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574 results on '"Pohl, Benjamin"'

202. Uses and limits of thermal indices: the case of Sahel

Author

Rome, Sandra, Moron, Vincent, Oueslati, Boutheina, Pohl, Benjamin, Fontaine, Bernard, Diedhiou, Arona, Laboratoire d'étude des transferts en hydrologie et environnement (LTHE), Centre National de la Recherche Scientifique (CNRS)-Institut National Polytechnique de Grenoble (INPG)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA), Centre européen de recherche et d'enseignement des géosciences de l'environnement (CEREGE), Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Collège de France (CdF)-Institut national des sciences de l'Univers (INSU - CNRS)-Aix Marseille Université (AMU)-Institut National de la Recherche Agronomique (INRA), Biogéosciences [UMR 6282] [Dijon] (BGS), Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique (CNRS), Chairs of the Organising and High-Level Committees: Hervé Le Treut and Jean Jouzel, CFCC15 Scientific Committee, chaired by Chris Field, ANR-13-SENV-0007,ACASIS,Alerte aux Canicules Au Sahel et à leurs Impacts sur la Santé(2013), Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Institut National Polytechnique de Grenoble (INPG)-Centre National de la Recherche Scientifique (CNRS), Institut de Recherche pour le Développement (IRD)-Institut National de la Recherche Agronomique (INRA)-Aix Marseille Université (AMU)-Collège de France (CdF (institution))-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Biogéosciences [UMR 6282] (BGS), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Rome, Sandra, Sociétés et Changements Environnementaux - Alerte aux Canicules Au Sahel et à leurs Impacts sur la Santé - - ACASIS2013 - ANR-13-SENV-0007 - Soc&Env - VALID, Institut National Polytechnique de Grenoble (INPG)-Centre National de la Recherche Scientifique (CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS), Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Collège de France (CdF (institution))-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS)-Institut National de la Recherche Agronomique (INRA), Laboratoire d'étude des transferts en hydrologie et environnement ( LTHE ), Centre National de la Recherche Scientifique ( CNRS ) -Institut National Polytechnique de Grenoble ( INPG ) -Institut national des sciences de l'Univers ( INSU - CNRS ) -Université Joseph Fourier - Grenoble 1 ( UJF ) -Observatoire des Sciences de l'Univers de Grenoble ( OSUG ), Université Joseph Fourier - Grenoble 1 ( UJF ) -Institut national des sciences de l'Univers ( INSU - CNRS ) -Centre National de la Recherche Scientifique ( CNRS ) -Université Grenoble Alpes ( UGA ) -Centre National de la Recherche Scientifique ( CNRS ) -Université Grenoble Alpes ( UGA ), Centre européen de recherche et d'enseignement de géosciences de l'environnement ( CEREGE ), Centre National de la Recherche Scientifique ( CNRS ) -Institut de Recherche pour le Développement ( IRD ) -Aix Marseille Université ( AMU ) -Collège de France ( CdF ) -Institut National de la Recherche Agronomique ( INRA ) -Institut national des sciences de l'Univers ( INSU - CNRS ), Biogéosciences [Dijon] ( BGS ), AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), ANR-13-SENV-0007,ACASIS,Alerte aux Canicules Au Sahel et à leurs Impacts sur la Santé ( 2013 ), and Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut National Polytechnique de Grenoble (INPG)-Centre National de la Recherche Scientifique (CNRS)

Subjects
[ SDE ] Environmental Sciences, [SDE] Environmental Sciences, heat wave, [SHS.GEO] Humanities and Social Sciences/Geography, Sahel, ACASIS, [SDE]Environmental Sciences, [SHS.GEO]Humanities and Social Sciences/Geography, Thermal extreme, [ SHS.GEO ] Humanities and Social Sciences/Geography
Abstract

