633 research outputs found
Forcing of Northern Hemisphere climate trends
The impact of observed global SST trends during the second half of the twentieth century on the Northern Hemisphere extratropical winter atmospheric circulation is investigated using ensembles of simulations with the Center for Ocean - Land - Atmosphere Studies ( COLA) atmospheric GCM. In contrast to some other studies, the simulated ensemble mean 500-hPa trends in the North Atlantic sector do not resemble the observed trend. However, the intraensemble variability of the trends is large, with the dominant structure of that variability resembling the Arctic Oscillation "annular mode.'' The model results are consistent with the interpretation that the observed trend is dominated by the forced signal in the Pacific - North America sector, while over the rest of the Northern Hemisphere, and especially the North Atlantic sector, the trend is primarily interdecadal timescale internal atmospheric noise with an annular structure.In order to diagnose the origins of the forced component of the model trend, a series of equilibrium response simulations is performed using constant-in-time SST anomalies with the structure of the trend superimposed on the annually varying climatological SST. It is found that the SST trend in the latitude belt from 20degreesS to 20degreesN is responsible for forcing much of the extratropical trend, and that the dominant tropical forcing is the SST trend in the Indian Ocean/western Pacific and eastern Pacific sectors. The idealized experiments show that the precipitation response in the Tropics is linearly related to the SST trend, and that the NH December - January February height response to SST anomalies in various regions is quasi-linear.Some additional analysis and interpretation is given. The extratropical response to low-latitude SST trends in the idealized experiments has characteristics reminiscent of Rossby wave trains forced by tropical deep convection. The intraensemble variability in the model's extratropical zonal mean height trend, which cannot be explained by external forcing, appears to be due to variability in the trends of midlatitude eddy stirring. The observed zonal mean trend also shows evidence of forcing by trends in the eddy stirring
Disentanglement of two harmonic oscillators in relativistic motion
We study the dynamics of quantum entanglement between two Unruh-DeWitt
detectors, one stationary (Alice), and another uniformly accelerating (Rob),
with no direct interaction but coupled to a common quantum field in (3+1)D
Minkowski space. We find that for all cases studied the initial entanglement
between the detectors disappears in a finite time ("sudden death"). After the
moment of total disentanglement the correlations between the two detectors
remain nonzero until late times. The relation between the disentanglement time
and Rob's proper acceleration is observer dependent. The larger the
acceleration is, the longer the disentanglement time in Alice's coordinate, but
the shorter in Rob's coordinate.Comment: 16 pages, 8 figures; typos added, minor changes in Secs. I and
Dynamics of Atom-Field Entanglement from Exact Solutions: Towards Strong Coupling and Non-Markovian Regimes
We examine the dynamics of bipartite entanglement between a two-level atom
and the electromagnetic field. We treat the Jaynes-Cummings model with a single
field mode and examine in detail the exact time evolution of entanglement,
including cases where the atomic state is initially mixed and the atomic
transition is detuned from resonance. We then explore the effects of other
nearby modes by calculating the exact time evolution of entanglement in more
complex systems with two, three, and five field modes. For these cases we can
obtain exact solutions which include the strong coupling regimes. Finally, we
consider the entanglement of a two-level atom with the infinite collection of
modes present in the intracavity field of a Fabre-Perot cavity. In contrast to
the usual treatment of atom-field interactions with a continuum of modes using
the Born-Markov approximation, our treatment in all cases describes the full
non-Markovian dynamics of the atomic subsystem. Only when an analytic
expression for the infinite mode case is desired do we need to make a weak
coupling assumption which at long times approximates Markovian dynamics.Comment: 12 pages, 5 figures; minor changes in grammar, wording, and
formatting. One unnecessary figure removed. Figure number revised (no longer
counts subfigures separately
Intensified Asian Summer Monsoon and its variability in a coupled model forced by increasing greenhouse gas concentrations
The Asian summer monsoon response to global warming is investigated by a transient green‐house warming integration with the ECHAM4/OPYC3 CGCM. It is demonstrated that increases of greenhouse gas concentrations intensify the Asian summer monsoon and its variability. The intensified monsoon results mainly from an enhanced land‐sea contrast and a northward shift of the convergence zone. A gradual increase of the monsoon variability is simulated from year 2030 onwards. It seems to be connected with the corresponding increase of the sea surface temperature variability over the tropical Pacific
Dynamical and cloud-radiation feedbacks in El Nino and greenhouse warming
An El Niño‐like steady response is found in a greenhouse warming simulation resulting from coupled ocean‐atmosphere dynamical feedbacks similar to those producing the present‐day El Niños. There is a strong negative cloud‐radiation feedback on the sea surface temperature (SST) anomaly associated with this enhanced eastern equatorial Pacific warm pattern. However, this negative feedback is overwhelmed by the positive dynamical feedbacks and cannot diminish the sensitivity of the tropical SST to enhanced greenhouse gas concentrations. The enhanced eastern‐Pacific warming in the coupled ocean‐atmosphere system suggests that coupled dynamics can strengthen this sensitivity
