1,721,151 research outputs found
How does informational heterogeneity affect the quality of forecasts?
No full textWe investigate a toy model of inductive interacting agents aiming to forecast a continuous, exogenous random variable E. Private information on E is spread heterogeneously across agents. Herding turns out to be the preferred forecasting mechanism when heterogeneity is maximal. However in such conditions aggregating information efficiently is hard even in the presence of learning, as the herding ratio rises significantly above the efficient market expectation of 1 and remarkably close to the empirically observed values. We also study how different parameters (interaction range, learning rate, cost of information and score memory) may affect this scenario and improve efficiency in the hard phase. © 2009 Elsevier B.V. All rights reserved
The Double-ITCZ Syndrome in Coupled General Circulation Models: The Role of Large-Scale Vertical Circulation Regimes
No full textITCZ, GCMThe double-intertropical convergence zone (DI) systematic error, affecting state-of-the-art coupled general circulation models (CGCMs), is examined in the multimodel Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) ensemble of simulations of the twentieth-century climate. The aim of this study is to quantify the DI error on precipitation in the tropical Pacific, with a specific focus on the relationship between the DI error and the representation of large-scale vertical circulation regimes in climate models. The DI rainfall signal is analyzed using a regime-sorting approach for the vertical circulation regimes. Through the use of this compositing technique, precipitation events are regime sorted based on the large-scale vertical motions, as represented by the midtropospheric Lagrangian pressure tendency omega(500) dynamical proxy. This methodology allows partition of the precipitation signal into deep and shallow convective components. Following the regime-sorting diagnosis, the total DI bias is split into an error affecting the magnitude of precipitation associated with individual convective events and an error affecting the frequency of occurrence of single convective regimes. It is shown that, despite the existing large intramodel differences, CGCMs can be ultimately grouped into a few homogenous clusters, each featuring a well-defined rainfall-vertical circulation relationship in the DI region. Three major behavioral clusters are identified within the AR4 models ensemble: two unimodal distributions, featuring maximum precipitation under subsidence and deep convection regimes, respectively, and one bimodal distribution, displaying both components. Extending this analysis to both coupled and uncoupled (atmosphere only) AR4 simulations reveals that the DI bias in CGCMs is mainly due to the overly frequent occurrence of deep convection regimes, whereas the error on rainfall magnitude associated with individual convective events is overall consistent with errors already present in the corresponding atmosphere stand-alone simulations. A critical parameter controlling the strength of the DI systematic error is identified in the model-dependent sea surface temperature (SST) threshold leading to the onset of deep convection (THR), combined with the average SST in the southeastern Pacific. The models featuring a THR that is systematically colder (warmer) than their mean surface temperature are more (less) prone to exhibit a spurious southern intertropical convergence zone
Changes in Tropical Cyclone Activity due to Global Warming: Results from a High-Resolution Coupled General Circulation Model
No full textcharacteristics of tropical cyclones (TCs). The analysis has been performed using scenario climate simulations carried out with a fully coupled high-resolution global general circulation model. The capability of the model to reproduce a reasonably realistic TC climatology has been assessed by comparing the model results from a simulation of the twentieth century with observations. The model appears to be able to simulate tropical cyclone-like vortices with many features similar to the observed TCs. The simulated TC activity exhibits realistic geographical distribution, seasonal modulation, and interannual variability, suggesting that the model is able to reproduce the major basic mechanisms that link TC occurrence with large-scale circulation. The results from the climate scenarios reveal a substantial general reduction of TC frequency when the atmospheric CO2 concentration is doubled and quadrupled. The reduction appears particularly evident for the tropical western North Pacific (WNP) and North Atlantic (ATL). In the NWP the weaker TC activity seems to be associated with reduced convective instabilities. In the ATL region the weaker TC activity seems to be due to both the increased stability of the atmosphere and a stronger vertical wind shear. Despite the generally reduced TC activity, there is evidence of increased rainfall associated with the simulated cyclones. Finally, the action of the TCs remains well confined to the tropical region and the peak of TC number remains equatorward of 20 degrees latitude in both hemispheres, notwithstanding the overall warming of the tropical upper ocean and the expansion poleward of warm SSTs
A diagnostic study of the Indian Ocean dipole mode in El Nino and non-El Nino years
No full textThe Indian Ocean dipole mode ( IODM) is examined by comparing the characteristics of oceanic and atmospheric circulations, heat budgets, and possible mechanisms of IODM between El Nino and non - El Nino years. Forty- year ECMWF Re- Analysis ( ERA- 40) data, Reynolds SST data, and ocean assimilation data from the Modular Ocean Model are used to form composites of the IODM that occur during El Nino ( 1972, 1982, and 1997) and non - El Nino ( 1961, 1967, and 1994) years. In El Nino years, two off- equatorial, anticyclonic circulations develop, associated with the increased pressure over the eastern Indian Ocean. The anticyclonic circulation over the Northern Hemisphere enhances the easterly component of the winds in the northwestern Indian Ocean. This enhanced easterly component increases the mixed layer temperature by inducing an anomalous westward ocean current that advects the warm mean mixed layer from the central to the western Indian Ocean. Meanwhile, the anticyclonic circulation over the southeastern Indian Ocean strengthens southeasterlies, thereby causing oceanic meridional and vertical advection of the cold mean temperature. Consequently, the IODM in El Nino years is characterized by the warming in the northwestern and the cooling in the southeastern Indian Ocean. In non - El Nino years, a monsoonlike wind flow increases the westerly and southeasterly components of the wind over the northwestern and southeastern Indian Ocean, respectively. Oceanic currents induced by these winds result in anomalous cold advection in both of these regions. In addition, the monsoonlike wind flow over the southeastern Indian Ocean enhances the anomalous latent and sensible heat fluxes in non - El Nino years. Hence, the cooling of the eastern tropical Indian Ocean, rather than the warming of the western Indian Ocean, becomes the major feature of the IODM during non - El Nino years
Impact of atmospheric horizontal resolution on El Nino Southern Oscillation forecasts
No full textENSO, Southern Oscillation, ForecastIn this paper, we assess the skill of a seasonal forecast system to predict El Nino Southern Oscillation (ENSO) anomalies and the impact that the horizontal resolution of the atmospheric component has on the forecasts of the growing phase of strong ENSO events. The analysis has been carried out on retrospective six-month forecasts for the period 1973-2001 performed with the Istituto Nazionale di Geofisica e Vulcanologia (INGV) coupled ocean-atmosphere general circulation model (SINTEX) in the framework of the DEMETER project. Each forecast is formed by an ensemble of nine integrations that start from perturbed initial conditions. In order to consider the possible impact of the seasonal cycle on the forecasts, four different dates of the year have been chosen as initial conditions. The results indicate that, in general, the model is reasonably good in predicting the distribution of the seasonal anomalies in the tropical Pacific. However, it is also shown that the model predictions underestimate substantially the amplitude of the anomalies during the developing phase of intense El Nino episodes. The forecasts of strong ENSO events in the period here considered have been repeated, increasing the horizontal resolution of the atmospheric component. In these experiments, both the ocean model component and the oceanic initial conditions are unchanged. During the decaying phase of the oscillation, the high-resolution and low-resolution versions of the model produce similar results and the forecasts are in general good agreement with the observations. During the developing phase of ENSO, the low-resolution version of the model is unable to sustain the growth of the coupled disturbance. With the low-resolution atmosphere, the initial perturbation of the coupled system decays and the anomalies in the tropical Pacific tend to vanish very quickly. With the high-resolution atmosphere, on the other hand, the model appears to be able to sustain the growth of the disturbance, improving the quality of the forecasts both for El Nino and La Nina cases
Tropical cyclone effects on Arctic Sea ice variability
No full textIn recent years increasing interest has been put on the role that intense Tropical Cyclones can play in the climate system. The following study is aimed at highlighting the effects of strong Tropical Cyclones over the Tropical Atlantic on the mean climate. Their composite effect on the surface winds is made apparent by a wide cyclonic perturbation that affects a large portion of the Atlantic tropical Ocean. Teleconnection patterns, which are visible in the Sea Level Pressure anomalies associated with this Tropical Composite Cyclone, appear to link the activity of the hurricanes to the Arctic Ocean. A significant negative correlation between the energy dissipated by hurricanes in the Tropical atmosphere and the sea ice cover along the Transpolar Drift Stream path, has also been found. Citation: Scoccimarro, E., S. Gualdi, and A. Navarra (2012), Tropical cyclone effects on Arctic Sea ice variability, Geophys. Res. Lett., 39, L17704, doi:10.1029/2012GL052987
Análisis de los escenarios climáticos presentes y futuros. In “Informe de valoración de los impactos de actividades antrópicas y del cambio climático”. Producto C – Doc. 25051-REL-T010.0. World Bank Tender: Adaptation to Climate Change Impacts on the Coastal Wetlands in the Gulf of Mexico (PR. P100438).
No full textGracias al nuevo conjunto de simulaciones climáticas para el siglo XXI, realizadas con los modelos de circulación global (GCM) que fueron producidos para el Proyecto de Intercomparación de Modelos Acoplados, fase cinco (CMIP5, Meehl y Bony, 2012) se da la posibilidad de investigar los cambios futuros en el clima promedio en el Sureste de México. El presente análisis toma en cuenta dos posibles trayectorias, RCP8.5 y RCP4.5, de las llamadas trayectorias representativas de concentración de gases de efecto invernadero (RCP) consideradas como escenarios potenciales e ilustrativos del futuro hasta llegar al 2100, en el CMIP5.
De entre la totalidad de posibles escenarios, el escenario RCP8.5 es el que toma en cuenta el índice más alto de incremento en las concentraciones de los gases de efecto invernadero (GEI), mientras que el escenario RCP4.5 se caracteriza por un índice de incremento moderado. El principal objetivo de este análisis es el de inspeccionar los cambios en la temperatura del aire, la precipitación y el nivel medio del mar bajo condiciones de calentamiento global, comparando los cambios calculados de la última parte del siglo XXI (2071-2100) con los últimos 30 años del período de datos históricos (1975-2004). También se presenta, una descripción general de los eventos extremos sobre la región de estudio
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The impact of the AMV on Eurasian summer hydrological cycle
Full text linkImpact studies of the Atlantic Multidecadal Variability (AMV) on the climate system are severely limited by the lack of sufficiently long observational records. Relying on a model-based approach is therefore mandatory to overcome this limitation. Here, a novel experimental setup, designed in the framework of the CMIP6-endorsed Decadal Climate Prediction Project, is applied to the CMCC climate model to analyse the remote climate impact of the AMV on the Northern Eurasian continent. Model results show that, during Boreal summer, an enhanced warming associated to a positive phase of the AMV, induces a hemispheric-scale wave-train response in the atmospheric circulation, affecting vast portions of Northern Eurasia. The overall AMV-induced response consists in an upper-tropospheric anomalous flows leading to a rainfall increase over Scandinavia and Siberia and to an intensified river runoff by the major Siberian rivers. A strengthening of Eurasian shelves’ stratification, broadly consistent with the anomalous river discharge, is found in the proximity of the river mouths during positive-AMV years. Considering that Siberian rivers (Ob’, Yenisei and Lena) account for almost half of the Arctic freshwater input provided by terrestrial sources, the implications of these findings for decadal variability and predictability of the Arctic environment are also discussed
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