Home ▶ Vol 108, No 3/4 (2012) ▶ Swart

Article Information Authors: Sebastiaan Swart1,2 Nicolette Chang1 Nicolas Fauchereau1 Warren Joubert1,2 Mike Lucas3 Thato Mtshali1 Alakendra Roychoudhury4 Alessandro Tagliabue1,2 Sandy Thomalla1 Howard Waldron2 Pedro M.S. Monteiro1 Affiliations: 1Southern Ocean Carbon and Climate Observatory, Natural Resources and the Environment, CSIR, Stellenbosch, South Africa 2Department of Oceanography, Marine Research Institute, University of Cape Town, Cape Town, South Africa 3Department of Zoology, University of Cape Town, Cape Town, South Africa 4Department of Earth Sciences, Stellenbosch University, Stellenbosch, South Africa Correspondence to: Pedro Monteiro Email: pmonteir@csir.co.za Postal address: PO Box 320, Stellenbosch 7599, South Africa How to cite this article: Swart S, Chang N, Fauchereau, et al. Southern Ocean Seasonal Cycle Experiment 2012: Seasonal scale climate and carbon cycle links. S Afr J Sci. 2012;108(3/4), Art. #1089, 3 pages. http://dx.doi.org/10.4102/ sajs.v108i3/4.1089 Copyright Notice: © 2012. The Authors. Licensee: AOSIS OpenJournals. This work is licensed under the Creative Commons Attribution License. ISSN: 0038-2353 (print) ISSN: 1996-7489 (online)