Notes on entropic characteristics of quantum channels
One of most important issues in quantum information theory concerns
transmission of information through noisy quantum channels. We discuss few
channel characteristics expressed by means of generalized entropies. Such
characteristics can often be dealt in line with more usual treatment based on
the von Neumann entropies. For any channel, we show that the -average output
entropy of degree is bounded from above by the -entropy of the
input density matrix. Concavity properties of the -entropy exchange are
considered. Fano type quantum bounds on the -entropy exchange are
derived. We also give upper bounds on the map -entropies in terms of the
output entropy, corresponding to the completely mixed input.Comment: 10 pages, no figures. The statement of Proposition 1 is explicitly
illustrated with the depolarizing channel. The bibliography is extended and
updated. More explanations. To be published in Cent. Eur. J. Phy
Pulsatile blood flow, shear force, energy dissipation and Murray's Law
BACKGROUND: Murray's Law states that, when a parent blood vessel branches into daughter vessels, the cube of the radius of the parent vessel is equal to the sum of the cubes of the radii of daughter blood vessels. Murray derived this law by defining a cost function that is the sum of the energy cost of the blood in a vessel and the energy cost of pumping blood through the vessel. The cost is minimized when vessel radii are consistent with Murray's Law. This law has also been derived from the hypothesis that the shear force of moving blood on the inner walls of vessels is constant throughout the vascular system. However, this derivation, like Murray's earlier derivation, is based on the assumption of constant blood flow. METHODS: To determine the implications of the constant shear force hypothesis and to extend Murray's energy cost minimization to the pulsatile arterial system, a model of pulsatile flow in an elastic tube is analyzed. A new and exact solution for flow velocity, blood flow rate and shear force is derived. RESULTS: For medium and small arteries with pulsatile flow, Murray's energy minimization leads to Murray's Law. Furthermore, the hypothesis that the maximum shear force during the cycle of pulsatile flow is constant throughout the arterial system implies that Murray's Law is approximately true. The approximation is good for all but the largest vessels (aorta and its major branches) of the arterial system. CONCLUSION: A cellular mechanism that senses shear force at the inner wall of a blood vessel and triggers remodeling that increases the circumference of the wall when a shear force threshold is exceeded would result in the observed scaling of vessel radii described by Murray's Law
Relations for certain symmetric norms and anti-norms before and after partial trace
Changes of some unitarily invariant norms and anti-norms under the operation
of partial trace are examined. The norms considered form a two-parametric
family, including both the Ky Fan and Schatten norms as particular cases. The
obtained results concern operators acting on the tensor product of two
finite-dimensional Hilbert spaces. For any such operator, we obtain upper
bounds on norms of its partial trace in terms of the corresponding
dimensionality and norms of this operator. Similar inequalities, but in the
opposite direction, are obtained for certain anti-norms of positive matrices.
Through the Stinespring representation, the results are put in the context of
trace-preserving completely positive maps. We also derive inequalities between
the unified entropies of a composite quantum system and one of its subsystems,
where traced-out dimensionality is involved as well.Comment: 11 pages, no figures. A typo error in Eq. (5.15) is corrected. Minor
improvements. J. Stat. Phys. (in press
Water vapor and the dynamics of climate changes
Water vapor is not only Earth's dominant greenhouse gas. Through the release
of latent heat when it condenses, it also plays an active role in dynamic
processes that shape the global circulation of the atmosphere and thus climate.
Here we present an overview of how latent heat release affects atmosphere
dynamics in a broad range of climates, ranging from extremely cold to extremely
warm. Contrary to widely held beliefs, atmospheric circulation statistics can
change non-monotonically with global-mean surface temperature, in part because
of dynamic effects of water vapor. For example, the strengths of the tropical
Hadley circulation and of zonally asymmetric tropical circulations, as well as
the kinetic energy of extratropical baroclinic eddies, can be lower than they
presently are both in much warmer climates and in much colder climates. We
discuss how latent heat release is implicated in such circulation changes,
particularly through its effect on the atmospheric static stability, and we
illustrate the circulation changes through simulations with an idealized
general circulation model. This allows us to explore a continuum of climates,
constrain macroscopic laws governing this climatic continuum, and place past
and possible future climate changes in a broader context.Comment: 22 pages, 11 figure
Daily life stress and the cortisol awakening response : testing the anticipation hypothesis
Acknowledgments We thank Paul Stewart for his contribution to data collection and Dr Matthew Jones for programming the handheld computers. Author Contributions Conceived and designed the experiments: WS DJP. Performed the experiments: DJP. Analyzed the data: WS. Wrote the paper: WS DJP.Peer reviewedPublisher PD
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