Thomalla S.J., Gilbert Ogunkoya, Vichi M., Swart S.
Abstract

One approach to deriving phytoplankton carbon biomass estimates (Cphyto) at appropriate scales is through optical products. This study uses a high-resolution glider data set in the Sub-Antarctic Zone (SAZ) of the Southern Ocean to compare four different methods of deriving Cphyto from particulate backscattering and fluorescence-derived chlorophyll (chl-a). A comparison of the methods showed that at low (<0.5 mg m−3) chlorophyll concentrations (e.g., early spring and at depth), all four methods produced similar estimates of Cphyto, whereas when chlorophyll concentrations were elevated one method derived higher concentrations of Cphyto than the others. The use of methods derived from particulate backscattering rather than fluorescence can account for cellular adjustments in chl-a:Cphytothat are not driven by biomass alone. A comparison of the glider chl-a:Cphyto ratios from the different optical methods with ratios from laboratory cultures and cruise data found that some optical methods of deriving Cphyto performed better in the SAZ than others and that regionally derived methods may be unsuitable for application to the Southern Ocean. A comparison of the glider chl-a:Cphyto ratios with output from a complex biogeochemical model shows that although a ratio of 0.02 mg chl-a mg C−1 is an acceptable mean for SAZ phytoplankton (in spring-summer), the model misrepresents the seasonal cycle (with decreasing ratios from spring to summer and low sub-seasonal variability). As such, it is recommended that models expand their allowance for variable chl-a:Cphyto ratios that not only account for phytoplankton acclimation to low light conditions in spring but also to higher optimal chl-a:Cphyto ratios with increasing growth rates in summer.

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Time-evolution of chl-a:Cphyto ratios at the surface (10 m) derived from the 30%POC method (solid lighter green top line), the B05 method (red line), the M13 method (blue line), and the S09 method (pink line). In addition, Chl-a:Cphyto ratios were calculated using the chl-a:POC ratio from the cruise data (which implies that all POC is phytoplankton specific) and is presented as 100%POC (darker green bottom dashed line). Included for comparision are (1) the chl-a:Cphyto ratios derived from the original equation from Sathyendranath et al. (2009) presented as S09original (purple line), (2) the satellite range of ratios from Behrenfeld et al. (2005) (black dotted lines) and (3) the ratios derived from the PELAGOS025 model (McKiver et al., 2015, extracted from the model for the same geographical co-ordinates as the glider transect in time but for a year 2011 simulation, solid black line). The inset shows a detail of the daily signal for the B05 method.

Time-evolution of chl-a:Cphyto ratios at the surface (10 m) derived from the 30%POC method (solid lighter green top line), the B05 method (red line), the M13 method (blue line), and the S09 method (pink line). In addition, Chl-a:Cphyto ratios were calculated using the chl-a:POC ratio from the cruise data (which implies that all POC is phytoplankton specific) and is presented as 100%POC (darker green bottom dashed line). Included for comparision are (1) the chl-a:Cphyto ratios derived from the original equation from Sathyendranath et al. (2009) presented as S09original (purple line), (2) the satellite range of ratios from Behrenfeld et al. (2005) (black dotted lines) and (3) the ratios derived from the PELAGOS025 model (McKiver et al., 2015, extracted from the model for the same geographical co-ordinates as the glider transect in time but for a year 2011 simulation, solid black line). The inset shows a detail of the daily signal for the B05 method.

