332 results on '"Pohl, Benjamin"'
Search Results
302. The Exchequer and King John : bureaucracy and the exercise of power, 1177-1216
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Booker, Daniel W. B., Pohl, Benjamin, and Smith, Brendan
- Abstract
This thesis examines the royal exchequer during the reign of King John (1199-1216) and the ways in which the institution's laws and routines shaped the exercise of power in the early thirteenth century. The exchequer, it shows, was a potent tool of political and financial discipline. Its laws and routines could be used by King John to coerce and intimidate his subjects into greater states of dependency or directed against those targeted for humiliation or destruction. Yet the exchequer also constrained the exercise of power in ways that carried profound consequences for royal government in England and Ireland during the thirteenth century. The exchequer, this thesis shows, had undergone a process of institutionalisation in the decades preceding John's coronation, developing a distinct identity and ideology which governed how its members should conceive of and conduct their duties. Institutionalisation and inertia, it will be argued, impacted upon King John's ability to reform and reorganise the exchequer and the financial system it oversaw. As his situation deteriorated, King John's behaviour was channelled into modes of behaviour which were both effective and in alignment with the exchequer's laws and routines; yet this same behaviour also contributed significantly towards uniting his subjects in common grievance, thus engendering social, political, and financial crises which ultimately dictated the tenor and development of royal government for the remainder of the thirteenth century. This thesis, therefore, raises fundamental questions over the nature of power and the contours of royal agency: it demands that we consider how rulers' behaviour was shaped by the institutions and officials which underpinned their rule.
- Published
- 2021
303. Daily Weather Types in February--June (1979--2016) and Temperature Variations in Tropical North Africa.
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MORON, VINCENT, OUESLATI, BOUTHEINA, and POHL, BENJAMIN
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SEA level , *MADDEN-Julian oscillation , *OCEAN-atmosphere interaction , *SEASONAL temperature variations , *ROSSBY waves - Abstract
This study investigates to what extent weather types (WTs) computed over tropical North Africa and the tropical North Atlantic Ocean (408W--408E, 08--308N) are relevant for documenting intraseasonal and interannual temperature variability in tropical North Africa (west of 378E, 28--278N). Nine WTs are extracted by using clustering analysis of the daily anomalies of sea level pressure and low-level 925-hPa winds from two reanalyses (NCEP--DOE and ERA-Interim) from 1979 to 2016. The analyses are carried out separately for February--March and for April--June, when temperatures reach their annual peak across most of the region. TheWTpatterns mix the effects of different multiscale phenomena, including the extratropical Rossby waves that travel on the northern edge of the domain (and are partly related to the North Atlantic Oscillation), the Madden--Julian oscillation, and Kelvin waves in the subequatorial zone. For each WT, warm (cold) minimum (TN) and maximum (TX) daily temperature anomalies tend to be systematically located east of cyclonic (anticyclonic) low-level circulation anomalies associated with the WT patterns. By modulating the greenhouse effect, the water vapor anomalies exert a major influence, leading to warm (cold) TX and TN anomalies associated with moister (drier) air, through advection from the tropical Atlantic or equatorial latitudes (the Sahara or northern latitudes) toward tropical North Africa. WTs are also useful for monitoring interannual variability of TX/TN anomalies mostly north of 108N in February--March, even if they greatly underestimate the long-term warming trend. Most WTs significantly raise or lower the probability of regional-scale heat peaks, defined as the crossing of the 90th percentile of daily TX or TN. [ABSTRACT FROM AUTHOR]
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- 2018
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304. Intraseasonal and synoptic modulation of diurnal surface solar radiation over Reunion island in the South-West Indian Ocean.
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Tang, Chao, Mialhe, Pauline, Pohl, Benjamin, Morel, Béatrice, Wild, Martin, Koseki, Shunya, Abiodun, Babatunde, Bessafi, Miloud, Lennard, Chris, Kumar Beeharry, Girish, Lollchund, Roddy, Cunden, Tyagaraja S.M., and Singh, Swati
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SOLAR surface , *OCEAN , *REUNIONS , *MADDEN-Julian oscillation , *TROPICAL cyclones - Published
- 2023
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305. Writing History in the Anglo-Norman World: Manuscripts, Makers and Readers, c.1066–c.1250.
