Descent of dense water masses along continental slopes

The formation of dense water over the continental shelf and its descent along the continental slope have been investigated both theoretically and experimentally. Models have been developed for slope fronts and dense filaments, with emphasis on the role of the bottom boundary layer. An analytical, two-layer, two-dimensional model is first presented for the development of dense slope fronts near the shelf-break. The effects of vertical viscosity are explored and two behavioral regimes identified. The most relevant regime is determined by the parameter F = (UQ/gεs)(f3/v)1/2 where UQ is the flux of newly created dense water per unit length of coastline, g is the gravitational acceleration, ε is the density anomaly, s is the bottom slope, f is the Coriolis parameter and v is the vertical viscosity. In both cases, the alongslope velocity in the lower layer increases away from the coast during geostrophic adjustment, with an accompanying growth in the downslop Ekman flux. When F is small, dense water production near the coast can be balanced by transport within the boundary layer, which extends down the slope as a shallow intrusion with an alongslope speed of gεs/f. However, when F is large this type of flow cannot provide sufficient downslope transport. Dense water then accumulates, causing the front to steepen while diminishing the influence of the bottom slope. There is a corresponding increase in alongslope speed, which eventually plateaus at (2f/v)1/2/UQ where the Ekman flux balances the production of new dense water. These behaviors are strongly supported by results from laboratory experiments and are consistent with the limited available observations of the Antarctic Slope Front. After moving off the shelf, the dense water mass may continue to move down the slope within the bottom boundary layer, or alternatively, form an isolated filament with a front on both sides. Theoretical solutions are developed for dense filaments both with and without an active upper layer. In the latter case, the influence of dissipation is investigated beginning with a simple bulk parameterization. This produces a filament which broadens as it moves down the slope, while its mean alongslope velocity increases with bottom slope and its horizontal shear decreases. More realistic boundary layer dynamics have also been incorporated using a similar approach to that described for slope fronts. The solutions compare well with results from laboratory experiments on relatively stable filaments. Implications of the study for deep water formation around Antarctica are discussed briefly

Separation and recirculation of the North Brazil Current

The North Brazil Current separates from the French Guiana coast in the western tropical North Atlantic and a portion of the flow retroflects to form a recirculation zone known as the Demerara Anticyclone. The remainder of the North Brazil Current continues northwestward to join the Guiana Current. This part of the flow may be particularly significant, since it carries a cross-gyre transport of heat and mass from the tropical Atlantic into the subtropical gyre. An extension of the recirculation model of Cessi (1988), in which a quasi-geostrophic flow was driven by potential vorticity anomalies along a boundary, has been used to investigate dynamics relevant to the separation and recirculation of the North Brazil Current. Unlike that of Cessi, the present model is two dimensional and consists of two recirculating gyres driven only by the western boundary current potential vorticity distribution. The recirculation zones were much smaller than those predicted by one dimensional models and since most of the streamlines passed through a diffusive boundary layer there was no evidence of homogenization of the potential vorticity field. The influence of diffusivity, boundary forcing and boundary orientation were considered. The western boundary layer was similar to a Munk layer over most of the parameter range investigated. Since the boundary forcing diffused into the interior, the meridional and zonal dimensions of the inertial part of the recirculation both had a weak power law dependency on the boundary forcing and diffusivity. When the western boundary was not aligned with the planetary vorticity gradient, westward drift distorted the shape of the recirculation

Start-Up Costs in American Research Universities

Our report briefly summarizes findings from the 2002 Cornell Higher Education Research Institute survey of start-up costs at the over 220 universities classified as Research and Doctoral universities by the Carnegie Foundation in 1994. It reports the mean start-up cost packages across institutions for new assistant professors and senior faculty, broken down by institutional type (public/private), Carnegie classification and field (biology, chemistry, engineering, physics and astronomy) and also discuses the sources of funding for start-up costs

Models of abyssal flow in basins separated by amid-ocean ridge

Numerical and laboratory models have been used to study source-driven flows in a system consisting of two basins (with sloping sidewalls) separated by a mid-ocean ridge. Numerical spin-up occurs via topographically modified Kelvin waves which propagate away from the source region around the outer perimeter of the model ocean. Energy is then carried along the ridge by topographic waves and westward by planetary waves. The resulting flow eventually concentrates in strong cyclonic circulation patterns, defined by regions of closed geostrophic contours in the lower latitude portion of each basin. When the deep water source is located at the latitude of closed geostrophic contours, there is no significant flow outside the closed contours. However, when it is located further toward polar regions, strong flow is evident up to the source latitude. There is a close correspondence with the laboratory model when similarity conditions are satisfied. One notable difference was a higher level of wave and eddy activity in the laboratory, particularly near the border between closed and blocked contour regions

Convectively driven coastal currents in a rotating basin

Density driven coastal currents were produced in the laboratory by differentially heating and cooling the end walls of a rotating rectangular cavity. After turning on the heat flux, intrusions propagated along the side walls of the cavity under an inertial buoyancy balance, with a geostrophic cross-stream balance. These boundary currents were internally stratified in temperature, while the environment during the early stages of development of the flow was isothermal. Rotational instabilities developed on the edge of the currents and broke to form cyclone-anticyclone eddy pairs. Measurements were made of the intrusion velocity, the temporal development of the width of the boundary currents, their internal thermal structure, and the characteristics of the unstable waves, including their growth rates, wavelengths, and phase speeds. Comparisons are made with previous field observations of the Leeuwin Current off Western and Southern Australia