Southern Ocean Seasonal Cycle Experiment 2012: Seasonal scale climate and carbon cycle links In This News and Views... Open Access • The experiment • References In early May 2012, South Africa will take delivery of a new polar research ship, the SA Agulhas II, representing a significant investment of R1.6 billion in polar infrastructure to further strengthen South Africa’s presence in the polar region, particularly in support of its stewardship of the Southern Ocean and Antarctic Treaty obligations. This investment follows closely on recent DST–CSIR infrastructure investments in the Centre for High Performance Computing, used to run global ocean–climate models, an ocean robotics observational capability and five other new research facilities. Together, these investments offer opportunities to enhance South Africa’s advanced numerical and technological capacity as well as its impact on Southern Hemisphere polar climate and ecosystem science by using its geographical advantage. We are planning the first scientific programme that will capitalise on these major infrastructure investments: the Southern Ocean Seasonal Cycle Experiment (SOSCEx). The Southern Ocean is arguably the main source of medium-term uncertainty in terms of the effectiveness of global CO2 mitigation plans. The reason for this is that the Southern Ocean plays both an important role in the uptake of anthropogenic CO2 (50% of all ocean uptake) as well as in the very large (90 Gt/Cy) natural CO2 exchange between the oceans and the atmosphere. The Southern Ocean is the only region where deep-ocean CO2 reservoirs (38 000 Gt C) exchange directly with the smaller atmospheric reservoir (700 Gt C). Moreover, although 85% of all ocean productivity is supported by nutrients derived from the Southern Ocean, little is known about the sensitivity of these carbon and nutrient fluxes to climate change driven adjustments or – most importantly – at what scales these links couple. One of the important gaps in the reliable prediction of the response of the Southern Ocean carbon cycle to climate change is its sensitivity to seasonal, subseasonal forcings (in time) and mesoscales (in space). The Southern Ocean Carbon and Climate Observatory (SOCCO), a CSIR-led consortium, is planning SOSCEx, which will be a new type of large-scale experiment. SOSCEx reflects a shift from the historical focus on ship-based descriptive Southern Ocean oceanography and living resource conservation, to system-scale dynamics studies spanning much greater time and space scales. Our limited grasp on these climate and climate-feedback sensitivities are linked to key knowledge gaps on the scales that link climate to carbon. Moreover, existing global models show only weak agreement in terms of the seasonal cycle of the upper-ocean in both physical and biogeochemical indicators such as primary productivity (Lenton A, Tilbrook B 2011, personal communication, Aug 01). The seasonal cycle is one of the strongest modes of variability in the primary productivity and the carbon cycle of the Southern Ocean. Additionally it reflects the coupling between climate forcing and important ecosystem responses, such as productivity.1 The ultimate sensitivity of the ecosystem over seasonal time scales is governed by their adapted phenological characteristics. However, climate forecast models are not able to reflect this seasonal timing reliably, which casts some doubt about our understanding of the scales at which climate and biogeochemistry are linked in the Southern Ocean. In addition, there appear to be important regional and basin differences in the way that carbon and ocean productivity respond to this otherwise regular seasonal forcing. Uncertainties in the understanding of the sensitivities of the biogeochemical cycles to changes in the climate forcing factors, hampers our ability to understand the long-term trends. Thus there is a need for a seasonal cycle experiment using autonomous platforms that focus on these scales (Figure 1), most importantly on the link between the physical forcing mechanisms and biogeochemical responses over the whole annual cycle. At a recent workshop2 convened to discuss the role of the seasonal cycle in coupling climate and carbon cycling in the Southern Ocean, two approaches were identified, (1) developing seasonal scale views to understand regional contrasts in the timing of the onset and evolution of phytoplankton blooms and (2) developing targeted ocean physics–carbon–ecosystem process studies in regional ‘hotspots’.23 For SOSCEx, it is hypothesised that climate change signals will be reflected in changes to the magnitude, timing and persistence of the seasonal cycle in mixed-layer physics and biogeochemistry and, in particular, the carbon cycle.2 We also propose that a high-resolution approach to advancing our understanding of the coupling of carbon and climate will reduce the uncertainty in projections of long-term trends in the ocean’s natural carbon fluxes and the anthropogenic carbon sink. FIGURE 1: A space�time plot showing relative scale magnitudes of a number of platforms (ships, instrumented moorings and gliders), the seasonal cycle and climate projections. This graphical representation emphasises that, even with both ships and moorings observational platforms, it is not possible to address questions on the seasonal cycle sensitivity of climate projections without using autonomous platforms. Ocean gliders are uniquely poised to bridge the spatial and temporal gap between ships and moorings � a bridge which critically covers the seasonal �window� in the Southern Ocean Seasonal Cycle Experiment. Recent studies highlight substantial variability in the response of Southern Ocean biological productivity to physical forcing on interannual, seasonal and subseasonal timescales.1,4,5 A key factor in the responses appears to be the variable control of phytoplankton growth by iron or light limitation.4 The spatially and temporally variable rates of buoyancy (heating) and momentum (wind mixing) forcing define the seasonal evolution of the light–iron status of the euphotic zone – the layer where photosynthesis occurs.5 These studies emphasise the importance of understanding the coupling of physics to biogeochemistry at subseasonal scales and submesoscales.6,7 The stark seasonal transition in the seasonal cycle phase between the Subtropical and Subantarctic zones1 provides an ideal test to understand this coupling. These frontal regions are believed to play pivotal roles in the variability of the mixed layer and vertical velocities caused by submesoscale processes that evolve at the fronts as a result of current meandering and eddy–eddy interaction.8 Researchers have now recognised that submesoscale structures account for approximately 50% of the total resolved variance7,8 observed from high-resolution satellite imagery, whilst vertical velocity variance shows a tenfold increase when