Abstract

The Southern Ocean forms a key component of the global carbon budget, taking up about 1.0 Pg C yr−1 of anthropogenic CO2 emitted annually (∼10.7 ± 0.5 Pg C yr−1 for 2012). However, despite its importance, it still remains undersampled with respect to surface ocean carbon flux variability, resulting in weak constraints for ocean carbon and carbon – climate models. As a result, atmospheric inversion and coupled physics-biogeochemical ocean models still play a central role in constraining the air-sea CO2fluxes in the Southern Ocean. A recent synthesis study (Lenton et al., 2013a), however, showed that although ocean biogeochemical models (OBGMs) agree on the mean annual flux of CO2 in the Southern Ocean, they disagree on both amplitude and phasing of the seasonal cycle and compare poorly to observations. In this study, we develop and present a methodological framework to diagnose the controls on the seasonal variability of sea-air CO2 fluxes in model outputs relative to observations. We test this framework by comparing the NEMO-PISCES ocean model ORCA2-LIM2-PISCES to the Takahashi 2009 (T09) CO2 dataset. Here we demonstrate that the seasonal cycle anomaly for CO2fluxes in ORCA2LP is linked to an underestimation of winter convective CO2 entrainment as well as the impact of biological CO2 uptake during the spring-summer season, relative to T09 observations. This resulted in sea surface temperature (SST) becoming the dominant driver of seasonal scale of the partial pressure of CO2 (pCO2) variability and hence of the differences in the seasonality of CO2 sea-air flux between the model and observations.

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Global ocean summer and winter air-sea CO2 flux climatologies contrasting Takahashi, 2009 (T09) observations for reference year 2000 (a–b), and NEMO-PISCES (1993–2006) (c–d), units mmol C m−2 day. It shows seasonal climatological biases between the model and observations in the Southern Ocean.

Global ocean summer and winter air-sea CO2 flux climatologies contrasting Takahashi, 2009 (T09) observations for reference year 2000 (a–b), and NEMO-PISCES (1993–2006) (c–d), units mmol C m−2 day. It shows seasonal climatological biases between the model and observations in the Southern Ocean.

Abstract

In the Sub-Antarctic Ocean elevated phytoplankton biomass persists through summer at a time when productivity is expected to be low due to iron limitation. Biological iron recycling has been shown to support summer biomass. In addition, we investigate an iron supply mechanism previously unaccounted for in iron budget studies. Using a 1-D biogeochemical model, we show how storm-driven mixing provides relief from phytoplankton iron limitation through the entrainment of iron beneath the productive layer. This effect is significant when a mixing transition layer of strong diffusivities (kz > 10−4 m2 s−1) is present beneath the surface-mixing layer. Such subsurface mixing has been shown to arise from interactions between turbulent ocean dynamics and storm-driven inertial motions. The addition of intraseasonal mixing yielded increases of up to 60% in summer primary production. These results stress the need to acquire observations of subsurface mixing and to develop the appropriate parameterizations of such phenomena for ocean-biogeochemical models.

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Comparisons of (a and b) primary production, (c and d) DFe, and (e and f) integrated PP, surface PP*64, MLD, and surface DFe between the 'SXLD surface mixed-layer deepening' and the 'subsurface mixing run'.

Comparisons of (a and b) primary production, (c and d) DFe, and (e and f) integrated PP, surface PP*64, MLD, and surface DFe between the ‘SXLD surface mixed-layer deepening’ and the ‘subsurface mixing run’.

Abstract

The Southern Ocean (SO) contributes most of the uncertainty in contemporary estimates of the mean annual flux of carbon dioxide CO2 between the ocean and the atmosphere. Attempts to reduce this uncertainty have aimed at resolving the seasonal cycle of the fugacity of CO2 (fCO2). We use hourly CO2 flux and driver observations collected by the combined deployment of ocean gliders to show that resolving the seasonal cycle is not sufficient to reduce the uncertainty of the flux of CO2 to below the threshold required to reveal climatic trends in CO2 fluxes. This was done by iteratively subsampling the hourly CO2 data set at various time intervals. We show that because of storm-linked intraseasonal variability in the spring-late summer, sampling intervals longer than 2 days alias the seasonal mean flux estimate above the required threshold. Moreover, the regional nature and long-term trends in storm characteristics may be an important influence in the future role of the SO in the carbon-climate system.

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The spatial variability of the FCO2 uncertainties, which arise from a uniform 10 day sampling period choice. The Southern Ocean is characterized with uncertainties of 10–25% (10–25 μmol m2 h1) at this sampling period.