International audience; Our main goal here is to analyse extreme heat waves (HWs) in the Sahel (13°N-18°N; 16°W-30°E), using different thermal indices. In the ACASIS project funded bythe French “Agence Nationale de la Recherche”, HWs analyses are characterized forthe first time during the hottest season in the Sahel, using the Global Summary of theDay (GSOD) synoptic observations during April-May-June 1973-2013. Such extremehigh temperatures are usually defined by 3 criteria: 1/ Their low probability ofoccurrence: less than the 10% of the days. The use of absolute thresholds,associated with heat budget and physiological impacts, could be an alternative(Seneviratne et al., 2012). 2/ Their intensity, i.e. their amplitude in terms of standarddeviations from the local climatology (Goubanova, 2007), or otherwise excesses overabsolute intensity thresholds. 3/ Their severity: extreme events causing socioeconomicor human losses, the notion of impact is thus of major importance for theirdefinition (Beniston et al., 2007).According to prior IPCC assessments (TAR, AR4 and SREX), it is very likely thatincreased maximum temperatures and enhanced probabilities of hot days occurrenceswill occur at the global scale. There is also medium confidence that warm spells/heatweave frequencies, lengths or intensities will increase in many regions (Cubasch et al.,2013). Because of its latitude, Sahel is especially concerned, with diurnaltemperatures often exceeding 40°C in boreal spring and, to a lesser extent, autumn.AR4 concluded that global land-surface air temperature (LSAT) had increased over theinstrumental period of record, with the warming rate approximately double thatreported over the oceans since 1979. AR5 confirms previous estimates: Global LSATincreased: 0.1005°C ± 0.01925°C per decade over 1901-2012 period while the risereaches 0.262°C ± 0.05°C per decade over 1979-2012 time period, according to theobservations of CRUTEM4.1.1.0, GHCNv3.2.0, GISS and Berkeley dataset average(Hartman et al., 2013). Changes in the length of the spells are observed and they arelikely to increase but this needs to be investigated further over Sahel where severeheat can have damaging consequences for societies.For instance, in Niamey (Niger) in April 2010 whereas Tmax reaching 47°C (Ringard etal., 2015), increased morbidity and mortality rates were observed, especially amongthe elderly and young children. Moreover, in some places, HWs can be associated withincreased rates of atmospheric pollution.Terminology is large: a “ hot spell” implies the hottest temperature during an extremeof temperature with a small duration (a few hours), small extension (around thestation) and a high frequency; a “warm spell” should be less frequent and covers alarger area; a “heat wave” (HW) exceeds thresholds of temperature most frequentlyand covers at least a whole region (~ 500-1000 km²). These thresholds can beabsolute (T° fixed locally) or relative, exceeding 5 °C as local standards for examplefor Expert Team on Climate Change Detection and Indices (ETCCDI, 2013). ACMAD(African Centre of Meteorological Applications for Development) classifies the extremetemperatures following five high-impact weather events; they broadcasts a mail alertwhen Tmax> 40°C. In ACASIS, Sahelian HW is defined (Rome et al. 2015) as aperiod of at least 3 consecutive days of extremely high (above the local 90thpercentile) daily heat index. The Heat Index (HI) (Steadman 1979, 1994),combining temperature and relative humidity, appears as the most appropriate index,suitable for tropical climate, which takes into account the human-perceived equivalenttemperature. The role of humidity is rather small in boreal spring across the Sahel,when high incoming solar radiation is combined with extremely dry soils, preventingcooling effect associated with latent heat flux. As for global scales, results show aclear warming trend over the last three decades, with a clear trend for HW events tobecome more frequent, last longer, cover larger areas and reach higher intensities(see also Oueslati et al. 2015). We then observe a decrease in cases of “Caution” HIand an increase in cases of “Danger” and “Extreme Danger” HI values.

Published
2015

203. An original way to evaluate daily rainfall variability simulated by a regional climate model: the case of South African austral summer rainfall

Author

Crétat , Julien, Pohl , Benjamin, Chateau Smith , Carmela, Vigaud , Nicolas, Richard , Yves, Nucleus for European Modeling of the Ocean (NEMO R&D ), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales [Toulouse] (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales [Toulouse] (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales [Toulouse] (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Biogéosciences [UMR 6282] (BGS), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), UFR Sciences de la Vie, de la Terre et de l'Environnement (Université de Bourgogne) (UFR SVTE), Université de Bourgogne (UB), International Research Institute for Climate and Society (IRI), Earth Institute at Columbia University, Columbia University [New York]-Columbia University [New York], Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Muséum national d'Histoire naturelle (MNHN)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales [Toulouse] (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales [Toulouse] (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Muséum national d'Histoire naturelle (MNHN)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), Biogéosciences [UMR 6282] [Dijon] (BGS), Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique (CNRS), Nucleus for European Modeling of the Ocean ( NEMO R&D ), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques ( LOCEAN ), Muséum National d'Histoire Naturelle ( MNHN ) -Université Pierre et Marie Curie - Paris 6 ( UPMC ) -Institut national des sciences de l'Univers ( INSU - CNRS ) -Centre National de la Recherche Scientifique ( CNRS ) -Muséum National d'Histoire Naturelle ( MNHN ) -Université Pierre et Marie Curie - Paris 6 ( UPMC ) -Institut national des sciences de l'Univers ( INSU - CNRS ) -Centre National de la Recherche Scientifique ( CNRS ), Biogéosciences [Dijon] ( BGS ), Université de Bourgogne ( UB ) -AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique ( CNRS ), UFR Sciences de la Vie, de la Terre et de l'Environnement (Université de Bourgogne) ( UFR SVTE ), Université de Bourgogne ( UB ), and International Research Institute for Climate and Society ( IRI )

Subjects
regional climate modelling, South Africa, [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology, WRF, [ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology, daily rainfall, cluster analysis
Abstract

