Watermasses as a unifying framework for understanding the Southern Ocean carbon cycle

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.

Download Full-text

On the impact of sea ice in a global ocean circulation model

Annals of Glaciology ◽

10.3189/s0260305500013884 ◽

1997 ◽

Vol 25 ◽

pp. 111-115 ◽

Cited By ~ 3

Author(s):

Achim Stössel

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Ocean Circulation ◽

General Circulation ◽

Thermohaline Circulation ◽

Circulation Model ◽

Deep Ocean ◽

Global Ocean ◽

Convective Activity ◽

The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

Download Full-text

Southern Ocean phytoplankton turnover in response to stepwise Antarctic cooling over the past 15 million years

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1600318113 ◽

2016 ◽

Vol 113 (25) ◽

pp. 6868-6873 ◽

Cited By ~ 22

Author(s):

James S. Crampton ◽

Rosie D. Cody ◽

Richard Levy ◽

David Harwood ◽

Robert McKay ◽

...

Keyword(s):

Southern Ocean ◽

Atmospheric Carbon Dioxide ◽

Large Scale ◽

Species Turnover ◽

Polar Front ◽

Extinction Rate ◽

The Past ◽

Extinction Rates ◽

Phytoplankton Communities ◽

Global Extinction

It is not clear how Southern Ocean phytoplankton communities, which form the base of the marine food web and are a crucial element of the carbon cycle, respond to major environmental disturbance. Here, we use a new model ensemble reconstruction of diatom speciation and extinction rates to examine phytoplankton response to climate change in the southern high latitudes over the past 15 My. We identify five major episodes of species turnover (origination rate plus extinction rate) that were coincident with times of cooling in southern high-latitude climate, Antarctic ice sheet growth across the continental shelves, and associated seasonal sea-ice expansion across the Southern Ocean. We infer that past plankton turnover occurred when a warmer-than-present climate was terminated by a major period of glaciation that resulted in loss of open-ocean habitat south of the polar front, driving non-ice adapted diatoms to regional or global extinction. These findings suggest, therefore, that Southern Ocean phytoplankton communities tolerate “baseline” variability on glacial–interglacial timescales but are sensitive to large-scale changes in mean climate state driven by a combination of long-period variations in orbital forcing and atmospheric carbon dioxide perturbations.

Download Full-text

Large-scale distribution of coccolithophores and Parmales in the surface waters of the Atlantic Ocean

Journal of the Marine Biological Association of the United Kingdom ◽

10.1017/s0025315416001740 ◽

2016 ◽

Vol 98 (3) ◽

pp. 567-579 ◽

Cited By ~ 4

Author(s):

Qingshan Luan ◽

Jianqiang Sun ◽

Jun Wang

Keyword(s):

Atlantic Ocean ◽

Southern Ocean ◽

Surface Waters ◽

Large Scale ◽

Close Association ◽

Polar Front ◽

Falkland Islands ◽

North East ◽

Algal Groups ◽

Latitudinal Patterns

Coccolithophores and Parmales are important functional groups of calcified and siliceous marine nanophytoplankton. Large-scale biogeographic distributions of the two groups were investigated based on 71 samples that were collected in the Atlantic Ocean. Using a scanning electron microscope, a total of 48 taxa of coccolithophores and eight taxa of Parmales were recorded, with Emiliania huxleyi, Tetraparma pelagica and Triparma strigata as the predominant forms. The highest abundances of coccolithophores (376 × 103 cells l−1) and Parmales (624 × 103 cells l−1) were observed in waters north-east of the Falkland Islands and the South Georgia Island, in close association with the Subantarctic Front and Polar Front, respectively. Three major biogeographic assemblages, i.e. the Falkland Shelf Assemblage, the Southern Ocean Assemblage and the Atlantic Ocean Assemblage, were revealed in cluster analysis. Additionally, canonical correspondence analysis indicated that temperature significantly affects the latitudinal patterns of the two algal groups. High abundances of Parmales were closely coupled with those of E. huxleyi in waters of the Southern Ocean with low temperature (<10°C). However, the number of coccolithophore species, along with the Shannon–Weaver diversity, significantly increased with elevated temperature, suggesting more diverse assemblages in tropical waters.

