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

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

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.

Abstract

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 the Southern Ocean Seasonal Cycle Experiment (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. The experiment 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.

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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.

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.

Martins, R., Roberts, M.J., Lett, C., Vidal, E.A.G., Moloney, C., Chang N., de Camargo, M.G.
Abstract

Annual landings of chokka squid (Loligo reynaudii), an important fishing resource for South Africa, fluctuate greatly, and are believed to be related to recruitment success. The ‘Westward Transport Hypothesis’ (WTH) attributes recruitment strength to variability in transport of newly hatched paralarvae from spawning grounds to the ‘cold ridge’ nursery region some 100–200 km to the west, where oceanographic conditions sustain high productivity. We used an individual-based model (IBM) coupled with a 3-D hydrodynamic model (ROMS) to test the WTH and assessed four factors that might influence successful transport – Release Area, Month, Specific Gravity (body density) and Diel Vertical Migration (DVM) – in numerical experiments that estimated successful transport of squid paralarvae to the cold ridge. A multifactor ANOVA was used to identify the primary determinants of transport success in the various experimental simulations. Among these, release area was found to be the most important, implying that adult spawning behaviour (i.e., birth site fidelity) may be more important than paralarval behaviour in determining paralarval transport variability. However, specific gravity and DVM were found to play a role by retaining paralarvae on the shelf and optimizing early transport, respectively. Upwelling events seem to facilitate transport by moving paralarvae higher in the water column and thus exposing them to faster surface currents.

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Garavelli L., Grass A., Grote B., Chang N., Smith M., Verley P.
Abstract

The two Cape hake species of the southern Benguela ecosystem, the shallow-water and deep-water hakes Merluccius capensis and M. paradoxus, are economically the most important marine resources in South Africa. Recruitment is a key process in the dynamics of marine organisms, yet very little is known about the early life history of Cape hakes, especially the location of spawning grounds and transport of eggs and larvae. For each species, ichthyoplankton dispersal off South Africa is simulated by coupling oceanographic simulations to an individual-based model in order to track virtual individuals. Results indicate that the most favorable spawning areas for transport to nursery areas are located off the south-western coast and the eastern Agulhas Bank, and highlight partly different drift routes followed by the two ichthyoplankton species off Cape Columbine. Transport from spawning to nursery areas is the highest in austral winter for a spawning depth ranging between 0 and 100 m. These modeling results are in broad agreement with available knowledge on the ecology of Cape hakes. The present work on Cape hakes complements previous modeling studies on anchovy and sardine in the same area. Taken together, these studies underline the correspondence between cross-shore (for hakes) or alongshore (for anchovy and sardine) transport mechanisms and the spawning strategies used by these key species of the southern Benguela ecosystem.

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Abstract

In the Ocean, the seasonal cycle is the mode that couples climate forcing to ecosystem response in production, diversity and carbon export. A better characterisation of the ecosystem’s seasonal cycle therefore addresses an important gap in our ability to estimate the sensitivity of the biological pump to climate change. In this study, the regional characteristics of the seasonal cycle of phytoplankton biomass in the Southern Ocean are examined in terms of the timing of the bloom initiation, its amplitude, regional scale variability and the importance of the climatological seasonal cycle in explaining the overall variance. The seasonal cycle was consequently defined into four broad zonal regions; the subtropical zone (STZ), the transition zone (TZ), the Antarctic circumpolar zone (ACZ) and the marginal ice zone (MIZ). Defining the Southern Ocean according to the characteristics of its seasonal cycle provides a more dynamic understanding of ocean productivity based on underlying physical drivers rather than climatological biomass. The response of the biology to the underlying physics of the different seasonal zones resulted in an additional classification of four regions based on the extent of inter-annual seasonal phase locking and the magnitude of the integrated seasonal biomass. This regionalisation contributes towards an improved understanding of the regional differences in the sensitivity of the Southern Oceans ecosystem to climate forcing, potentially allowing more robust predictions of the effects of long term climate trends.

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A schematic summarising the response of phytoplankton biomass to the underlying physics of the different seasonal regimes. Regions in blue represent regions of low ( 0.4) (Region A, light blue) or low seasonal cycle reproducibility (R2  0.25 mgm−3) with either high seasonal cycle reproducibility (Region C, dark green) or low seasonal cycle reproducibility (Region D, light green). Mean (1998–2007) frontal positions are shown for the STF (red), the SAF (black), the PF (orange) and the SACCF (blue).

A schematic summarising the response of phytoplankton biomass to the underlying physics of the different seasonal regimes. Regions in blue represent regions of low (< 0.25 mgm−3) chlorophyll concentration with either high seasonal cycle reproducibility (R2 > 0.4) (Region A, light blue) or low seasonal cycle reproducibility (R2 < 0.4) (Region B, dark blue). Regions in green represent regions of high chlorophyll concentration ( > 0.25 mgm−3) with either high seasonal cycle reproducibility (Region C, dark green) or low seasonal cycle reproducibility (Region D, light green). Mean (1998–2007) frontal positions are shown for the STF (red), the SAF (black), the PF (orange) and the SACCF (blue).

Martins, R.S., Roberts, M.J., Chang N., Verley, P., Moloney C.L., Vidal, E.A.G.
Abstract

Specific gravity is an important parameter in the dispersal of marine zooplankton, because the velocity of currents, and therefore the speed of transport, is usually greatest near the surface. For the South African chokka squid (Loligo reynaudii), recruitment is thought to be influenced by the successful transport of paralarvae from the spawning grounds to a food-rich feature known as the cold ridge some 100–200 km away. The role of paralarval specific gravity on such transport is investigated. Specific gravity ranged from 1.0373 to 1.0734 g cm−3 during the yolk-utilization phase, implying that paralarvae are always negatively buoyant, regardless of yolk content. The data were incorporated into a coupled individual-based model (IBM)—Regional Ocean Modelling System model. The output showed that dispersal was dominantly westward towards the cold ridge. Also, modelled paralarval vertical distribution suggested that hydrodynamic turbulence was an important factor in dispersal. The negative buoyancy of early chokka squid paralarvae may reduce the risk of paralarvae being advected off the eastern Agulhas Bank and into the open ocean, where food is less abundant, so specific gravity may be important in enhancing the survival and recruitment of chokka squid.

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Blanke, B., Penven, P., Roy, C., Chang N., Kokoszka, F.
Abstract

This study analyzes the oceanic pathway connecting the Agulhas Bank to the southern Benguela upwelling system by means of a quantitative Lagrangian interpretation of the velocity field calculated by a high-resolution numerical simulation of the ocean around the southwestern tip of Africa. The regional ocean model is forced with National Centers for Environmental Prediction surface winds over 1993–2006 and offers a relevant numerical platform for the investigation of the variability of the water transferred between both regions, both on seasonal and intraseasonal time scales. We show that the intensity of the connection fluctuates in response to seasonal wind variability in the west coast upwelling system, whereas intraseasonal anomalies are mostly related to the organization of the eddy field along the southwestern edge of the Agulhas Bank. Though the study only considers passive advection processes, it may provide useful clues about the strategy adopted by anchovies in their selection of successful spawning location and period. The pathway under investigation is of major interest for the ecology of the southern Benguela upwelling system because it connects the spawning grounds on the Agulhas Bank with the nursery grounds located on the productive upwelling off the west coast.

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