Changes in Southern Ocean Biogeochemistry and the Potential Impact on pH-Sensitive Planktonic Organisms

Abstract. Sensitivities of the oceanic biological pump within the GISS climate modeling system are explored here. Results are presented from twin control simulations of the air-sea CO2 gas exchange using two different ocean models coupled to the same atmosphere. The two ocean models (Russell ocean model and Hybrid Coordinate Ocean Model, HYCOM) use different vertical coordinate systems, and therefore different representations of column physics. Both variants of the GISS climate model are coupled to the same ocean biogeochemistry module (the NASA Ocean Biogeochemistry Model, NOBM) which computes prognostic distributions for biotic and abiotic fields that influence the air-sea flux of CO2 and the deep ocean carbon transport and storage. In particular, the model differences due to remineralization rate changes are compared to differences attributed to physical processes modeled differently in the two ocean models such as ventilation, mixing, eddy stirring and vertical advection. The Southern Ocean emerges as a key region where the CO2 flux is as sensitive to biological parameterizations as it is to physical parameterizations. Mixing in the Southern Ocean is shown to be a~good indicator of the magnitude of the biological pump efficiency regardless of physical model choice.

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Modeling the physical drivers of the decadal variability of the Southern Ocean carbon uptake

10.5194/egusphere-egu21-8654 ◽

2021 ◽

Author(s):

Lavinia Patara ◽

Torge Martin ◽

Ivy Frenger ◽

Jan Klaus Rieck ◽

Chia-Te Chien

Keyword(s):

Southern Ocean ◽

Decadal Variability ◽

Open Ocean ◽

Global Ocean ◽

Carbon Uptake ◽

Positive Trend ◽

The Past ◽

Rate Of Increase ◽

Ocean Biogeochemistry ◽

Ocean Carbon

Observational estimates point to pronounced changes of the Southern Ocean carbon uptake in the past decades, but the mechanisms are still not fully understood. In this study we assess physical drivers of the Southern Ocean carbon uptake variability in a suite of global ocean biogeochemistry models with 0.5&#186;, 0.25&#186; and 0.1&#186; horizontal resolution as well as in a 3-member ensemble performed with an Earth System Model (ESM) sharing the same ocean biogeochemistry model. The ocean models show a positive trend of the Southern Ocean CO2 uptake in the past decades, with a weakening of its rate of increase in the 1990s. The 0.1&#186; model exhibits the strongest trend in the Southern Ocean carbon uptake.&#160;Different physical drivers of the carbon uptake variability and of its trends (such as changes in stratification, ventilation, overturning circulation, and SST) are analyzed. A particular focus of this study is to assess the role of open-ocean polynyas in driving Southern Ocean carbon uptake. Open-ocean polynyas in the Southern Ocean have pronounced climate fingerprints, such as reduced sea-ice coverage, heat loss by the ocean and enhanced bottom water formation, but their role for the Southern Ocean carbon uptake has been as yet little studied. To this end we analyze conjunctly ESM simulations and an ocean-only sensitivity experiment where open-ocean polynyas are artificially created by perturbing the Antarctic freshwater runoff. We find that enhanced CO2 outgassing takes place during the polynya opening, because old carbon-rich waters come in contact with the atmosphere. The concomitant increased uptake of anthropogenic CO2 partially compensates the CO2 outgassing. When the polynya closes, the ocean CO2 uptake increases significantly, possibly fueled by abundant nutrients and higher alkalinity brought to the surface during the previous convective phase. Our results suggest that open-ocean polynyas could have a significant impact on the Southern Ocean CO2 uptake and could thus modulate its decadal variability.&#160;

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Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

Biogeosciences Discussions ◽

10.5194/bgd-10-15033-2013 ◽

2013 ◽

Vol 10 (9) ◽

pp. 15033-15076 ◽

Cited By ~ 1

Author(s):

K. B. Rodgers ◽

O. Aumont ◽

S. E. Mikaloff Fletcher ◽

Y. Plancherel ◽

L. Bopp ◽

...

Keyword(s):

Carbon Cycle ◽

Southern Ocean ◽

Ad Hoc ◽

Global Ocean ◽

Nutrient Concentrations ◽

Carbon Uptake ◽

Carbon Cycle Model ◽

Ocean Biogeochemistry ◽

Ocean Carbon ◽

The Impact

Abstract. Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model. The objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000–2006, with differences of order 0.9 Pg C yr−1 over the region south of 45° S. Wind stirring impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other species associated with ocean biogeochemistry. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong sensitivity of nutrient concentrations exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry, through its impact over the Southern Ocean.

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A data assimilating model for estimating Southern Ocean biogeochemistry

Journal of Geophysical Research Oceans ◽

10.1002/2016jc012650 ◽

2017 ◽

Vol 122 (9) ◽

pp. 6968-6988 ◽

Cited By ~ 43

Author(s):

A. Verdy ◽

M. R. Mazloff

Keyword(s):

Southern Ocean ◽

Ocean Biogeochemistry

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Variations of ocean biogeochemistry in a transient deglacial simulation with MPI-ESM

10.5194/egusphere-egu21-3073 ◽

2021 ◽

Author(s):

Bo Liu ◽

Katharina D. Six ◽

Tatiana Ilyina ◽

Thomas Extier

Keyword(s):

Southern Ocean ◽

Net Primary Production ◽

Water Discharge ◽

Ice Sheet ◽

Meridional Overturning Circulation ◽

Earth System ◽

Water Pulse ◽

Melt Water ◽

Ocean Biogeochemistry ◽

The Impact

Variations in ocean-atmosphere carbon exchange, in response to varying physical and biogeochemical ocean states, is one of the major causes of the glacial-interglacial atmospheric CO2 changes. Most of the existing modelling studies use time-slice simulations with Earth System Models to quantify the proposed mechanisms, such as the impact of a weakened Southern Ocean westerlies and a massive discharge of freshwater from ice sheet melting on the deglacial atmospheric CO2 rise. We present the variations of ocean biogeochemistry in a transient deglaciation (21 &#8211; 10 kB.P.) simulation using the Max Planck Institute Earth System Model. We force the model with reconstructions of atmospheric greenhouse gas concentrations, orbital parameters, ice sheet and dust deposition. In line with the physical ocean component, we account for the automatic adjustment of all marine biogeochemical tracers in response to changing bathymetry and coastlines that relate to deglacial melt water discharge and isostatic adjustment. We include a new representation of the stable carbon isotope (13C) in the ocean biogeochemical component to evaluate the simulation against &#948;13C records from sediment cores.The model reproduces several proposed oceanic CO2 outgassing mechanisms. First, the net primary production (NPP) in the North Atlantic Ocean dramatically decrease (by 40 &#8211; 80%) during the first melt water pulse (15 &#8211; 14 kB.P.) which is caused by the weakening in the strength of the Atlantic Meridional Overturning Circulation from 21 to 3 Sv. However, globally the oceanic NPP only slightly decreases by 8% as oceanic NPP in the South Hemisphere increases during the same period. Second, during the melt water pulse in the Southern Ocean the ventilation of intermediate waters, which has high DIC content and low alkalinity concentration, is slightly enhanced. Third, the surface alkalinity decreases due to dilution and due to episodic shifts between CaCO3 production and opal production by phytoplankton. Lastly, CO2 solubility decreases with increasing deglacial sea surface temperature. The increase of surface pCO2 caused by the above mechanisms is, however, smaller than that of the prescribed atmospheric CO2. Thus, the ocean is a weak carbon sink in this deglacial simulation.

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