Surface geostrophic currents across the Antarctic circumpolar current in Drake Passage from 1992 to 2004

The Southern Ocean plays an important role in the global overturning circulation as a significant proportion of deep water is converted into intermediate and deeper water masses in this region. Recently, a secular trend has been reported in wind stress around the Southern Ocean and it is thought theoretically that the strength of the ACC is closely related to wind stress, so one consequence should be a corresponding increase in ACC transport and hence changes in the rate of the global overturning. There are no long-term data sets of ACC transport and so we must examine other data that may also respond to changing wind stress. Here we calculate surface currents in Drake Passage every seven days over 11.25 years from 1992 to 2004. We combine surface velocity anomalies calculated from satellite altimeter sea surface heights with measured surface currents. Since 1992, the UK has regularly occupied WOCE hydrographic section SR1b across the ACC in Drake Passage. From seven hydrographic sections surface currents are estimated by referencing relative geostrophic velocities from CTD sections with current measurements made by shipboard and lowered acoustic Doppler current profilers. Combining the seven estimates of surface currents with the altimeter data reduces bias in the estimates of average currents over time through Drake Passage and we show that surface current anomalies estimated by satellite and in situ observations are in good agreement. The strongest surface currents are found in the Subantarctic and Polar Fronts with average speeds of 50 cm/s and 35 cm/s, respectively and are inversely correlated, so that maximum westward flow in one corresponds to minimum westward flow in the other. The average cross-sectional weighted surface velocity from 1992 to 2004 is 16.7 ± 0.2 cm/s. A spectral analysis of the average surface current has only weakly increasing energy at higher frequencies and there is no dominant mode of variability. The standard deviation of the seven day currents is 0.68 cm/s and a running 12 month average has only a slightly smaller standard deviation of 0.52 ± 0.16 cm/s. The southern annular mode (SAM) measures the circumpolar average of wind stress and like the surface currents its spectrum has slightly increased energy at frequencies greater than 1 cpy. A cospectral analysis of these, averaging cospectra of five slightly overlapping 36 month segments improve statistical reliability, suggests that there is coherence between them at 1 cpy with the currents leading changes in the Southern annular mode. We conclude that the SAM and average Drake Passage surface currents are weakly correlated with no dominant co-varying modes, and hence predicting Southern Ocean transport variability from the SAM is not likely to give significant results and that secular trends in surface currents are likely to be masked by weekly and interannual variability.     

Sediment velocity estimation using iterative 3-D migrations of short offset seismic reflection data in deep water

In deep ocean settings where water depth greatly exceeds the source-to-receiver length, the geometry is insufficient for accurate determinations of velocity from reflection-moveout. However, velocities are crucial for estimates of physical properties and image processing. Focusing analyses with conventional post-stack two-dimensional migration improves images, but does not produce geologically meaningful velocities except in the special case of a two-dimensional earth. For the more general case of the three-dimensional earth there is no a priori method to determine the degree of geometrical complexity. We present a technique using a short-offset three-dimensional (3-D) data set over the 5 km deep trench west of the Lesser Antilles. These data illustrate highly sensitive post-stack 3-D focusing analyses (± 20 m s^–1 interval velocities), and the relationship of these seismically derived velocities to rock velocities. In our Barbados example we were able to establish the presence of a widespread 80-160 m thick low-velocity zone at and above the main low-angle fault. This observation suggests the water-rich décollement leaks water into the overlying sections. Also evident is a low-velocity section associated with turbidite sands. These results are confirmed with sparse logging data and well samples. Deep-water short offset 3-D experiments provide a potentially effective approach for velocity estimation, replacing the operational complexity of long-offsets with simpler short-offset techniques. In areas of structural complications and abundant diffracted energy, it is a surprisingly accurate method, utilizing the high fidelity 3-D wavefield and the information carried in zero-offset diffraction ellipsoids. The velocity used to properly collapse a diffraction ellipsoid is explicitly the velocity of propagation in the media since the travel path is known exactly. Thus, the derived velocities should closely represent rock velocities, unlike the 2-D case where the propagation geometry is not known.     

