Abstract

The Atlantic sector of the Southern Ocean is characterized by markedly different frontal zones with specific seasonal and sub-seasonal dynamics. Demonstrated here is the effect of iron on the potential maximum productivity rates of the phytoplankton community. A series of iron addition productivity versus irradiance (PE) experiments utilizing a unique experimental design that allowed for 24h incubations were performed within the austral summer of 2015/16 to determine the photosynthetic parameters αB, PBmax and Ek. Mean values for each photosynthetic parameter under iron-replete conditions were 1.46 ± 0.55 (μg (μg Chl a)−1 h−1 (μM photons m−2 s−1)−1) for αB, 72.55 ± 27.97 (μg (μg Chl a)−1 h−1) for PBmax and 50.84 ± 11.89 (μM photons m−2 s−1) for Ek, whereas mean values under the control conditions were 1.25 ± 0.92 (μg (μg Chl a)−1 h−1 (μM photons m−2 s−1)−1) for αB, 62.44 ± 36.96 (μg (μg Chl a)−1 h−1) for PBmax and 55.81 ± 19.60 (μM photons m−2 s−1) for Ek. There were no clear spatial patterns in either the absolute values or the absolute differences between the treatments at the experimental locations. When these parameters are integrated into a standard depth-integrated primary production model across a latitudinal transect, the effect of iron addition shows higher levels of primary production south of 50°S, with very little difference observed in the subantarctic and polar frontal zone. These results emphasize the need for better parameterization of photosynthetic parameters in biogeochemical models around sensitivities in their response to iron supply. Future biogeochemical models will need to consider the combined and individual effects of iron and light to better resolve the natural background in primary production and predict its response under a changing climate.

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Depth-integrated primary production for each transect, including the mean for each treatment, the mean absolute differences between and the mean percentage difference between the treatments.

Depth-integrated primary production for each transect, including the mean for each treatment, the mean absolute differences between and the mean percentage difference between the treatments.

Hayley Evers-King, Victor Martinez-Vicente, Robert J. W. brewin, Giorgio Dall'Olmo, Anna E. Hickman, Thomas Jackson, Tihomir S. Kostadinov, Hajo Krasemann, Hubert Loisel, Rüdiger Röttgers, Shovonlal Roy, Dariusz Stramski, Sandy Thomalla, Trevor Platt and Shubha Sathyendranath
Abstract

Particulate Organic Carbon (POC) plays a vital role in the ocean carbon cycle. Though relatively small compared with other carbon pools, the POC pool is responsible for large fluxes and is linked to many important ocean biogeochemical processes. The satellite ocean-color signal is influenced by particle composition, size, and concentration and provides a way to observe variability in the POC pool at a range of temporal and spatial scales. To provide accurate estimates of POC concentration from satellite ocean color data requires algorithms that are well validated, with uncertainties characterized. Here, a number of algorithms to derive POC using different optical variables are applied to merged satellite ocean color data provided by the Ocean Color Climate Change Initiative (OC-CCI) and validated against the largest database of in situ POC measurements currently available. The results of this validation exercise indicate satisfactory levels of performance from several algorithms (highest performance was observed from the algorithms of Loisel et al., 2002Stramski et al., 2008) and uncertainties that are within the requirements of the user community. Estimates of the standing stock of the POC can be made by applying these algorithms, and yield an estimated mixed-layer integrated global stock of POC between 0.77 and 1.3 Pg C of carbon. Performance of the algorithms vary regionally, suggesting that blending of region-specific algorithms may provide the best way forward for generating global POC products.

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POC estimated using the five candidate algorithms applied to a monthly composite of OC-CCI data from May 2005 (A) Stramski et al. (2008) (Rrs), (B) Stramski et al. (2008) (bbp), (C) Loisel et al. (2002), (D) Gardner et al. (2006), (E) Kostadinov et al. (2016), and (F) POC associated with an extracted transect through the Atlantic at 20o W for each algorithm, and the associated [Chl] from the OC-CCI data.

POC estimated using the five candidate algorithms applied to a monthly composite of OC-CCI data from May 2005 (A) Stramski et al. (2008) (Rrs), (B) Stramski et al. (2008) (bbp), (C) Loisel et al. (2002), (D) Gardner et al. (2006), (E) Kostadinov et al. (2016), and (F) POC associated with an extracted transect through the Atlantic at 20o W for each algorithm, and the associated [Chl] from the OC-CCI data.