The spatial variability of the FCO2 uncertainties, which arise from a uniform 10 day sampling period choice. The Southern Ocean is characterized with uncertainties of 10–25% (10–25 μmol m2 h1) at this sampling period.

Meredith, M., Swart S., Monteiro P.M.S., et al.
Abstract

The Southern Ocean exerts a disproportionately strong influence on global climate, so determining its changing state is of key importance in understanding the planetary-scale system. This is a consequence of the connectedness of the Southern Ocean, which links the other major ocean basins and is a site of strong lateral fluxes of climatically important tracers. It is also a consequence of processes occurring within the Southern Ocean, including the vigorous overturning circulation that leads to the formation of new water masses, and to the strong exchange of carbon, heat, and other climatically relevant properties at the ocean surface. However, determining the state of the Southern Ocean in a given year is even more problematic than for other ocean basins, due to the paucity of observations. Nonetheless, using the limited data available, some key aspects of the state of the Southern Ocean in 2014 can be ascertained.

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BAMS Sate of the Climate 2014 cover

BAMS Sate of the Climate 2014 cover

Thomalla S.J., Dr Marie-Fanny Racault, Swart S., Monteiro P.M.S.
Abstract

In the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.

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Time series of (a) modelled MLD and water column integrated NPP (mg C m-2 d-1), (b) modelled respiration (mg C m-2 d-1) (Sverdrup 1953), with standard mean error (shaded area), (c) same as for (c) but for NCP (mg C m-2 d-1), and (d) f-ratio approximation of the export efficiency (PP/mean NCP) (solid line).

Time series of (a) modelled MLD and water column integrated NPP (mg C m-2 d-1), (b) modelled respiration (mg C m-2 d-1) (Sverdrup 1953), with standard mean error (shaded area), (c) same as for (c) but for NCP (mg C m-2 d-1), and (d) f-ratio approximation of the export efficiency (PP/mean NCP) (solid line).

Thomalla S.J., Dr Marie-Fanny Racault, Swart S., Monteiro P.M.S.
Abstract

In the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.

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Mozorov, E., Tarakanov, R., Ansorge I. J., Swart S.
Abstract

In December 2003, a hydrographic section across the Drake Passage was carried out by R/V Akademik Sergei Vavilov from King George Island to Tierra del Fuego with a Seabird 911 and lowered Doppler current (ADCP) profilers. A total of 25 stations were occupied across the passage from the surface to the bottom. The geostrophic water transport by the Antarctic Circumpolar Current (ACC) above the bottom reference level is estimated at 111 Sv (1 Sv = 106 m3/s), while the transport above the 3000 dbar reference level is equal to 97 Sv. These values are close to the smallest ones in the record of measurements of water transport through the Drake Passage since 1975. The geostrophic velocities are compared with the LADCP and shipborne ADCP measurements.

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Section of geostrophic velocities to the bottom in the Drake Passage

Section of geostrophic velocities to the bottom in the Drake Passage

Abstract

In the Southern Ocean there is increasing evidence that seasonal to sub-seasonal temporal scales, meso- and submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. In this study, high-resolution glider data (3 hourly, 2km horizontal resolution), from ~6 months of sampling (spring through summer) in the Sub-Antarctic Zone, is used to assess 1) the different forcing mechanisms driving variability in upper ocean physics and 2) how these may characterize the seasonal cycle of phytoplankton production. Results highlight the important role meso- to submesoscale features have in driving vertical stratification and early phytoplankton bloom initiations in spring by increasing light exposure. In summer, the combined role of solar heat flux, mesoscale features and subseasonal storms on the extent of the mixed layer is proposed to regulate both light and iron to the upper ocean at appropriate time scales for phytoplankton growth, thereby sustaining the bloom for an extended period through to late summer. This study highlights the need for climate models to resolve both meso- to submesoscale and subseasonal processes in order to accurately reflect the phenology of the phytoplankton community and understand the sensitivity of ocean primary productivity to climate change.