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Pohl, Benjamin
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ANGLO-Norman literature , *NONFICTION - Published
- 2019
306. The Cambridge Companion to the Age of William the Conqueror
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Pohl, Benjamin, editor
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- 2022
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307. Variability in flood frequency in sub-Saharan Africa: The role of large-scale climate modes of variability and their future impacts.
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Ekolu, Job, Dieppois, Bastien, Tramblay, Yves, Villarini, Gabriele, Slater, Louise J., Mahé, Gil, Paturel, Jean-Emmanuel, Eden, Jonathan M., Moulds, Simon, Sidibe, Moussa, Camberlin, Pierre, Pohl, Benjamin, and van de Wiel, Marco
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EMERGENCY management , *MODES of variability (Climatology) , *FLOOD risk , *ECONOMIC equilibrium , *TWENTY-first century - Abstract
• Climate variability explains 30–90% of flood variability across Sub-Saharan Africa. • East/southern African flood variability mainly driven by Pacific and Indian Oceans. • West/central African flood variability mainly driven by Atlantic and Mediterranean. • Flood occurrences could vary by ± 10–50 % from 2015 to 2100 due to climate variability. Sub-Saharan Africa (SSA) is strongly affected by flood hazards, endangering human lives and economic stability. However, the role of internal climate modes of variability in driving fluctuations in SSA flood occurrence remains poorly documented and understood. To address this gap, we quantify the relative and combined contribution of large-scale climate drivers to seasonal and regional flood occurrence using a new 65-year daily streamflow dataset, sea-surface temperatures derived from observations, and 12 Single Model Initial-condition Large Ensembles (SMILEs) from the Coupled Model Intercomparison Project Phases 5 and 6. We find significant relationships between floods and large-scale climate variability across SSA, with climatic drivers accounting for 30–90 % of the variability in floods. Notably, western, central, and the summer-rain region of southern Africa display stronger teleconnections to large-scale climate variability in comparison to East Africa and the winter-rain region of South Africa, where regional circulation patterns and human activities may play a more important role. In southern and eastern Africa, floods are mainly influenced by teleconnections with the Pacific and Indian Oceans, while in western and central Africa, teleconnections with the Atlantic Ocean and Mediterranean Sea play a larger role. We also find that the number of floods is projected to fluctuate by ± 10–50 % during the 21st century in response to different sequences of key modes of climate variability. We also note that the relative contributions of large-scale climate variability to future flood risks are generally consistent across all SMILEs. Our findings thus provide valuable information for long-term disaster risk reduction and management. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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308. Millennial‐scale deglaciation across the European Alps at the transition between the Younger Dryas and the Early Holocene – evidence from a new cosmogenic nuclide chronology.
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Protin, Marie, Schimmelpfennig, Irene, Mugnier, Jean‐Louis, Buoncristiani, Jean‐François, Le Roy, Melaine, Pohl, Benjamin, Moreau, Luc, Aumaître, Georges, Bourlès, Didier, and Keddadouche, Karim
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YOUNGER Dryas , *COSMOGENIC nuclides , *GLACIERS , *ALPINE glaciers , *LITTLE Ice Age , *GLACIAL melting , *HOLOCENE Epoch - Abstract
Reconstructing the spatial and temporal response of mountain glaciers to rapid climate change in the past provides access to the effects of current climate change. Yet, the spatial and temporal variability of past glacier fluctuations is not fully understood. In this study, we focus on the timing of glacier fluctuations in the European Alps during the Younger Dryas/Early Holocene (YD/EH) transition. In an effort to elucidate whether glacier fluctuations were synchronous during this period, we present a new chronology of the Alpine Talèfre glacier, based on 14 new 10Be ages of moraines and roches moutonnées. The retreat of Talèfre glacier was initiated during the mid‐YD (~12.4 ka), then it experienced a gradual retreat punctuated by at least three oscillations until ∼11 ka before shrinking substantially within its Little Ice Age limits (13th−19th centuries). Comparison of our findings with published glacier chronologies in the Alpine region highlights broadly synchronous behaviour of glaciers across the Alps between 12 and 10 ka. The coeval glacier fluctuations at a regional scale suggest that common regional climate conditions had a major impact on Alpine glacier variations during the YD/EH transition. The similarity of glacier behaviour and independent temperature records in both the Alpine region and the northern high latitudes suggests a teleconnection between these regions, but differences in the amplitude of the mean annual temperature signals relative to summer temperature indicate pronounced changes in seasonality between the YD and the EH. [ABSTRACT FROM AUTHOR]
- Published
- 2021
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309. Statistical hydrology for evaluating peatland water table sensitivity to simple environmental variables and climate changes application to the mid-latitude/altitude Frasne peatland (Jura Mountains, France).
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Bertrand, Guillaume, Ponçot, Alex, Pohl, Benjamin, Lhosmot, Alexandre, Steinmann, Marc, Johannet, Anne, Pinel, Sébastien, Caldirak, Huseyin, Artigue, Guillaume, Binet, Philippe, Bertrand, Catherine, Collin, Louis, Magnon, Geneviève, Gilbert, Daniel, Laggoun-Deffarge, Fatima, and Toussaint, Marie-Laure
- Abstract
Peatlands are habitats for a range of fragile flora and fauna species. Their eco-physicochemical characteristics make them as outstanding global carbon and water storage systems. These ecosystems occupy 3% of the worldwide emerged land surface but represent 30% of the global organic soil carbon and 10% of the global fresh water volumes. In such systems, carbon speciation depends to a large extent on specific redox conditions which are mainly governed by the depth of the water table. Hence, understanding their hydrological variability, that conditions both their ecological and biogeochemical functions, is crucial for their management, especially when anticipating their future evolution under climate change. This study illustrates how long-term monitoring of basic hydro-meteorological parameters combined with statistical modeling can be used as a tool to evaluate i) the horizontal (type of peat), ii) vertical (acrotelm/catotelm continuum) and iii) future hydrological variability. Using cross-correlations between meteorological data (precipitation, potential evapotranspiration) and water table depth (WTD), we primarily highlight the spatial heterogeneity of hydrological reactivity across the Sphagnum -dominated Frasne peatland (French Jura Mountain). Then, a multiple linear regression model allows performing hydrological projections until 2100, according to regionalized IPCC RCP4.5 and 8.5 scenarios. Although WTD remains stable during the first half of 21th century, seasonal trends beyond 2050 show lower WTD in winter and markedly greater WTD in summer. In particular, after 2050, more frequent droughts in summer and autumn should occur, increasing WTD. These projections are completed with risk evaluations for peatland droughts until 2100 that appear to be increasing especially for transition seasons, i.e. May–June and September–October. Comparing these trends with previous evaluations of phenol concentrations in water throughout the vegetative period, considered as a proxy of plant functioning intensity, highlights that these hydrological modifications during transitional seasons could be a great ecological perturbation, especially by affecting Sphagnum metabolism. Unlabelled Image • Ten years of Frasne peatland hydrometeorological was performed. • Cross-correlation highlights spatial hydrological reactivity heterogeneities. • Multiple Linear Modeling was performed under RCP4.5 and RCP8.5 till 2100. • We observe a stable mean WTD under RCP4.5 but a significant increase under RCP8.5. • Seasonal variability of WTD is expected to increase, especially for RCP8.5. • Both RCP's imply greater summer WTD, potentially impacting the Sphagnum metabolism. [ABSTRACT FROM AUTHOR]
- Published
- 2021
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310. Numerical simulation of surface solar radiation over Southern Africa. Part 2: projections of regional and global climate models.
