99 research outputs found
A study of the sensitivity of ENSO to the mean climate
International audienceWe study the dependence of the simulated ENSO on the mean simulated climate in the HadCM3 GCM and attempt to understand its relation with results from intermediate-complexity models (ICMs). Our aim is to bridge an existing gap between results from complex GCMs and from more readily understandable ICMs, and thereby to improve our process-based prediction skills of the potential sensitivity of observed ENSO properties (amplitude, frequency and pattern) to climate change. Although there is a suggestion that surface ENSO processes are dominating the response in HadCM3, our work also shows that the complex changes in simulated climate can have contrasting effects on the ENSO and the net result may not be robust
Mass-loaded spherical accretion flows
We have calculated the evolution of spherical accretion flows undergoing mass-loading from embedded clouds through either conduction or hydrodynamical ablation. We have observed the effect of varying the ratios of the mass-loading timescale and the cooling timescale to the ballistic crossing timescale through the mass-loading region.
We have also varied the ratio of the potential energy of a particle injected into the flow near the outer region of mass-loading to the temperature at which a minimum occurs in the cooling curve. The two types of mass-loading produce qualitatively different types of behaviour in the accretion flow, since mass-loading through conduction requires the ambient gas to be hot, whereas mass ablation from clumps occurs throughout the flow. Higher ratios of injected to accreted mass typically occur with hydrodynamical ablation, in agreement with previous work on wind-blown bubbles and supernova remnants. We find that mass-loading damps the radiative overstability of such flows, in agreement with our earlier work. If the mass-loading is high enough it can stabilize the accretion shock at a constant radius, yielding an almost isothermal subsonic post-shock flow. Such solutions may be relevant to cooling flows onto massive galaxies. Mass-loading can also lead to the formation of isolated shells of high temperature material, separated by gas at cooler temperatures
The effects of ram-pressure stripping on the internal kinematics of simulated spiral galaxies
We investigate the influence of ram-pressure stripping on the internal gas
kinematics of simulated spiral galaxies. Additional emphasis is put on the
question of how the resulting distortions of the gaseous disc are visible in
the rotation curve and/or the full 2D velocity field of galaxies at different
redshifts. A Milky-Way type disc galaxy is modelled in combined
N-body/hydrodynamic simulations with prescriptions for cooling, star formation,
stellar feedback, and galactic winds. This model galaxy moves through a
constant density and temperature gas, which has parameters similar to the
intra-cluster medium (ICM). Rotation curves (RCs) and 2D velocity fields of the
gas are extracted from these simulations in a way that follows the procedure
applied to observations of distant, small, and faint galaxies as closely as
possible. We find that the appearance of distortions of the gaseous disc due to
ram-pressure stripping depends on the direction of the acting ram pressure. In
the case of face-on ram pressure, the distortions mainly appear in the outer
parts of the galaxy in a very symmetric way. In contrast, in the case of
edge-on ram pressure we find stronger distortions. The 2D velocity field also
shows signatures of the interaction in the inner part of the disc. At angles
smaller than 45 degrees between the ICM wind direction and the disc, the
velocity field asymmetry increases significantly compared to larger angles.
Compared to distortions caused by tidal interactions, the effects of
ram-pressure stripping on the velocity field are relatively low in all cases
and difficult to observe at intermediate redshift in seeing-limited
observations. (abridged)Comment: 9 pages, 11 figures, accepted for publication in A&
On the influence of ram-pressure stripping on the star formation of simulated spiral galaxies
We investigate the influence of ram-pressure stripping on the star formation
and the mass distribution in simulated spiral galaxies. Special emphasis is put
on the question where the newly formed stars are located. The stripping radius
from the simulation is compared to analytical estimates. Disc galaxies are
modelled in combined N-body/hydrodynamic simulations (GADGET-2) with
prescriptions for cooling, star formation, stellar feedback, and galactic
winds. These model galaxies move through a constant density and temperature
gas, which has parameters comparable to the intra-cluster medium (ICM) in the
outskirts of a galaxy cluster (T=3 keV ~3.6x10^7 K and rho=10^-28 g/cm^3). With
this numerical setup we analyse the influence of ram-pressure stripping on the
star formation rate of the model galaxy. We find that the star formation rate
is significantly enhanced by the ram-pressure effect (up to a factor of 3).
Stars form in the compressed central region of the galaxy as well as in the
stripped gas behind the galaxy. Newly formed stars can be found up to hundred
kpc behind the disc, forming structures with sizes of roughly 1 kpc in diameter
and with masses of up to 10^7 M_sun. As they do not possess a dark matter halo
due to their formation history, we name them 'stripped baryonic dwarf'
galaxies. We also find that the analytical estimate for the stripping radius
from a Gunn & Gott (1972) criterion is in good agreement with the numerical
value from the simulation. Like in former investigations, edge-on systems lose
less gas than face-on systems and the resulting spatial distribution of the gas
and the newly formed stars is different.Comment: 8 pages, 7 figures, accepted for publication in A&
Self-similar evolution of wind-blown bubbles with mass loading by hydrodynamic ablation
We present similarity solutions for adiabatic bubbles that are blown by winds
having time independent mechanical luminosities and that are each mass-loaded
by the hydrodynamic ablation of distributed clumps. The mass loading is
`switched-on' at a specified radius (with free-expansion of the wind interior
to this point) and injects mass at a rate per unit volume proportional to
M^delta r^lambda where delta = 4/3 (1) if the flow is subsonic (supersonic)
with respect to the clumps. In the limit of negligible mass loading a
similarity solution found by Dyson (1973) for expansion into a smooth ambient
medium is recovered. The presence of mass loading heats the flow, which leads
to a reduction in the Mach number of the supersonic freely-expanding flow, and
weaker jump conditions across the inner shock. In solutions with large mass
loading, it is possible for the wind to connect directly to the contact
discontinuity without first passing through an inner shock. For a solution that
gives the mass of swept-up ambient gas to be less than the sum of the masses of
the wind and ablated material, lambda < -2. Maximum possible values for the
ratio of ablated mass to wind mass occur when mass loading starts very close to
the bubble center and when the flow is supersonic with respect to the clumps
over the entire bubble radius. The maximum temperature in the bubble often
occurs near the onset of mass loading, and in some cases can be many times
greater than the post-inner-shock temperature. Our solutions are relevant to eg
stellar wind-blown bubbles, galactic winds, etc. This work complements Pittard
et al (2001) where it was assumed that clumps were evaporated through
conductive energy transport.Comment: 13 pages, 7 figures, to be published in A&
Description and evaluation of NorESM1-F: a fast version of the Norwegian Earth System Model (NorESM)
A new computationally efficient version of the Norwegian Earth System Model
(NorESM) is presented. This new version (here termed NorESM1-F) runs about
2.5Â times faster (e.g., 90 model years per day on current hardware) than the
version that contributed to the fifth phase of the Coupled Model
Intercomparison project (CMIP5), i.e., NorESM1-M, and is therefore
particularly suitable for multimillennial paleoclimate and carbon cycle
simulations or large ensemble simulations. The speed-up is primarily a result
of using a prescribed atmosphere aerosol chemistry and a tripolar ocean–sea
ice horizontal grid configuration that allows an increase of the ocean–sea
ice component time steps. Ocean biogeochemistry can be activated for fully
coupled and semi-coupled carbon cycle applications. This paper describes the
model and evaluates its performance using observations and NorESM1-M as
benchmarks. The evaluation emphasizes model stability, important large-scale
features in the ocean and sea ice components, internal variability in the
coupled system, and climate sensitivity. Simulation results from NorESM1-F in
general agree well with observational estimates and show evident
improvements over NorESM1-M, for example, in the strength of the meridional
overturning circulation and sea ice simulation, both important metrics in
simulating past and future climates. Whereas NorESM1-M showed a slight global
cool bias in the upper oceans, NorESM1-F exhibits a global warm bias. In
general, however, NorESM1-F has more similarities than dissimilarities
compared to NorESM1-M, and some biases and deficiencies known in NorESM1-M
remain.</p
Daisyworld: a review
Daisyworld is a simple planetary model designed to show the long-term effects of coupling between life and its environment. Its original form was introduced by James Lovelock as a defense against criticism that his Gaia theory of the Earth as a self-regulating homeostatic system requires teleological control rather than being an emergent property. The central premise, that living organisms can have major effects on the climate system, is no longer controversial. The Daisyworld model has attracted considerable interest from the scientific community and has now established itself as a model independent of, but still related to, the Gaia theory. Used widely as both a teaching tool and as a basis for more complex studies of feedback systems, it has also become an important paradigm for the understanding of the role of biotic components when modeling the Earth system. This paper collects the accumulated knowledge from the study of Daisyworld and provides the reader with a concise account of its important properties. We emphasize the increasing amount of exact analytic work on Daisyworld and are able to bring together and summarize these results from different systems for the first time. We conclude by suggesting what a more general model of life-environment interaction should be based on
Metal Enrichment Processes in the Intra-Cluster Medium
We present numerical simulations of galaxy clusters which include interaction
processes between the galaxies and the intra-cluster gas. The considered
interaction processes are galactic winds and ram-pressure stripping, which both
transfer metal-enriched interstellar medium into the intra-cluster gas and
hence increase its metallicity. We investigate the efficiency and time
evolution of the interaction processes by simulated metallicity maps, which are
directly comparable to those obtained from X-ray observations. We find that
ram-pressure stripping is more efficient than quiet (i.e. non-starburst driven)
galactic winds in the redshift interval between 1 and 0. The expelled metals
are not mixed immediately with the intra-cluster gas, but inhomogeneities are
visible in the metallicity maps. Even stripes of higher metallicity that a
single galaxy has left behind can be seen. The spatial distribution of the
metals transported by ram-pressure stripping and by galactic winds are very
different for massive clusters: the former process yields a centrally
concentrated metal distribution while the latter results in an extended metal
distribution.Comment: accepted for publication in A&A Letters, 4 pages, 2 figure
Metal enrichment processes
There are many processes that can transport gas from the galaxies to their
environment and enrich the environment in this way with metals. These metal
enrichment processes have a large influence on the evolution of both the
galaxies and their environment. Various processes can contribute to the gas
transfer: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy
interactions and others. We review their observational evidence, corresponding
simulations, their efficiencies, and their time scales as far as they are known
to date. It seems that all processes can contribute to the enrichment. There is
not a single process that always dominates the enrichment, because the
efficiencies of the processes vary strongly with galaxy and environmental
properties.Comment: 18 pages, 8 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 17; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
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