Cross-shelf eddy heat transport in a wind-free coastal ocean undergoing winter time cooling

A steady state cross-shelf density gradient of a wind-free coastal ocean undergoing winter time cooling is found for cooling and geometries which do not vary in the along-shelf direction. The steady state cross-shelf density gradient exists even when the average density of the water continues to increase. The steady state density gradient dan be attained in less than a winter for parameters appropriate to the mid-Atlantic Eight. The cross-shelf eddy-driven buoyancy fluxes which cause this steady state gradient are found to depend critically on bottom friction and bottom slope, and the coastal polyna solutions of Chapman and Gawarkiewicz [1997] are significantly modified by this dependence in the limit of polynas with a large alongshore extent. Bottom friction retards the cross-shelf propagation of eddies, so that the buoyancy transport is no longer carried by self-advecting eddy pairs but mixed across the shelf by interacting eddies. The eddy interaction changes the length scale of the eddies until it is the lesser of the Rhines arrest scale or an analogous frictional arrest scale. The estimates of the steady state cross-shelf density gradient are found to compare well with numerical model results

Enhancement of wind-driven upwelling and downwelling by alongshore bathymetric variability

Steady wind-driven flow along a shelf of changing width is described with a frictional barotropic model valid in the limit of small Rossby and Burger number. In these limits, an alongshore wind drives enhanced onshelf transport in a coastal ocean if the shelf widens downwind, and the change in shelf width only affects the flow in the direction of Kelvin wave propagation ( downwave\u27\u27) from the change in shelf width. There is enhanced onshore transport of cold, nutrient-laden bottom water if the winds favor upwelling and the shelf narrows in the direction of Kelvin wave propagation. This enhanced transport extends a considerable distance away from the change in shelf width but becomes concentrated near the shelf break far from the change in width. Isobath curvature on the scale of the shelf width significantly modifies local cross-shelf transport. The cross-shelf transport of nutrient-rich water during upwelling is expected to be enhanced from Point Eugenia to La Jolla, San Luis Obispo to Monterey, and Point Reyes to Cape Mendocino on the west coast of North America

Variability of black hole accretion discs: The cool, thermal disc component

We extend the model of King et al. (2004) for variability in black hole accretion discs, by taking proper account of the thermal properties of the disc. Because the degree of variability in the King et al. (2004) model depends sensitively on the ratio of disc thickness to radius, H/R, it is important to follow the time-dependence of the local disc structure as the variability proceeds. In common with previous authors, we develop a one-zone model for the local disc structure. We agree that radial heat advection plays an important role in determining the inner disc structure, and also find limit-cycle behaviour. When the stochastic magnetic dynamo model of King et al. (2004) is added to these models, we find similar variability behaviour to before. We are now better placed to put physical constraints on model parameters. In particular, we find that in order to be consistent with the low degree of variability seen in the thermal disc component of black hole binaries, we need to limit the energy density of the poloidal field that can be produced by local dynamo cells in the disc to less than a few percent of the energy density of the dynamo field within the disc itself.Comment: 18 pages, 17 figures, accepted by MNRA

Effects of Radiation Forces on the Frequency of Gravitomagnetic Precession Near Neutron Stars

Gravitomagnetic precession near neutron stars and black holes has received much recent attention, particularly as a possible explanation of 15--60 Hz quasi-periodic brightness oscillations (QPOs) from accreting neutron stars in low-mass X-ray binaries, and of somewhat higher-frequency QPOs from accreting stellar-mass black holes. Previous analyses of this phenomenon have either ignored radiation forces or assumed for simplicity that the radiation field is isotropic, and in particular that there is no variation of the radiation field with angular distance from the rotational equatorial plane of the compact object. However, in most realistic accretion geometries (e.g., those in which the accretion proceeds via a geometrically thin disk) the radiation field depends on latitude. Here we show that in this case radiation forces typically have an important, even dominant, effect on the precession frequency of test particles in orbits that are tilted with respect to the star's rotational equator. Indeed, we find that even for accretion luminosities only a few percent of the Eddington critical luminosity, the precession frequency near a neutron star can be changed by factors of up to $\sim 10$. Radiation forces must therefore be included in analyses of precession frequencies near compact objects, in such varied contexts as low-frequency QPOs, warp modes of disks, and trapped oscillation modes. We discuss specifically the impact of radiation forces on models of low-frequency QPOs involving gravitomagnetic precession, and show that such models are rendered much less plausible by the effects of radiation forces.Comment: 15 pages LaTeX including three figures, submitted to Ap

