Australian-Antarctic Basin Research Articles

Impact of the Mertz Glacier Tongue calving on dense water formation and export

Antarctic Bottom Water (AABW) is a critical component of the global climate system, occupying the abyssal layer of the World Ocean and driving the lower limb of the global meridional overturning circulation. Around East Antarctica, the dense shelf water (DSW) precursor to AABW is predominantly formed by enhanced sea ice formation in coastal polynyas. The dominant source region of AABW supply to the Australian-Antarctic Basin is the Adélie and George V Land coast, in particular, polynyas formed in the western lee of the Mertz Glacier Tongue (MGT) and the grounded iceberg B9b over the Adélie and the Mertz Depressions, respectively. The calving of the MGT, which occurred on 12-13 February 2010, dramatically changed the environment for producing DSW. Here, we assess its impact using a state-of-the-art ice-ocean model. The model shows that oceanic circulation and sea ice production in the region changes immediately after the calving event, and that the DSW export is reduced by up to 23%.

An almost‐free barotropic mode in the Australian‐Antarctic Basin

The Australian‐Antarctic Basin (AAB) is known for its high levels of intraseasonal variability; sea‐surface height variability exceeds background values by factors of 2 over thousands of kilometers. This paper addresses the hypothesis that this variability is caused by trapping of barotropic energy by the basin geometry. Analysis of a multi‐year integration of a shallow‐water model shows that the variability is dominated by a single, large‐scale statistical mode that is highly coherent over the entire AAB. The flow associated with this mode is northwestward along the Southeast Indian Ridge, southward in the Kerguelen Abyssal Plain, and eastward in the southern AAB. The mode is interpreted as an almost‐free topographically trapped mode, as it is confined by contours of potential vorticity that almost entirely enclose the AAB. The apex of the Wilkes Abyssal Plain represents the strongest barrier to the modal circulation: here velocities are strongest, making it a key area for dissipation of kinetic energy through bottom friction and eddy viscosity.

The cyclonic circulation in the Australian–Antarctic basin simulated by an eddy-resolving general circulation model

Flow structure in the Australian–Antarctic basin is investigated using an eddy-resolving general ocean circulation model and validated with iceberg and middepth float trajectories. A cyclonic circulation system between the Antarctic Circumpolar Current and Antarctic Slope Current consists of a large-scale gyre in the west (80–110° E) and a series of eddies in the east (120–150° E). The western gyre has an annual mean westward transport of 22 Sv in the southern limb. Extending west through the Princess Elizabeth Trough, 5 Sv of the gyre recirculates off Prydz Bay and joins the western boundary current off the Kerguelen Plateau. Iceberg trajectories from QuickScat and ERS-1/2 support this recirculation and the overall structure of the Antarctic Slope Current against isobath in the model. Argo float trajectories also reveal a consistent structure of the deep westward slope current. This study indicates the presence of a large cyclonic circulation in this basin, which is comparable to the Weddell and Ross gyres.

Observations of the Fawn Trough Current over the Kerguelen Plateau from instrumented elephant seals

Due to its great meridional extent and relatively shallow depths, the Kerguelen Plateau constitutes a major barrier to the eastward flowing Antarctic Circumpolar Current in the Indian sector of the Southern Ocean. While most of the Antarctic Circumpolar Current transport is deflected north of the Kerguelen Islands, the remainder (∼ 50 Sv, 1 Sv = 10 6 m 3 s − 1 ) must pass south of the islands, most probably through the Fawn and Princess Elizabeth Troughs. However, the paucity of finely resolved quasi-synoptic hydrographic data in this remote and infrequently sampled area has limited the progress in our knowledge of the regional circulation. Since 2004, a new approach using elephant seals from the Kerguelen Islands as autonomous oceanographic profilers has provided new information on the hydrography over the Kerguelen Plateau, covering the entire Antarctic Zone between the Polar Front and Antarctica, with a mean along-track resolution of about 25 km. These finely resolved bio-logged data revealed details of a strong northeastward current found across the Fawn Trough (sill depth: 2600 m; 56°S, 78°E). This so-called Fawn Trough Current transports cold Antarctic waters found mostly south of the Elan Bank, between the Ice Limit (58°S) and the Antarctic Divergence (64°S) in the eastern Enderby Basin, toward the Australian–Antarctic Basin. Our analysis also demonstrates that the Deep Western Boundary Current, which carries cold Antarctic water along the eastern flank of the southern Kerguelen Plateau collides with Fawn Trough Current at the outlet of the Fawn Trough sill. In other words, the Fawn Trough constitutes a veritable bottleneck, channelling the quasi-totality of the Antarctic Circumpolar flow found south of the Polar Front. Thanks to the unprecedented fine resolution of seal-borne data, a branch of flow centered at the Winter Water isotherm of 1 °C is also revealed along the northern escarpment of the Elan Bank, and then along the southern edge of Heard Island. Further analysis of different supplementary data reveals a complex circulation pattern in the entire Enderby Basin, with several distinctive branches of flow being strongly controlled by prominent topographic features such as the Southwest Indian Ridge, Conrad Rise, Elan Bank, and Kerguelen Plateau. This newly emerged frontal structure refines considerably previous large-scale circulation schematics of the area.

