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The dramatic calving of the Mertz Glacier Tongue in 2010, triggered by the impact of iceberg B09B, reshaped the oceanographic regime across the Mertz Polynya and Commonwealth Bay, regions where high salinity shelf water (HSSW) is formed, the precursor to Antarctic bottom water (AABW). Here we compare post-calving observations with high-resolution ocean modelling which suggest that this reconfiguration has led to the development of a new polynya off Commonwealth Bay, where HSSW production continues by the grounding of B09B. Our findings demonstrate how local changes in icescape can impact formation of AABW, with implications for large-scale ocean circulation and climate.

We performed a metagenomic survey (6.6Gbp of 454 sequence data) of Southern Ocean (SO) microorganisms during the austral summer of 2007-2008, examining the genomic signatures of communities across a latitudinal transect from Hobart (44°S) to the Mertz Glacier, Antarctica (67°S). Operational taxonomic units (OTUs) of the SAR11 and SAR116 clades and the cyanobacterial genera Prochlorococcus and Synechococcus were strongly overrepresented north of the Polar Front (PF). Conversely, OTUs of the Gammaproteobacterial Sulfur Oxidizer-EOSA-1 (GSO-EOSA-1) complex, the phyla Bacteroidetes and Verrucomicrobia and order Rhodobacterales were characteristic of waters south of the PF. Functions enriched south of the PF included a range of transporters, sulfur reduction and histidine degradation to glutamate, while branched-chain amino acid transport, nucleic acid biosynthesis and methionine salvage were overrepresented north of the PF. The taxonomic and functional characteristics suggested a shift of primary production from cyanobacteria in the north to eukaryotic phytoplankton in the south, and reflected the different trophic statuses of the two regions. The study provides a new level of understanding about SO microbial communities, describing the contrasting taxonomic and functional characteristics of microbial assemblages either side of the PF. © 2012 Society for Applied Microbiology and Blackwell Publishing Ltd.

The ubiquitous SAR11 bacterial clade is the most abundant type of organism in the worldĝ€™s oceans, but the reasons for its success are not fully elucidated. We analysed 128 surface marine metagenomes, including 37 new Antarctic metagenomes. The large size of the data set enabled internal transcribed spacer (ITS) regions to be obtained from the Southern polar region, enabling the first global characterization of the distribution of SAR11, from waters spanning temperatures ĝ̂'2 to 30°C. Our data show a stable co-occurrence of phylotypes within both ĝ€̃ tropicalĝ€™ (>20°C) and ĝ€̃ polarĝ€™ (<10°C) biomes, highlighting ecological niche differentiation between major SAR11 subgroups. All phylotypes display transitions in abundance that are strongly correlated with temperature and latitude. By assembling SAR11 genomes from Antarctic metagenome data, we identified specific genes, biases in gene functions and signatures of positive selection in the genomes of the polar SAR11ĝ€"genomic signatures of adaptive radiation. Our data demonstrate the importance of adaptive radiation in the organismĝ€™s ability to proliferate throughout the worldĝ€™s oceans, and describe genomic traits characteristic of different phylotypes in specific marine biomes. © 2012 EMBO and Macmillan Publishers Limited All rights reserved.

A 7 year time series of austral summer expendable bathythermograph (XBT) sections between Tasmania and Antarctica is used to describe the subsurface structure of Southern Ocean interannual temperature anomalies. Comparison of the discontinuous XBT record with a continuous (weekly average) satellite-based SST data set confirms that the XBT sampling is adequate to resolve interannual variability. Significant correlations are found between changes in surface and subsurface temperature along much of the section. In the Subantarctic zone, surface and subsurface changes are coherent because deep convection in winter homogenizes the upper 600 m, and the summer mixed layer temperature reflects the preconditioning of the previous winter. At other latitudes the regions of high correlation generally extend well beyond the depth of direct atmospheric influence (i.e., the depth of the winter mixed layer). Negative (positive) temperature anomalies at interannual timescales are coherent with poleward (equatorward) displacements of fronts. The simplest explanation for the correspondence between deep, vertically coherent temperature anomalies and the location of ACC fronts is that the observed features reflect meridional shifts of temperature gradients associated with the fronts. The link between temperature anomalies and front displacements suggests Southern Ocean sea surface temperature (SST) anomalies primarily reflect a dynamical response of the ocean to wind or other forcing rather than surface-trapped features produced by anomalous air-sea heat flux. The XBT record contains both interannual and longer (unresolved) period signals, which cannot be isolated in the short discontinuous XBT time series. Spectral analysis of the 17 year SST data set reveals roughly equal variability in 2-7 year and >7 year bands.