Effects of atrazine and linuron on photosynthesis and growth of the macrophytes, Potamogeton perfoliatus L. and Myriophyllum spicatum L. in an estuarine environment

Phytotoxicities of the herbicides, atrazine and linuron, were evaluated for two species of submersed vascular plants (Potamogeton perfoliatus, L. Myriophyllum spicatum, L.) which, until the late 1960s, had been abundant in Chesapeake Bay. Plants were grown in 50-liter laboratory microcosms, containing filtered estuarine water and sediments for a period of 5 weeks and then treated with atrazine or linuron at initial concentrations of 0, 5, 50, 100, 500 and 1000 gmg/liter. Plant responses were measured primarily in terms of apparent O₂ production, P₃, and above-ground biomass for 4 weeks post treatment. In general, at ≥ 50 gmg/liter there was a significant depression in P_a for both species and herbicides. However, M. spicatum appeared to be less sensitive, with a significant enhancement in P_a of this species at 5 gmg/liter, and linuron was slightly more effective than atrazine at reducing P_a for both species. Treatment effects on biomass generally paralleled those for P_a. In spite of relatively constant atrazine concentrations (84–89 % remaining at termination), both species exhibited evidence of photosynthetic recovery 2–3 weeks after treatment at concentrations ≤ 100 gmg/liter. Using an exponential dose-response model, I₅₀ (concentration for 50 % photosynthetic inhibition), ranged from 45–117 gmg/liter for all experiments. In general, in situ concentrations of atrazine and linuron in Chesapeake Bay and its tributaries appear to be sufficient to result in small reductions in P_a (2–10%, estimated from dose-response model) during a typical growing season. While such effects may be important for the survival of otherwise stressed plant populations, they suggest that these herbicides, per se, were not the cause of the general decline in abundance of these plants.     

Linear stability and stable modes of geostrophic fronts

Abstract

A Rayleigh integral is used to prove that an unbounded geostrophic front of uniform potential vorticity is stable with respect to small perturbations of arbitrary wavelength. The ageostrophic theory developed in this study yields a stable, near-inertial, long trapped mode. Recent oceanic observations of the increase in the energy of the inertial peak in the vicinity of fronts support the existence of this inertial trapped mode. In addition the theory yields a geostrophic mode which is expected to become unstable when the potential vorticity is not uniform.     

Present-day geodynamics of the circum-Adriatic region: An overview

This volume builds on presentations made at two thematic sessions on “The Active Tectonics of the Circum-Adriatic Region” at the EGU 2007 and 2008 General Assemblies in Vienna. In both sessions, contributions were invited from workers incorporating structural, geophysical, seismological, remote sensing, geodetic and thermochronological methods to better understand the crustal evolution and ongoing development of one of Europe's most interesting tectonic regions. The Adriatic region occupies a central position within the geodynamic framework of the Central Mediterranean. It is surrounded by actively deforming belts of diverse character including transpressional, contractional, strike-slip and extensional domains, collectively defining the structurally complex and kinematically varied boundaries of the Adria plate. Because many countries occupy the circum-Adriatic region, international collaborative research into the tectonic activity and associated natural hazards of the region is relevant and important to European society. In this context, the papers presented in this volume provide important new data that bear on various aspects of the active tectonics of the circum-Adriatic region. These contributions typically draw on multidisciplinary structural, geophysical and geochronological datasets to draw new conclusions concerning the spatial and temporal evolution of Adria's bounding deformation zones, especially the southeastern and southern Italian regions and NE Adriatic region. In this overview, we provide a state-of-the-art summary of different aspects of present-day Adria geodynamics to provide a common background for the individual contributions that follow.     