18 pages; International audience; We discuss the value of a clustering approach as a tool for evaluating daily rainfall output from climate models. Ascendant hierarchical clustering is used to evaluate how well South African recurrent daily rainfall patterns are simulated during the austral summer (December to February 1970–1971 to 1998–1999). A set of 35-km regional climate simulations, run with the WRF model and driven by the ERA40 reanalysis, is chosen as a case study. Six recurrent patterns are identified and compared to the observed clusters obtained by applying the same methodology to 5352 daily rain gauge records. Two of the WRF clusters describe either a persistent and widespread dryness (65% of the days) or patterns similar to the seasonal mean rainfall gradient (13% of the days). The four remaining WRF clusters (∼20% of the days) are wetter; they describe the weakening, conservation or strengthening of the average rainfall gradient. The WRF cluster rainfall patterns and their associated circulation match the observed clusters rather well, but their frequency of occurrence is greatly overestimated by WRF during dry events, and underestimated for near-normal rainfall conditions. The weak model biases found at the seasonal timescale conceal strongly biased intraseasonal rainfall variability. The WRF-simulated rainfall patterns are then temporally or spatially projected on to the observed clusters. Spatial projection proves to be the more useful of these two approaches in quantifying model skill by assessing both the temporal co-variability between WRF and observations, and the rainfall biases of the model with or without temporal dephasing. The WRF model simulates transient rainfall activity partially out of phase with observations, which induces large rainfall biases when temporal dephasing is not removed. Rainfall biases are significantly reduced, however, when temporal dephasing is removed. The clustering approach therefore proves its efficiency to highlight climate model strengths and deficiencies.

Published
2015
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204. Books Fit for a King: The Presentation Copies of Martin Bucer's De regno Christi (London, British Library, Royal MS. 8 B. VII) and Johannes Sturm's De periodis (Cambridge, Trinity College, II.12.21 and London, British Library, C.24.e.5)

Author

Tether, Leah and Pohl, Benjamin

Abstract

This article discusses the presentation copies of two sixteenth-century works, Martin Bucer’s De regno Christi and Johannes Sturm’s De periodis, both of which were sent in fine copies by Bucer to John Cheke in 1550. The covering letter that accompanied these books survives today at Corpus Christi College, Cambridge, whilst the presentation copy of De regno Christi intended for King Edward VI is British Library, Royal MS. 8 B. VII. The circumstances surrounding these books, their production and transmission can be reconstructed in unusual and intriguing detail. This article presents several new and important discoveries, including the identification of the two presentation copies of Sturm’s De periodis mentioned by Bucer in his letter (today London, British Library, C.24.e.5 and Cambridge, Trinity College, II.12.21), as well as the tentative attribution of the binding of Royal MS. 8 B. VII to Bucer’s collaborator, the Strasbourg-based printer Remigius Guidon. An in-depth analysis of these artefacts and their codicological features confirms that TCC II.12.21 was intended for Edward VI, whereas BL C.24.e.5 can be identified, for the first time, as a book designed for and received by Princess Elizabeth. The dynamics governing the production and exchange of presentation copies between the Edwardian court and the Protestant reformers serve to paint a sharply focused picture of Bucer’s activities around 1550, that is, at a key moment of religious change in England.

Published
2015

205. REWRITING THE GESTA NORMANNORUM DUCUMIN THE FIFTEENTH CENTURY: SIMON DE PLUMETOT'S BREVIS CRONICA COMPENDIOSA DUCUM NORMANNIE

Author

POHL, BENJAMIN and ALLEN, RICHARD

Abstract

This article is dedicated to Liesbeth van Houts, editor of the Gesta Normannorum ducum, generous mentor, colleague, and friend.This article offers an analysis, edition, and translation of the Brevis cronica compendiosa ducum Normannie, a historiographical account of the dukes of Normandy and their deeds, written at the turn of the fifteenth century by the Norman jurist and man of letters, Simon de Plumetot (1371–1443). Having all but escaped the attention of modern scholars, this study is the first to examine and publish the Brevis cronica. It not only demonstrates that the work is of greater importance than its rather scrappy form might at first suggest, but it also looks to place the text within the broader context of Simon's literary and bibliophilic practices and to determine its raison d’être. In doing so, it argues that the Brevis cronicawas perhaps created as part of a much larger historiographical project, namely an extended chronicle of Normandy, written in the vernacular, the text of which is now lost. By exploring these important issues, the article sheds new light on a wide range of topics, from early humanist book collecting to the writing of history in France in the later Middle Ages.

Published
2020
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218. Temperature changes in the mid- and high-latitudes of the Southern Hemisphere

Author

Richard, Yves, Rouault, Mathieu, Pohl, Benjamin, Crétat, Julien, Duclot, I., Taboulot, S., Reason, C.J.C., Macron, Clémence, Buiron, D., Biogéosciences [UMR 6282] [Dijon] (BGS), Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique (CNRS), Department of Oceanography [Cape Town], University of Cape Town, Nansen-Tutu Centre for Marine Environmental Research, Météo-France [Paris], Météo France, Laboratoire de glaciologie et géophysique de l'environnement (LGGE), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS), Funding from CNRS, NRF, Nansen-Tutu Centre for Marine Environmental Research and ACCESS., Centre National de la Recherche Scientifique (CNRS)-Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement, Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Centre National de la Recherche Scientifique (CNRS), Biogéosciences [Dijon] ( BGS ), Université de Bourgogne ( UB ) -AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique ( CNRS ), Department of Oceanography, Laboratoire de glaciologie et géophysique de l'environnement ( LGGE ), Observatoire des Sciences de l'Univers de Grenoble ( OSUG ), and Université Joseph Fourier - Grenoble 1 ( UJF ) -Institut national des sciences de l'Univers ( INSU - CNRS ) -Centre National de la Recherche Scientifique ( CNRS ) -Université Grenoble Alpes ( UGA ) -Université Joseph Fourier - Grenoble 1 ( UJF ) -Institut national des sciences de l'Univers ( INSU - CNRS ) -Centre National de la Recherche Scientifique ( CNRS ) -Université Grenoble Alpes ( UGA ) -Centre National de la Recherche Scientifique ( CNRS )