Remotely forced nearshore upwelling in southern California

[1] Alongshore winds in Baja California strongly influence nearshore temperatures hundreds of kilometers to the north at Point Loma, San Diego, California, on timescales of a week to a year. The time lag between wind and temperature is consistent with first mode coastal trapped wave phase speed. The nearshore cross-shelf circulation forced by the coastal trapped waves is, at least much of the year, oppositely directed at the surface and bottom. No relation is found between the winds and temperature for periods greater than a year. It is argued that similar results may be found elsewhere in the Southern California Bight. The relationship between stratification and bottom temperature varies over the 1.3 years of data, but for much of the time, warmer bottom waters are associated with even warmer surface waters and thus stronger stratification. The effects of the remotely forced cross-shelf exchange on coastal pollution, nutrient dynamics, and larval transport are briefly discussed

Dynamics of wind-driven upwelling and relaxation between Monterey Bay and Point Arena: Local-, regional-, and gyre-scale controls

In north and central California, equatorward winds drive equatorward flows and the upwelling of cold dense water over the shelf during the midspring and summer upwelling season. When the winds temporarily weaken, the upwelling flows between Point Reyes and Point Arena relax,\u27\u27 becoming strongly poleward over the shelf. Analytical and numerical models are used to describe the effect of alongshore variability of winds, bathymetry, and basin-scale pressure gradients on the strength of upwelling and its relaxation. Alongshore winds weaken to the south of Point Reyes, and the shelf becomes narrower from Point Reyes to Monterey Bay. Both of these lead to reduced upwelling at and to the north of Point Reyes, causing an alongshore gradient of temperature and density on the shelf. These alongshore gradients lead to an along-isobath pressure gradient over the shelf that drive the relaxation flows. A simple analytical model is used to explain the dynamics, magnitude, and structure of the relaxation flows. The modeling also suggests that the depth of origin of the upwelled waters, and thus their temperature, is controlled by the along-isobath pressure gradient that exists over the continental slope. This along-slope pressure gradient is also responsible for the California undercurrent in this region. This pressure gradient is not generated in a model of the Californian coast extending from 32 degrees N to 42 degrees N and integrated for several months, suggesting it is caused by dynamics whose spatial or temporal scales are larger than the Californian coast and/or longer than several months

Externally-polluted white dwarfs with dust disks

We report Spitzer Space Telescope photometry of eleven externally-polluted white dwarfs. Of the nine stars for which we have IRAC photometry, we find that GD 40, GD 133 and PG 1015+161 each has an infrared excess that can be understood as arising from a flat, opaque, dusty disk. GD 56 also has an infrared excess characteristic of circumstellar dust, but a flat-disk model cannot reproduce the data unless there are grains as warm as 1700 K and perhaps not even then. Our data support the previous suggestion that the metals in the atmosphere of GD 40 are the result of accretion of a tidally-disrupted asteroid with a chondritic composition.Comment: ApJ, in pres

Radiation induced warping of protostellar accretion disks

We examine the consequences of radiatively driven warping of accretion disks surrounding pre-main-sequence stars. These disks are stable against warping if the luminosity arises from a steady accretion flow, but are unstable at late times when the intrinsic luminosity of the star overwhelms that provided by the disk. Warps can be excited for stars with luminosities of around 10 solar luminosities or greater, with larger and more severe warps in the more luminous systems. A twisted inner disk may lead to high extinction towards stars often viewed through their disks. After the disk at all radii becomes optically thin, the warp decays gradually on the local viscous timescale, which is likely to be long. We suggest that radiation induced warping may account for the origin of the warped dust disk seen in Beta Pictoris, if the star is only around 10-20 Myr old, and could lead to non-coplanar planetary systems around higher mass stars.Comment: 12 pages, including 3 figures. ApJ Letters, in pres