Ecological Risk Assessment for seabird interactions in Western and Central Pacific longline fisheries

The risk of seabird–fishery interactions in the Western and Central Pacific Ocean (WCPO) was examined by analysing the overlap of seabird distributions with tuna and swordfish pelagic longline fisheries managed by the Western and Central Pacific Fisheries Commission (WCPFC) and its constituent members. The study used spatially-explicit Productivity–Susceptibility Analysis (PSA). Key data inputs were species productivity, fishing effort, likelihood of capture and species density by region. The outputs tailored results to the needs of fisheries- and wildlife-managers, indicating areas of greatest risk of species interactions, species of greatest concern for population impacts, and the flags or fisheries most likely to contribute to the risk. Large albatross species were found to be most likely to suffer population effects when exposed to longline fishing activity, followed by the larger petrels from the genuses Procellaria, Macronectes and Pterodroma. A mixture of coastal states with nesting seabird populations in their Exclusive Economic Zones (New Zealand, Australia and United States of America), distant water fishing nations (Japan, Taiwan) and flags of convenience (Vanuatu) contributed 90% of the risk to seabird populations. Recommendations include enhancing the level of fisheries observer monitoring in areas indicated as high to medium risk for seabird interactions, and consideration of spatial management tools, such as more intensive or more stringent seabird bycatch mitigation requirements in high- to medium-risk areas. The methods used, and similar studies conducted in the Atlantic Ocean could lead to improved targeting of monitoring resources, and greater specificity in the needs for seabird-mitigation measures. This will assist in reducing seabird mortality in longline fishing operations and with more effective use of resources for fishery managers in both domestic fisheries and RFMOs.     

A search for propylene oxide and glycine in Sagittarius B2 (LMH) and Orion

We have used the Mopra Telescope to search for glycine and the simple chiral molecule propylene oxide in the Sgr B2 (LMH) and Orion KL, in the 3-mm band. We have not detected either species, but have been able to put sensitive upper limits on the abundances of both molecules. The 3σ upper limits derived for glycine conformer I are  3.7 × 10¹⁴ cm⁻²  in both Orion-KL and Sgr B2 (LMH), comparable to the reported detections of conformer I by Kuan et al. However, as our values are 3σ upper limits rather than detections we conclude that this weighs against confirming the detection of Kuan et al. We find upper limits for the glycine II column density of  7.7 × 10¹² cm⁻²  in both Orion-KL and Sgr B2 (LMH), in agreement with the results of Combes et al. The results presented here show that glycine conformer II is not present in the extended gas at the levels detected by Kuan et al. for conformer I. Our ATCA results have ruled out the detection of glycine (both conformers I and II) in the compact hot core of the LMH at the levels reported, so we conclude that it is unlikely that Kuan et al. have detected glycine in either Sgr B2 or Orion-KL. We find upper limits for propylene oxide abundance of  3.0 × 10¹⁴ cm⁻²  in Orion-KL and  6.7 × 10¹⁴ cm⁻²  in Sgr B2 (LMH). We have detected fourteen features in Sgr B2 and four features in Orion-KL which have not previously been reported in the interstellar medium, but have not been able to plausibly assign these transitions to any carrier.     

Salinity on the southeastern Dundas Tableland,Victoria

Historical evidence of early salinity, vegetation and landuse changes, and pedological studies have been used in formulating a new model for salinity processes acting on the Dundas Tableland in southwestern Victoria. Contrary to previous assumptions, salinity in this area was a feature of the pre‐European landscape and was noted in the earliest surveys and journals. Analysis of historical records show an initial post‐settlement increase in the tree numbers, followed by a rapid decline much later than previously assumed. Accumulation of salts in the regolith may be attributed to marine incursions during the Miocene and Pliocene, the extensive weathering to develop a deep regolith, and wind‐blown and cyclic salt accumulation. A trend analysis of historical streamflow and bore hydrograph records does not indicate rising groundwater levels. The pedological features of duplex and sodic soil profiles support a history of prolonged seasonal waterlogging. A model with seasonal lateral flow of water through the upper regolith can better account for the spread of salinity than the rising groundwater hypothesis. By control of waterlogging, land managers could improve soil structure, enhance root growth and soil water use, as well as inhibit the spread of salinity.     

Improved representation of ocean heat content in energy balance models

Climatic Change - Anomaly-diffusing energy balance models (AD-EBMs) are routinely employed to analyze and emulate the warming response of both observed and simulated Earth systems. We demonstrate a...     

Color imaging of Mars by the High Resolution Imaging Science Experiment (HiRISE)

HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE.