Heat balance and eddies in the Peru-Chile current system   总被引：2，自引：1，他引：1  

François Colas  James C. McWilliams  Xavier Capet  Jaison Kurian 《Climate Dynamics》2012,39(1-2):509-529

The Peru-Chile current System (PCS) is a region of persistent biases in global climate models. It has strong coastal upwelling, alongshore boundary currents, and mesoscale eddies. These oceanic phenomena provide essential heat transport to maintain a cool oceanic surface underneath the prevalent atmospheric stratus cloud deck, through a combination of mean circulation and eddy flux. We demonstrate these behaviors in a regional, quasi-equilibrium oceanic model that adequately resolves the mesoscale eddies with climatological forcing. The key result is that the atmospheric heating is large (>50 W m^?2) over a substantial strip >500 km wide off the coast of Peru, and the balancing lateral oceanic flux is much larger than provided by the offshore Ekman flux alone. The atmospheric heating is weaker and the coastally influenced strip is narrower off Chile, but again the Ekman flux is not sufficient for heat balance. The eddy contribution to the oceanic flux is substantial. Analysis of eddy properties shows strong surface temperature fronts and associated large vorticity, especially off Peru. Cyclonic eddies moderately dominate the surface layer, and anticyclonic eddies, originating from the nearshore poleward Peru-Chile Undercurrent (PCUC), dominate the subsurface, especially off Chile. The sensitivity of the PCS heat balance to equatorial intra-seasonal oscillations is found to be small. We demonstrate that forcing the regional model with a representative, coarse-resolution global reanalysis wind product has dramatic and deleterious consequences for the oceanic circulation and climate heat balance, the eddy heat flux in particular.  相似文献   

Strategies to mitigate aliasing of loading signals while estimating GPS frame parameters   总被引：7，自引：5，他引：2  

Xavier?CollilieuxEmail author  Tonie?van?Dam  Jim?Ray  David?Coulot  Laurent?Métivier  Zuheir?Altamimi 《Journal of Geodesy》2012,86(1):1-14

Although GNSS techniques are theoretically sensitive to the Earth center of mass, it is often preferable to remove intrinsic origin and scale information from the estimated station positions since they are known to be affected by systematic errors. This is usually done by estimating the parameters of a linearized similarity transformation which relates the quasi-instantaneous frames to a long-term frame such as the International Terrestrial Reference Frame (ITRF). It is well known that non-linear station motions can partially alias into these parameters. We discuss in this paper some procedures that may allow reducing these aliasing effects in the case of the GPS techniques. The options include the use of well-distributed sub-networks for the frame transformation estimation, the use of site loading corrections, a modification of the stochastic model by downweighting heights, or the joint estimation of the low degrees of the deformation field. We confirm that the standard approach consisting of estimating the transformation over the whole network is particularly harmful for the loading signals if the network is not well distributed. Downweighting the height component, using a uniform sub-network, or estimating the deformation field perform similarly in drastically reducing the amplitude of the aliasing effect. The application of these methods to reprocessed GPS terrestrial frames permits an assessment of the level of agreement between GPS and our loading model, which is found to be about 1.5 mm WRMS in height and 0.8 mm WRMS in the horizontal at the annual frequency. Aliased loading signals are not the main source of discrepancies between loading displacement models and GPS position time series.  相似文献   

Eddy formation in the bays of Kamchatka and fluxes to the open ocean     

L’Her  Alexandre  Reinert  Markus  Prants  Sergey  Carton  Xavier  Morvan  Mathieu 《Ocean Dynamics》2021,71(5):601-612

Ocean Dynamics - The Eastern Kamchatka Current (EKC) is the western boundary current of the North Pacific subpolar gyre. Southeast of the Kamchatka Peninsula lies a large anticyclonic eddy, the...  相似文献   

Microlensing by cosmic strings     

Konrad Kuijken  Xavier Siemens  Tanmay Vachaspati 《Monthly notices of the Royal Astronomical Society》2008,384(1):161-164

We consider the signature and detectability of gravitational microlensing of distant quasars by cosmic strings. Because of the simple image configuration such events will have a characteristic lightcurve, in which a source would appear to brighten by exactly a factor of 2, before reverting to its original apparent brightness. We calculate the optical depth and event rate, and conclude that current predictions and limits on the total length of strings on the sky imply optical depths of ≲ 10⁻⁸ and event rates of fewer than one event per 10⁹ sources per year. Disregarding those predictions but replacing them with limits on the density of cosmic strings from the cosmic microwave background fluctuation spectrum, leaves only a small region of parameter space (in which the sky contains about 3 × 10⁵ strings with deficit angle of the order of 0.3 milli-seconds) for which a microlensing survey of exposure 10⁷ source years, spanning a 20–40-year period, might reveal the presence of cosmic strings.  相似文献   