Download Full-text

Sea Level Anomalies in the Southern Ocean due to Thermohaline Variability

10.5194/egusphere-egu21-14301 ◽

2021 ◽

Author(s):

Fabien Roquet ◽

Marlen Kolbe ◽

Etienne Pauthenet ◽

David Nerini

Keyword(s):

Southern Ocean ◽

Sea Level Rise ◽

Sea Level ◽

Salt Content ◽

Principal Component ◽

Polar Front ◽

Functional Principal Component Analysis ◽

Global Ocean ◽

Regional Variability ◽

Steric Height

<div> <div> <div> <p>The Southern Ocean is responsible for the majority of the global oceanic heat uptake which contributes to global sea level rise. At the same time, ocean temperature does not change everywhere at the same rate and salinity changes are also associated with sea level variability. Changes in heat and salt content drive together variations in the steric height that differ importantly in both time and space. This study investigates steric height variability in the Southern Ocean from 2008 to 2017 by analysing temperature and salinity variations obtained from global ocean reanalyses. The thermohaline variability is decomposed on so-called thermohaline modes using a functional Principal Component Analysis (fPCA). Thermohaline modes provide a natural basis on which to decompose the joint temperature-salinity vertical profiles into a sum of vertical modes weighted by their respective principal components. Steric height was computed in the reanalyses and related to the principal component using a Multiple Linear Regression (MLR) model. Trends in steric height are found to differ significantly between subtropical and subpolar regions, simultaneously which with a shift from a thermohaline stratification dominated by the first "thermocline" mode in the North to the second "saline" mode in the South. The Polar Front appears as a natural boundary between the two regions, where steric height variations are minimized. Since 2008, steric height has dropped close to the Antarctic continent, while subtropical waters farther north have mostly risen due to increased heat storage. While the dominant cause for the significant sea level rise south of 30S remains freshwater discharge from glaciers and ice sheets, thermohaline variability produces sizeable regional variability in the rate of sea level rise.</p> </div> </div> </div>

Download Full-text

Intercomparison of the Southern Ocean Circulations in IPCC Coupled Model Control Simulations

Journal of Climate ◽

10.1175/jcli3869.1 ◽

2006 ◽

Vol 19 (18) ◽

pp. 4560-4575 ◽

Cited By ~ 123

Author(s):

Joellen L. Russell ◽

Ronald J. Stouffer ◽

Keith W. Dixon

Keyword(s):

Southern Ocean ◽

Large Scale ◽

Heat Storage ◽

Climate Model ◽

Model Simulation ◽

Coupled Model ◽

Heat Fluxes ◽

Wind Forcing ◽

Transient Climate Response ◽

Model Control

Abstract The analyses presented here focus on the Southern Ocean as simulated in a set of global coupled climate model control experiments conducted by several international climate modeling groups. Dominated by the Antarctic Circumpolar Current (ACC), the vast Southern Ocean can influence large-scale surface climate features on various time scales. Its climatic relevance stems in part from it being the region where most of the transformation of the World Ocean’s water masses occurs. In climate change experiments that simulate greenhouse gas–induced warming, Southern Ocean air–sea heat fluxes and three-dimensional circulation patterns make it a region where much of the future oceanic heat storage takes place, though the magnitude of that heat storage is one of the larger sources of uncertainty associated with the transient climate response in such model projections. Factors such as the Southern Ocean’s wind forcing, heat, and salt budgets are linked to the structure and transport of the ACC in ways that have not been expressed clearly in the literature. These links are explored here in a coupled model context by analyzing a sizable suite of preindustrial control experiments associated with the forthcoming Intergovernmental Panel on Climate Change’s Fourth Assessment Report. A framework is developed that uses measures of coupled model simulation characteristics, primarily those related to the Southern Ocean wind forcing and water mass properties, to allow one to categorize, and to some extent predict, which models do better or worse at simulating the Southern Ocean and why. Hopefully, this framework will also lead to increased understanding of the ocean’s response to climate changes.

Download Full-text

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.