numerical simulation resolutions increase from 6 km to 1 km.9 This high variance underscores the potential magnitude of the vertical nutrient supply and gas transport at small scales that impact on the overall primary productivity. The experiment (Back to top) SOSCEx has four main themes which address the same intraseasonal and submesoscale questions which link the carbon cycle to climate variability: mixed-layer stratification dynamics; CO2 and O2 gas exchange with the atmosphere; carbon export from the mixed layer; and the bio-optics linking water column inherent optical properties to outgoing-satellite visible irradiance. In SOSCEx, concurrent in-situ observational and modelling approaches are used to investigate the sampling scales necessary for in-situ observations and testing hypotheses at larger temporal and spatial scales. The modelling experiments will initially be used in two ways. The first is an ’upstream mode’ in which the model is a surrogate database that enables us – a priori – to try to determine optimal strategies for positioning gliders (Figure 2) by deploying virtual gliders in the model and investigating the effects of different sampling strategies on their recovery in order to anticipate the scales of variability we can expect. By contrast, the second way, a ‘downstream mode’, will use the model after the experiment in order to test new model parameterisations and to compare modelled and observed scales of variability. The research plan is coordinated to span approaches that encompass in-situ and remotely sensed observations, modelling and laboratory experiments. FIGURE 2: Gliders will be deployed south of Gough Island (in September 2012) and then allowed to proceed downstream with the major Southern Ocean fronts (blue lines), before being retrieved at the GoodHope line (between December 2012 and February 2013) or allowed to continue to Marion Island if battery energy can be sustained. The fronts are depicted as the mean position, between 2007 and 2008, derived from satellite altimetry data. The bathymetry is underlaid (greyscale shading in metres below sea level). The experiment is planned to take place around the three annual logistical trips of the ship, starting with the spring voyage to Gough Island, the mid-summer voyage to the South African National Antarctic Expedition (SANAE) base and the autumn voyage to Marion Island with little (5–10 days) additional time required above the existing schedule. In-situ sampling will begin in September 2012 during the austral spring relief voyage from Cape Town to Gough Island (located at 40.2°S, 10°E). The four iRobot® SeagliderTM units (Figure 2) will be deployed from the ship (1) south of the Subtropical Front, (2) in the Subantarctic Zone, (3) at the Subantarctic Front and (4) in the northern Polar Frontal Zone. The seagliders will be programmed to profile the water column from the surface to a depth of 1000 m and at a nominal horizontal resolution of every 4 km (4 dives/day). Carbon–explorer floats will be deployed at the same locations as each glider and two Liquid Robotics wave–glider units will be deployed in the Subantarctic Zone and Polar Frontal Zone to sample CO2 and oxygen to derive their fluxes. Each of the units will then be intercepted during the summer SANAE poleward or equatorward legs and finally retrieved in the autumn on the Marion Island trip. Full-depth water column profiles, including clean casts for iron chemistry, will be undertaken at each release and interception location. This sampling will provide us with the biogeochemical and physics boundary conditions of the system, such as CO2 levels, nutrients, iron concentrations, phytoplankton species composition and bio-optics. It will also allow for calibration and quality checks of the data received from the temperature, salinity, oxygen, backscatter, fluorescence and photosynthetically active radiation sensors housed on the gliders. Concurrent on-board bio-assay incubation experiments will examine the limiting factor of phytoplankton growth to be determined (light or iron) and contribute to improving model parameterisation in the Southern Ocean. The water column process stations will be supplemented by six profiles to 1000 m along the meridional axis of the sampling. Continuous remotely sensed data will be collected and used as an independent, large-scale data set before, during and after the period of the experiment. These data include, for example, sea surface height and temperature data to inform on the dynamics of ocean circulation, while ocean colour data will be used as an indicator of primary production and biomass accumulation in the surface waters. SOSCEx provides a new and unprecedented opportunity to gain a better understanding of the links between climate drivers and ecosystem productivity and climate feedbacks in the Southern Ocean. This combined high-resolution approach to both observations and modelling experiments will permit us, for the first time, to address some key questions relating to the physical nature of the Southern Ocean and its carbon cycle. References (Back to top) 1. Thomalla SJ, Fauchereau N, Swart S, Monteiro PMS. Regional scale characteristics of the seasonal cycle of chlorophyll in the Southern Ocean. Biogeosciences. 2011;8:2849–2866. http://dx.doi.org/10.5194/bg-8-2849-2011 2. Monteiro PMS, Boyd P, Bellerby R. Role of the seasonal cycle in coupling climate and carbon cycling in the Subantarctic zone. Eos Trans AGU. 2011;92(28):235. http://dx.doi.org/10.1029/2011EO280007 3. Sallée J, Speer K, Rintoul S, Wijffels S. Southern Ocean thermocline ventilation. J Phys Oceanogr. 2010;40:509–529. http://dx.doi.org/10.1175/2009JPO4291.1 4. Fauchereau N, Tagliabue A, Bopp L, Monteiro PMS. The response of phytoplankton biomass to transient mixing events in the Southern Ocean. Geophys Res Lett. 2011;38:L17601. http://dx.doi.org/10.1029/2011GL048498 5. Lévy M, Klein P, Ben Jelloul M. New production stimulated by high-frequency winds in a turbulent mesoscale eddy field. Geophys Res Lett. 2009;36:L16603. http://dx.doi.org/10.1029/2009GL039490 6. Klein P, Lapeyre G. The oceanic vertical pump induced by mesoscale and submesoscale turbulence. Ann Rev Mar Sci. 2009;1:351–375. http://dx.doi.org/10.1146/annurev.marine.010908.163704 7. Glover DM, Doney SC, Nelson NB, Wallis A. Submesoscale anisotropy (fronts, eddies, and filaments) as observed near Bermuda with ocean color data. Paper presented at: 2008 Ocean Sciences Meeting. Proceedings of the 2008 Ocean Sciences Meeting; 2008 March 02–07; Orlando, Florida, USA. 8. Taylor JR, Ferrari R. Ocean fronts trigger high latitude phytoplankton blooms. Geophys Res Lett. 2011;38:L23601. http://dx.doi.org/10.1029/2011GL049312 9. Klein P, Hua BL, Lapeyre G, Capet X, Le Gentil S, Sasaki H. Upper ocean turbulence from high 3-D resolution simulations. J Phys Oceanogr. 2008;38:1748–1763. http://dx.doi.org/10.1175/2007JPO3773.1

