Swart S., Speich, S.
Abstract

Swart et al. (2010) applied altimetry data to the gravest empirical mode south of Africa to yield a 16 year time series of temperature and salinity sections. In this study we use these thermohaline sections to derive weekly estimates of heat content (HC) and salt content (SC) at the GoodHope meridional transect of the Antarctic Circumpolar Current (ACC). These estimates compare favorably to observed data. The resulting 16 year time series of HC and SC estimates are used to explain the subsurface thermohaline variability at each ACC front and frontal zone. The variability at the Subantarctic Zone (SAZ) is principally driven by the presence of Agulhas Rings, which occur in this region approximately 2.7 times per annum and are responsible for the longest and highest scales of observed variability. The variability of the SAZ is responsible for over 50% and 60% of the total ACC HC and SC variability, respectively. Poleward of the SAZ, the variability is largely determined by the influence of the local topography on the fronts of the region and can be explained by the conservation of potential vorticity. Wavelet analysis is conducted on the time series of meridionally integrated HC and SC in each ACC front and frontal zone, revealing a consistent seasonal mode that becomes more dominant toward the southern limit of the ACC. The lower-frequency signals are compared with two dominant modes of variability in the Southern Ocean. The Southern Annular Mode correlates well with the HC and SC anomaly estimates at the Antarctic Polar Front, while the Southern Oscillation Index appears to have connections to the variability found in the very southern domains of the ACC.

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The propagation of an Agulhas Ring into the SAZ is identified using satellite altimetry data. These northerly originating features cause sharp changes in climate and biogeochemical variables in the SAZ.

The propagation of an Agulhas Ring into the SAZ is identified using satellite altimetry data. These northerly originating features cause sharp changes in climate and biogeochemical variables in the SAZ.

Swart S., Speich, S., Ansorge I. J., Lutjeharms, J. R. E.
Abstract

Hydrographic transects of the Antarctic Circumpolar Current (ACC) south of Africa are projected into baroclinic stream function space parameterized by pressure and dynamic height. This produces a two-dimensional gravest empirical mode (GEM) that captures more than 97% of the total density and temperature variance in the ACC domain. Weekly maps of absolute dynamic topography data, derived from satellite altimetry, are combined with the GEM to obtain a 16 year time series of temperature and salinity fields. The time series of thermohaline fields are compared with independent in situ observations. The residuals decrease sharply below the thermocline and through the entire water column the mean root-mean-square (RMS) error is 0.15°C, 0.02, and 0.02 kg m−3 for temperature, salinity, and density, respectively. The positions of ACC fronts are followed in time using satellite altimetry data. These locations correspond to both the observed and GEM-based positions. The available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar. This is compared with velocity measurements from repeat hydrographic transects at the GoodHope line. The net accumulated transports of the ACC, derived from these different methods are within 1–3 Sv of each other. Similarly, GEM-produced cross-sectional velocities at 300 dbar compare closely to the observed data, with the RMS difference not exceeding 0.03 m s−1. The continuous time series of thermohaline fields, described here, are further exploited to understand the dynamic nature of the ACC fronts in the region, and which is given by Swart and Speich (2010).

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Location of the altimetry-determined ACC fronts (black lines) overlaid onto the surface geostrophic velocities (in colour).

Location of the altimetry-determined ACC fronts (black lines) overlaid onto the surface geostrophic velocities (in colour).

