Topographically generated steady currents in barotropic turbulence

Abstract

Steady currents develop in oceanic turbulence above topography even in the absence of steady forcing. Mesoscale steady currents are correlated with mesoscale topography with anticyclonic eddies above topographic bumps, and large scale westward flows develop when β is non-zero. The relationship between those two kinds of steady currents, as well as their dependence on various parameters, is studied using a barotropic quasi-geostrophic channel model. The percentage of steady energy is found to depend on the forcing, friction and topography in a non-monotonic fashion. For example, the percentage of steady currents grows with the energy level in the linear regime (low energies) and decreases when the energy level increases in the nonlinear regime (high energies). Mesoscale steady currents are the energy source for the steady westward flow U, and therefore U is the maximum when large scale and mesoscale currents are of the same order of magnitude. This happens when the ratio S of the large scale slope βH/f ₀ and the mesoscale rms topographic slope α is of order one. U decreases for both small and large values of S.     

A new approach to define surface/sub-surface transition in gravel beds

The vertical structure of river beds varies temporally and spatially in response to hydraulic regime, sediment mobility, grain size distribution and faunal interaction. Implicit are changes to the active layer depth and bed porosity, both critical in describing processes such as armour layer development, surface-subsurface exchange processes and siltation/ sealing. Whilst measurements of the bed surface are increasingly informed by quantitative and spatial measurement techniques (e.g., laser displacement scanning), material opacity has precluded the full 3D bed structure analysis required to accurately define the surface-subsurface transition. To overcome this problem, this paper provides magnetic resonance imaging (MRI) data of vertical bed porosity profiles. Uniform and bimodal (?? _g = 2.1) sand-gravel beds are considered following restructuring under sub-threshold flow durations of 60 and 960 minutes. MRI data are compared to traditional 2.5D laser displacement scans and six robust definitions of the surface-subsurface transition are provided; these form the focus of discussion.     

The shape of the dark matter halo in the early-type galaxy NGC 2974

We present H  i observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected H  i disc in this galaxy is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape.
We construct mass models of NGC 2974 by combining the H  i rotation curve with the central kinematics of the ionized gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionized gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the H  i gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and modified Newtonian dynamics, which fit the data marginally worse.
The mass-to-light ratio M / L _I increases in NGC 2974 from 4.3 M_⊙/L_⊙, _I at one effective radius to 8.5 M_⊙/L_{⊙, I} at 5  R _e. This increase of M / L already suggests the presence of dark matter: we find that within 5  R _e at least 55 per cent of the total mass is dark.     

A model-based study of ice and freshwater transport variability along both sides of Greenland

We investigate some aspects of the variability of the Arctic freshwater content during the 1965–2002 period using the DRAKKAR eddy admitting global ocean/sea-ice model (12 km resolution in the Arctic). A comparison with recent mooring sections shows that the model realistically represents the major advective exchanges with the Arctic basin, through Bering, Fram and Davis Straits, and the Barents Sea. This allows the separate contributions of the inflows and outflows across each section to be quantified. In the model, the Arctic freshwater content variability is explained by the sea-ice flux at Fram and the combined variations of ocean freshwater inflow (at Bering) and outflow (at Fram and Davis). At all routes, except trough Fram Strait, the freshwater transport variability is mainly accounted for by the liquid component, with small contributions from the sea-ice flux. The ocean freshwater transport variability through both Davis and Fram is controlled by the variability of the export branch (Baffin Island Current and East Greenland Current, respectively), the variability of the inflow branches playing a minor role. We examine the respective role of velocity and salinity fluctuations in the variability of the ocean freshwater transport. Fram and Davis Straits offer a striking contrast in this regard. Freshwater transport variations across Davis Strait are completely determined by the variations of the total volume flux (0.91 correlation). On the other hand, the freshwater transport through Fram Strait depends both on variations of volume transport and salinity. As a result, there is no significant correlation between the variability of freshwater flux at Fram and Davis, although the volume transports on each side of Greenland are strongly anti-correlated (−0.84). Contrary to Davis Strait, the salinity of water carried by the East Greenland Current through Fram Strait varies strongly due to the ice-ocean flux north of Greenland.     