Reader Comments

Before posting a comment, read our privacy policy.

Post a comment (login required)

 

1. Spatial and temporal scales of chlorophyll variability using high-resolution glider data
H.J. Little, M. Vichi, S.J. Thomalla, S. Swart
Journal of Marine Systems  vol: 187  first page: 1  year: 2018  
doi: 10.1016/j.jmarsys.2018.06.011

2. Modelled estimates of spatial variability of iron stress in the Atlantic sector of the Southern Ocean
Thomas J. Ryan-Keogh, Sandy J. Thomalla, Thato N. Mtshali, Hazel Little
Biogeosciences  vol: 14  issue: 17  first page: 3883  year: 2017  
doi: 10.5194/bg-14-3883-2017

3. Southern Ocean phytoplankton dynamics and carbon export: insights from a seasonal cycle approach
Sandy J. Thomalla, Marcel Du Plessis, Nicolas Fauchereau, Isabelle Giddy, Luke Gregor, Stephanie Henson, Warren R. Joubert, Hazel Little, Pedro M. S. Monteiro, Thato Mtshali, Sarah Nicholson, Thomas J. Ryan-Keogh, Sebastiaan Swart
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  vol: 381  issue: 2249  year: 2023  
doi: 10.1098/rsta.2022.0068

4. The seasonal cycle of mixed layer dynamics and phytoplankton biomass in the Sub-Antarctic Zone: A high-resolution glider experiment
S. Swart, S.J. Thomalla, P.M.S. Monteiro
Journal of Marine Systems  vol: 147  first page: 103  year: 2015  
doi: 10.1016/j.jmarsys.2014.06.002

5. The Fundamental Contribution of Phytoplankton Spectral Scattering to Ocean Colour: Implications for Satellite Detection of Phytoplankton Community Structure
Lisl Robertson Lain, Stewart Bernard
Applied Sciences  vol: 8  issue: 12  first page: 2681  year: 2018  
doi: 10.3390/app8122681

6. BIOPERIANT12: a mesoscale-resolving coupled physics–biogeochemical model for the Southern Ocean
Nicolette Chang, Sarah-Anne Nicholson, Marcel du Plessis, Alice D. Lebehot, Thulwaneng B. Mashifane, Tumelo C. Moalusi, Precious Mongwe, Pedro M. S. Monteiro
Geoscientific Model Development  vol: 18  issue: 18  first page: 6415  year: 2025  
doi: 10.5194/gmd-18-6415-2025

7. A sea of potential: The politics of global ocean observations
Jessica Lehman
Political Geography  vol: 55  first page: 113  year: 2016  
doi: 10.1016/j.polgeo.2016.09.006

8. Seasonal development of iron limitation in the sub-Antarctic zone
Thomas J. Ryan-Keogh, Sandy J. Thomalla, Thato N. Mtshali, Natasha R. van Horsten, Hazel J. Little
Biogeosciences  vol: 15  issue: 14  first page: 4647  year: 2018  
doi: 10.5194/bg-15-4647-2018

9. Storms drive outgassing of CO2 in the subpolar Southern Ocean
Sarah-Anne Nicholson, Daniel B. Whitt, Ilker Fer, Marcel D. du Plessis, Alice D. Lebéhot, Sebastiaan Swart, Adrienne J. Sutton, Pedro M. S. Monteiro
Nature Communications  vol: 13  issue: 1  year: 2022  
doi: 10.1038/s41467-021-27780-w

10. Mesoscale features and phytoplankton biomass at the GoodHope line in the Southern Ocean during austral summer
S Swart, SJ Thomalla, PMS Monteiro, IJ Ansorge
African Journal of Marine Science  vol: 34  issue: 4  first page: 511  year: 2012  
doi: 10.2989/1814232X.2012.749811

11. The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations
M. D. du Plessis, S. Swart, L. C. Biddle, I. S. Giddy, P. M. S. Monteiro, C. J. C. Reason, A. F. Thompson, S.‐A. Nicholson
Journal of Geophysical Research: Oceans  vol: 127  issue: 4  year: 2022  
doi: 10.1029/2021JC017760

12. Southern Ocean Seasonal Restratification Delayed by Submesoscale Wind–Front Interactions
Marcel du Plessis, Sebastiaan Swart, Isabelle J. Ansorge, Amala Mahadevan, Andrew F. Thompson
Journal of Physical Oceanography  vol: 49  issue: 4  first page: 1035  year: 2019  
doi: 10.1175/JPO-D-18-0136.1

