What drives the seasonality of air–sea CO2 fluxes in the ice-free zone of the Southern Ocean: A 1D coupled physical–biogeochemical model approach

Abstract. A watermass-based framework is presented for a quantitative understanding of the processes controlling the cycling of carbon in the Southern Ocean. The approach is developed using a model simulation of the global carbon transports within the ocean and with the atmosphere. It is shown how the watermass framework sheds light on the interplay between biology, air-sea gas exchange, and internal ocean transport including diapycnal processes, and the way in which this interplay controls the large-scale ocean-atmosphere carbon exchange. The simulated pre-industrial regional patterns of DIC distribution and the global distribution of the pre-industrial air-sea CO2 fluxes compare well with other model results and with results from an ocean inversion method. The main differences are found in the Southern Ocean where the model presents a stronger CO2 outgassing south of the polar front, a result of the upwelling of DIC-rich deep waters into the surface layer. North of the subantarctic front the typical temperature-driven solubility effect produces a net ingassing of CO2. The biological controls on surface CO2 fluxes through primary production is generally smaller than the temperature effect on solubility. Novel to this study is also a Lagrangian trajectory analysis of the meridional transport of DIC. The analysis allows to evaluate the contribution of separate branches of the global thermohaline circulation (identified by watermasses) to the vertical distribution of DIC throughout the Southern Ocean and towards the global ocean. The most important new result is that the overturning associated with Subantarctic Mode Waters sustains a northward net transport of DIC (15.7×107 mol/s across 30° S). This new finding, which has also relevant implications on the prediction of anthropogenic carbon redistribution, results from the specific mechanism of SAMW formation and its source waters whose consequences on tracer transports are analyzed for the first time in this study.

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Factors affecting interdecadal variability of air–sea CO2 fluxes in the tropical Pacific, revealed by an ocean physical–biogeochemical model

Climate Dynamics ◽

10.1007/s00382-019-04766-5 ◽

2019 ◽

Vol 53 (7-8) ◽

pp. 3985-4004

Author(s):

Feng Tian ◽

Rong-Hua Zhang ◽

Xiujun Wang

Keyword(s):

Tropical Pacific ◽

Interdecadal Variability ◽

Biogeochemical Model ◽

Co2 Fluxes ◽

Factors Affecting ◽

The Tropical Pacific

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On modeling the Southern Ocean Phytoplankton Functional Types

10.5194/bg-2019-289 ◽

2019 ◽

Author(s):

Svetlana N. Losa ◽

Stephanie Dutkiewicz ◽

Martin Losch ◽

Julia Oelker ◽

Mariana A. Soppa ◽

...

Keyword(s):

Southern Ocean ◽

General Circulation ◽

Circulation Model ◽

Biogeochemical Model ◽

Massachusetts Institute Of Technology ◽

Pigment Analysis ◽

Functional Types ◽

Model Configuration ◽

Institute Of Technology

Abstract. This study highlights recent advances and challenges of applying coupled physical-biogeochemical modeling for investigating the distribution of the key phytoplankton groups in the Southern Ocean, an area of strong interest for understanding biogeochemical cycling and ecosystem functioning under present climate change. Our simulations of the phenology of various Phytoplankton Functional Types (PFTs) are based on a version of the Darwin biogeochemical model coupled to the Massachusetts Institute of Technology (MIT) general circulation model (Darwin-MITgcm). The ecological module version was adapted for the Southern Ocean by: 1) improving coccolithophores abundance relative to the original model by introducing a high affinity for nutrients and an ability to escape grazing control for coccolithophores; 2) including two different (small vs. large) size classes of diatoms; and 3) accounting for two distinct life stages for Phaeocystis (single cell vs. colonial). This new model configuration describes best the competition and co-occurrence of the PFTs in the Southern Ocean. It improves significantly relative to an older version the agreement of the simulated abundance of the coccolithophores and diatoms with in situ scanning electron microscopy observations in the Subantarctic Zone as well as with in situ diatoms and haptophytes (including coccolithophores and Phaeocystis) chlorophyll a concentrations within the Patagonian Shelf and along the Western Antarctic Peninsula obtained by diagnostic pigment analysis. The modeled Southern Ocean PFT dominance also agrees well with satellite-based PFT information.

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Sedimentary and atmospheric sources of iron around South Georgia, Southern Ocean: a modelling perspective

Biogeosciences ◽

10.5194/bg-11-1981-2014 ◽

2014 ◽

Vol 11 (7) ◽

pp. 1981-2001 ◽

Cited By ~ 16

Author(s):

I. Borrione ◽

O. Aumont ◽

M. C. Nielsdóttir ◽

R. Schlitzer

Keyword(s):

Southern Ocean ◽

Phytoplankton Bloom ◽

Biogeochemical Model ◽

Dust Deposition ◽

Data Set ◽

Western Atlantic ◽

Atlantic Sector ◽

South Georgia ◽

The North ◽

Mean Square Errors

Abstract. In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia. Multiple sources, including shallow sediments and atmospheric dust deposition, are thought to introduce iron to the region. However, the relative importance of each source is still unclear, owing in part to the scarcity of dissolved iron (dFe) measurements in the South Georgia region. In this study, we combine results from a recently published dFe data set around South Georgia with a coupled regional hydrodynamic and biogeochemical model to further investigate iron supply around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north (i.e. downstream) of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of ~0.02 nM and ~0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and very shallow shelf-sediments (depths < 20 m). Dust deposition exerts almost no effect on surface chlorophyll a concentrations. Other sources of iron such as run-off and glacial melt are not represented explicitly in the model, however we discuss their role in the local iron budget.

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The Macquarie Island [LoFlo2G] high-precision continuous atmospheric carbon dioxide record

10.5194/amt-2018-300 ◽

2018 ◽

Cited By ~ 1

Author(s):

Ann R. Stavert ◽

Rachel M. Law ◽

Marcel van der Schoot ◽

Ray L. Langenfelds ◽

Darren A. Spencer ◽

...

Keyword(s):

Carbon Dioxide ◽

Southern Ocean ◽

High Precision ◽

New Record ◽

Large Scale ◽

Atmospheric Co2 ◽

Spatial And Temporal Variability ◽

Co2 Fluxes ◽

Macquarie Island ◽

Uncertainty Estimates

Abstract. The Southern Ocean (south of 30° S) is a key global scale sink of carbon dioxide (CO2). However, the isolated and inhospitable nature of this environment has restricted the number of oceanic and atmospheric CO2 measurements in this region. This has limited the scientific community’s ability to investigate trends and seasonal variability of the sink. Compared to regions further north, the near-absence of terrestrial CO2 exchange and strong large-scale zonal mixing demands unusual inter-site measurement precision to help distinguish the presence of mid-to-high latitude ocean exchange from large CO2 fluxes transported southwards in the atmosphere. Here we describe a continuous, in-situ, ultra-high-precision, Southern Ocean region CO2 record, which ran at Macquarie Island (54°37’ S, 158°52’ E) from 2005–2016 using a LoFlo2 instrument, along with its calibration strategy, uncertainty analysis and baseline filtering procedures. Uncertainty estimates calculated for minute and hourly frequency data range from 0.01 to 0.05 μmol mol−1 depending on averaging period and application. Higher precisions are applicable when comparing MQA LoFlo measurements to those of similar instruments on the same internal laboratory calibration scale and more uncertain values are applicable when comparing to other networks. Baseline selection is designed to remove measurements that are influenced by local, Macquarie Island, CO2 sources, with effective removal achieved using a within-minute CO2 standard deviation metric. Additionally, measurements that are influenced by CO2 fluxes from Australia or other southern hemisphere land masses are effectively removed using model-simulated radon concentration. A comparison with flask records of atmospheric CO2 at Macquarie Island highlights the limitation of the flask record (due to corrections for storage time and limited temporal coverage) when compared to the new high-precision, continuous record; the new record shows much less noisy seasonal variations than the flask record. As such this new record is ideal for improving our understanding of the spatial and temporal variability of the Southern Ocean CO2 flux particularly when combined with data from similar instruments at other Southern Hemispheric locations.

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Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment

10.5194/bg-2017-219 ◽

2017 ◽

Cited By ~ 1

Author(s):

Thomas W. Trull ◽

Abraham Passmore ◽

Diana M. Davies ◽

Tim Smit ◽

Kate Berry ◽

...

Keyword(s):

Southern Ocean ◽

Biogeochemical Model ◽

Biogenic Carbonate ◽

Antarctic Waters ◽

Calcite Saturation ◽

Ocean Biogeochemical Model ◽

Ocean Ecosystems ◽

Negative Impacts ◽

Abundant Phytoplankton ◽

The Impact

Abstract. The Southern Ocean provides a vital service by absorbing about one sixth of humankind's annual emissions of CO2. This comes with a cost – an increase in ocean acidity that is expected to have negative impacts on ocean ecosystems. The reduced ability of phytoplankton and zooplankton to precipitate carbonate shells is a clearly identified risk. The impact depends on the significance of these organisms in Southern Ocean ecosystems, but there is very little information on their abundance or distribution. To quantify their presence, we used coulometric measurement of particulate inorganic carbonate (PIC) on particles filtered from surface seawater into two size fractions: 50–1000 μm to capture foraminifera (the most important biogenic carbonate forming zooplankton) and 1–50 μm to capture coccolithophores (the most important biogenic carbonate forming phytoplankton). Ancillary measurements of biogenic silica (BSi) and particulate organic carbon (POC) provided context, as estimates of the abundance of diatoms (the most abundant phytoplankton in polar waters), and total microbial biomass, respectively. Results for 9 transects from Australia to Antarctica in 2008–2015 showed low levels of PIC compared to northern hemisphere polar waters. Coccolithophores slightly exceeded the biomass of diatoms in Subantarctic waters, but their abundance decreased more than 30-fold poleward, while diatom abundances increased, so that on a molar basis PIC was only 1 % of BSi in Antarctic waters. This limited importance of coccolithophores in the Southern Ocean is further emphasized in terms of their associated POC, representing less than 1 % of total POC in Antarctic waters and less than 10 % in Subantarctic waters. NASA satellite ocean colour based PIC estimates were in reasonable agreement with (though somewhat higher than) the shipboard results in Subantarctic waters, but greatly over-estimated PIC in Antarctic waters. Contrastingly, the NASA Ocean Biogeochemical Model (NOBM) shows coccolithophores as overly restricted to Subtropical and northern Subantarctic waters. The cause of the strong southward decrease in PIC abundance in the Southern Ocean is not yet clear. Poleward decrease in pH is small and while calcite saturation decreases strongly southward it remains well above saturation (> 2). Nitrate and phosphate variations would predict a poleward increase. Temperature and competition with diatoms for limiting iron appear likely to be important. While the future trajectory of coccolithophore distributions remains uncertain, their current low abundances suggest small impacts on overall Southern Ocean pelagic ecology.

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The sensitivity of pCO<sub>2</sub> reconstructions in the Southern Ocean to sampling scales: a semi-idealized model sampling and reconstruction approach

10.5194/bg-2021-344 ◽

2022 ◽

Author(s):

Laique Merlin Djeutchouang ◽

Nicolette Chang ◽

Luke Gregor ◽

Marcello Vichi ◽

Pedro Manuel Scheel Monteiro

Keyword(s):

Machine Learning ◽

Southern Ocean ◽

Seasonal Cycle ◽

Temporal Frequency ◽

Gradient Boosting ◽

Biogeochemical Model ◽

Clear Understanding ◽

Surface Ocean ◽

Temporal Sampling ◽

Model Domain

Abstract. The Southern Ocean is a complex system yet is sparsely sampled in both space and time. These factors raise questions about the confidence in present sampling strategies and associated machine learning (ML) reconstructions. Previous studies have not yielded a clear understanding of the origin of uncertainties and biases for the reconstructions of the partial pressure of carbon dioxide (pCO2) at the surface ocean (pCO2ocean). Here, we examine these questions by investigating the sensitivity of pCO2ocean reconstruction uncertainties and biases to a series of semi-idealized observing system simulation experiments (OSSEs) that simulate spatio-temporal sampling scales of surface ocean pCO2 in ways that are comparable to ocean CO2 observing platforms (Ship, Waveglider, Carbon-float, Saildrone). These experiments sampled a high spatial resolution (±10 km) coupled physical and biogeochemical model (NEMO-PISCES) within a sub-domain representative of the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean. The reconstructions were done using a two-member ensemble approach that consisted of two machine learning (ML) methods, (1) the feed-forward neural network and (2) the gradient boosting machines. With the baseline observations being from the simulated ships mimicking observations from the Surface Ocean CO2 Atlas (SOCAT), we applied to each of the scale-sampling simulation scenarios the two-member ensemble method ML2, to reconstruct the full sub-domain pCO2ocean and assess the reconstruction skill through a statistical comparison of reconstructed pCO2ocean and model domain mean. The analysis shows that uncertainties and biases for pCO2ocean reconstructions are very sensitive to both the spatial and temporal scales of pCO2 sampling in the model domain. The four key findings from our investigation are the following: (1) improving ML-based pCO2 reconstructions in the Southern Ocean requires simultaneous high resolution observations of the meridional and the seasonal cycle (< 3 days) of pCO2ocean; (2) Saildrones stand out as the optimal platforms to simultaneously address these requirements; (3) Wavegliders with hourly/daily resolution in pseudo-mooring mode improve on Carbon-floats (10-day period), which suggests that sampling aliases from the low temporal frequency have a greater negative impact on their uncertainties, biases and reconstruction means; and (4) the present summer seasonal sampling biases in SOCAT data in the Southern Ocean may be behind a significant winter bias in the reconstructed seasonal cycle of pCO2ocean.

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Rapid establishment of the CO<sub>2</sub> sink associated with Kerguelen's bloom observed during the KEOPS2/OISO20 cruise

Biogeosciences Discussions ◽

10.5194/bgd-11-17543-2014 ◽

2014 ◽

Vol 11 (12) ◽

pp. 17543-17578 ◽

Cited By ~ 7

Author(s):

C. Lo Monaco ◽

N. Metzl ◽

F. D'Ovidio ◽

J. Llort ◽

C. Ridame

Keyword(s):

Southern Ocean ◽

Vertical Mixing ◽

Growing Season ◽

Atmospheric Co2 ◽

Co2 Fluxes ◽

Kerguelen Plateau ◽

Iron Source ◽

Iron Fertilization ◽

Co2 Sink ◽

The Impact

Abstract. Iron and light are the main factors limiting the biological pump of CO2 in the Southern Ocean. Iron fertilization experiments have demonstrated the potential for increased uptake of atmospheric CO2, but little is known about the evolution of fertilized environnements. This paper presents observations collected in one of the largest phytoplankton bloom of the Southern Ocean sustained by iron originating from the Kerguelen Plateau. We first complement previous studies by investigating the mechanisms that control air–sea CO2 fluxes over and downstream of the Kerguelen Plateau at the onset of the bloom based on measurements obtained in October–November 2011. These new observations show the rapid establishment of a strong CO2 sink in waters fertilized with iron as soon as vertical mixing is reduced. The magnitude of the CO2 sink was closely related to chlorophyll a and iron concentrations. Because iron concentration strongly depends on the distance from the iron source and the mode of delivery, we identified lateral advection as the main mechanism controlling air–sea CO2 fluxes downtream the Kerguelen Plateau during the growing season. In the southern part of the bloom, situated over the Plateau (iron source), the CO2 sink was stronger and spatially more homogeneous than in the plume offshore. However, we also witnessed a substantial reduction in the uptake of atmospheric CO2 over the Plateau following a strong winds event. Next, we used all the data available in this region in order to draw the seasonal evolution of air–sea CO2 fluxes. The CO2 sink is rapidly reduced during the course of the growing season, which we attribute to iron and silicic acid depletion. South of the Polar Front, where nutrients depletion is delayed, we suggest that the amplitude and duration of the CO2 sink is mainly controlled by vertical mixing. The impact of iron fertilization on air–sea CO2 fluxes is revealed by comparing the uptake of CO2 integrated over the productive season in the bloom, between 1 and 1.5 mol C m−2 yr−1, and in the iron-poor HNLC waters, where we found a typical value of 0.4 mol C m−2 yr−1. Extrapolating our results to the ice-free Southern Ocean (~50–60° S) suggests that iron fertilization of the whole area would increase the contemporay oceanic uptake of CO2 by less than 0.1 Pg C yr−1, i.e., less than 1% of the current anthropogenic CO2 emissions.