Insights into preservation of fossil plant cuticles using thermally assisted hydrolysis methylation

The chemical composition of Cretaceous leaf remains showing exceptionally well preserved cuticles was investigated using pyrolysis gas chromatography–mass spectrometry (Py-GC–MS) and thermally assisted hydrolysis methylation (THM)-GC–MS. Samples of Coniferales (Frenelopsis) and Ginkgoales (Nehvizdya penalveri) leaf remains were collected from freshwater and coastal marine depositional environments. Material for pyrolysis included (i) untreated leaves and cuticles obtained after extraction from mineral rock matrix and bleaching, (ii) kerogen fraction from both materials, (iii) non-hydrolysable fraction from kerogen. The THM-GC–MS data from untreated leaves and bleached cuticles show that the fossil cuticle geopolymer essentially released aliphatic components upon thermal treatment, with a dominance of fatty acids (FAs) and n-alkanes/n-alkenes. The FAs are essentially resistant to bleaching and remain after solvent extraction. They occur mainly as short chain compounds ranging from C₆ to C₁₆ and with maximum abundance at C₈–C₉. The n-alkanes/n-alkenes from kerogen and the non-hydrolysable residue occur mainly as short chain compounds in the range C₁₀–C₁₆, with the highest abundance at C₉–C₁₂. The THM-GC–MS pyrograms of the fossil cuticles differ from those of cutan from fresh living plants. They support the preservation model via polymerization of monomers derived from cutin or from unsaturated cell FAs.     

Stratification and water mass formation in the Arctic Ocean: some implications for the nutrient distribution

The mixing processes and the water formations (transformations) in the Arctic Ocean are reviewed and their influence on the stratification discussed. The relations between the stratification and the nutrient distribution are examined. The interactions between drifting sea ice and advected warmer and nutrient-rich waters favour an early biological activity. By contrast, in the central Arctic Ocean and over comparably deep shelf areas such as the northern Barents Sea, the possibilities for large productivity are more limited because of late melting, less nutrient supply, and in the central Arctic, less available light. The sedimentation of organic matter on the shelves and the remineralisation into cold, dense waters formed by brine rejection and draining off the shelves lead to a loss of nutrients to the deep waters, which must be compensated for by advection of nutrient rich waters to the Arctic Ocean.
Possible effects of a reduction of the river run-off on the stratification and the nutrient distribution are discussed.     

Embracing postcolonial geographies: Contributions by Dame Evelyn Stokes to the development of postcolonial geography in New Zealand

Abstract: This paper acknowledges Dame Evelyn Stokes’ introduction of postcolonial critical analysis of Māori/Pākehā relationships before postcolonialism was established as a field of theoretical enquiry. The paper reviews research I have done on some of the issues faced by the Mashantucket Pequots in the wider context of the contribution Dame Evelyn Stokes was making to similar debates in New Zealand. She emphasized that Māori research and knowledge provide a different world view and that this world view needs to be taken into account in any geographical study of New Zealand. Only in this way will it be possible to avoid the continued marginalization of Māoris from nation building in their own country.     

Seismic Microzonation: A Comparison between Geotechnical and Seismological Approaches in Pointe-à-Pitre (French West Indies)

Pointe-à-Pitre, the main city of Guadeloupe in the French West Indies, has on several occasions been partially destroyed by major historical earthquakes. Moreover, a post-seismic assessment of the damage from the 1985 Montserrat earthquake indicates that the town is prone to site effects. Consequently, from 1996 to 1998, BRGM conducted a seismic microzonation study based on geotechnical and geological data. At the same time, three seismological studies were being conducted – two based on earthquake recordings using a time-series analysis and the classical spectral ratio (CSR) method (CETE/LCPC and BRGM), and the third based on noise measurement at 400 points using the horizontal-to-vertical noise ratio (HVNR) method (CETE/LCPC). The objective of this paper is not to carry out a new microzonation study by taking into account all the results, but rather to show in what respects the results of these different methods are in agreement or not. A comparison of the results of the seismological studies with the geotechnical microzonation shows that they are in fairly good agreement, albeit with some discrepancies. The results indicate that the seismological methods and the geotechnical data are highly complementary and should be used together in compiling seismic transfer-function microzonation maps. This revised version was published online in July 2006 with corrections to the Cover Date.