Circulation and water masses of the southwest Pacific: WOCE Section P11, Papua New Guinea to Tasmania

The circulation near the western boundary of the South Pacific is described on the basis of water properties and geostrophic velocities measured on a meridional section along 155E through the East Australian and Coral Basins. The section was occupied in winter 1993 as part of the World Ocean Circulation Experiment (WOCE, Section P11S). The primary objective of P11 was to quantify the zonal flows entering and leaving the western boundary of the basin. The primary inflow to the Tasman and Coral seas is supplied by the South Equatorial Current (SEC), which crosses the P11 section as a wide band of westward flow between 14 and 18S with a total geostrophic transport of 55 Sv (1 Sv = 106 m3/s) relative to the bottom. The SEC bifurcates at the Australian coast near 18S: 29 Sv turns south to feed the East Australian Current (EAC), and 26 Sv recirculates in the Gulf of Papua New Guinea as a low-latitude western boundary current (the Great Barrier Reef and New Guinea Coastal Undercurrents, GBRUC/NGCUC). The NGCUC closes the tropical gyre, and carries South Pacific water around the Louisiade Archipelago and through the Solomon Sea to the equator. The core of the EAC lies over the continental slope between 18S and 30S. A system of deep-reaching, recirculating eddies or gyres is located offshore of the EAC. At 30S the EAC separates from the coast, and the steeply sloping isopycnals associated with the current persist from the surface to the bottom. The total geostrophic transport of the EAC after separation is 57 Sv relative to the bottom. After separation from the coast, more than half of the EAC (33 Sv) recirculates north and then west, crossing P11 again at 28S. The remainder (24 Sv) continues east as a meandering jet across the Tasman Sea (the Tasman Front). The circulation in the southern part of the Tasman Sea is dominated by transient eddies and standing gyres. An anticyclonic circulation facilitates the exchange of water between the Tasman Sea and the Southern Ocean. About 10 Sv of subantarctic water spreads north to 36-38S, then recirculates back to the west to merge with a weak southward flow of modified subtopical water near the Tasmanian coast. The circulation in the deeper layers consists of a weak northward deep western boundary current, a cyclonic recirculation filling the Tasman Basin, and a net export of about 3 Sv of deep water to the Coral Sea. The transport of mode and intermediate water in the low-latitude western boundary current crossing P11 is similar to the transport in these density classes further upstream in the subtropical gyre at 32S. This suggests that the mode and intermediate waters entering the Pacific from the south to compensate the export through the Indonesian passages are carried north to the tropical western Pacific primarily along isopycnals

Close Binary Progenitors of Long Gamma Ray Bursts

The strong dependence of the neutrino annihilation mechanism on the mass accretion rate makes it difficult to explain the LGRBs with duration in excess of 100 seconds as well as the precursors separated from the main gamma-ray pulse by few hundreds of seconds. Even more difficult is to explain the Swift observations of the shallow decay phase and X-ray flares, if they indeed indicate activity of the central engine for as long as 10,000 seconds. These data suggest that some other, most likely magnetic mechanisms have to be considered. The magnetic models do not require the development of accretion disk within the first few seconds of the stellar collapse and hence do not require very rapidly rotating stellar cores at the pre-supernova state. This widens the range of potential LGRB progenitors. In this paper, we re-examine the close binary scenario allowing for the possibility of late development of accretion disks in the collapsar model and investigate the available range of mass accretion rates, black hole masses, and spins. A particularly interesting version of the binary progenitor involves merger of a WR star with an ultra-compact companion, neutron star or black hole. In this case we expect the formation of very long-lived accretion disks, that may explain the phase of shallow decay and X-ray flares observed by Swift. Similarly long-lived magnetic central engines are expected in the current single star models of LGRB progenitors due to their assumed exceptionally fast rotation.Comment: Submitted to MNRA

Shearwater Foraging in the Southern Ocean: The Roles of Prey Availability and Winds

Background Sooty (Puffinus griseus) and short-tailed (P. tenuirostris) shearwaters are abundant seabirds that range widely across global oceans. Understanding the foraging ecology of these species in the Southern Ocean is important for monitoring and ecosystem conservation and management. Methodology/Principal Findings Tracking data from sooty and short-tailed shearwaters from three regions of New Zealand and Australia were combined with at-sea observations of shearwaters in the Southern Ocean, physical oceanography, near-surface copepod distributions, pelagic trawl data, and synoptic near-surface winds. Shearwaters from all three regions foraged in the Polar Front zone, and showed particular overlap in the region around 140°E. Short-tailed shearwaters from South Australia also foraged in Antarctic waters south of the Polar Front. The spatial distribution of shearwater foraging effort in the Polar Front zone was matched by patterns in large-scale upwelling, primary production, and abundances of copepods and myctophid fish. Oceanic winds were found to be broad determinants of foraging distribution, and of the flight paths taken by the birds on long foraging trips to Antarctic waters. Conclusions/Significance The shearwaters displayed foraging site fidelity and overlap of foraging habitat between species and populations that may enhance their utility as indicators of Southern Ocean ecosystems. The results highlight the importance of upwellings due to interactions of the Antarctic Circumpolar Current with large-scale bottom topography, and the corresponding localised increases in the productivity of the Polar Front ecosystem