Swart S., Speich, S., Ansorge I. J., Goni, G., Lutjeharms, J. R. E.
Abstract

Data from five CTD and 18 XBT sections are used to estimate the baroclinic transport (referenced to 2500 dbar) of the ACC south of Africa. Surface dynamic height is derived from XBT data by establishing an empirical relationship between vertically integrated temperature and surface dynamic height calculated from CTD data. This temperature-derived dynamic height data compare closely with dynamic heights calculated from CTD data (average RMS difference = 0.05 dyn m). A second empirical relationship between surface dynamic height and cumulative baroclinic transport is defined, allowing us to study a more extensive time series of baroclinic transport derived from upper ocean temperature sections. From 18 XBT transects of the ACC, the average baroclinic transport, relative to 2500 dbar, is estimated at 90 ± 2.4 Sv. This estimate is comparable to baroclinic transport values calculated from CTD data. We then extend the baroclinic transport time-series by applying an empirical relationship between dynamic height and cumulative baroclinic transport to weekly maps of absolute dynamic topography derived from satellite altimetry, between 14 October 1992 and 23 May 2007. The estimated mean baroclinic transport of the ACC, obtained this way, is 84.7 ± 3.0 Sv. These transports agree well with simultaneous in-situ estimates (RMS difference in net transport = 5.2 Sv). This suggests that sea level anomalies largely reflect baroclinic transport changes above 2500 dbar.

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A 15 year time series of transports (in Sverdrups) of the ACC and associated fronts as derived from satellite altimetry using proxy techniques.

A 15 year time series of transports (in Sverdrups) of the ACC and associated fronts as derived from satellite altimetry using proxy techniques.

Abstract

A detailed hydrographic and biological survey was carried out in the region of the South-West Indian Ridge during April 2004. Altimetry and hydrographic data have identified this region as an area of high flow variability. Hydrographic data revealed that here the Subantarctic Polar Front (SAF) and Antarctic Polar Front (APF) converge to form a highly intense frontal system. Water masses identified during the survey showed a distinct separation in properties between the northwestern and southeastern corners. In the north-west, water masses were distinctly Subantarctic (>8.5°C, salinity >34.2), suggesting that the SAF lay extremely far to the south.
In the southeast corner water masses were typical of the Antarctic zone, showing a distinct subsurface temperature minimum of <2.5°C. Total integrated chl-a concentration during the survey ranged from 4.15 to 22.81 mg chl-a m–2, with the highest concentrations recorded at stations occupied in the frontal region. These data suggest that the region of the South-West Indian Ridge represents not only an area of elevated biological activity but also acts as a strong biogeographic barrier to the spatial distribution of zooplankton.

Altimetry data showing sea-surface height anomalies (in cm) for the period 6–24 April 2004.

Altimetry data showing sea-surface height anomalies (in cm) for the period 6–24 April 2004.

Abstract

This study was undertaken to characterise the seasonal cycle of air–sea fluxes of carbon dioxide (CO2) in the southern Benguela upwelling system off the South African west coast. Samples were collected from six monthly cross-shelf cruises in the St. Helena Bay region during 2010. CO2 fluxes were calculated from pCO2 derived from total alkalinity and dissolved inorganic carbon and scatterometer-based winds. Notwithstanding that it is one of the most biologically productive eastern boundary upwelling systems in the global ocean, the southern Benguela was found to be a very small net annual CO2 sink of -1.4 ± 0.6 mol C/m2 per year (1.7 Mt C/year). Regional primary productivity was offset by nearly equal rates of sediment and sub-thermocline remineralisation flux of CO2, which is recirculated to surface waters by upwelling. The juxtaposition of the strong, narrow near-shore out-gassing region and the larger, weaker offshore sink resulted in the shelf area being a weak CO2 sink in all seasons but autumn (-5.8, 1.4 and -3.4 mmol C/m2 per day for summer, autumn and winter, respectively).

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Southern Benguela air-sea CO2 flux

Southern Benguela air-sea CO2 flux (mmol/m2 per day) along the St. Helena Bay Monitoring line for daily winds (right). Red is outgassing and blue is ingassing. The dot markers show the sample location and date. The average daily flux is plotted on the left.