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Glider sections of (a) temperature (°C), (b) stratification and (c) chlorophyll-a concentration (mg m-3) during the 'spring bloom initiation phase' of SOSCEx. The MLD is depicted using a white curve.

Glider sections of (a) temperature (°C), (b) stratification and (c) chlorophyll-a concentration (mg m-3) during the ‘spring bloom initiation phase’ of SOSCEx. The MLD is depicted using a white curve.

Ansorge I. J., Jackson, J., Reid, K., Durgadoo, J., Swart S., Eberenz, S.
Abstract

Mesoscale eddies and meanders have been shown to be one of the dominant sources of flow variability in the world’s ocean. One example of an isolated eddy hotspot is the South-West Indian Ridge (SWIR). Several investigations have shown that the SWIR and the corresponding planetary potential vorticity field (f/H) exert a strong influence on the location and dynamics of the Antarctic Circumpolar Current (ACC), resulting in substantial fragmentation of the jets downstream of the ridge. The easterly extension of this eddy corridor appears to be restricted to the deep channel separating the Conrad Rise from the Del Cano and Crozet Plateau. However, while the fate of eddies formed at the SWIR has been widely investigated and the frontal character of this eastward extension is well known, the zone of diminishing variability that extends southwards to approximately 60S remains poorly sampled. Using a combination of Argo, AVISO and NCEP/NCAR datasets, the character of this eddy corridor as a conduit for warm core eddies to move across the ACC into the Antarctic zone is investigated. In this study, we track a single warm-core eddy as it moves southwards from an original position of 31E, 50 20′ S to where it dissipates 10 months later in the Enderby Basin at 561200 S. An Argo float entrained within the eddy confirms that its water masses are consistent with water found within the Antarctic Polar Frontal Zone north of the APF. Latent and sensible heat fluxes are on average 8 W/m2 and 10 W/m2 greater over the eddy than directly east of this feature. It is estimated that the eddy lost an average of 5 W/m2 of latent heat and 5 W/m2 of sensible heat over a 1-year period, an amount capable of melting approximately 0.92 m of sea ice. In addition, using an eddy tracking algorithm a total of 28 eddies are identified propagating southwards, 25 of which are anti-cyclonic in rotation. Based on the new Argo float data, combined with AVISO and NCEP/NCAR datasets, these results suggest that the southward passage of warm-core eddies act as vehicles transporting heat, salt and biota southwards across the ACC and into the eastern boundary of the Weddell gyre.

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Sea surface height variability for the period 2000-2009. The trajectory of a positive anomaly is identified with a solid black line

Sea surface height variability for the period 2000-2009. The trajectory of a positive anomaly is identified with a solid black line

Macey, A.I., Ryan-Keogh T J, Richier, S., Moore, C.M., Bibby, T.S.
Abstract

Iron availability influences phytoplankton physiology and growth over more than one-third of the surface oceans, with recent evidence even indicating iron stress during and following the latter stages of the spring bloom in the high latitude North Atlantic. The mechanistic basis of the phytoplankton physiological responses used for diagnosing iron stress and the broader ecophysiological consequences of iron stress within natural phytoplankton communities still remain unclear. We describe photosynthetic macromolecular and physiological responses of natural phytoplankton communities both in situ and within factorial nutrient-addition (iron and nitrogen) experiments over a seasonal growth cycle in the subpolar North Atlantic. The abundance of quantified photosynthetic proteins increased under relief of iron stress, with the synthesis of the associated protein catalytic complexes accounting for, ~ 3% of inorganic nitrogen drawdown. However, no significant differences in the stoichiometries of the photosynthetic complexes were observed, suggesting that re-modeling of the photosynthetic electron transport chain was not a significant influence on the community-level ecophysiological responses to iron stress. In marked contrast, iron stress resulted in significant increases in the cellular ratios of chlorophyll to the photosynthetic catalysts, including photosystem II (PSII), alongside a marked increase in PSII normalized chlorophyll fluorescence. Characteristic depressions in apparent photosynthetic energy conversion efficiencies in iron-limited oceanic regions are thus likely driven by a significant accumulation of partially energetically uncoupled chlorophyll-binding complexes. Such iron-stress–induced chlorophyll-binding proteins may contribute, ~ 40% of the total chlorophyll pool during iron-stressed periods.