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Tang, Chao, Morel, Béatrice, Wild, Martin, Pohl, Benjamin, Abiodun, Babatunde, Lennard, Chris, and Bessafi, Miloud
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SOLAR radiation , *ATMOSPHERIC models , *SOLAR surface , *LONG-range weather forecasting , *CLOUDINESS - Abstract
In the second part of this study, possible impacts of climate change on Surface Solar Radiation (SSR) in Southern Africa (SA) are evaluated. We use outputs from 20 regional climate simulations from five Regional Climate Models (RCM) that participate in the Coordinated Regional Downscaling Experiment program over the African domain (CORDEX-Africa) along with their 10 driving Global Climate Models (GCM) from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Multi-model mean projections of SSR trends are consistent between the GCMs and their nested RCMs. However, this consistency is not found for each GCM/RCM setup. Over the centre of SA, GCMs and RCMs project a statistically significant increase in SSR by 2099 of about + 1 W/m2 per decade in RCP4.5 (+ 1.5 W/m2 per decade in RCP8.5) during the DJF season in their multi-model means. Over Eastern Equatorial Africa (EA-E) a statistically significant decrease in SSR of about − 1.5 W/m2 per decade in RCP4.5 (− 2 W/m2 per decade in RCP8.5) is found in the ensemble means in DJF, whereas in JJA SSR is predicted to increase by about + 0.5 W/m2 per decade under RCP4.5 (+ 1 W/m2 per decade in RCP8.5). SSR projections are fairly similar between RCP8.5 and RCP4.5 before 2050 and then the differences between those two scenarios increase up to about 1 W/m2 per decade with larger changes in RCP8.5 than in RCP4.5 scenario. These SSR evolutions are generally consistent with projected changes in Cloud Cover Fraction over SA and may also related to the changes in atmosphere water vapor content. SSR change signals emerge earlier out of internal variability estimated from reanalyses (European Centre for Medium-Range Weather Forecasts Reanalysis ERA-Interim, ERAIN) in DJF in RCMs than in GCMs, which suggests a higher sensitivity of RCMs to the forcing RCP scenarios than their driving GCMs in simulating SSR changes. Uncertainty in SSR change projections over SA is dominated by the internal climate variability before 2050, and after that model and scenario uncertainties become as important as internal variability until the end of the 21st century. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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311. Predicting the seasonal evolution of southern African summer precipitation in the DePreSys3 prediction system.
- Author
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Monerie, Paul-Arthur, Robson, Jon, Dong, Buwen, Dieppois, Bastien, Pohl, Benjamin, and Dunstone, Nick
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SOUTHERN oscillation , *TELECONNECTIONS (Climatology) , *PRECIPITATION variability , *METEOROLOGICAL precipitation , *CONVERGENCE (Meteorology) ,EL Nino - Abstract
We assess the ability of the DePreSys3 prediction system to predict austral summer precipitation (DJF) over southern Africa, defined as the African continent south of 15°S. DePresys3 is a high resolution prediction system (at a horizontal resolution of ~ 60 km in the atmosphere in mid-latitudes and of the quarter degree in the Ocean) and spans the long period 1959–2016. We find skill in predicting interannual precipitation variability, relative to a long-term trend; the anomaly correlation skill score over southern Africa is greater than 0.45 for the first summer (i.e. lead month 2–4), and 0.37 over Mozambique, Zimbabwe and Zambia for the second summer (i.e. lead month 14–16). The skill is related to the successful prediction of the El-Nino Southern Oscillation (ENSO), and the successful simulation of ENSO teleconnections to southern Africa. However, overall skill is sensitive to the inclusion of strong La-Nina events and also appears to change with forecast epoch. For example, the skill in predicting precipitation over Mozambique is significantly larger for the first summer in the 1990–2016 period, compared to the 1959–1985 period. The difference in skill in predicting interannual precipitation variability over southern Africa in different epochs is consistent with a change in the strength of the observed teleconnections of ENSO. After 1990, and consistent with the increased skill, the observed impact of ENSO appears to strengthen over west Mozambique, in association with changes in ENSO related atmospheric convergence anomalies. However, these apparent changes in teleconnections are not captured by the ensemble-mean predictions using DePreSys3. The changes in the ENSO teleconnection are consistent with a warming over the Indian Ocean and modulation of ENSO properties between the different epochs, but may also be associated with unpredictable atmospheric variability. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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312. Numerical simulation of surface solar radiation over Southern Africa. Part 1: Evaluation of regional and global climate models.