Program Design and Student Outcomes in Graduate Education

Doctoral programs in the humanities and related social sciences are characterized by high attrition and long time-to-degree. In response to these long-standing problems, the Andrew W. Mellon Foundation launched the Graduate Education Initiative (GEI) to improve the structure and organization of PhD programs, and in turn reduce attrition and shorten time-to-degree. Over a 10-year period starting in 1991, the Foundation provided a total of $80 million to 51 departments at 10 major research universities. This paper estimates the impact of the GEI on attrition rates and time-to-degree. Our analysis is based on a competing-risk duration model and student-level data spanning the start of the GEI, including data on students at a set of control departments. We estimate that, on average, the GEI had modest impacts on student outcomes in the expected directions: reducing attrition rates, reducing time-to-degree, and increasing completion rates. The overall impacts of the GEI appear to have been driven in part by reductions in cohort size, increases in financial aid, and increases in student quality

Information Inertia

We study how information about an asset affects optimal portfolios and equilibrium asset prices when investors are not sure about the model that predicts future asset values and thus treat the information as ambiguous. We show that this ambiguity leads to optimal portfolios that are insensitive to news even though there are no information processing costs or other market frictions. In equilibrium, we show that stock prices may not react to public information that is worse than expected and this mispricing of bad news leads to profitable trading strategies based on public information

Convection in a rotating cavity

Many large scale flows in the ocean are driven by an imposed longitudinal density gradient and the resulting buoyancy-driven flow is both influenced by the Earth's rotation and has a low aspect ratio (i.e. the characteristic vertical scale of the motion is small compared to the characteristic horizontal scale of the motion). The essential features of such flows were incorporated into a laboratory model, by differentially heating and cooling the vertical end walls of a low aspect ratio, rectangular cavity rotating about a vertical axis through its centre. When heating and cooling were initiated at the respective vertical end walls of the cavity, a hot current formed along the surface and a cold current along the bottom. These moved out from each end wall into the interior of the tank, but were confined to the sidewalls (model coastlines) by the effects of rotation. Initially the currents propagated under a balance between buoyancy and inertial forces, with an unstable balance between buoyancy and Coriolis forces in the cross-stream direction. Drag forces eventually slowed the the propagation speeds. The currents were internally stratified in temperature, and became unstable as a result of a rotationally dominated instability, driven by both the potential energy associated with the temperature difference between the currents and the isothermal environment and the velocity shear across the current. The flows were analogous to buoyancy driven coastal currents such as the East Greenland Current, the Norwegian Coastal Current and the Leeuwin Current off Western Australia. As an experiment progressed, the instabilities on the currents grew and broke to produce eddies which eventually filled the cavity. The timescales for development of the stratification within the cavity were found to be dependent on the end wall temperatures, but independent of rotation. In its statistically steady state the mean circulation consisted of baroclinic boundary currents superimposed on two basin-scale counter-rotating gyres and a nearly linear vertical temperature gradient. These observations can be explained in terms of potential vorticity dynamics in the presence of a relative slope between isopotential surfaces and horizontal boundaries. Measurements of the potential vorticity were made in the laboratory flow and the quantity proved to be a very effective dynamical tracer. The steady state flow may have interesting implications for the large scale circulation of the oceans

How to get to Australia … more than 50,000 years ago

[Extract] Over just the past few years, new archaeological findings have revealed the lives of early Aboriginal Australians in the Northern Territory’s Kakadu potentially as early as 65,000 years ago, from the Kimberley and Pilbara regions of Western Australia by about 50,000 years ago, and the Flinders Ranges of South Australia by around 49,000 years ago. But how was it even possible for people to get to Australia in the first place? And how many people must have made it to Australia to explain the diversity of Aboriginal people today? In a study published in Quaternary Science Reviews this week, we use new environmental reconstructions, voyage simulations, and genetic population estimates to show for the first time that colonisation of Australia by 50,000 years ago was achieved by a globally significant phase of purposeful and coordinated marine voyaging

Regulation of cell death in mitotic neural progenitor cells by asymmetric distribution of prostate apoptosis response 4 (PAR-4) and simultaneous elevation of endogenous ceramide

Cell death and survival of neural progenitor (NP) cells are determined by signals that are largely unknown. We have analyzed pro-apoptotic signaling in individual NP cells that have been derived from mouse embryonic stem cells. NP formation was concomitant with elevated apoptosis and increased expression of ceramide and prostate apoptosis response 4 (PAR-4). Morpholino oligonucleotide-mediated antisense knockdown of PAR-4 or inhibition of ceramide biosynthesis reduced stem cell apoptosis, whereas PAR-4 overexpression and treatment with ceramide analogs elevated apoptosis. Apoptotic cells also stained for proliferating cell nuclear antigen (a nuclear mitosis marker protein), but not for nestin (a marker for NP cells). In mitotic cells, asymmetric distribution of PAR-4 and nestin resulted in one nestin(−)/PAR-4(+) daughter cell, in which ceramide elevation induced apoptosis. The other cell was nestin(+), but PAR-4(−), and was not apoptotic. Asymmetric distribution of PAR-4 and simultaneous elevation of endogenous ceramide provides a possible mechanism underlying asymmetric differentiation and apoptosis of neuronal stem cells in the developing brain