Uncertainties in European summer precipitation changes: role of large scale circulation   总被引：2，自引：2，他引：0  

Julien Boé  Laurent Terray  Christophe Cassou  Julien Najac 《Climate Dynamics》2009,33(2-3):265-276

Climate models suggest that anthropogenic emissions are likely to induce an important drying during summer over most of Europe in the late 21st century. However, the amplitude of the associated decrease in precipitation strongly varies among the different climate models. In order to reduce this spread, it is first necessary to identify its causes and the associated physical mechanisms. Consequently, the focus of this paper is to better estimate the role of large scale circulation (LSC) in precipitation changes over Europe using a multi-model framework and then to characterize the LSC changes using the weather regime paradigm. We show that LSC changes directly lead to a decrease of precipitation over northwestern Europe. This circulation-driven decrease in rainfall is mainly linked to an increase (decrease) of the occurrence of positive (negative) phase of the North Atlantic Oscillation regime. LSC is also responsible for a significant part of the models spread in precipitation changes over these regions. Over southern Europe, the role of LSC changes on multi-model mean precipitation changes is generally weak. We also show that the precipitation anomalies directly induced by LSC modifications seem to be further amplified through local feedbacks.  相似文献   

Sources of spread in simulations of Arctic sea ice loss over the twenty-first century     

Julien Boé  Alex Hall  Xin Qu 《Climatic change》2010,99(3-4):637-645

We show that intermodel variations in the anthropogenically-forced evolution of September sea ice extent (SSIE) in the Arctic stem mainly from two factors: the baseline climatological sea ice thickness (SIT) distribution, and the local climate feedback parameter. The roles of these two factors evolve over the course of the twenty-first century. The SIT distribution is the most important factor in current trends and those of coming decades, accounting for roughly half the intermodel variations in SSIE trends. Then, its role progressively decreases, so that around the middle of the twenty-first century the local climate feedback parameter becomes the dominant factor. Through this analysis, we identify the investments in improved simulation of Arctic climate necessary to reduce uncertainties both in projections of sea ice loss over the coming decades and in the ultimate fate of the ice pack.  相似文献   

Evaporation estimates from Nasser Lake, Egypt, based on three floating station data and Bowen ratio energy budget   总被引：2，自引：1，他引：1  

Mohamed Elsawwaf  Patrick Willems  Andrea Pagano  Jean Berlamont 《Theoretical and Applied Climatology》2010,100(3-4):439-465

Nasser Lake is located in a hyper-arid region in the south of Egypt. Evaporation is by far the most important factor in explaining the water losses from the lake. To obtain better management scenarios for Nasser Lake, an accurate estimation of the lake evaporation losses thus is essential. This paper presents an update of previous evaporation estimates, making use of local meteorological and hydrological data collected from instrumented platforms (floating weather stations) at three locations on the lake: at Raft, Allaqi, and Abusembel (respectively at 2, 75, and 280 km upstream of the Aswan High Dam). Results from six conventional evaporation quantification methods were compared with the values obtained by the Bowen ratio energy budget method (BREB). The results of the BREB method showed that there is no significant difference between the evaporation rates at Allaqi and Abusembel. At Raft, higher evaporation rates were obtained, which were assumed to be overestimated due to the high uncertainty of the Bowen ratio (BR) parameter. The average BR value at Allaqi and Abusembel was used to eliminate this overestimates evaporation. Variance-based sensitivity and uncertainty analyses on the BREB results were conducted based on quasi-Monte Carlo sequences (Latin Hypercube sampling). The standard deviation of the total uncertainty on the BREB evaporation rate was found to be 0.62 mm day^?1. The parameter controlling the change in stored energy, followed by the BR parameter, was found to be the most sensitive parameters. Several of the six conventional methods showed substantial bias when compared with the BREB method. These were modified to eliminate the bias. When compared to the BREB-based values, the Penman method showed most favorably for the daily time scale, while for the monthly scale, the Priestley–Taylor and the deBruin–Keijman methods showed best agreement. Differences in mean evaporation estimates of these methods (against the BREB method) were found to be in the range 0.14 and 0.36 mm day^?1. All estimates were based calculations at the daily time scale covering a 10-year period (1995–2004).  相似文献   

European floods during the winter 1783/1784: scenarios of an extreme event during the ‘Little Ice Age’   总被引：1，自引：0，他引：1  

Rudolf Brázdil  Gaston R. Demarée  Mathias Deutsch  Emmanuel Garnier  Andrea Kiss  Jürg Luterbacher  Neil Macdonald  Christian Rohr  Petr Dobrovolný  Petr Kolář  Kateřina Chromá 《Theoretical and Applied Climatology》2010,100(1-2):163-189

The Lakagígar eruption in Iceland during 1783 was followed by the severe winter of 1783/1784, which was characterised by low temperatures, frozen soils, ice-bound watercourses and high rates of snow accumulation across much of Europe. Sudden warming coupled with rainfall led to rapid snowmelt, resulting in a series of flooding phases across much of Europe. The first phase of flooding occurred in late December 1783–early January 1784 in England, France, the Low Countries and historical Hungary. The second phase at the turn of February–March 1784 was of greater extent, generated by the melting of an unusually large accumulation of snow and river ice, affecting catchments across France and Central Europe (where it is still considered as one of the most disastrous known floods), throughout the Danube catchment and in southeast Central Europe. The third and final phase of flooding occurred mainly in historical Hungary during late March and early April 1784. The different impacts and consequences of the above floods on both local and regional scales were reflected in the economic and societal responses, material damage and human losses. The winter of 1783/1784 can be considered as typical, if severe, for the Little Ice Age period across much of Europe.  相似文献   

Impacts of warm and cold situations in the Mediterranean basins on the West African monsoon: observed connection patterns (1979–2006) and climate simulations     

Bernard Fontaine  Javier Garcia-Serrano  Pascal Roucou  Belen Rodriguez-Fonseca  Teresa Losada  Fabrice Chauvin  Sébastien Gervois  Sivarajan Sijikumar  Paolo Ruti  Serge Janicot 《Climate Dynamics》2010,35(1):95-114

Using both empirical and numerical ensemble approaches this study focuses on the Mediterranean/West African relationship in northern summer. Statistical analyses utilize skin temperature, sea surface temperature, in situ and satellite rainfall, outgoing longwave radiation (OLR) observations and reanalyzed data winds and specific humidity on isobaric surfaces. Numerical investigations are based on a large set of sensitivity experiments performed on four atmospheric general circulation models (AGCM): ARPEGE-Climat3, ECHAM4, LMDZ4 and UCLA7.3. Model outputs are compared to observations, discussed model by model and with an ensemble (multi-model) approach. As in previous studies the anomalous Mediterranean warm events are associated with specific impacts over the African monsoon region, i.e., a more intense monsoon, enhanced flux convergence and ascendances around the ITCZ, a strengthening of low level moisture advection and a more northward location of ascending motion in West Africa. The results show also new features (1) thermal variability observed in the two Mediterranean basins has unalike impacts, i.e. the western Mediterranean covaries with convection in Gulf of Guinea, while the eastern Mediterranean can be interpreted as Sahelian thermal-forcing; (2) although observations show symmetry between warming and cooling, modelling evidences only support the eastern warming influence; (3) anomalous East warm situations are associated with a more northward migration of the monsoon system accompanied by enhanced southwertely flow and weakened northeasterly climatological wind; (4) the multi-model response shows that anomalous East warm surface temperatures generate an enhancement of the overturning circulation in low and high levels, an increase in TEJ (Tropical Eeasterly Jet) and a decrease in AEJ (African Eeasterly Jet).  相似文献   

Summer interactions between weather regimes and surface ocean in the North-Atlantic region     

Virginie Guemas  David Salas-Mélia  Masa Kageyama  Hervé Giordani  Aurore Voldoire  Emilia Sanchez-Gomez 《Climate Dynamics》2010,34(4):527-546

This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-h frequency in solar heat flux (6 h for the other forcing fields) over the 1959–2001 period. The model has 124 vertical levels with a vertical resolution of 1 m near the surface and 500 m at the bottom. This ocean forced experiment is used to assess the impact of the North Atlantic weather regimes on the surface ocean. Composites of sea surface temperature (SST) anomalies associated with each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related to each weather regime in the ocean-forced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a negative feedback on the persistence of the NAO-regime and favours the transition from the Atlantic Ridge regime to the NAO-regime and from the Atlantic Low regime toward the Blocking regime.  相似文献   

Relationship between climate change and vegetation distribution in the Mediterranean mountains: Manzanares Head valley, Sierra De Guadarrama (Central Spain)     

Arturo García-Romero  Julio Muñoz  Nuria Andrés  David Palacios 《Climatic change》2010,100(3-4):645-666

This work analyzes the consequences of climate change in the distribution of the Mediterranean high-mountain vegetation. A study area was chosen at the Sierra de Guadarrama, in the center of the Iberian Peninsula (1,795 to 2,374 m asl). Climate change was analyzed from the record of 18 variables regarding temperature, rainfall and snowfall over the period 1951–2000. The permanence of snow cover (1996–2004), landforms stability and vegetation distribution in 5 years (1956, 1972, 1984, 1991 and 1998) were all analyzed. The Nival Correlation Level of the different vegetation classes was determined through their spatial and/or temporal relationship with several climatologic variables, snow cover duration and landforms. In order to quantify trends and major change processes, areas and percent changes were calculated, as well as Mean Annual Transformation Indices and Transition Matrices. The findings reveal that in the first part of the study period (up to the first half of the 1970s) the temperature rise in the mid-winter months caused the reduction of some classes of nival vegetation, while others expanded, favored by high rainfall, decrease in both maximum temperatures and summer aridity, and longer snow cover duration. The second part of the study period was characterized by the consolidation of the increase in all thermal variables, along with an important reduction in rainfall volume and snow cover duration. As a result, herbaceous plants, which are highly correlated with a long snow permanence and abundance of melting water, have been replaced by leguminous shrubs which grow away from the influence of snow, and which are steadily becoming denser.  相似文献   

Potential changes in extremes and links with the Southern Annular Mode as simulated by a multi-model ensemble     

Claudio G. Menéndez  Andrea F. Carril 《Climatic change》2010,98(3-4):359-377

We assess the likely changes in climate extremes under enhanced greenhouse gases over the southern extratropics, with emphasis in southern South America and sub-Antarctic seas, through the analysis of extreme indices measured from models participating in the IPCC 4th Assessment Report. We discuss how the anthropogenic climate change under A1B scenario influences both the patterns of mean change of extreme indices and the likelihood of occurrence of severe extreme indices. The likelihood of occurrence of a year with a large number of days with “warm” minimum temperatures is estimated to increase by a factor of 4 by the end of this century over most of the southern extratropics. By that time, the risk of “severe” precipitation intensity is projected to rise in most areas with the exception of the subtropical anticyclones, which experience particularly strong drying. Over the Southern Ocean this likelihood has increased to over 60%. Corresponding estimates of the changing likelihood for very long dry spells show a banded structure with positive ratios to the north of about 50° S and negative ratios in the sub Antarctic seas. In southern South America this risk about doubled between present and future climates. Then, we explore if the Southern Annular Mode influences the occurrence of severe extreme indices during the period 2070–2099. Its positive phase inhibits the extremely warm minimum temperatures in the Southern Ocean, with the exception of the eastern Bellingshausen Sea, and favors severe frost days to the north of the Ross Sea. Temperature indices show very little change induced by the SAM to the north of 50° S. Severe dry spells are inhibited during the positive phase along the sub Antarctic seas, while the mid-latitudes, including most of Patagonia, show the opposite behaviour. The Southern Ocean reveals a non-uniform distribution with both increases and decreases in the occurrence of heavier precipitation during positive SAM.  相似文献   

Dynamics of sea-surface temperature anomalies in the Southern Ocean diagnosed from a 2D mixed-layer model     

Frédéric Vivier  Daniele Iudicone  Fabiano Busdraghi  Young-Hyang Park 《Climate Dynamics》2010,34(2-3):153-184

We analyze the processes responsible for the generation and evolution of sea-surface temperature anomalies observed in the Southern Ocean during a decade based on a 2D diagnostic mixed-layer model in which geostrophic advection is prescribed from altimetry. Anomalous air–sea heat flux is the dominant term of the heat budget over most of the domain, while anomalous Ekman heat fluxes account for 20–40% of the variance in the latitude band 40°?60°S. In the ACC pathway, lateral fluxes of heat associated with anomalous geostrophic currents are a major contributor, dominating downstream of several topographic features, reflecting the influence of eddies and frontal migrations. A significant fraction of the variability of large-scale SST anomalies is correlated with either ENSO or the SAM, each mode contributing roughly equally. The relation between the heat budget terms and these climate modes is investigated, showing in particular that anomalous Ekman and air–sea heat fluxes have a co-operating effect (with regional exceptions), hence the large SST response associated with each mode. It is further shown that ENSO- or SAM-locked anomalous geostrophic currents generate substantial heat fluxes in all three basins with magnitude comparable with that of atmospheric forcings for ENSO, and smaller for the SAM except for limited areas. ENSO-locked forcings generate SST anomalies along the ACC pathway, and advection by mean flows is found to be a non-negligible contribution to the heat budget, exhibiting a wavenumber two zonal structure, characteristic of the Antarctic Circumpolar Wave. By contrast SAM-related forcings are predominantly zonally uniform along the ACC, hence smaller zonal SST gradients and a lesser role of mean advection, except in the SouthWest Atlantic. While modeled SST anomalies are significantly correlated with observations over most of the Southern Ocean, the analysis of the data-model discrepancies suggests that vertical ocean physics may play a significant role in the nonseasonal heat budget, especially in some key regions for mode water formation.  相似文献   

Subfilter-scale Fluxes over a Surface Roughness Transition. Part I: Measured Fluxes and Energy Transfer Rates     

Matthew A. Carper  Fernando Porté-Agel 《Boundary-Layer Meteorology》2008,126(1):157-179

A wind-tunnel experiment was designed and carried out to study the effect of a surface roughness transition on subfilter-scale (SFS) physics in a turbulent boundary layer. Specifically, subfilter-scale stresses are evaluated that require parameterizations and are key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer. The surface transition considered in this study consists of a sharp change from a rough, wire-mesh covered surface to a smooth surface. The resulting magnitude jump in aerodynamic roughnesses, M = ln(z ₀₁/z ₀₂), where z ₀₁ and z ₀₂ are the upwind and downwind aerodynamic surface roughnesses respectively, is similar to that of past experimental studies in the atmospheric boundary layer. The two-dimensional velocity fields used in this study are measured using particle image velocimetry and are acquired at several positions downwind of the roughness transition as well as over a homogeneous smooth surface. Results show that the SFS stress, resolved strain rate and SFS transfer rate of resolved kinetic energy are dependent on the position within the boundary layer relative to the surface roughness transition. A mismatch is found in the downwind trend of the SFS stress and resolved strain rate with distance from the transition. This difference of behaviour may not be captured by some eddy-viscosity type models that parameterize the SFS stress tensor as proportional to the resolved strain rate tensor. These results can be used as a benchmark to test the ability of existing and new SFS models to capture the spatial variability SFS physics associated with surface roughness heterogeneities.  相似文献