- Author
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Tang, Chao, Morel, Béatrice, Wild, Martin, Pohl, Benjamin, Abiodun, Babatunde, and Bessafi, Miloud
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SOLAR radiation , *COMPUTER simulation , *ATMOSPHERIC models , *GENERAL circulation model - Abstract
This study evaluates the performance of climate models in reproducing surface solar radiation (SSR) over Southern Africa (SA) by validating five Regional Climate Models (RCM, including CCLM4, HIRHAM5, RACMO22T, RCA4 and REMO2009) that participated in the Coordinated Regional Downscaling Experiment program over Africa (CORDEX-Africa) along with their ten driving General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 5 over SA. The model simulated SSR was thereby compared to reference data from ground-based measurements, satellite-derived products and reanalyses over the period 1990-2005. Results show that (1) the references obtained from satellite retrievals and reanalyses overall overestimate SSR by up to 10 W/m2 on average when compared to ground-based measurements from the Global Energy Balance Archive, which are located mainly over the eastern part of the southern African continent. (2) Compared to one of the satellite products (Surface Solar Radiation Data Set—Heliosat Edition 2; SARAH-2): GCMs overestimate SSR over SA in terms of their multi-model mean by about 1 W/m2 (compensation of opposite biases over sub-regions) and 7.5 W/m2 in austral summer and winter respectively; RCMs driven by GCMs show in their multimodel mean underestimations of SSR in both seasons with Mean Bias Errors (MBEs) of about − 30 W/m2 in austral summer and about − 14 W/m2 in winter compared to SARAH-2. This multi-model mean low bias is dominated by the simulations of the CCLM4, with negative biases up to − 76 W/m2 in summer and − 32 W/m2 in winter. (3) The discrepancies in the simulated SSR over SA are larger in the RCMs than in the GCMs. (4) In terms of trend during the "brightening" period 1990-2005, both GCMs and RCMs (driven by European Centre for Medium-Range Weather Forecasts Reanalysis ERA-Interim, short as ERAINT and GCMs) simulate an SSR trend of less than 1 W/m2 per decade. However, variations of SSR trend exist among different references data. (5) For individual RCM models, their SSR bias fields seem rather insensitive with respect to the different lateral forcings provided by ERAINT and various GCMs, in line with previous findings over Europe. (6) Biases in SSR are overall qualitatively consistent with those in total cloud cover. The information obtained in present study is of crucial importance for understanding future climate projections of SSR and for relevant impact studies. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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313. The Haskins Society Journal 26: 2014. Studies in Medieval History
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Pohl, Benjamin, contributor, Matlis, Corinna, contributor, Henley, Georgia, contributor, Halfond, Gregory, contributor, Jasperse, Jitske, contributor, Gathagan, Laura L., editor, La Rocca, Maria Cristina, contributor, Jones, Owain Wyn, contributor, Bobrycki, Shane, contributor, Keynes, Simon, contributor, Roche, Thomas, contributor, Heebll-Holm, Thomas K., contributor, and North, William, editor
- Published
- 2015
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314. Trade & Travel
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Oksanen, Eljas, Pohl, Benjamin, and Department of Cultures
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Europe ,GIS mapping ,Shipping ,England ,economic history ,Historical Communications ,615 History and Archaeology ,Historical roads ,Inland navigation - Published
- 2022
315. Warfare and Violence
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Strickland, Matthew and Pohl, Benjamin
- Abstract
Introducing the volume’s fourth and final thematic strand (Cultural Perspectives), this chapter offers a study of warfare and violence in the age of William the Conqueror. Following a general introduction, it scrutinises the justifications of William’s wars and the history of hostilities and their limitations in Normandy. This is followed by a discussion of rebellion against ducal and royal rule contextualised within contemporary cultures of warfare and violence in northern France. The chapter concludes with studies of castles, conquests, and conduct in the Anglo-Norman world of the eleventh century.
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- 2022
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316. Bias correction of dynamically downscaled precipitation to compute soil water deficit for explaining year-to-year variation of tree growth over northeastern France.
- Author
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Boulard, Damien, Castel, Thierry, Camberlin, Pierre, Sergent, Anne-Sophie, Asse, Daphné, Bréda, Nathalie, Badeau, Vincent, Rossi, