789 research outputs found
Seasonal Antarctic pressure variability during the twentieth century from spatially complete reconstructions and CAM5 simulations
As most permanent observations in Antarctica started in the 1950s, understanding Antarctic climate variations throughout the twentieth century remains a challenge. To address this issue, the non-summer multi-decadal variability in pressure reconstructions poleward of 60°S is evaluated and assessed in conjunction with climate model simulations throughout the twentieth and early twenty-first centuries to understand historical atmospheric circulation variability over Antarctica. Austral autumn and winter seasons show broadly similar patterns, with negative anomalies in the early twentieth century (1905–1934), positive pressure anomalies in the middle twentieth century (1950–1980), and negative pressure anomalies in the most recent period (1984–2013), consistent with concurrent trends in the SAM index. In autumn, the anomalies are significant in the context of estimates of interannual variability and reconstruction uncertainty across most of the Antarctic continent, and the reconstructed patterns agree best with model-generated patterns when the simulation includes the forced response to tropical sea surface temperatures and external radiative forcing. In winter and spring, the reconstructed anomalies are less significant and are consistent with internal atmospheric variability alone. The specific role of tropical SST variability on pressure trends in these seasons is difficult to assess due to low reconstruction skill in the region of strongest tropical teleconnections, the large internal atmospheric variability, and uncertainty in the SST patterns themselves. Indirect estimates of pressure variability, whether through sea ice reconstructions, proxy records, or improved models and data assimilation schemes, will help to further constrain the magnitude of internal variability relative to the forced responses expected from SST trends and external radiative forcing
Computer-Assisted Proofs of Some Identities for Bessel Functions of Fractional Order
We employ computer algebra algorithms to prove a collection of identities
involving Bessel functions with half-integer orders and other special
functions. These identities appear in the famous Handbook of Mathematical
Functions, as well as in its successor, the DLMF, but their proofs were lost.
We use generating functions and symbolic summation techniques to produce new
proofs for them.Comment: Final version, some typos were corrected. 21 pages, uses svmult.cl
THE AMUNDSEN SEA LOW Variability, Change, and Impact on Antarctic Climate
The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases
HP-LT rocks exhumed during intra-oceanic subduction: the example of the Escambray massif (Cuba).
High-Presssure metabasites embedded in a serpentinite or metasedimentary matrix from the Sancti Spiritus dome (Escambray massif, Central Cuba) have been studied in order to better understand the origine and the evolution of the Northern Carribean boundary plate during the Cretaceous, in a global subduction context. Geochemical analyses (major, trace elements and isotopes) of the high pressure rocks show that they could be partially derived from the Cretaceous calc-alkaline arc described in Central Cuba, these were probably incorporated in the subduction zone by tectonic erosion. The High-Pressure rocks record a prograde path from the epidote bearing amphibolite facies to the barroisite bearing eclogite facies (P = 19 ± 2 Kbar, T = 590 ± 90 °C). These metabasites show evidence of retrogression starting from the glaucophane bearing eclogite facies to the lawsonite bearing blueschist facies. Therefore, these HP/LT rocks are characterized by a counter-clockwise cooling P/T path, which can be explained by the exhumation of HP rocks while the subduction was still active. Concordant geochronological data (Rb/Sr and Ar/Ar) suggest that the main exhumation of HP/LT rocks from the Sancti Spiritus dome occurred 70 Ma ago by top to SW thrusting. The retrogressed trajectory of these rocks, means that the northeast subduction of the Farallon plate continued after 70Ma. The final exhumation can be correlated with the beginning of the collision between the Bahamas platform and the Cretaceous island arc that induced a change of the subduction kinematic
Itinerant electron metamagnetism in LaCoSi
The strongly exchange enhanced Pauli paramagnet LaCoSi is found to
exhibit an itinerant metamagnetic phase transition with indications for
metamagnetic quantum criticality. Our investigation comprises magnetic,
specific heat, and NMR measurements as well as ab-initio electronic structure
calculations. The critical field is about 3.5 T for and 6 T for , which is the lowest value ever found for rare earth intermetallic
compounds. In the ferromagnetic state there appears a moment of about 0.2
/Co at the Co-sites, but sigificantly smaller moments at the 4d
and Co-sites.Comment: 11 pages, 5 figures, PRB Rapid Communication, in prin
A twentieth century perspective on summer Antarctic pressure change and variability and contributions from tropical SSTs and ozone depletion
During the late 20th 33 Century, the Antarctic atmospheric circulation has changed
and significantly influenced the overall Antarctic climate, through processes including a
poleward shift of the circumpolar westerlies. However, little is known about the full
spatial pattern of atmospheric pressure over the Antarctic continent prior to 1979. Here
we investigate surface pressure changes across the entire Antarctic continent back to
1905 by developing a new summer pressure reconstruction poleward of 60°S. We find
that only across East Antarctica are the recent pressures significantly lower than pressures
in the early 20th 40 century; we also discern periods of significant positive pressure trends in
the early 20th 41 century across the coastal South Atlantic sector of Antarctica. Climate
model simulations reveal that both tropical sea surface temperature variability and other
radiative forcing mechanisms, in addition to ozone depletion, have played an important
role in forcing the recent observed negative trends
Motional effects of single trapped atomic/ionic qubit
We investigate theoretical decoherence effects of the motional degrees of
freedom of a single trapped atomic/ionic electronically coded qubit. For single
bit rotations from a resonant running wave laser field excitation, we found the
achievable fidelity to be determined by a single parameter characterized by the
motional states. Our quantitative results provide a useful realistic view for
current experimental efforts in quantum information and computing.Comment: 3 fig
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
A guided tour of asynchronous cellular automata
Research on asynchronous cellular automata has received a great amount of
attention these last years and has turned to a thriving field. We survey the
recent research that has been carried out on this topic and present a wide
state of the art where computing and modelling issues are both represented.Comment: To appear in the Journal of Cellular Automat
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
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