Multiple jets of the Antarctic circumpolar current South of Australia

Maps of the gradient of sea surface height (SSH) and sea surface temperature (SST) reveal that the Antarctic Circumpolar Current (ACC) consists of multiple jets or frontal filaments. The braided and patchy nature of the gradient fields seems at odds with the traditional view, derived from hydrographic sections, that the ACC is made up of three continuous circumpolar fronts. By applying a nonlinear fitting procedure to 638 weekly maps of SSH gradient (del SSH), it is shown that the distribution of maxima in del SSH (i.e., fronts) is strongly peaked at particular values of absolute SSH (i.e., streamlines). The association between the jets and particular streamlines persists despite strong topographic and eddy - mean flow interactions, which cause the jets to merge, diverge, and fluctuate in intensity along their path. The SSH values corresponding to each frontal branch are nearly constant over the sector of the Southern Ocean between 100 degrees E and 180 degrees. The front positions inferred from SSH agree closely with positions inferred from hydrographic sections using traditional water mass criteria. Recognition of the multiple branches of the Southern Ocean fronts helps to reconcile differences between front locations determined by previous studies. Weekly maps of SSH are used to characterize the structure and variability of the ACC fronts and filaments. The path, width, and intensity of the frontal branches are influenced strongly by the bathymetry. The "meander envelopes" of the fronts are narrow on the northern slope of topographic ridges, where the sloping topography reinforces the beta effect, and broader over abyssal plains

Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 1. Mean circumpolar paths

High resolution hydrographic sections and maps of the gradient of sea surface height (SSH) reveal that the Antarctic Circumpolar Current (ACC) consists of multiple jets or frontal filaments. Here we use a 15 year time series of SSH observations to determine the circumpolar structure and distribution of the ACC fronts. The jets are consistently aligned with particular streamlines along the entire circumpolar path, confirming and extending the results of an earlier study restricted to the region south of Australia. The intensity of the fronts (as measured by the cross-front gradient of SSH) varies along the fronts and the individual branches merge and diverge, often in response to interactions with bathymetry. Maps of absolute velocity at 1000 m depth derived from Argo trajectories confirm the existence of multiple current cores throughout the Southern Ocean. High resolution hydrographic sections and profiles of temperature and salinity from Argo floats are used to show that the front locations derived from fitting SSH contours to maps of SSH gradient are consistent with locations inferred from the traditional criteria based on water mass properties, suitably modified to account for multiple frontal branches. Three regions are examined in detail: the Crozet Plateau, the Kerguelen Plateau and the Scotia Sea. These examples show how recognition of the multiple jets of the ACC can help resolve discrepancies between previous studies of ACC fronts

Rapid development and persistence of a massive Antarctic sea ice tongue

An extraordinary sea ice tongue developed near 85 degrees E over a period of 30 days in April-May 2002. The ice tongue extended to the north more than 800 km from the surrounding ice edge and covered an area greater than 200,000 km 2. Satellite measurements of ice extent and roughness characteristics demonstrate that the tongue persisted as a distinct feature throughout the winter. Remote sensing observations between 1978 and 2004 confirm that ice tongues occur frequently at this location, although the 2002 tongue was particularly pronounced. We show that ocean currents and winds conspire to favor the development of ice tongues at this location. Mean streamlines of the southern part of the Antarctic Circumpolar Current turn sharply to the north near 85 degrees E after passing through the Princess Elisabeth Trough. The edge and northern limit of the ice tongue correspond well with the pattern of mean streamlines. Mean winds in April - May have a dominant southerly component in this location, favoring offshore advection of ice; year- to- year variability in the prominence of the tongue is largely caused by variations in the wind, with northerly ( southerly) anomalies inhibiting ( promoting) development of a sea ice tongue. Ice drift is strongly northward along the axis of the tongue, suggesting the feature is formed by advection of ice from the south rather than by in situ thermodynamic ice formation. The northward current and sea ice tongue at 85 degrees E are associated with higher biomass at all trophic levels than observed elsewhere in east Antarctica

Tab. 1: Ar40/Ar39 results from Kuyul terrane, northeastern Russia

The Kamchatka Peninsula of northeastern Russia is located along the northwestern margin of the Bering Sea and consists of zones of complexly deformed accreted terranes. Along the northern portion of the peninsula, progressing from then orthwestem Bering Sea inland the Olyutorskiy, Ukelayat, and Koryak superterranes area acreted to the Okhotsk-Chukotsk volcanic-plutonic bell in northern-most Kamchatka. A sedimentary sequence of Albian to Maastrichtian age overlap terranes and units of the Koryak superterrane and constrains their accretion time with this region of the North America plate. Ophiolite complexes, widespread within the Koryak superterrane, are associated with serpentinite melanges and some of the ophiolite terranes include large portions of weakly serpentinized hyperbasites, layered gabbro, sheeted dikes, and pillow basalts outcropping as internally coherent blocks within a sheared melange matrix. Interpretation of magnetic anomalies allow the correlation of the Ukelayat with the West Kamchatka and Sredinny Range superterranes. The Olyutorskiy composite terrane may be correlated with the central and southern Kamchatka Peninsula Litke, Eastern Ranges and Vetlov composite terranes. The most "out-board" of the central and southern Kamchatka Peninsula terranes is the Kronotsky composite terrane, weil exposed along the Kamchatka, Kronotsky and Shipunsky Capes. Using regional geological constraints, paleomagnetism, and plate kinematic models for the Pacific basin a regional model can be proposed in which accretion of the Koryak composite terrane to the North America plate occurs during the Campanian-Maastrichtian, followed by the accretion of the Olyutorskiy composite terrane in the Middle Eocene, and the Late Oligocene-Early Miocene collision of the Kronotsky composite terrane. A revised age estimate of a key overlapping sedirnentary sequence of the Koryak superterrane, calibrated with new Ar40/Ar39 data, supports its Late Cretaceous accretion age