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Explicit silicate cycling in the Kiel Marine Biogeochemistry Model version 3 (KMBM3) embedded in the UVic ESCM version 2.9

Geoscientific Model Development ◽

10.5194/gmd-14-7255-2021 ◽

2021 ◽

Vol 14 (12) ◽

pp. 7255-7285

Author(s):

Karin Kvale ◽

David P. Keller ◽

Wolfgang Koeve ◽

Katrin J. Meissner ◽

Christopher J. Somes ◽

...

Keyword(s):

Southern Ocean ◽

Primary Production ◽

Large Scale ◽

Climate Model ◽

Net Primary Production ◽

Biogeochemical Model ◽

Earth System ◽

New Model ◽

Model Version ◽

Fully Coupled

Abstract. We describe and test a new model of biological marine silicate cycling, implemented in the Kiel Marine Biogeochemical Model version 3 (KMBM3), embedded in the University of Victoria Earth System Climate Model (UVic ESCM) version 2.9. This new model adds diatoms, which are a key component of the biological carbon pump, to an existing ecosystem model. This new model combines previously published parameterizations of a diatom functional type, opal production and export with a novel, temperature-dependent dissolution scheme. Modelled steady-state biogeochemical rates, carbon and nutrient distributions are similar to those found in previous model versions. The new model performs well against independent ocean biogeochemical indicators and captures the large-scale features of the marine silica cycle to a degree comparable to similar Earth system models. Furthermore, it is computationally efficient, allowing both fully coupled, long-timescale transient simulations and “offline” transport matrix spinups. We assess the fully coupled model against modern ocean observations, the historical record starting from 1960 and a business-as-usual atmospheric CO2 forcing to the year 2300. The model simulates a global decline in net primary production (NPP) of 1.4 % having occurred since the 1960s, with the strongest declines in the tropics, northern midlatitudes and Southern Ocean. The simulated global decline in NPP reverses after the year 2100 (forced by the extended RCP8.5 CO2 concentration scenario), and NPP returns to 98 % of the pre-industrial rate by 2300. This recovery is dominated by increasing primary production in the Southern Ocean, mostly by calcifying phytoplankton. Large increases in calcifying phytoplankton in the Southern Ocean offset a decline in the low latitudes, producing a global net calcite export in 2300 that varies only slightly from pre-industrial rates. Diatom distribution moves southward in our simulations, following the receding Antarctic ice front, but diatoms are outcompeted by calcifiers across most of their pre-industrial Southern Ocean habitat. Global opal export production thus drops to 75 % of its pre-industrial value by 2300. Model nutrients such as phosphate, silicate and nitrate build up along the Southern Ocean particle export pathway, but dissolved iron (for which ocean sources are held constant) increases in the upper ocean. This different behaviour of iron is attributed to a reduction of low-latitude NPP (and consequently, a reduction in both uptake and export and particle, including calcite scavenging), an increase in seawater temperatures (raising the solubility of particulate iron) and stratification that “traps” the iron near the surface. These results are meant to serve as a baseline for sensitivity assessments to be undertaken with this model in the future.

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