Download Full-text

Modulation of ocean acidification by decadal climate variability in the Gulf of Alaska

Communications Earth & Environment ◽

10.1038/s43247-021-00254-z ◽

2021 ◽

Vol 2 (1) ◽

Author(s):

Claudine Hauri ◽

Rémi Pagès ◽

Andrew M. P. McDonnell ◽

Malte F. Stuecker ◽

Seth L. Danielson ◽

...

Keyword(s):

Ocean Acidification ◽

Ocean Circulation ◽

Large Scale ◽

Dissolved Inorganic Carbon ◽

Model Simulation ◽

Gulf Of Alaska ◽

Sea Surface Height ◽

Sea Surface ◽

Global Ocean ◽

Wind Stress Curl

AbstractUptake of anthropogenic carbon dioxide from the atmosphere by the surface ocean is leading to global ocean acidification, but regional variations in ocean circulation and mixing can dampen or accelerate apparent acidification rates. Here we use a regional ocean model simulation for the years 1980 to 2013 and observational data to investigate how ocean fluctuations impact acidification rates in surface waters of the Gulf of Alaska. We find that large-scale atmospheric forcing influenced local winds and upwelling strength, which in turn affected ocean acidification rate. Specifically, variability in local wind stress curl depressed sea surface height in the subpolar gyre over decade-long intervals, which increased upwelling of nitrate- and dissolved inorganic carbon-rich waters and enhanced apparent ocean acidification rates. We define this sea surface height variability as the Northern Gulf of Alaska Oscillation and suggest that it can cause extreme acidification events that are detrimental to ecosystem health and fisheries.

Download Full-text

Meridional transport of salt in the global ocean from an eddy-resolving model

Ocean Science ◽

10.5194/os-10-243-2014 ◽

2014 ◽

Vol 10 (2) ◽

pp. 243-255 ◽

Cited By ~ 34

Author(s):

A. M. Treguier ◽

J. Deshayes ◽

J. Le Sommer ◽

C. Lique ◽

G. Madec ◽

...

Keyword(s):

Large Scale ◽

Global Analysis ◽

Salt Transport ◽

Global Ocean ◽

Necessary Condition ◽

Meridional Transport ◽

Mean Velocity ◽

Instability Waves ◽

Tropical Instability Waves ◽

Ocean Salinity

Abstract. The meridional transport of salt is computed in a global eddy-resolving numerical model (1/12° resolution) in order to improve our understanding of the ocean salinity budget. A methodology is proposed that allows a global analysis of the salinity balance in relation to surface water fluxes, without defining a "freshwater anomaly" based on an arbitrary reference salinity. The method consists of a decomposition of the meridional transport into (i) the transport by the time–longitude–depth mean velocity, (ii) time–mean velocity recirculations and (iii) transient eddy perturbations. Water is added (rainfall and rivers) or removed (evaporation) at the ocean surface at different latitudes, which creates convergences and divergences of mass transport with maximum and minimum values close to ±1 Sv. The resulting meridional velocity effects a net transport of salt at each latitude (±30 Sv PSU), which is balanced by the time–mean recirculations and by the net effect of eddy salinity–velocity correlations. This balance ensures that the total meridional transport of salt is close to zero, a necessary condition for maintaining a quasi-stationary salinity distribution. Our model confirms that the eddy salt transport cannot be neglected: it is comparable to the transport by the time–mean recirculation (up to 15 Sv PSU) at the poleward and equatorial boundaries of the subtropical gyres. Two different mechanisms are found: eddy contributions are localized in intense currents such as the Kuroshio at the poleward boundary of the subtropical gyres, while they are distributed across the basins at the equatorward boundaries. Closer to the Equator, salinity–velocity correlations are mainly due to the seasonal cycle and large-scale perturbations such as tropical instability waves.

Download Full-text

UK Global Ocean GO6 and GO7: a traceable hierarchy of model resolutions

Geoscientific Model Development ◽

10.5194/gmd-11-3187-2018 ◽

2018 ◽

Vol 11 (8) ◽

pp. 3187-3213 ◽

Cited By ~ 44

Author(s):

David Storkey ◽

Adam T. Blaker ◽

Pierre Mathiot ◽

Alex Megann ◽

Yevgeny Aksenov ◽

...

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Large Scale ◽

Coupled Model ◽

Weddell Sea ◽

Global Ocean ◽

Near Surface ◽

Ice Shelves ◽

Surface Circulation ◽

The Uk

Abstract. Versions 6 and 7 of the UK Global Ocean configuration (known as GO6 and GO7) will form the ocean components of the Met Office GC3.1 coupled model and UKESM1 earth system model to be used in CMIP61 simulations. The label “GO6” refers to a traceable hierarchy of three model configurations at nominal 1, 1∕4 and 1/12∘ resolutions. The GO6 configurations are described in detail with particular focus on aspects which have been updated since the previous version (GO5). Results of 30-year forced ocean-ice integrations with the 1/4∘ model are presented, in which GO6 is coupled to the GSI8.1 sea ice configuration and forced with CORE22 fluxes. GO6-GSI8.1 shows an overall improved simulation compared to GO5-GSI5.0, especially in the Southern Ocean where there are more realistic summertime mixed layer depths, a reduced near-surface warm and saline biases, and an improved simulation of sea ice. The main drivers of the improvements in the Southern Ocean simulation are tuning of the vertical and isopycnal mixing parameters. Selected results from the full hierarchy of three resolutions are shown. Although the same forcing is applied, the three models show large-scale differences in the near-surface circulation and in the short-term adjustment of the overturning circulation. The GO7 configuration is identical to the GO6 1/4∘ configuration except that the cavities under the ice shelves are opened. Opening the ice shelf cavities has a local impact on temperature and salinity biases on the Antarctic shelf with some improvement in the biases in the Weddell Sea.

Download Full-text

Side effects and accounting aspects of hypothetical large-scale Southern Ocean iron fertilization

Biogeosciences Discussions ◽

10.5194/bgd-7-2949-2010 ◽

2010 ◽

Vol 7 (2) ◽

pp. 2949-2995 ◽

Cited By ~ 3

Author(s):

A. Oschlies ◽

W. Koeve ◽

W. Rickels ◽

K. Rehdanz

Keyword(s):

Carbon Sequestration ◽

Side Effects ◽

Southern Ocean ◽

Atmospheric Carbon Dioxide ◽

Large Scale ◽

Climate Model ◽

N2o Emissions ◽

Co2 Fluxes ◽

Iron Fertilization ◽

Fertilization Experiments

Abstract. Recent suggestions to slow down the increase in atmospheric carbon dioxide have included ocean fertilization by addition of the micronutrient iron to Southern Ocean surface waters, where a number of natural and artificial iron fertilization experiments have shown that low ambient iron concentrations limit phytoplankton growth. Using a coupled carbon-climate model with the marine biology's response to iron addition calibrated against data from natural iron fertilization experiments, we examine biogeochemical side effects of a hypothetical large-scale Southern Ocean Iron Fertilization (OIF) that need to be considered when attempting to account for possible OIF-induced carbon offsets. In agreement with earlier studies our model simulates an OIF-induced increase in local air-sea CO2 fluxes by about 60 GtC over a 100-year period, which amounts to about 40% of the OIF-induced increase in organic carbon export. Offsetting CO2 return fluxes outside the region and after stopping the fertilization at 1, 7, 10, 50, and 100 years are quantified for a typical accounting period of 100 years. For continuous Southern Ocean iron fertilization, the return flux outside the fertilized area cancels about 8% of the fertilization-induced CO2 air-sea flux within the fertilized area on a 100-yr timescale. This "leakage" effect has a similar radiative impact as the simulated enhancement of marine N2O emissions. Other side effects not yet discussed in terms of accounting schemes include a decrease in Southern Ocean oxygen levels and a simultaneous shrinking of tropical suboxic areas, and accelerated ocean acidification in the entire water column in the Southern Ocean on the expense of reduced globally averaged surface water acidification. A prudent approach to account for the OIF-induced carbon sequestration would account for global air-sea CO2 fluxes rather than for local fluxes into the fertilized area only. However, according to our model, this would underestimate the potential for offsetting CO2 emissions by about 20% on a 100 year accounting timescale. We suggest that a fair accounting scheme applicable to both terrestrial and marine carbon sequestration has to be based on emission offsets rather than on changes in individual carbon pools.

Download Full-text