13. Evaluation of Satellite and Reanalysis Wind Products with In Situ Wave Glider Wind Observations in the Southern Ocean
Kevin M. Schmidt, Sebastiaan Swart, Chris Reason, Sarah-Anne Nicholson
Journal of Atmospheric and Oceanic Technology  vol: 34  issue: 12  first page: 2551  year: 2017  
doi: 10.1175/JTECH-D-17-0079.1

14. Deriving a Proxy for Iron Limitation From Chlorophyll Fluorescence on Buoyancy Gliders
Thomas J. Ryan-Keogh, Sandy J. Thomalla
Frontiers in Marine Science  vol: 7  year: 2020  
doi: 10.3389/fmars.2020.00275

15. Submesoscale processes promote seasonal restratification in the Subantarctic Ocean
M. du Plessis, S. Swart, I. J. Ansorge, A. Mahadevan
Journal of Geophysical Research: Oceans  vol: 122  issue: 4  first page: 2960  year: 2017  
doi: 10.1002/2016JC012494

16. The Southern Ocean mixed layer and its boundary fluxes: fine-scale observational progress and future research priorities
Sebastiaan Swart, Marcel D. du Plessis, Sarah-Anne Nicholson, Pedro M. S. Monteiro, Lilian A. Dove, Sandy Thomalla, Andrew F. Thompson, Louise C. Biddle, Johan M. Edholm, Isabelle Giddy, Karen J. Heywood, Craig Lee, Amala Mahadevan, Geoff Shilling, Ronald Buss de Souza
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  vol: 381  issue: 2249  year: 2023  
doi: 10.1098/rsta.2022.0058

17. Using Optical Sensors on Gliders to Estimate Phytoplankton Carbon Concentrations and Chlorophyll-to-Carbon Ratios in the Southern Ocean
Sandy J. Thomalla, A. Gilbert Ogunkoya, Marcello Vichi, Sebastiaan Swart
Frontiers in Marine Science  vol: 4  year: 2017  
doi: 10.3389/fmars.2017.00034

18. Seasonal Depletion of the Dissolved Iron Reservoirs in the Sub‐Antarctic Zone of the Southern Atlantic Ocean
T. N. Mtshali, N. R. van Horsten, S. J. Thomalla, T. J. Ryan‐Keogh, S.‐A. Nicholson, A. N. Roychoudhury, E. Bucciarelli, G. Sarthou, A. Tagliabue, P. M. S. Monteiro
Geophysical Research Letters  vol: 46  issue: 8  first page: 4386  year: 2019  
doi: 10.1029/2018GL081355

19. GliderTools: A Python Toolbox for Processing Underwater Glider Data
Luke Gregor, Thomas J. Ryan-Keogh, Sarah-Anne Nicholson, Marcel du Plessis, Isabelle Giddy, Sebastiaan Swart
Frontiers in Marine Science  vol: 6  year: 2019  
doi: 10.3389/fmars.2019.00738

20. Atmospheric Rivers Contribute to Summer Surface Buoyancy Forcing in the Atlantic Sector of the Southern Ocean
Johan M. Edholm, Sebastiaan Swart, Marcel D. Plessis, Sarah‐Anne Nicholson
Geophysical Research Letters  vol: 49  issue: 17  year: 2022  
doi: 10.1029/2022GL100149

21. The ocean carbon sink – impacts, vulnerabilities and challenges
C. Heinze, S. Meyer, N. Goris, L. Anderson, R. Steinfeldt, N. Chang, C. Le Quéré, D. C. E. Bakker
Earth System Dynamics  vol: 6  issue: 1  first page: 327  year: 2015  
doi: 10.5194/esd-6-327-2015

22. High-resolution view of the spring bloom initiation and net community production in the Subantarctic Southern Ocean using glider data
Sandy J. Thomalla, Marie-Fanny Racault, Sebastiaan Swart, Pedro M. S. Monteiro
ICES Journal of Marine Science  vol: 72  issue: 6  first page: 1999  year: 2015  
doi: 10.1093/icesjms/fsv105

23. Storm‐Driven pCO2 Feedback Weakens the Response of Air‐Sea CO2 Fluxes in the Sub‐Antarctic Southern Ocean
Tesha Toolsee, Sarah‐Anne Nicholson, Pedro M. S. Monteiro
Geophysical Research Letters  vol: 51  issue: 9  year: 2024  
doi: 10.1029/2023GL107804