Abstract

Eleven incubation experiments were conducted in the South Atlantic sector of the Southern Ocean to investigate the relationship between new production (ρNO), regenerated production (ρNH+), and total carbon production (ρC) as a function of varying light. The results show substantial variability in the photosynthesis–irradiance (P vs E) parameters, with phytoplankton communities at stations that were considered iron (Fe)-limited showing low maximum photosynthetic capacity (Pmax) and low quantum efficiency of photosynthesis (αB) for ρNO3, but high Pmax and αB for ρNH4, with consequently low export efficiency. Results at stations likely relieved of Fe stress (associated with shallow bathymetry and the marginal ice zone) showed the highest rates of Pmax and αB for ρNO3 and ρC. To establish the key factors influencing the variability of the photosynthetic parameters, a principal components analysis was performed on P vs E parameters, using surface temperature, chlorophyll-a concentration, ambient nutrients, and an index for community size structure. Strong covariance between ambient nitrate (NO3) and αB for ρNO3 suggests that Fe and possibly light co-limitation affects the ability of phytoplankton in the region to access the surplus NOreservoir. However, the observed relationships between community structure and the P vs E parameters suggest superior performance by smaller-sized cells, in terms of resource acquisition and Fe limitation, as the probable driver of smaller-celled phytoplankton communities that have reduced photosynthetic efficiency and which require higher light intensities to saturate uptake. A noticeable absence in covariances between chlorophyll-a and αB, between Pmax and αB, and between temperature and αB may have important implications for primary-production models, although the absence of some expected relationships may be a consequence of the small dataset and low range of variability. However, significant relationships were observed between ambient NO3 and αB for ρNO3, and between the light-saturation parameter Ek for ρNO3 and the phytoplankton community’s size structure, which imply that Fe and light co-limitation drives access to the surplus NO3 reservoir and that larger-celled communities are more efficient at fixing NO3 in low light conditions. Although the mean Pmax results for ρC were consistent with estimates of global  production from satellite chlorophyll measurements, the range of variability was large. These results highlight the need for more-advanced primary-production models that take into account a diverse range of environmental and seasonal drivers of photosynthetic responses.

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Spatial distribution map of  Maximum photosynthetic capacity (PB max) for carbon uptake

Spatial distribution map of Maximum photosynthetic capacity (PB max) for carbon uptake

Abstract

Traditionally, the mechanism driving the seasonal restratification of the Southern Ocean mixed layer (ML) is thought to be the onset of springtime warming. Recent developments in numerical modelling and North Atlantic observations have shown that submesoscale ML eddies (MLE) can drive a restratifying flux to shoal the deep winter ML prior to solar heating at high latitudes. The impact of submesoscale processes on the intra-seasonal variability of the Subantarctic ML is still relatively unknown. We compare five months of glider data in the Subantarctic Zone to simulations of a 1-D mixing model to show that the magnitude of restratification of the ML cannot be explained by heat, freshwater and momentum fluxes alone. During early spring, we estimate that periodic increases in the vertical buoyancy flux by MLEs caused small increases in stratification, despite predominantly down-front winds that promote the destruction of stratification. The timing of seasonal restratification was consistent between 1-D model estimates and the observations. However, during up-front winds, the strength of springtime stratification increased over two-fold compared to the 1-D model, with a rapid shoaling of the MLD from >200 m to <100 m within a few days. The ML stratification is further modified under a negative Ekman buoyancy flux during down-front winds, resulting in the destruction of ML stratification and deepening the MLD. These results propose the importance of submesoscale buoyancy fluxes enhancing seasonal restratification and mixing of the Subantarctic ML.

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Marcel_F1

a) NCEP wind stress (N m-2) co-located to the time and position of glider profiles and used for the PWP model run. b) MERRA 6-hourly net heat flux (QNET ) at the ocean surface (red (blue) indicates ocean heating (cooling)) indicates a strong diurnal structure with a net positive heat flux into the ML. c) Time series of stratification (N2) averaged for the upper 100 m for SG573 (green line) and the PWP model (magenta line). Shaded areas are as in Figure 3.

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’.

Ryan-Keogh T J, Liza M. DeLizo, Walker O. Smith Jr., Peter N. Sedwick, Dennis J. McGillicuddy Jr., C. Mark Moore, Thomas S. Bibby
Abstract

The bioavailability of iron influences the distribution, biomass and productivity of phytoplankton in the Ross Sea, one of the most productive regions in the Southern Ocean. We mapped the spatial and temporal extent and severity of iron-limitation of the native phytoplankton assemblage using long- (>24 h) and short-term (24 h) iron-addition experiments along with physiological and molecular characterisations during a cruise to the Ross Sea in December–February 2012. Phytoplankton increased their photosynthetic efficiency in response to iron addition, suggesting proximal iron limitation throughout most of the Ross Sea during summer. Molecular and physiological data further indicate that as nitrate is removed from the surface ocean the phytoplankton community transitions to one displaying an iron-efficient photosynthetic strategy characterised by an increase in the size of photosystem II (PSII) photochemical cross section (σPSII) and a decrease in the chlorophyll-normalised PSII abundance. These results suggest that phytoplankton with the ability to reduce their photosynthetic iron requirements are selected as the growing season progresses, which may drive the well-documented progression from Phaeocystis antarctica- assemblages to diatom-dominated phytoplankton. Such a shift in the assemblage-level photosynthetic strategy potentially mediates further drawdown of nitrate following the development of iron deficient conditions in the Ross Sea.

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Spatial distribution of iron stress response with concurrent variable fluorescence changes in long-term iron addition experiments.

Spatial distribution of iron stress response with concurrent variable fluorescence changes in long-term iron addition experiments.

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

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