into the ocean interior through intermediate, deep and bottom water formation. Here we present first results of the Southern Ocean chapter of RECCAP2, which is the Global Carbon Project&#8217;s second systematic study on Regional Carbon Cycle Assessment and Processes. In the Southern Ocean chapter, we aim to assess the Southern Ocean carbon sink 1985-2018 from a wide range of available models and data sets, and to identify patterns of regional and temporal variability, model limitations and future challenges.We gathered global and regional estimates of the air-sea CO2 flux over the period 1985-2018 from global ocean biogeochemical models, surface pCO2-based data products, and data-assimilated models. The analysis on the Southern Ocean quantified geographical patterns in the annual mean and seasonal amplitude of air-sea CO2 flux, with results presented here aggregated to the level of large-scale ocean biomes.Considering the suite of observed and modelled estimates, we found that the subtropical seasonally stratified (STSS) biome stands out with the largest air-sea CO2 flux per area and a seasonal cycle with largest ocean uptake of CO2 in winter, whereas the ice (ICE) biome is characterized by a large ensemble spread and a pronounced seasonal cycle with the largest ocean uptake of CO2 in summer. Connecting these two, the subpolar seasonally stratified (SPSS) biome has intermediate flux densities (flux per area), and most models have difficulties simulating the seasonal cycle with strongest uptake during the summer months.Our analysis also reveals distinct differences between the Atlantic, Pacific and Indian sectors of the aforementioned biomes. In the STSS, the Indian sector contributes most to the ocean carbon sink, followed by the Atlantic and then Pacific sectors. This hierarchy is less pronounced in the models than in the data-products. In the SPSS, only the Atlantic sector exhibits net CO2 uptake in all years, likely linked to strong biological production. In the ICE biome, the Atlantic and Pacific sectors take up more CO2 than the Indian sector, suggesting a potential role of the Weddell and Ross Gyres.These first results confirm the global relevance of the Southern Ocean carbon sink and highlight the strong regional and interannual variability of the Southern Ocean carbon uptake in connection to physical and biogeochemical processes.

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Deep ocean ventilation, carbon isotopes, marine sedimentation and the deglacial CO2 rise

Climate of the Past ◽

10.5194/cp-7-771-2011 ◽

2011 ◽

Vol 7 (3) ◽

pp. 771-800 ◽

Cited By ~ 76

Author(s):

T. Tschumi ◽

F. Joos ◽

M. Gehlen ◽

C. Heinze

Keyword(s):

Southern Ocean ◽

Ocean Circulation ◽

Isotope Composition ◽

Deep Ocean ◽

Atmospheric Co2 ◽

Carbon Cycle Model ◽

Ocean Ventilation ◽

Marine Sedimentation ◽

Sediment Formation ◽

Ocean Carbon

Abstract. The link between the atmospheric CO2 level and the ventilation state of the deep ocean is an important building block of the key hypotheses put forth to explain glacial-interglacial CO2 fluctuations. In this study, we systematically examine the sensitivity of atmospheric CO2 and its carbon isotope composition to changes in deep ocean ventilation, the ocean carbon pumps, and sediment formation in a global 3-D ocean-sediment carbon cycle model. Our results provide support for the hypothesis that a break up of Southern Ocean stratification and invigorated deep ocean ventilation were the dominant drivers for the early deglacial CO2 rise of ~35 ppm between the Last Glacial Maximum and 14.6 ka BP. Another rise of 10 ppm until the end of the Holocene is attributed to carbonate compensation responding to the early deglacial change in ocean circulation. Our reasoning is based on a multi-proxy analysis which indicates that an acceleration of deep ocean ventilation during early deglaciation is not only consistent with recorded atmospheric CO2 but also with the reconstructed opal sedimentation peak in the Southern Ocean at around 16 ka BP, the record of atmospheric δ13CCO2, and the reconstructed changes in the Pacific CaCO3 saturation horizon.

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