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Macey-paper-fig

Incubation Experiment 4 (IE4) is shown as a typical example of the response of the phytoplankton community to the addition of Fe (white symbols) in relation to control samples (black symbols). (a–c) changes in the apparent photochemistry of PSII (Fv : Fm), chlorophyll a (μg L-1), and nitrate concentrations (μmol L-1). (d–f) changes in Fm normalized to chlorophyll (Fm : Chl), and accumulation of the peptide PsbA (a subunit of the photosynthetic complex PSII) normalized to total protein (fmol (μg protein)-1) and as total molar concentration (pmol L-1). a.u., arbitrary units.

Swart S., Liu, J., Bhaskar, P., Newman, L., Finney, K., Meredith, M., Schofield, O.
Abstract

The first Southern Ocean Observing System (SOOS) Asian Workshop was successfully held in Shanghai, China in May 2013, attracting over 40 participants from six Asian nations and widening exposure to the objectives and plans of SOOS. The workshop was organized to clarify Asian research activities currently taking place in the Southern Ocean and to discuss, amongst other items, the potential for collaborative efforts with and between Asian countries in SOOS-related activities. The workshop was an important mechanism to initiate discussion, understanding and collaborative avenues in the Asian domain of SOOS beyond current established efforts. Here we present some of the major outcomes of the workshop covering the principle themes of SOOS and attempt to provide a way forward to achieve a more integrated research community, enhance data collection and quality, and guide scientific strategy in the Southern Ocean.

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Map of the Southern Ocean and approximate location of regular shipping transects maintained by Asian nations.

Map of the Southern Ocean and approximate location of regular shipping transects maintained by Asian nations.

Liu, J., Swart S., Bhaskar, P., Newman, L., Meredith, M., Schofield, O., Jianfeng, HE.
Abstract

SOOS must be a fully integrated and coordinated international system with infrastructure, resources and investment from all nations involved in the Southern Ocean research and observations. This was the motivation behind the organization of the SOOS Asian workshop. The objective of the SOOS Asian Workshop was to highlight the activities of Asian countries currently engaged in Southern Ocean research and observations relevant to the SOOS science strategy, and to stimulate discussion and foster further involvement from Asian countries in the SOOS activities.

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The Southern Ocean Observing System

The Southern Ocean Observing System

Tagliabue, A., Sallee, J. B., Bowie, A. R., Levy M., Swart S., Boyd. P. W.
Abstract

Low levels of iron limit primary productivity across much of the Southern Ocean. At the basin scale, most dissolved iron is supplied to surface waters from subsurface reservoirs, because land inputs are spatially limited. Deep mixing in winter together with year-round diffusion across density surfaces, known as diapycnal diffusion, are the main physical processes that carry iron-laden subsurface waters to the surface. Here, we analyse data on dissolved iron concentrations in the top 1,000 m of the Southern Ocean, taken from all known and available cruises to date, together with hydrographic data to determine the relative importance of deep winter mixing and diapycnal diffusion to dissolved iron fluxes at the basin scale. Using information on the vertical distribution of iron we show that deep winter mixing supplies ten times more iron to the surface ocean each year, on average, than diapycnal diffusion. Biological observations from the sub-Antarctic sector suggest that following the depletion of this wintertime iron pulse, intense iron recycling sustains productivity over the subsequent spring and summer. We conclude that winter mixing and surface-water iron recycling are important drivers of temporal variations in Southern Ocean primary production.

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A schematic representation of the seasonal variability in Southern Ocean Fe cycling

A schematic representation of the seasonal variability in Southern Ocean Fe cycling

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