33 research outputs found
A mechanism for Pacific interdecadal resonances
Pacific interdecadal variability (PIV) is an important largeâscale climate phenomenon. There is growing evidence that PIV contains three spectral resonances: a decadal (13 ± 1âyear) spectral peak, bidecadal (20 ± 5âyear) resonance, and a pentadecadal (60 ± 10âyear) resonance. Although much has been clarified about mechanisms behind PIV, there are still many open questions about the origin of these resonant modes (especially the pentadecadal mode). We describe dynamics in the Pacific basin by a toy (delayed oscillator) model that sheds light on the nature of these resonant peaks. The model suggests that unlike the bidecadal resonance, which results from local atmosphereâocean coupling in the extratropics, the pentadecadal and possibly also decadal resonances result from atmospheric and oceanic teleconnections between the extratropics and tropics. We show that a tiny coupling between extratropics and tropics through the ocean tunnel is sufficient to trigger the pentadecadal oscillation in the Pacific basin. Our model also explains (a) the observed threeâperiod locking between the bidecadal and pentadecadal modes and (b) the synchronization of anomalies in the central eastern tropics and central North Pacific with the opposite relative sign. We conclude that the role of oceanic teleconnections is probably underestimated in the current literature on PIV
Quasi-normal modes, area spectra and multi-horizon spacetimes
We suggest an interpretation for the highly damped QNM frequencies of the
spherically symmetric multi-horizon spacetimes (Reissner-Nordstrom,
Schwarzschild-deSitter, Reissner-Nordstrom-deSitter) following Maggiore's
proposal about the link between the asymptotic QNM frequencies and the black
hole thermodynamics. We show that the behavior of the asymptotic frequencies is
easy to understand if one assumes that all of the horizons have the same
equispaced area spectra. The QNM analysis is then consistent with the choice of
the area spectra to be the one originally proposed for the black hole's horizon
by Bekenstein: A=8\pi n (in Planck units). The interpretation of the highly
damped QNM frequencies in the multi-horizon case is based on the similar
grounds as in the single horizon (Schwarzschild) case, but it has some new
features that are discussed in the paper.Comment: 8 pages, v2: no physics changed, some references added, few sentences
added in the discussion part
Generic master equations for quasi-normal frequencies
Generic master equations governing the highly-damped quasi-normal frequencies
[QNFs] of one-horizon, two-horizon, and even three-horizon spacetimes can be
obtained through either semi-analytic or monodromy techniques. While many
technical details differ, both between the semi-analytic and monodromy
approaches, and quite often among various authors seeking to apply the
monodromy technique, there is nevertheless widespread agreement regarding the
the general form of the QNF master equations. Within this class of generic
master equations we can establish some rather general results, relating the
existence of "families" of QNFs of the form omega_{a,n} = (offset)_a + i n
(gap) to the question of whether or not certain ratios of parameters are
rational or irrational.Comment: 23 pages; V2: Minor additions, typos fixed. Matches published versio
The causal structure of spacetime is a parameterized Randers geometry
There is a by now well-established isomorphism between stationary
4-dimensional spacetimes and 3-dimensional purely spatial Randers geometries -
these Randers geometries being a particular case of the more general class of
3-dimensional Finsler geometries. We point out that in stably causal
spacetimes, by using the (time-dependent) ADM decomposition, this result can be
extended to general non-stationary spacetimes - the causal structure (conformal
structure) of the full spacetime is completely encoded in a parameterized
(time-dependent) class of Randers spaces, which can then be used to define a
Fermat principle, and also to reconstruct the null cones and causal structure.Comment: 8 page
Quasi-normal frequencies: Key analytic results
The study of exact quasi-normal modes [QNMs], and their associated
quasi-normal frequencies [QNFs], has had a long and convoluted history -
replete with many rediscoveries of previously known results. In this article we
shall collect and survey a number of known analytic results, and develop
several new analytic results - specifically we shall provide several new QNF
results and estimates, in a form amenable for comparison with the extant
literature. Apart from their intrinsic interest, these exact and approximate
results serve as a backdrop and a consistency check on ongoing efforts to find
general model-independent estimates for QNFs, and general model-independent
bounds on transmission probabilities. Our calculations also provide yet another
physics application of the Lambert W function. These ideas have relevance to
fields as diverse as black hole physics, (where they are related to the damped
oscillations of astrophysical black holes, to greybody factors for the Hawking
radiation, and to more speculative state-counting models for the Bekenstein
entropy), to quantum field theory (where they are related to Casimir energies
in unbounded systems), through to condensed matter physics, (where one may
literally be interested in an electron tunelling through a physical barrier).Comment: V1: 29 pages; V2: Reformatted, 31 pages. Title changed to reflect
major additions and revisions. Now describes exact QNFs for the double-delta
potential in terms of the Lambert W function. V3: Minor edits for clarity.
Four references added. No physics changes. Still 31 page
Ecoregions in the Mediterranean Sea Through the Reanalysis of Phytoplankton Functional Types and Carbon Fluxes
In this work we produced a longâterm reanalysis of the phytoplankton community structure in the Mediterranean Sea and used it to define ecoregions. These were based on the spatial variability of the phytoplankton type fractions and their influence on selected carbon fluxes. A regional ocean color product of four phytoplankton functional types (PFTs; diatoms, dinoflagellates, nanophytoplankton, and picophytoplankton) was assimilated into a coupled physicalâbiogeochemical model of the Mediterranean Sea (Proudman Oceanographic Laboratory Coastal Ocean Modelling SystemâEuropean Regional Seas Ecosystem Model, POLCOMSâERSEM) by using a 100âmember ensemble Kalman filter, in a reanalysis simulation for years 1998â2014. The reanalysis outperformed the reference simulation in representing the assimilated ocean color PFT fractions to total chlorophyll, although the skill for the ocean color PFT concentrations was not improved significantly. The reanalysis did not impact noticeably the reference simulation of not assimilated in situ observations, with the exception of a slight bias reduction for the situ PFT concentrations, and a deterioration of the phosphate simulation. We found that the Mediterranean Sea can be subdivided in three PFTâbased ecoregions, derived from the spatial variability of the PFT fraction dominance or relevance. Picophytoplankton dominates the largest part of open ocean waters; microphytoplankton dominates in a few, highly productive coastal spots near largeâriver mouths; nanophytoplankton is relevant in intermediateâproductive coastal and Atlanticâinfluenced waters. The trophic and carbon sedimentation efficiencies are highest in the microphytoplankton ecoregion and lowest in the picophytoplankton and nanophytoplankton ecoregions. The reanalysis and regionalization offer new perspectives on the variability of the structure and functioning of the phytoplankton community and related biogeochemical fluxes, with foreseeable applications in Blue Growth of the Mediterranean Sea
SST dynamics at different scales: evaluating the oceanographic model resolution skill to represent SST processes in the Southern Ocean
In this study we demonstrate the many strengths of scale analysis: we use it to evaluate the Nucleus for European Modelling of the Ocean (NEMO) model skill in representing sea surface temperature (SST) in the Southern Ocean (SO) by comparing three model resolutions: 1/12°, 1/4° and 1°. We show that whilst 4â5 times resolution scale is sufficient for each model resolution to reproduce the magnitude of satellite Earth Observation (EO) SST spatial variability to within ±10%, the representation of ⌠100 km SST variability patterns is substantially (e.g âŒ50% at 750 km) improved by increasing model resolution from 1° to 1/12°. We also analysed the dominant scales of the SST model input drivers (shortâwave radiation, airâsea heat fluxes, wind stress components, wind stress curl, bathymetry) variability with the purpose of determining the optimal SST model input driver resolution. The SST magnitude of variability is shown to scale with two power law regimes separated by a scaling break at âŒ200 km scale. The analysis of the spatial and temporal scales of dominant SST driver impact helps to interpret this scaling break as a separation between two different dynamical regimes: the (relatively) fast SST dynamics below âŒ200 km governed by eddies, fronts, Ekman upwelling and airâsea heat exchange, whilst above âŒ200 km the SST variability is dominated by longâterm (seasonal and supraâseasonal) modes and the SST geography
Stringy Space-Time Foam and High-Energy Cosmic Photons
In this review, I discuss briefly stringent tests of Lorentz-violating
quantum space-time foam models inspired from String/Brane theories, provided by
studies of high energy Photons from intense celestial sources, such as Active
Galactic Nuclei or Gamma Ray Bursts. The theoretical models predict
modifications to the radiation dispersion relations, which are quadratically
suppressed by the string mass scale, and time delays in the arrival times of
photons (assumed to be emitted more or less simultaneously from the source),
which are proportional to the photon energy, so that the more energetic photons
arrive later. Although the astrophysics at the source of these energetic
photons is still not understood, and such non simultaneous arrivals, that have
been observed recently, might well be due to non simultaneous emission as a
result of conventional physics effects, nevertheless, rather surprisingly, the
observed time delays can also fit excellently the stringy space-time foam
scenarios, provided the space-time defect foam is inhomogeneous. The key
features of the model, that allow it to evade a plethora of astrophysical
constraints on Lorentz violation, in sharp contrast to other field-theoretic
Lorentz-violating models of quantum gravity, are: (i) transparency of the foam
to electrons and in general charged matter, (ii) absence of birefringence
effects and (iii) a breakdown of the local effective lagrangian formalism.Comment: 26 pages Latex, 4 figures, uses special macros. Keynote Lecture in
the International Conference "Recent Developments in Gravity" (NEB14),
Ioannina (Greece) June 8-11 201
Semi-analytic results for quasi-normal frequencies
The last decade has seen considerable interest in the quasi-normal
frequencies [QNFs] of black holes (and even wormholes), both asymptotically
flat and with cosmological horizons. There is wide agreement that the QNFs are
often of the form omega_n = (offset) + i n (gap), though some authors have
encountered situations where this behaviour seems to fail. To get a better
understanding of the general situation we consider a semi-analytic model based
on a piecewise Eckart (Poeschl-Teller) potential, allowing for different
heights and different rates of exponential falloff in the two asymptotic
directions. This model is sufficiently general to capture and display key
features of the black hole QNFs while simultaneously being analytically
tractable, at least for asymptotically large imaginary parts of the QNFs. We
shall derive an appropriate "quantization condition" for the asymptotic QNFs,
and extract as much analytic information as possible. In particular, we shall
explicitly verify that the (offset)+ i n (gap) behaviour is common but not
universal, with this behaviour failing unless the ratio of rates of exponential
falloff on the two sides of the potential is a rational number. (This is
"common but not universal" in the sense that the rational numbers are dense in
the reals.) We argue that this behaviour is likely to persist for black holes
with cosmological horizons.Comment: V1: 28 pages, no figures. V2: 3 references added, no physics changes.
V3: 29 pages, 9 references added, no physics changes; V4: reformatted, now 27
pages. Some clarifications, comparison with results obtained by monodromy
techniques. This version accepted for publication in JHEP. V5: Minor typos
fixed. Compatible with published versio
Propagation of light in area metric backgrounds
The propagation of light in area metric spacetimes, which naturally emerge as
refined backgrounds in quantum electrodynamics and quantum gravity, is studied
from first principles. In the geometric-optical limit, light rays are found to
follow geodesics in a Finslerian geometry, with the Finsler norm being
determined by the area metric tensor. Based on this result, and an
understanding of the non-linear relation between ray vectors and wave covectors
in such refined backgrounds, we study light deflection in spherically symmetric
situations, and obtain experimental bounds on the non-metricity of spacetime in
the solar system.Comment: 18pp, no figures, Journal versio