Spectral simulation of vector random fields with stationary Gaussian increments in <Emphasis Type="Italic">d</Emphasis>-dimensional Euclidean spaces     

Daisy?Arroyo  Xavier?EmeryEmail author 《Stochastic Environmental Research and Risk Assessment (SERRA)》2017,31(7):1583-1592

This paper addresses the problem of simulating multivariate random fields with stationary Gaussian increments in a d-dimensional Euclidean space. To this end, one considers a spectral turning-bands algorithm, in which the simulated field is a mixture of basic random fields made of weighted cosine waves associated with random frequencies and random phases. The weights depend on the spectral density of the direct and cross variogram matrices of the desired random field for the specified frequencies. The algorithm is applied to synthetic examples corresponding to different spatial correlation models. The properties of these models and of the algorithm are discussed, highlighting its computational efficiency, accuracy and versatility.  相似文献   

Process-based 2DH morphodynamic modeling of tidal inlets: A comparison with empirical classifications and theories     

Alphonse Nahon  Xavier Bertin  André B. Fortunato  Anabela Oliveira 《Marine Geology》2012

The economical and ecological importance of tidal inlets has fostered the development of empirical tools for inlet management during the last century. This study aims at confronting these empirical theories with results obtained with a process-based numerical model, MORSYS2D. This 2DH morphodynamic modeling system is applied to an idealized tidal inlet/lagoon system with different combinations of significant wave height, tidal range and tidal prism. The numerical model predictions are compared to the empirical models of Hayes, Bruun, O'Brien and FitzGerald and to morphologies observed at natural tidal inlets. The results present good accordance with observations as well as with some key behaviors predicted with the empirical theories. The predicted morphologies satisfy the relation of O'Brien between the tidal prism and the cross-sectional area, the model reproduces the conceptual model of sand by-passing by ebb-tidal delta breaching of FitzGerald and the classifications of Hayes and Bruun are generally respected. However, some inconsistencies between model results and Hayes classification highlight the limitations of applying this classification, which only considers the yearly-averaged significant wave height and tidal range, to a single tidal inlet case.  相似文献   

Relationships between southern Chilean varved lake sediments,precipitation and ENSO for the last 600 years     

Xavier Boës  Nathalie Fagel 《Journal of Paleolimnology》2008,39(2):237-252

In this paper, the relationships between paleo-precipitation and the regional influence of El Nino Southern Oscillation (ENSO) in South America are assessed from a high-resolution calendar varve-thickness record. Two short laminated sediment cores (53 and 61 cm length) from Lago Puyehue (40° S) are analysed by continuous varve measurements through the last 600 years. The calendar varve years are determined by the occurrence of graded planktonic-rich layers. The annual sediment accumulation rates are reconstructed by using the standard varve-counting methods on thin sections. The 1980–2000 varve-thickness record is interpreted in terms of climate through correlation with limnological and local monthly instrumental climate databases. The comparison between the standardized varve thickness with the instrumental records reveals a strong correlation (r = 0.75, р = 0.07) between the total varve thickness and the austral autumn/winter precipitation. We argue that strong austral winter winds and precipitation are the forcing factors for the seasonal turn-over and phytoplankton increase in the lake sediments. During strong El Nino events the precipitation and the winds decrease abnormally, hence reducing the thickness of the biogenic sediments deposited after the winter turn-over. Our results show one significant regional maximum peak of winter precipitation (>900 mm) in the mid 20th century and a significant period with lower winter precipitation (<400 mm) before the 15th century, i.e., the late Medieval Warm Period. The first peak in the mid 20th century is confirmed by the regional precipitation database. The influence of ENSO cycles over the last 600 years is assessed by spectral analysis in Fagel et al. (2007). The possible influence of the regional volcanism and/or the seismic activity on the local climate record is also discussed. This is the sixth in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M. -F. Loutre and E. Chapron.  相似文献   

Environmental Radioxenon Levels in Europe: a Comprehensive Overview     

Paul R. J. Saey  Clemens Schlosser  Pascal Achim  Matthias Auer  Anders Axelsson  Andreas Becker  Xavier Blanchard  Guy Brachet  Luis Cella  Lars-Erik De Geer  Martin B. Kalinowski  Gilbert Le Petit  Jenny Peterson  Vladimir Popov  Yury Popov  Anders Ringbom  Hartmut Sartorius  Thomas Taffary  Matthias Zähringer 《Pure and Applied Geophysics》2010,167(4-5):499-515

Activity concentration data from ambient radioxenon measurements in ground level air, which were carried out in Europe in the framework of the International Noble Gas Experiment (INGE) in support of the development and build-up of a radioxenon monitoring network for the Comprehensive Nuclear-Test-Ban Treaty verification regime are presented and discussed. Six measurement stations provided data from 5 years of measurements performed between 2003 and 2008: Longyearbyen (Spitsbergen, Norway), Stockholm (Sweden), Dubna (Russian Federation), Schauinsland Mountain (Germany), Bruyères-le-Châtel and Marseille (both France). The noble gas systems used within the INGE are designed to continuously measure low concentrations of the four radioxenon isotopes which are most relevant for detection of nuclear explosions: ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe with a time resolution less than or equal to 24 h and a minimum detectable concentration of ¹³³Xe less than 1 mBq/m³. This European cluster of six stations is particularly interesting because it is highly influenced by a high density of nuclear power reactors and some radiopharmaceutical production facilities. The activity concentrations at the European INGE stations are studied to characterise the influence of civilian releases, to be able to distinguish them from possible nuclear explosions. It was found that the mean activity concentration of the most frequently detected isotope, ¹³³Xe, was 5–20 mBq/m³ within Central Europe where most nuclear installations are situated (Bruyères-le-Châtel and Schauinsland), 1.4–2.4 mBq/m³ just outside that region (Stockholm, Dubna and Marseille) and 0.2 mBq/m³ in the remote polar station of Spitsbergen. No seasonal trends could be observed from the data. Two interesting events have been examined and their source regions have been identified using atmospheric backtracking methods that deploy Lagrangian particle dispersion modelling and inversion techniques. The results are consistent with known releases of a radiopharmaceutical facility.  相似文献   

The impact of agriculture on solute and suspended sediment load on a Mediterranean watershed after intense rainstorms     

Luís Outeiro  Xavier Úbeda  Joaquim Farguell 《地球表面变化过程与地形》2010,35(5):549-560

The solute and suspended‐sediment load following five rainstorms (2005–2007) with varied intensities were studied at the Vernegà experimental watershed, north‐western Spain. Two land‐use areas are located within this watershed, the upstream one (forest) with 160 ha a 100% forested area, and the downstream one (agricultural) with 97 ha being 9 ha conventional agricultural field and 88 ha forest. This study investigates the capacity of each land‐use to yield water, suspended sediment concentration (SSC) and dissolved solid concentration (DSC). The hypothesis is that DSC and SSC from the agricultural area are greater than DSC and SSC of the forest area. Results showed that the agriculture area produced significantly greater mean DSC than in the forest area, the main contribution was the Ca²⁺ (24·68 ± 46·52 mg l^?1) ion at the agricultural area. A long‐term sediment production rate at the agricultural outlet was calculated (69·1 tonnes per 100 years) based on the total sediment discharge (TSD) and the recurrence interval of the largest event of the five rainstorms (October 2005). Geographic information system (GIS) spatial data layers of the watershed were produced to determine the relation of tracks, landforms, slopes and forest management to SSC yield in the forest outlet (133·89 ± 308·14 mg l^?1) during the five rainstorms. Agriculture practices are the main cause of soil erosion at the study area. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Multi-Gaussian kriging and simulation in the presence of an uncertain mean value   总被引：1，自引：1，他引：0  

Xavier Emery 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(2):211-219

The multi-Gaussian model is used in geostatistical applications to predict functions of a regionalized variable and to assess uncertainty by determining local (conditional to neighboring data) distributions. The model relies on the assumption that the regionalized variable can be represented by a transform of a Gaussian random field with a known mean value, which is often a strong requirement. This article presents two variations of the model to account for an uncertain mean value. In the first one, the mean of the Gaussian random field is regarded as an unknown non-random parameter. In the second model, the mean of the Gaussian field is regarded as a random variable with a very large prior variance. The properties of the proposed models are compared in the context of non-linear spatial prediction and uncertainty assessment problems. Algorithms for the conditional simulation of Gaussian random fields with an uncertain mean are also examined, and problems associated with the selection of data in a moving neighborhood are discussed.  相似文献