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’.

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).

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.

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

Achterberg, E.P., Moore, C.M., Henson, S.A., Steigenberger, S., Stohl, A., Eckhardt, S., Avendano, L.C., Cassidy, M., Hembury, D., Lucas M., Ryan-Keogh T J, Et al.
Abstract

Aerosol deposition from the 2010 eruption of the Icelandic volcano Eyjafjallajökull resulted in significant dissolved iron (DFe) inputs to the Iceland Basin of the North Atlantic. Unique ship-board measurements indicated strongly enhanced DFe concentrations (up to 10 nM) immediately under the ash plume. Bioassay experiments performed with ash collected at sea under the plume also demonstrated the potential for associated Fe release to stimulate phytoplankton growth and nutrient drawdown. Combining Fe dissolution measurements with modeled ash deposition suggested that the eruption had the potential to increase DFe by > 0.2 nM over an area of up to 570,000 km2 . Although satellite ocean color data only indicated minor increases in phytoplankton abundance over a relatively constrained area, comparison of in situ nitrate concentrations with historical records suggested that ash deposition may have resulted in enhanced major nutrient drawdown. Our observations thus suggest that the 2010 Eyjafjallajökull eruption resulted in a significant perturbation to the biogeochemistry of the Iceland Basin.

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fig3_depo

(a) Modeled DFe enhancement (nM) as a result of ash deposition (15 April to 23 May) using midrange estimates of salt layer thickness (20 nm) of volcanic particles as obtained through leaching experiments. Contours mark 0.2nM DFe enhancement. The dashed line is the cruise track (May 2010). (b) The proportion of the Iceland Basin (assumed to be a region ~1 x 106 km2) receiving DFe inputs from ash (15 April to 23 May) using minimum (solubility 0.042%) and maximum (salt layer coating of 90nm thickness) estimates of Fe content of volcanic particles.

Ryan-Keogh T J, Macey, A.I., Lucas M., Steigenberger, S.S., Stinchcombe, M.C., Achterberg, E.P., Bibby, T.S., Moore, C.M.
Abstract

The high-latitude North Atlantic (HLNA) is characterized by a marked seasonal phytoplankton bloom, which removes the majority of surface macronutrients. However, incomplete nitrate depletion is frequently observed during summer in the region, potentially reflecting the seasonal development of an iron (Fe) limited phytoplankton community. In order to investigate the seasonal development and spatial extent of iron stress in the HLNA, nutrient addition experiments were performed during the spring (May) and late summer (July and August) of 2010. Grow-out experiments (48–120 h) confirmed the potential for iron limitation in the region. Short-term (24 h) incubations further enabled high spatial coverage and mapping of phytoplankton physiological responses to iron addition. The difference in the apparent maximal photochemical yield of photosystem II (PSII) (Fv : Fm) between nutrient (iron) amended and control treatments (Δ(Fv : Fm)) was used as a measure of the relative degree of iron stress. The combined observations indicated variability in the seasonal cycle of iron stress between different regions of the Irminger and Iceland Basins of the HLNA, related to the timing of the annual bloom cycle in contrasting biogeochemical provinces. Phytoplankton iron stress developed during the transition from the prebloom to peak bloom conditions in the HLNA and was more severe for larger cells. Subsequently, iron stress was reduced in regions where macronutrients were depleted following the bloom. Iron availability plays a significant role in the biogeochemistry of the HLNA, potentially lowering the efficiency of one of the strongest biological carbon pumps in the ocean.

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HLNA Fig 8

(a) In situ chlorophyll data (μg L-1) and relative degree of Fe stress (Δ(Fv:Fm)+2.0 Fe), (b) in situ DIN (μmol L-1) data and Δ(Fv:Fm)+2.0 Fe and in situ DIN and different in net chlorophyll growth rate following Fe addition (ΔμChl (d-1)) relative to time of peak of bloom. Superimposed on panel (c) conceptualised model of bloom dynamics, demonstrating two different post-bloom scenarios (low DIN and high DIN) associated with different degrees of Fe stress and iron limited growth rates.

Giddy, I., Swart S., Tagliabue, A.
Abstract

The canonical C/N/P ratio of 106/16/1 in phytoplankton has been instrumental in our understanding of ocean biogeochemical cycles and the development of numerical models as it couples the cycling of C to nutrients. However, this ratio can show marked variability and the processes driving these trends are still uncertain. There are, in particular, two main hypotheses to explain N/P ratios that deviate from 16/1. Firstly, it is postulated that species have specific, yet distinct, ratios that are averaged out over large spatial and temporal scales (Weber and Deutsch, 2010). Alternatively, varying optimal growth strategies resulting from physiological adaptation to environmental conditions could drive N/P variability, which simply averages out as 16/1 under current environmental conditions (Klausmeier et al., 2004). To address these hypotheses, we examine seasonal changes in the NO3- to PO43- ratio (via the geochemical tracer N*) on a section between Cape Town and Antarctica, where macronutrients are not fully depleted. Overall, we find roles for both species composition and physiology in driving the seasonal changes in N* depending on the location. Both mechanisms could act in concert and physiology was generally more important in regions undergoing large changes in phytoplankton biomass. Better understanding the driving mechanisms behind changes in the Southern Ocean N/P ratio is important as its signal is exported to low latitudes, having major impacts on global biogeochemical cycles.

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The concentration of Chl-a (mg.m) for the region of the GH line extracted from Globcolor data are provided for (a) early summer (December 2010) and (b) late summer (February 2011).

The concentration of Chl-a (mg.m) for the region of the GH line extracted from Globcolor data are provided for (a) early summer (December 2010) and (b) late summer (February 2011).

Abstract

Two sets of high-resolution subsurface hydrographic and underway surface chlorophyll a (Chl a) measurements are used, in conjunction with satellite remotely sensed data, to investigate the upper layer oceanography (mesoscale features and mixed layer depth variability) and phytoplankton biomass at the GoodHope line south of Africa, during the 2010–2011 austral summer. The link between physical parameters of the upper ocean, specifically frontal activity, to the spatially varying in situ and satellite measurements of Chl a concentrations is investigated. The observations provide evidence to show that the fronts act to both enhance phytoplankton biomass as well as to delimit regions of similar chlorophyll concentrations, although the front–chlorophyll relationships become obscure towards the end of the growing season due to bloom advection and ‘patchy’ Chl a behaviour. Satellite ocean colour measurements are compared to in situ chlorophyll measurements to assess the disparity between the two sampling techniques. The scientific value of the time-series of oceanographic observations collected at the GoodHope line between 2004 to present is being realised. Continued efforts in this programme are essential to better understand both the physical and biogeochemical dynamics of the upper ocean in the Atlantic sector of the Southern Ocean.

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Relationship between in situ and Globcolour Chl a concentrations at the GH line in December (black) and February (grey). The 1:1 slope is depicted by the grey line.

Relationship between in situ and Globcolour Chl a concentrations at the GH line in December (black) and February (grey). The 1:1
slope is depicted by the grey line.

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