The Southern Annular Mode and opposite-phased Basin Mode of the Southern Ocean circulation

Over the Southern Ocean the dominant modes of the atmospheric field are known as the Southern Annular Mode (SAM) or Antarctic Oscillation, and the Pacific South American (PSA) pattern. Statistical analysis of sea surface wind (SSW) from satellite observation revealed two leading modes of SAM-like and PSA patterns. In the high latitudes, the SAM-like pattern of the SSW had a large amplitude over the Bellingshausen Basin and Australian-Antarctic Basin, with opposite phase between the two basins. On the intraseasonal time scale, large-scale sea surface height (SSH) also had notable variability, showing a basin-scale anti-phase mode over the two basins. To explain the response of oceanic variations to these atmospheric modes, we analyzed the relationship between the dominant modes of wind stress and large-scale SSH on the intraseasonal time scale. The SAM-like pattern of wind stress was correlated with the SSH variation over the two basins. The SSH basin mode was most simply explained by a simple barotropic response to the SAM-like mode of wind stress, with the curl of opposite phase between the two basins. We conclude that the zonal asymmetry of the wind field of the SAM plays an important role in driving the antiphase SSH basin modes.

A recount of Ross Sea waters

Abstract Oceanographic observations within the Ross Sea have grown dramatically in recent years, both in number and quality. This has prompted a parallel recount of the circulation and structure of all water masses in the southwestern continental margins of the Pacific Ocean. A high-resolution set of horizontal property distributions was combined into a new climatology, which in turn is the basis of a fine volumetric θ − S census of all Ross Sea water masses. Inshore of the shelf break (700 m isobath) the Ross Sea volume (25×10 4 km 3 ) partitioning into layers of neutral density ( γ n ) is: 25% in the top layer ( γ n −3 ) of Antarctic Surface Water (AASW), 22% in the middle layer (28.00 kg m −3 γ n −3 ) of oceanic thermocline water (12%) and Modified Circumpolar Deep Water (MCDW; 10%), and 53% in the bottom layer ( γ n >28.27 kg m −3 ) of Shelf Water (SW, θ θ >−1.85 °C; 22%), precursor of the Antarctic Bottom Water (AABW) offshore. AASW flows westward along the outer shelf and southward along the eastern coast past Cape Colbeck. Oceanic thermocline waters (28.00 kg m −3 γ n −3 ) cross the shelf break west of 170°W and follow southward paths along banks, shoaling progressively to near the sea surface of the central and western inner shelves. Winter buoyancy loss converts AASW and oceanic thermocline water into denser types of MCDW and SW, which move cyclonically toward the sills of major troughs. The census shows a continuous mode of relatively dense MCDW (28.10 kg m −3 γ n −3 ) directly linked to the Circumpolar Deep Water (CDW) offshore, effectively ventilating and freshening the deep ocean at levels below the salinity maximum and above AABW. MCDW outflows from the Ross Sea shelf are traced to the Antarctic Circumpolar Current: near 155°E, into the Australian-Antarctic Basin, along the northern flank of the Southwest Pacific Ridge, and to near 135°W, past the eastern end of the Ross Gyre. Two sources of salty SW are inferred near the coastal polynyas in the Terra Nova Bay and western Ross Ice Shelf areas. The latter contributes to a major outflow of Ice SW that reaches the shelf break along 180°. Vertical mixing of MCDW and SW produce MSW over the shelf, continuing over the sills as new AABW. Export of low salinity AABW ( S

Warming and Freshening in the Abyssal Southeastern Indian Ocean*

Abstract Warming and freshening of abyssal waters in the eastern Indian Ocean between 1994/95 and 2007 are quantified using data from two closely sampled high-quality occupations of a hydrographic section extending from Antarctica northward to the equator. These changes are limited to abyssal waters in the Princess Elizabeth Trough and the Australian–Antarctic Basin, with little abyssal change evident north of the Southeast Indian Ridge. As in previous studies, significant cooling and freshening is observed in the bottom potential temperature–salinity relations in these two southern basins. In addition, analysis on pressure surfaces shows abyssal warming of about 0.05°C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1°C with freshening of about 0.005 in the abyssal Australian–Antarctic Basin. These 12-yr differences are statistically significant from zero at 95% confidence intervals over the bottom few to several hundred decibars of the water column in both deep basins. Both warming and freshening reduce the density of seawater, contributing to the vertical expansion of the water column. The changes below 3000 dbar in these basins suggest local contributions approaching 1 and 4 cm of sea level rise, respectively. Transient tracer data from the 2007 occupation qualitatively suggest that the abyssal waters in the two southern basins exhibiting changes have significant components that have been exposed to the ocean surface within the last few decades, whereas north of the Southeast Indian Ridge, where changes are not found, the component of abyssal waters that have undergone such ventilation is much reduced.

Rapid freshening of Antarctic Bottom Water formed in the Indian and Pacific oceans

Repeat hydrographic sections occupied in 1995 and 2005 reveal a rapid decline in the salinity and density of Antarctic Bottom Water throughout the Australian Antarctic Basin. The basin‐wide shift of the deep potential temperature‐salinity (θ − S) relationship reflects freshening of both the Indian and Pacific sources of Antarctic Bottom Water. The θ − S curves diverge for waters cooler than −0.1°C, corresponding to a layer up to 1000 m thick over the Antarctic continental slope and rise. Changes over the last decade are in the same direction but more rapid than those observed between the late 1960s and the 1990s. When combined with recent observations of similar freshening of North Atlantic Deep Water, these results demonstrate that dense water formed in both hemispheres is freshening in response to changes in the high latitude freshwater balance and rapidly transmitting the signature of changes in surface climate into the deep ocean.

Wind-Forced Intraseasonal Sea Level Variability of the Extratropical Oceans

Seven years' worth of sea level observations from the TOPEX/Poseidon altimeters and wind observations from the European Remote-Sensing Satellites (ERS-1/2) scatterometers were used to investigate the dynamics of large-scale intraseasonal sea level variability at mid- and high latitudes. Coherent patterns of sea level variability with spatial scales of 1000 km and timescales of 20 days to 1 year are identified in three particular regions: the Bellingshausen Basin west of Drake Passage, the Australian–Antarctic Basin, and the central North Pacific Ocean (30°–50°N) near the date line. Significant coherence is found between wind stress curl and the sea level variability. A simple barotropic vorticity equation, in which the time rate of relative vorticity variation is balanced by the wind stress curl and a damping term, is used to simulate the sea level anomalies with a significant degree of correlation with the observations. Although the coherence between the simulation and the observation is significant from periods of 30 days to 1 yr, the variance of the simulation is too low at periods shorter than 100 days. This is probably caused by the errors in the wind forcing as well as the other terms neglected in the vorticity equation. The simulation requires a damping timescale generally longer than 20 days, consistent with theoretical estimates of the dissipation timescales of a frictional bottom boundary layer. In the Bellingshausen Basin, the phase of the coherence between sea level and wind stress curl also shows dependence on frequency according to the dissipation mechanism and estimated damping timescales. However, the results at the other two locations are less conclusive.

Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough, Antarctica

Hydrographic, current meter and ADCP data collected during two recent cruises in the South Indian Ocean ( RRS Discovery cruise 200 in February 1993 and RRS Discovery cruise 207 in February 1994) are used to investigate the current structure within the Princess Elizabeth Trough (PET), near the Antarctic continent at 85°E, 63–66°S. This gap in topography between the Kerguelen Plateau and the Antarctic continent, with sill depth 3750 m, provides a route for the exchange of Antarctic Bottom Water between the Australian–Antarctic Basin and the Weddell–Enderby Basin. Shears derived from ADCP and hydrographic data are used to deduce the barotropic component of the velocity field, and thus the volume transports of the water masses. Both the Southern Antarctic Circumpolar Current Front (SACCF) and the Southern Boundary of the Antarctic Circumpolar Current (SB) pass through the northern PET (latitudes 63 to 64.5°S) associated with eastward transports. These are deep-reaching fronts with associated bottom velocities of several cm s -1. Antarctic Bottom water (AABW) from the Weddell–Enderby Basin is transported eastwards in the jets associated with these fronts. The transport of water with potential temperatures less than 0°C is 3 (±1) Sv. The SB is shown to meander in the PET, caused by the cyclonic gyre immediately west of the PET in Prydz Bay. The AABW therefore also meanders before continuing eastwards. In the southern PET (latitudes 64.5 to 66°S) a bottom intensified flow of AABW is observed flowing west. This AABW has most likely formed not far from the PET, along the Antarctic continental shelf and slope to the east. Current meters show that speeds in this flow have an annual scalar mean of 10 cm s -1. The transport of water with potential temperatures less than 0°C is 20 (±3) Sv. The southern PET features westward flow throughout the water column, since the shallower depths are dominated by the flow associated with the Antarctic Slope Front. Including the westward flow of bottom water, the total westward transport of the whole water column in the southern PET is 45 (±6) Sv.

Formation and circulation of the water masses between the southern Indian Ocean and Antarctica: Results from δ<SUP>18</SUP>O

We present measurements of the stable isotopes of oxygen ( 18 O and 16 O) from seawater samples collected during the Antarctic Deep Outflow Experiment (ADOX) cruises in the Southern Ocean and southern Indian Ocean, February to March, 1993 and 1994. The data are used in conjunction with hydrographic data to infer characteristics of the formation and circulation of water masses found in the region. The waters on the continental shelf of Antarctica (adjacent to the Princess Elizabeth Trough; PET) are isotopically the lightest found due to the injection of about 1% of glacial meltwater, and are probably advected to the region from farther east by the current associated with the Antarctic Slope Front. They appear to be locally disassociated from the Antarctic Surface Water and Winter Water (WW) farther north in the PET. The WW of the Enderby Basin is isotopically lighter than the PET WW and also fresher, indicating the presence of an additional component of glacial meltwater or high-latitude precipitation. North of the Antarctic Circumpolar Current, the surface δ 18 O values correlate strongly with salinity, but extrapolate to an apparent freshwater endmember which is isotopically too light to be reasonable; advection and mixing of the water masses dominate over the local water balance at this location. The Subantarctic Mode Water of the southern Indian Ocean lies on the line of the surface waters in salinity-δ 18 O space due to its local formation by deep convection. The Antarctic Intermediate Water also lies on this line, but it is known that the major sources for this water mass are outside the region. Consequently this observation does not necessarily imply formation by local deep convection. The δ 18 O measurements have shown that the major sources for PET Antarctic Bottom Water (AABW) lie outside the Weddell and Ross seas. Based on the potential temperature-salinity of the PET AABW we further deduce that the PET itself is not the formation location of a significant amount of AABW. Its major component is advected to the region from the Australian-Antarctic Basin farther east.

Sources and transport of the Deep Western Boundary Current East of the Kerguelen Plateau

East of the Kerguelen Plateau, a deep western boundary current in the Australian‐Antarctic Basin brings cold dense waters north from the margins of Antarctica. Geostrophic velocities referenced to acoustic Doppler current profiler data, both shipboard and lowered, suggest the flow is unidirectional throughout the water column with estimated northwestward transport below potential temperature 1°C of 28 ± 7 × 106 m³s−1 and 49 ± 9 × 106 m³s−1 respectively. Hydrographie and acoustic Doppler current profiler data show that the deep boundary current is supplied by northward flow emerging from the confluence of westward flow along the Antarctic continental slope and eastward flow of Weddell Basin waters through the Princess Elizabeth Trough to the south of the Kerguelen Plateau.