Subjects
[ SDE.MCG ] Environmental Sciences/Global Changes, Sub-Antarctic islands, climate change, sea surface temperature, [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology, [SDE.MCG]Environmental Sciences/Global Changes, regionalization, temperature, Southern Hemisphere, Southern Annular Mode, [ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology
Abstract

16 pages; International audience; A Hierarchical Ascending Classification is used to regionalize monthly temperature anomalies measured at 24 weather stations in Antarctica and the Sub-Antarctic and mid-latitude southern islands from 1973 to 2002. Three principal regions are identified that are geographically coherent: Eastern Antarctica, the Antarctic Peninsula and the Sub- Antarctic and mid-latitude islands. Within each region, consistent trends are observed: namely, stationary temperatures in 'East-Antarctica'; a robust warming in the 'Sub-Antarctic and mid-latitude islands', most pronounced in austral summer (nearly 0.5 °C per decade); and a strong but more recent warming in the 'Antarctic Peninsula'. Austral summer temperature anomalies are related to (1) the Southern Annular Mode (SAM) indexes computed using two reanalysis products (20th Century Reanalyses and ERA40) over two periods (1958-2002 and 1973-2002), (2) the seasonal frequencies of four recurrent daily weather regimes identified with a k-means algorithm applied on the 500hPa geopotential height (DJF 1958-2002) and (3) HadSST2 sea surface temperature (SST) anomalies (DJF 1958-2002). East-Antarctica interannual temperature anomalies are associated with the SAM variability. In the Antarctic Peninsula, only the long-term trend is common with the SAM. The SAM does impact significantly the temperature anomalies of the Sub-Antarctic and midlatitude islands. Trend and interannual variability of the islands' temperatures are associated with the nearby SST. For the Indian Ocean stations, warming in the Agulhas Current system could also have led to these changes.

Published
2013
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220. Recurrent daily rainfall patterns over South Africa and associated dynamics during the core of the austral summer

Author

Crétat , Julien, Richard , Yves, Pohl , Benjamin, Mathieu , Rouault, Reason , Chris, Fauchereau , Nicolas, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Department of Oceanography, University of Cape Town, CSIR / NRE, CHPC, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), and Department of Oceanography [Cape Town]

Subjects
South Africa, [ SDE.MCG ] Environmental Sciences/Global Changes, [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology, [SDE.MCG]Environmental Sciences/Global Changes, atmospheric dynamics, [ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology, ENSO, hierarchical clustering, daily rainfall patterns
Abstract

International audience; This paper investigates the influence of some modes of climate variability on the spatio-temporal rainfall variability over South Africa during the core of the rainy season (December to February; DJF). All analyses are based directly on the rainfall field instead of atmospheric processes and dynamics. An original agglomerative hierarchical clustering approach is used to classify daily rainfall patterns recorded at 5352 stations from DJF 1971 to DJF 1999. Five clusters are retained for analysis. Amongst them, one cluster looks most like the rainfall and circulation mean picture. Another one, representing 37% of the days, describes strong negative rainfall anomalies over South Africa resulting from a regional barotropic trough-ridge-trough wave structure and moisture divergence. These dry anomalies are more frequent in El Niño years (with a 0.75 correlation between ENSO and the seasonal occurrences of that cluster) and are associated with a weakening and / or a regional shift of the Walker-type circulation and the subtropical jet stream. Three clusters, which describe respectively a decrease, an increase and a conservation of the seasonal mean eastward rainfall gradient, are all associated with strong cyclonic moisture flux anomalies centered over Angola or northern Botswana and anomalous convergence located over the largest positive rainfall anomalies, suggesting the importance of thermal low pressures. One of them is clearly associated with tropical-temperate-troughs. The rainfall patterns of these 3 clusters tend to propagate eastward, and their seasonal frequencies tend to increase in La Niña years.

Published
2012
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221. Testing WRF capability in simulating the atmospheric water cycle over East Africa

Author

Pohl , Benjamin, Crétat , Julien, Camberlin , Pierre, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), ANR-08-VULN-0008,PICREVAT,Prévisibilité de l'information climatique pour la réduction de la vulnérabilité de l'agriculture tropicale ( 2008 ), Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), and ANR-08-VULN-0008,PICREVAT,Prévisibilité de l'information climatique pour la réduction de la vulnérabilité de l'agriculture tropicale(2008)

Subjects
[ SDE.MCG ] Environmental Sciences/Global Changes, [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology, [SDE.MCG]Environmental Sciences/Global Changes, WRF, rainfall, water cycle, [ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology, regional climate modeling, East Africa, Physics::Atmospheric and Oceanic Physics, Physics::Geophysics
Abstract

International audience; Uncertainties in simulating the seasonal mean atmospheric water cycle in Equatorial East Africa are quantified using 58 one-year-long experiments performed with the Weather Research and Forecasting model (WRF). Tested parameters include physical parameterizations of atmospheric convection, cloud microphysics, planetary boundary layer, land-surface model and radiation schemes, as well as land-use categories, lateral forcings (ERA-Interim and ERA40 reanalyses), and domain geometry (size and vertical resolution). Results show that (i) uncertainties, defined as the differences between the experiments, are larger than the biases; (ii) the parameters exerting the largest influence on simulated rainfall are, in order of decreasing importance, the shortwave radiation scheme, the land-surface model, the domain size, followed by convective schemes and land-use categories; (iii) cloud microphysics, lateral forcing reanalysis, the number of vertical levels and planetary boundary layer schemes appear to be of lesser importance at the seasonal scale. Though persisting biases (consisting of conditions that are too wet over the Indian Ocean and the Congo Basin and too dry over eastern Kenya) prevail in most experiments, several configurations simulate the regional climate with reasonable accuracy.