 

 

Abstract

In this study we use the southern Benguela upwelling system to investigate the role of nutrient and carbon stoichiometry on carbonate dynamics in eastern boundary upwelling systems. Six months in 2010 were sampled along a cross-shelf transect. Data were classified into summer, autumn, and winter. Nitrate, phosphate, dissolved inorganic carbon, and total alkalinity ratios were used in a stoichiometric reconstruction model to determine the contribution of biogeochemical processes on a parcel of water as it upwelled. Deviations from the Redfield ratio were dominated by denitrification and sulfate reduction in the subsurface waters. The N:P ratio was lowest (7.2) during autumn once anoxic waters had formed. Total alkalinity (TA) generation by anaerobic remineralization decreased pCO2 by 227 μatm. Ventilation during summer and winter resulted in elevated N:P ratios (12.3). We propose that anaerobic production of TA has an important regional effect in mitigating naturally high CO2 and making upwelled waters less corrosive.

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Marine Carbonate fluxes in the southern Benguela: A schematic showing the magnitude of processes contributing to DIC and TA fluxes. Black numbers represent DIC and gray TA. The solid/dashed/dotted lines represent the thermocline and its intensity. Increases in DIC throughout all seasons were largely due to aerobic remineralization (RM). Large TA gains in autumn were due to benthic processes: denitrification (DN), sulfate reduction (SR), and calcite dissolution (CD). Strong primary production (PP) in summer reduced the surface DIC, while calcification (CL) in autumn resulted in decreased TA.

Marine Carbonate fluxes in the southern Benguela: A schematic showing the magnitude of processes contributing to DIC and TA fluxes. Black numbers represent DIC and gray TA. The solid/dashed/dotted lines represent the thermocline and its intensity. Increases in DIC throughout all seasons were largely due to aerobic remineralization (RM). Large TA gains in autumn were due to benthic processes: denitrification (DN), sulfate reduction (SR), and calcite dissolution (CD). Strong primary production (PP) in summer reduced the surface DIC, while calcification (CL) in autumn resulted in decreased TA.

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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Caption

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

As part of the Bonus-GoodHope (BGH) campaign, 15N-labelled nitrate, ammonium and urea uptake measurements were made along the BGH transect from Cape Town to ~60° S in late austral summer, 2008. Our results are categorised according to distinct hydrographic regions defined by oceanic fronts and open ocean zones. High regenerated nitrate uptake rate in the oligotrophic Subtropical Zone (STZ) resulted in low f-ratios (f = 0.2) with nitrogen uptake being dominated by ρurea, which contributed up to 70 % of total nitrogen uptake. Size fractionated chlorophyll data showed that the greatest contribution (>50 %) of picophytoplankton (<2 μm) were found in the STZ, consistent with a community based on regenerated production. The Subantarctic Zone (SAZ) showed the greatest total integrated nitrogen uptake (10.3 mmol m−2 d−1), mainly due to enhanced nutrient supply within an anticyclonic eddy observed in this region. A decrease in the contribution of smaller size classes to the phytoplankton community was observed with increasing latitude, concurrent with a decrease in the contribution of regenerated production. Higher f-ratios observed in the SAZ (f = 0.49), Polar Frontal Zone (f= 0.41) and Antarctic Zone (f = 0.45) relative to the STZ (f = 0.24), indicate a higher contribution of NO3-uptake relative to total nitrogen and potentially higher export production. High ambient regenerated nutrient concentrations are indicative of active regeneration processes throughout the transect and ascribed to late summer season sampling. Higher depth integrated uptake rates also correspond with higher surface iron concentrations. No clear correlation was observed between carbon export estimates derived from new production and 234Th flux. In addition, export derived from 15N estimates were 2–20 times greater than those based on 234Th flux. Variability in the magnitude of export is likely due to intrinsically different methods, compounded by differences in integration time scales for the two proxies of carbon export.

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Cruise track during the Bonus Goodhope 2008 campaign in the Atlantic Southern Ocean. Red dots indicate the sampling positions for 15N uptake experiments. The hydrographic fronts, Subtropical Front (STF), Sub-Antarctic Front (SAF), Polar Front (PF), South Antarctic Circumpolar Current Front (SAccF) and Southern Boundary (Sbdy) are indicated by dotted white lines.

Cruise track during the Bonus Goodhope 2008 campaign in the Atlantic Southern Ocean. Red dots indicate the sampling positions for 15N uptake experiments. The hydrographic fronts, Subtropical Front (STF), Sub-Antarctic Front (SAF), Polar Front (PF), South Antarctic Circumpolar Current Front (SAccF) and Southern Boundary (Sbdy) are indicated by dotted white lines.

Thomalla S.J., Waldron H., Lucas M., Read, J.F., Ansorge, I.J., Pakhamov, E.
Abstract

During the 1999 Marion Island Oceanographic Survey (MIOS 4) in late austral summer, a northbound and reciprocal southbound transect were taken along the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands and 31° S. The sections crossed a number of major fronts and smaller mesoscale features and covered a wide productivity spectrum from subtropical to subantarctic waters. Associated with the physical survey were measurements of size fractionated chlorophyll, nutrients and nitrogen (NO3, NH4 and urea) uptake rates. Subtropical waters were characterised by low chlorophyll concentrations (max = 0.27.3 mg m−3 dominated by pico-phytoplankton cells (> 81%) and very low f-ratios (< 0.1), indicative of productivity based almost entirely on recycled ammonium and urea. Micro-phytoplankton growth was limited by the availability of NO3 (< 0.5 mmol m−3 and Si(OH)4 (< 1.5 mmol m−3 through strong vertical stratification preventing the upward flux of nutrients into the euphotic zone. Biomass accumulation of small cells was likely controlled by micro-zooplankton grazing. In subantarctic waters, total chlorophyll concentrations increased (max = 0.74 mg m−3 relative to the subtropical waters and larger cells became more prevalent, however smaller phytoplankton cells and low f-ratios (< 0.14) still dominated, despite sufficient NO3 availability. The results from this study favour Si(OH)4 limitation, light-limited deep mixing and likely Fe deficiency as the dominant mechanisms controlling significant new production by micro-phytoplankton. The percentage of micro-phytoplankton cells and rates of new production did however increase at oceanic frontal regions (58.6% and 11.22%, respectively), and in the region of the Prince Edward archipelago (61.4% and 14.16%, respectively). Here, water column stabilization and local Fe-enrichment are thought to stimulate phytoplankton growth rates. Open ocean regions such as these provide important areas for local but significant particulate organic carbon export and biological CO2draw-down in an overall high nutrient low chlorophyll Southern Ocean.

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 Cruise tracks overlaid on bathymetry of the Northbound transect, showing XBT station positions as black dots. Together with the reciprocal Southbound transect, showing CTD station positions as white circles. The cruise tracks followed the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands (PEI) and 31_ S. Productivity stations are shown as pink circles and labelled (NP1–NP6). Black arrows mark the position of the Agulhas current (AC), the Agulhas Return Current (ARC) and the Antarctic Circumpolar current (ACC). Grey lines indicate the mean frontal positions of the Sub Tropical Front (STF), the Sub Antarctic Front (SAF) and the Polar Front (PF) according to Orsi et al. (1995).

Cruise tracks overlaid on bathymetry of the Northbound transect, showing XBT station positions as black dots. Together with the reciprocal Southbound transect, showing CTD station positions as white circles. The cruise tracks followed the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands (PEI) and 31_ S. Productivity stations are shown as pink circles and labelled (NP1–NP6). Black arrows mark the position of the Agulhas current (AC), the Agulhas Return Current (ARC) and the Antarctic Circumpolar current (ACC). Grey lines indicate the mean
frontal positions of the Sub Tropical Front (STF), the Sub Antarctic Front (SAF) and the Polar Front (PF) according to Orsi et al. (1995).

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