The oldest magnetic anomalies in the Australian‐Antarctic Basin: Are they isochrons?

We present a revised tectonic interpretation of Australia and Antarctica incorporating new magnetic data off of Wilkes Land, Antarctica, for the earliest period of seafloor spreading on the Southeast Indian Ridge, from the Late Cretaceous to Early Tertiary. Reconstructions based on our revised anomaly identifications are characterized by a surprisingly large amount of continental overlap, involving the South Tasman Rise, Tasmania, and the Victoria Land region of Antarctica. The overlap is due to continental extension and/or deformation of oceanic crust in the Australian‐Antarctic Basin. The second hypothesis would imply that the magnetic anomalies older than chron 31 in the Australian‐Antarctic Basin are not isochrons. We also find evidence for a previously unrecognized period of extremely slow spreading on the Southeast Indian Ridge from chron 31 (68.7 Ma) to chron 24 (53.3 Ma) in the Australian‐Antarctic Basin. In the western end of the basin we have tentatively identified a hiatus in the generation of crustal accretion associated with a minimum sustainable threshold half spreading rate of 1.5 mm/yr. In addition, we recognize a new fracture zone on the Antarctic plate, the Vincennes fracture zone, as the conjugate to the Perth fracture zone on the Australian plate, in the western Australian‐Antarctic Basin. These fracture zones record the initial NW‐SE opening direction between Australia and Antarctica.

The Maastrichtian-Danian radiation of triserial and biserial planktic foraminifera: Testing phylogenetic and adaptational hypotheses in the (micro)fossil record

The earliest Tertiary planktic foraminiferal taxonomic diversification has long been regarded as one of the best examples of adaptive radiation within the microfossil record. However, the morphologic, phylogenetic, and paleoecologic data required to validate this interpretation have never been adequately synthesized. In order to determine whether or not the Maastrichtian through Danian evolution of the triserial and biserial planktic foraminiferal genera Guembelitria, Woodringina, and Chiloguembelina can be identified as an example of adaptive radiation, both morphologic and paleoecologic data were gathered. Phylogenetic analyses of twelve species based on 52 morphologic characters representing all ontogenetic stages indicate that all three genera belong to a single monophyletic clade. Within this clade, Guembelitria appears to constitute a monophyletic sister group of Woodringina-Chiloguembelina. Although there is compelling evidence for recognizing Chiloguembelina as a subsidiary monophyletic group, both Fitch and Wagner parsimony analyses suggest that Woodringina most likely represents a morphologic grade within a larger Woodringina-Chiloguembelina clade. Ecologic diversification within this monophyletic group was assessed by tracing species-specific changes in preferred depth habitat as reflected by variations in the δ 18O and δ 13C isotopic composition of populations from Nye Kløv (Denmark), Brazos River (Texas), and ODP Site 738C (Kerguelen Plateau, Australian-Antarctic Basin), along with addition data from the published literature. Results indicate that Guembelitria cretacea, Guembelitria trifolia, Woodringina hornerstownensis, and Chiloguembelina morsei occupied near surface (mixed layer) planktic habitats whereas Guembelitria danica, Chiloguembelina crinita, Chiloguembelina midwayensis, and Chiloguembelina waiparaensis preferred intermediate depth habitats; probably near the base of the thermocline. Since no isotopic data were used in the phylogenetic analyses, direct comparison of these paleoecologic and phylogenetic data permits the reconstruction of certain aspects of this clade's ecologic history. While these patterns of morphologic change and habitat shift are congruent, instances of speciation appear to have occurred much more frequently than instances of habitat diversification. This finding is contrary to the “newly vacated ecospace” model of adaptive radiation, but is consistent with adaptive radiation via the “taxon pulse” model. Moreover, since there seems to be no evidence indicating that habitat diversification within this clade took place at an atypically rapid pace during the earliest Danian, these data also suggest that intermediate-depth planktic foraminiferal niches may have been occupied by K/T survivor populations during the interval from Danian planktic foraminiferal biozones P0 to P1a—a position supported by increasing body of independent evidence. By combining morphologic, stratigraphic, and ecologic studies within an explicitly phylogenetic framework (as illustrated by this study), micropaleontologists can develop tools for probing many different types of macroevolutionary phenomena.