Published
2011
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227. Spatial distribution of precipitation annual cycles over South Africa in 10 CORDEX regional climate model present-day simulations

Author

Favre, Alice, primary, Philippon, Nathalie, additional, Pohl, Benjamin, additional, Kalognomou, Evangelia-Anna, additional, Lennard, Christopher, additional, Hewitson, Bruce, additional, Nikulin, Grigori, additional, Dosio, Alessandro, additional, Panitz, Hans-Juergen, additional, and Cerezo-Mota, Ruth, additional

Published
2015
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234. Désagrégation numérique de précipitations en Afrique australe et dynamique atmosphérique associée

Author

Crétat, Julien, Pohl, Benjamin, Drobinski, Philippe, Richard, Yves, Roura, Denis, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS Paris)-École normale supérieure - Paris (ENS Paris), Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS Paris), and Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS)-École des Ponts ParisTech (ENPC)-École polytechnique (X)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC)

Subjects
désagrégation dynamique, [SDE.MCG] Environmental Sciences/Global Changes, [SDU.STU.CL] Sciences of the Universe [physics]/Earth Sciences/Climatology, [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology, [SDE.MCG]Environmental Sciences/Global Changes, dynamique atmosphérique, Afrique australe, Modèle de Circulation Régionale WRF, précipitations
Abstract

Cette étude examine les capacités du modèle Advanced Weather Research Forecast (WRF) à simuler les champs saisonniers de précipitations et la dynamique atmosphérique associée en Afrique australe pendant le cœur de la saison des pluies d'été sud-africaines, de décembre 1993 à février 1994. Sa sensibilité à la paramétrisation physique est aussi analysée en termes de quantités précipitées et de types de pluie. WRF simule convenablement les principales structures pluviométriques saisonnières, notamment les Zones de Convergence Inter-Tropicale et Sud Indienne, bien que les pluies y soient sous-estimées, et le gradient ouest-est observé de la région tropicale en Afrique du Sud. Les régions où le modèle simule le plus de pluies, en particulier sur les reliefs, sont celles où la convergence d'humidité et la vitesse verticale moyenne couche sont les plus fortes. Les quantités précipitées et le type de convection sont davantage dépendants de la paramétrisation de la convection que de celles de la couche limite et de la microphysique. Parmi les tests effectués, le schéma de convection Grell s'avère être le plus performant, simulant à la fois des quantités précipitées comparables aux observations en Afrique du Sud et des systèmes pluviogènes convectifs cohérents avec la saison simulée.

Published
2010

235. Impact de la topographie et de la circulation atmosphérique sur l’îlot de chaleur urbain en situation de canicule (Dijon, France)

Author

Crétat, Julien, Richard, Yves, Planchon, Olivier, Emery, Justin, Poupelin, Melissa, Rega, Mario, Pergaud, Julien, Joly, Daniel, Diallo-Dudek, Julita, Roy, Damien, Granjon, Ludovic, Pohl, Benjamin, Crétat, Julien, Richard, Yves, Planchon, Olivier, Emery, Justin, Poupelin, Melissa, Rega, Mario, Pergaud, Julien, Joly, Daniel, Diallo-Dudek, Julita, Roy, Damien, Granjon, Ludovic, and Pohl, Benjamin

Abstract

Canicules et fortes chaleurs induisent un stress thermique potentiellement accru en milieu urbain. Nous examinons ici la combinaison de ces différents éléments à Dijon, à partir d’un réseau dense de stations avec des mesures horaires sur la période 2014-2021. Pour cela, nous mettons en œuvre une analyse (i) de la circulation atmosphérique synoptique et locale et (ii) des déterminants géophysiques (occupation du sol et topographie). Les cinq canicules détectées persistent 4 à 5 jours et sont associées à des situations de blocage atmosphérique de large échelle favorisant le développement d’inversions thermiques. Sur les 24 nuits étudiées : 60% sont caractérisées par un Îlot de Chaleur Urbain (ICU) excédant +3°C, une inversion thermique souvent supérieure à 0,5°C/100 m et un vent faible (<2 m/s); 30% par un ICU plafonnant à +2°C, un gradient adiabatique et un vent non négligeable (>2 m/s); 10% par un faible ICU, une faible inversion thermique et des conditions de vent variables. Des statistiques comparables sont obtenues par jours de fortes chaleurs (105 jours). Canicules et fortes chaleurs sont associées à deux structures contrastées en fonction des conditions de vent. Un vent non négligeable (>2 m/s) contribue à ventiler l’excès de chaleur de la ville et à limiter le contrôle de la topographie. En résultent des températures très homogènes sur l’ensemble de l’aire d’étude. Au contraire, un vent faible (<2 m/s) maximise le contrôle de l’occupation du sol et de la topographie sur la température de l’air. En résulte un excès de chaleur en ville. La plaine, à l’est, est relativement plus fraîche que le plateau à l’ouest, de même qu’un axe traversant l’agglomération le long du talweg et du cours d’eau (vallée de l’Ouche). Cet axe frais naturel limite l’ICU ou, a minima, favorise de relatifs Îlots de Fraîcheur Urbains nocturnes. Cette étude montre la pertinence de l’analyse combinée d’un réseau de mesures de la température de l’air, de la circulation atmosphérique et des descripteurs géophysiques pour mettre à jour les déterminants de la température de l’air et la spatialiser.

Published
2023
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236. L'Oscillation de Madden-Julian et la variabilité pluviométrique régionale en Afrique Subsaharienne

Author

Pohl, Benjamin, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Université de Bourgogne, Pierre Camberlin(camber@u-bourgogne.fr), Centre de Recherches de Climatologie (CRC), and Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS)

Subjects
atmospheric dynamics, intraseasonal variability, interactions d'échelle, Sub-Saharan Africa, variabilité intrasaisonnière, rainfall, dynamique atmosphérique, climate diagnostic, Afrique Subsaharienne, [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology, diagnostic climatique, Madden-Julian Oscillation, pluviométrie, scale interactions, [ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology, Oscillation de Madden-Julian
Abstract

The role of the Madden-Julian Oscillation ("MJO") on rainfall variability in Sub-Saharan Africa is examined, based on daily rain-gauge records and the NCEP-DOE AMIP-II reanalyses. The convective and dynamical signal associated with the MJO is extracted using two differing methodologies, the BMRC daily indices (Wheeler & Hendon 2004) on the one hand, and a Local Mode Analysis ("LMA", Goulet & Duvel 2000) on the other hand. The temporal variability of the MJO (in terms of period, amplitude, seasonality and location of the convective anomalies) is first analysed. Though the overall amplitude of the signal is not related to El Niño, the oscillations occurring under El Niño (La Niña) conditions tend to be shorter (longer). Surface conditions such as sea surface temperature and the seasonal cycle in the Tropics are two features that play a predominant role on the location of the associated convective clusters. The response of the African rainfall to the MJO is then examined for 3 distinct regional indices. The rainy season(s) is (are) in each case significantly triggered. In Equatorial East Africa Kenya and northern Tanzania) the Highlands show the succession of a dry and a humid phase over the MJO cycle. The associated rain-causing mechanisms mainly involve deep convection. In the eastern plains and on the coast, the peak of rainfall is out-of-phase and results from a strengthening of the trade winds over the Indian basin. A significant influence of the MJO is also detected over Southern Africa (south of 15°S) and depends on an intraseasonal modulation of the SW Indian Ocean heights. Anticyclonic circulations over Tropical Africa favour northerly anomalies that advect moisture over the region from the tropical Indian Ocean. Over West Africa, the MJO is responsible of recurrent dry conditions, alternating with a weak wet phase. The latter does not seem to result from deep convection only. The case of the East African Long Rains (March through May) is finally considered to investigate scale interactions depending on the MJO. At the "shorter" timescales, the diurnal cycle of convection is modified, from one phase of the MJO to another. The onset of the rains and the extremely wet events are also strongly locked on the intraseasonal cycle. At the "longer" timescales, a significant fraction of the interannual variability of the Long Rains is statistically related to the MJO seasonal amplitude during the corresponding trimester. The MJO therefore "adds" rainfall over East Africa, rather than simply enhancing the differences between the humid and the wet phases of the Long Rains.; Le rôle de l'Oscillation de Madden-Julian ("MJO") sur la variabilité pluviométrique de l'Afrique Subsaharienne est ici examiné, à l'aide de relevés pluviométriques quotidiens et des réanalyses NCEP-DOE AMIP-II. Le signal convectif et dynamique associé à la MJO est extrait à l'aide de deux méthodes statistiques, les indices journaliers du BMRC (Wheeler & Hendon 2004) et une analyse en modes locaux ("LMA", Goulet & Duvel 2000). Dans un premier temps, la variabilité temporelle de l'oscillation est abordée à l'échelle de la ceinture tropicale (période, amplitude, saisonnalité, localisation des anomalies convectives). Si l'amplitude globale du signal n'apparaît pas reliée à El Niño, les oscillations se produisant lors des années El Niño (La Niña) tendent à être plus courtes (longues). Les conditions de surface (dont les température de surface marine) et le cycle annuel sont également des paramètres influant fortement sur la localisation des anomalies convectives les plus marquées associées à la MJO. Dans un second temps, la réponse de la pluviométrie à la MJO est examinée dans 3 ensembles régionaux de l'Afrique Subsaharienne. Dans les 3 cas examinés, la (les) saison(s) des pluies est (sont) significativement affectée(s). Sur l'Afrique de l'Est Equatoriale (Kenya, nord de la Tanzanie), les Hautes Terres d'Afrique montrent l'alternance d'une phase humide et d'une phase sèche au cours du cycle de la MJO. Les mécanismes pluviogènes font intervenir essentiellement la convection atmosphérique profonde. Sur les Basses Terres de l'est en revanche, et sur les plaines littorales, le pic de pluie est enregistré en opposition de phase par rapport aux Hautes Terres, et est relié à un renforcement des alizés depuis l'océan Indien. Une influence significative est également trouvée en Afrique Australe (sud de 15°S), et résulte d'un renforcement de l'anticyclone des Mascareignes, qui favorise via une circulation anticyclonique sur l'Afrique, un apport d'humidité sur la région depuis l'océan Indien tropical. Sur l'Afrique de l'Ouest enfin, la MJO est avant tout impliquée dans une baisse récurrente de la pluviométrie, et très secondairement seulement dans une amplification des cumuls journaliers. Cette dernière ne semble pas résulter uniquement de processus convectifs. L'exemple des Long Rains d'Afrique de l'Est (mars à mai) est ensuite utilisé pour explorer les interactions d'échelle dont la MJO est à l'origine. Aux pas de temps "courts", le cycle diurne de la convection est sensiblement modifié d'une phase de la MJO à l'autre ; les dates de démarrage de la saison des pluies et les évènements pluviométriques exceptionnels montrent également un calage très bon sur la phase de la MJO. Aux pas de temps "longs", une partie significative de la variabilité interannuelle des Long Rains est statistiquement corrélée à l'amplitude saisonnière moyenne de la MJO, qui semble dont rajouter régulièrement de la pluie sur la région plutôt que d'amplifier seulement les écarts entre les phases les plus humides et les plus sèches.

Published
2007

237. Impact of topography and land cover on air temperature space-time variability in an urban environment with contrasted topography (Dijon, France, 2014–2021).

Author

Crétat, Julien, Richard, Yves, Pohl, Benjamin, Emery, Justin, Dudek, Julita, Roy, Damien, Pergaud, Julien, Rega, Mario, Poupelin, Mélissa, Joly, Daniel, Thévenin, Thomas, Marquès, Eva, and Masson, Valéry

Subjects
*ATMOSPHERIC temperature, *LAND cover, *AUTUMN, *SPACETIME, *SPRING, *TOPOGRAPHY, *SUMMER
Abstract

The influence of topography and land cover on air temperature space-time variability is examined in an urban environment with contrasted topography through simple and multiple linear regression (SLR and MLR) models, ran for each hour of the period 2014–2021, to explain spatial patterns of air temperature measured by a dense network. The SLR models reveal a complementary influence of topography and land cover, with the largest influence during daytime and nighttime, respectively. The MLR significantly improves upon the SLR models despite persistent intensity errors at night and spatial errors in the early morning. Topography influences air temperatures all year round, with temperature decreasing with height during the day and frequent thermal inversions at night (up to 30% of the time). Impervious surfaces are more influential in summer and early fall, especially during the late afternoon for the fraction covered by buildings and during the early night for the distance from the city centre. They contribute to increase air temperature close to the city centre and where the fraction covered by buildings is large. By contrast, vegetation contributes to cool air temperature during the night, especially in spring and early summer for field crops, summer and early fall for forests, and late fall and winter for low vegetation. Our framework proves to be a low-cost and efficient way to assess how strongly and how recurrently the static surface conditions influence air temperature along the annual and diurnal cycles. It is easily transposable to other areas and study fields. [ABSTRACT FROM AUTHOR]

Published
2024
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238. Climate co-variability between South America and Southern Africa at interannual, intraseasonal and synoptic scales.

Author

Puaud, Yohan, Pohl, Benjamin, Fauchereau, Nicolas, Macron, Clémence, and Beltrando, Gérard

Subjects
*MADDEN-Julian oscillation, *SYNOPTIC climatology, *MULTIVARIATE analysis, *PRINCIPAL components analysis, *STATISTICAL correlation, SOUTH American climate
Abstract

This paper investigates and quantifies co-variability between large-scale convection in the South American and Southern African sectors at different timescales (interannual, intraseasonal and synoptic), during the austral summer seasons (November-February) from 1979 to 2012. Multivariate analyses (Canonical Correlation Analysis and Principal Component Analysis) are applied to daily outgoing longwave radiation (OLR, used as a proxy for atmospheric convection) anomalies to extract the principal modes of variability and co-variability in each and between both regions, filtered to consider the appropriate time-scales. At the interannual timescale, results confirm the predominant role of El Niño Southern Oscillation (ENSO), favoring enhanced convection over both southeastern Brazil and northern Argentina on the one hand, and tropical Africa and the western Indian Ocean on the other hand. At the intraseasonal timescale, the leading mode of co-variability is related to modulations of large-scale atmospheric convection over most of South America, and 10 days later, tropical Southern Africa. This mode accounts for the impacts of the Madden-Julian-oscillation (MJO) over these regions: identifying robust co-variability at the intraseasonal timescale between both regions require thus to consider a temporal shift between the two sectors. At the synoptic scale, however, co-variability consists mostly of a synchronous modulation of the large-scale atmospheric convection over the South American and Southern African sectors. This results from the development of concomitant Rossby waves forming a continuous wave train over the South Atlantic in the mid-latitudes, affecting both the South Atlantic and South Indian Convergence Zones. Among the days when convection shows significant anomalies (30 % of the total days in each sector), this synchronous mode occurs about 25 % of the time, individual Rossby waves modulating convection over one single region only during the remaining 75 % events. Another mode of co-variability, involving a single Rossby wave modulating the convection first over the Americas, and 4 days later over Africa, appears as sensibly weaker than the synchronous mode, suggesting that the 'wave train' mode occurs more frequently than the development and propagation of a single wave that could affect both regions. [ABSTRACT FROM AUTHOR]

Published
2017
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239. Characterization of Heat Waves in the Sahel and Associated Physical Mechanisms.

Author

Oueslati, Boutheina, Pohl, Benjamin, Moron, Vincent, Rome, Sandra, and Janicot, Serge

Subjects
*HEAT waves (Meteorology), *SURFACE energy, *HEAT budget (Geophysics), *CLOUDINESS, *ATMOSPHERIC circulation, *ATMOSPHERIC water vapor
Abstract

Great effort is made to address heat waves (HWs) in developed countries because of their devastating impacts on society, economy, and environment. However, HWs are still understudied over developing countries. This is particularly true in West Africa, and especially in the Sahel, where temperatures recurrently reach critical values, such as during the 2010 HW event in the western Sahel. This work aims at characterizing the Sahelian HWs during boreal spring seasons (April-May-June) and understanding the mechanisms associated with such extreme events. Over the last three decades, Sahelian HWs have been becoming more frequent, lasting longer, covering larger areas, and reaching higher intensities. The physical mechanisms associated with HWs are examined to assess the respective roles of atmospheric dynamics and radiative and turbulent fluxes by analyzing the surface energy budget. Results suggest that the greenhouse effect of water vapor is the main driver of HWs in the western Sahel, increasing minimum temperatures by enhanced downward longwave radiation. Atmospheric circulation plays an important role in sustaining these warm anomalies by advecting moisture from the Atlantic Ocean and the Guinean coasts into the Sahel. Maximum temperature anomalies are mostly explained by increased downward shortwave radiation due to a reduction in cloud cover. Interannual variability of HWs is affected by the delayed impact of El Niño-Southern Oscillation (ENSO), with anomalous temperature warming following warm ENSO events, resulting from an amplified water vapor feedback. [ABSTRACT FROM AUTHOR]

Published
2017
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243. CORRIGENDUM

Author

Macron, Clémence, primary, Pohl, Benjamin, additional, Richard, Yves, additional, and Bessafi, Miloud, additional

Published
2014
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250. Storylines of Sahel Precipitation Change: Roles of the North Atlantic and Euro‐Mediterranean Temperature

Author

Monerie, Paul‐Arthur, Biasutti, Michela, Mignot, Juliette, Mohino, Elsa, Pohl, Benjamin, and Zappa, Giuseppe

Abstract

Future changes in Sahel precipitation are uncertain because of large differences among projections from various models. In order to explore this uncertainty, we use a storyline approach which seeks to identify alternative plausible evolutions of Sahel precipitation and their driving factors. By analyzing projections from the CMIP6 climate models, we show that changes in North Atlantic and in Euro‐Mediterranean temperatures explain up to 60% of the central Sahel precipitation change uncertainty. We then construct several storylines of Sahel precipitation change based on future plausible changes in North Atlantic and Euro‐Mediterranean temperatures. In one storyline, an amplified warming of both the North Atlantic and the Euro‐Mediterranean areas promotes a northward shift of the West African monsoon, increasing precipitation over the central Sahel, while, in the opposite storyline, a moderate warming in both regions is associated with a small change in precipitation over the central Sahel and a decrease in precipitation over the western Sahel, at the end of the 21st century. These results indicate that Sahel precipitation uncertainty will not be substantially reduced unless the uncertainty in the future warming of the North Atlantic and the Euro‐Mediterranean areas is constrained. Variations in the strength of the West African Monsoon (WAM) circulation have societal impacts on around 80 million people from Senegal to Chad. However, future projections of the WAM precipitation are uncertain for the end of the 21st century because of strong differences from one climate model to another. We show here that sources of uncertainty in Sahel precipitation depend on changes in North Atlantic and Euro‐Mediterranean temperature, relative to changes in global mean surface air temperature. We show that differences in how climate models simulate the future warming over the North Atlantic and Euro‐Mediterranean area explain a large proportion of the uncertainty in precipitation change over the central Sahel. We provide a method that could be helpful at selecting models for impact studies, based on their sensitivity to climate change over the North Atlantic ocean and Mediterranean Sea. Future changes in Sahel precipitation are uncertainChanges in North Atlantic and Euro‐Mediterranean temperatures explain up to 60% of the Sahel precipitation change uncertaintyUncertainty in changes in Sahel precipitation is associated with uncertainty in the future northward shift of the Saharan heat low Future changes in Sahel precipitation are uncertain Changes in North Atlantic and Euro‐Mediterranean temperatures explain up to 60% of the Sahel precipitation change uncertainty Uncertainty in changes in Sahel precipitation is associated with uncertainty in the future northward shift of the Saharan heat low

Published
2023
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