Repeat hydrographic sections across the Antarctic Circumpolar Current are used to derive an empirical relationship between upper ocean temperature and the baroclinic transport stream function. Cross validation shows this relationship can be used to infer baroclinic transport (above and relative to 2500 dbar) from temperature measurements with an error of a few per cent. The mean transport distribution derived from 31 austral summer expendable bathythermograph (XBT) sections over a 6 year period consists of westward flow immediately south of Tasmania, a broad band of strong eastward flow between 50degrees and 55degreesS, and three cores of eastward flow south of 55degreesS. By defining a second empirical relationship between surface dynamic height and cumulative transport a continuous time series of baroclinic transport is derived from altimeter measurements of sea surface height. Transports derived from altimetry in this way agree well with simultaneous in situ estimates (root mean square error in net transport is 4x10(6) m(3) s(-1)), suggesting sea level anomalies largely reflect baroclinic changes above 2500 dbar. The 10 day sampling of the altimeter transport time series shows the irregular XBT sampling aliases variability at unresolved timescales. The standard deviation of net transport above and relative to 2500 m is 4.3x10(6) m(3) s(-1). The variability in net transport is largest (2.7x10(6) m(3) s(-1)) in the quasi-annual band (periods of 4 months to 1.5 years), slightly smaller (2.3x10(6) m(3) s(-1)) in the mesoscale band (<4 months), and smallest in the interannual band (>1.5 years, 1.5x10(6) m(3) s(-1)). Changes in transport are correlated with local changes in both wind stress and wind stress curl in the quasi-annual and interannual bands, but the transport time series is too short to draw significant conclusions.

Baroclinic transport variability of the Antarctic Circumpolar Current (ACC) near 140 degreesE is estimated from six: occupations of a repeat section occupied as part of the World Ocean Circulation Experiment (WOCE section SR3). The mean top-to-bottom volume transport is 147+/-10 Sv (mean +/-1 standard deviation), relative to a deep reference level consistent with water mass properties and float trajectories. The location and transport of the main fronts of the ACC are relatively steady: the Subantarctic Front carries 105+/-7 Sv at a mean latitude between 51.0 and 52 degreesS; the northern branch of the Polar Front carries 5+/-5 Sv to the east between 53 degrees and 54 degreesS; the southern Polar Front carries 24+/-3 Sv eastward at 59 degreesS; and two cores of the southern ACC front at 62 degrees and 64 degreesS carry 18+/-3 and 11+/-3 Sv, respectively. The variability in net property transports is largely due to variability of currents north of the ACC, in particular, an outflow of 8+/-13 Sv of mater from the Tasman Sea and a deep anticyclonic recirculation carrying 22+/-8 Sv in the Subantarctic Zone. Variability of net baroclinic volume transport is similar in magnitude to that measured at Drake Passage. In density layers, transport variability is small in deep layers, but significant (range of 4 to 16 Sv) in the Subantarctic Mode Water. Variability of eastward heat transport across SR3 is significant (range of 139 degreesC Sv, or 0.57 x 10(15) W, relative to 0 degreesC) and large relative to meridional heat flux in the Southern Hemisphere subtropical gyres. Heat transport changes are primarily due to variations in the westward flow of relatively warm water across the northern end of the section. Weak (strong) westward flow and large (small) eastward heat flux coincides with equatorward (poleward) displacements of the latitude of zero wind stress curl.

A mean synoptic view of the subantarctic front (SAF) is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross-stream coordinate that moves with the current. The cross-stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar (d780). By averaging absolute and baroclinic velocity measurements in stream coordinates, the spatial smoothing that results from Eulerian averaging of measurements of a meandering current is avoided. The mean SAF velocity profile is composed of one central peak and several smaller peaks. In the central peak, the 2-yr mean absolute velocity from current meters reaches 52 cm/s at 420 dbar; the mean baroclinic velocity from six CTD sections reaches 34 cm/s. The SAF flow is coherent at all levels, reaches the seafloor, and is at least 220 km wide. The crossstream structure of baroclinic and absolute transport of the SAF has been characterized for the first time. The integrated mean transport is 116+/-10 (x 10^6 m3/s), of which approximately 14% is barotropic, where barotropic transport is defined to be the velocity at the deepest current meter (3320 dbar) multiplied by the water depth. Comparison of the current meter measurements with hydrographic sections suggest this estimate is likely a lower bound on the absolute transport, as part of the 'cold side' of the SAF is not sampled by the moored array. Linear conditions for baroclinic and barotropic instability of the mean flow are satisfied at the array, consistent with earlier work that showed baroclinic conversion is the dominant mechanism for eddy growth at the array, with weaker barotropic conversion also occurring.

Two year time series measurements of current velocity and temperature in the Subantarctic Front (SAF) south of Australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the Antarctic Circumpolar Current (ACC) and further insight into the variability of the ACC. The SAF was found to be an energetic, meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm/s at 1150 dbar. A daily-varying coordinate frame which follows the direction of ow allowed mesoscale variability of the SAF to be isolated from variability due to meandering of the front and proved very successful for examining eddy uxes. Vertically averaged cross-stream eddy heat flux was 11.3 kW/m^2 poleward and significantly different from zero at the 95% confidence interval for fluctuations in the 2-90 day band. Zonally integrated, this eddy heat flux (=0.9 PW) is more than large enough to balance heat lost south of the Polar Front and is as large as cross-SAF fluxes found in Drake Passage. Cross-stream eddy momentum fluxes were small, not significantly different from zero and of indeterminate sign, but tending to decelerate the mean flow. A relationship between vertical motion and meander phase identified in the Gulf Stream was found to hold for the SAF. Eddy kinetic energy levels were similar to those in Drake Passage and southeast of New Zealand. Eddy potential energy was up to an order of magnitude larger than other ACC sites, most likely because meandering of the front is more common here. Baroclinic conversion was found to be the dominant mechanism by which eddies grow south of Australia. Typical eddy growth rate is estimated to be 30 days, approximately twice as fast as in Drake Passage, consistent with eddy energy growing rapidly downstream.

Coastal polynyas in the Adelie Depression are an important source of Antarctic Bottom Water to the Australian-Antarctic Basin. We present time series (April 1998 to May 1999 and August 1999 to February 2000) of data from temperature-salinity sensors, in both the Adelie Depression and the known outflow region of the Adelie Sill, to describe the annual cycle of shelf water densities. From April through September, salinification beneath the polynya produces dense shelf waters. During September–October, shelf water densities in the depression peak at 27.94 kg m3, and the cooling and freshening signature of Ice Shelf Water is observed north of Buchanan Bay. In November–December, shelf water densities decrease as intrusions of warm and relatively fresh modified Circumpolar Deep Water enter east of the Ade´lie Sill. From January–March the surface layer is conditioned by the cooling of the atmosphere, which overturns the upper water column. At the Ade´lie Sill, observed daily mean currents were approximately 10 cm s1 with intense instantaneous currents greater than 50 cm s1 at the sill depth. Using an idealized outflow region with a rectangular cross-sectional area (6 106 m2), we present the first estimates of shelf water export by potential density class. Assuming shelf water with a minimum density of 27.88 kg m3 has sufficient negative buoyancy for downslope mixing and a fourfold volume increase (1:3 mixing ratio) from entrainment, the dense shelf water export of 0.1–0.5 Sv results in an annual average production of bottom water in this region of between 0.4 and 2.0 Sv. The wide range in bottom water estimate results from data limitations, and a narrowing of this range requires further mooring observations.

Coastal polynyas in the Adelie Depression are an important source of Antarctic Bottom Water to the Australian-Antarctic Basin. We present time series (April 1998 to May 1999 and August 1999 to February 2000) of data from temperature-salinity sensors, in both the Adelie Depression and the known outflow region of the Adelie Sill, to describe the annual cycle of shelf water densities. From April through September, salinification beneath the polynya produces dense shelf waters. During September–October, shelf water densities in the depression peak at 27.94 kg m3, and the cooling and freshening signature of Ice Shelf Water is observed north of Buchanan Bay. In November–December, shelf water densities decrease as intrusions of warm and relatively fresh modified Circumpolar Deep Water enter east of the Ade´lie Sill. From January–March the surface layer is conditioned by the cooling of the atmosphere, which overturns the upper water column. At the Ade´lie Sill, observed daily mean currents were approximately 10 cm s1 with intense instantaneous currents greater than 50 cm s1 at the sill depth. Using an idealized outflow region with a rectangular cross-sectional area (6 106 m2), we present the first estimates of shelf water export by potential density class. Assuming shelf water with a minimum density of 27.88 kg m3 has sufficient negative buoyancy for downslope mixing and a fourfold volume increase (1:3 mixing ratio) from entrainment, the dense shelf water export of 0.1–0.5 Sv results in an annual average production of bottom water in this region of between 0.4 and 2.0 Sv. The wide range in bottom water estimate results from data limitations, and a narrowing of this range requires further mooring observations.

A mean synoptic view of the subantarctic front (SAF) is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross-stream coordinate that moves with the current. The cross-stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar (d780). By averaging absolute and baroclinic velocity measurements in stream coordinates, the spatial smoothing that results from Eulerian averaging of measurements of a meandering current is avoided. The mean SAF velocity profile is composed of one central peak and several smaller peaks. In the central peak, the 2-yr mean absolute velocity from current meters reaches 52 cm/s at 420 dbar; the mean baroclinic velocity from six CTD sections reaches 34 cm/s. The SAF flow is coherent at all levels, reaches the seafloor, and is at least 220 km wide. The crossstream structure of baroclinic and absolute transport of the SAF has been characterized for the first time. The integrated mean transport is 116+/-10 (x 10^6 m3/s), of which approximately 14% is barotropic, where barotropic transport is defined to be the velocity at the deepest current meter (3320 dbar) multiplied by the water depth. Comparison of the current meter measurements with hydrographic sections suggest this estimate is likely a lower bound on the absolute transport, as part of the 'cold side' of the SAF is not sampled by the moored array. Linear conditions for baroclinic and barotropic instability of the mean flow are satisfied at the array, consistent with earlier work that showed baroclinic conversion is the dominant mechanism for eddy growth at the array, with weaker barotropic conversion also occurring.

Two year time series measurements of current velocity and temperature in the Subantarctic Front (SAF) south of Australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the Antarctic Circumpolar Current (ACC) and further insight into the variability of the ACC. The SAF was found to be an energetic, meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm/s at 1150 dbar. A daily-varying coordinate frame which follows the direction of ow allowed mesoscale variability of the SAF to be isolated from variability due to meandering of the front and proved very successful for examining eddy uxes. Vertically averaged cross-stream eddy heat flux was 11.3 kW/m^2 poleward and significantly different from zero at the 95% confidence interval for fluctuations in the 2-90 day band. Zonally integrated, this eddy heat flux (=0.9 PW) is more than large enough to balance heat lost south of the Polar Front and is as large as cross-SAF fluxes found in Drake Passage. Cross-stream eddy momentum fluxes were small, not significantly different from zero and of indeterminate sign, but tending to decelerate the mean flow. A relationship between vertical motion and meander phase identified in the Gulf Stream was found to hold for the SAF. Eddy kinetic energy levels were similar to those in Drake Passage and southeast of New Zealand. Eddy potential energy was up to an order of magnitude larger than other ACC sites, most likely because meandering of the front is more common here. Baroclinic conversion was found to be the dominant mechanism by which eddies grow south of Australia. Typical eddy growth rate is estimated to be 30 days, approximately twice as fast as in Drake Passage, consistent with eddy energy growing rapidly downstream.

Responses by marine top predators to environmental variability have previously been almost impossible to observe directly. By using animal-mounted instruments simultaneously recording movements, diving behavior, and in situ oceanographic properties, we studied the behavioral and physiological responses of southern elephant seals to spatial environmental variability throughout their circumpolar range. Improved body condition of seals in the Atlantic sector was associated with Circumpolar Deep Water upwelling regions within the Antarctic Circumpolar Current, whereas High-Salinity Shelf Waters or temperature/salinity gradients under winter pack ice were important in the Indian and Pacific sectors. Energetic consequences of these variations could help explain recently observed population trends, showing the usefulness of this approach in examining the sensitivity of top predators to global and regional-scale climate variability.

Responses by marine top predators to environmental variability have previously been almost impossible to observe directly. By using animal-mounted instruments simultaneously recording movements, diving behavior, and in situ oceanographic properties, we studied the behavioral and physiological responses of southern elephant seals to spatial environmental variability throughout their circumpolar range. Improved body condition of seals in the Atlantic sector was associated with Circumpolar Deep Water upwelling regions within the Antarctic Circumpolar Current, whereas High-Salinity Shelf Waters or temperature/salinity gradients under winter pack ice were important in the Indian and Pacific sectors. Energetic consequences of these variations could help explain recently observed population trends, showing the usefulness of this approach in examining the sensitivity of top predators to global and regional-scale climate variability.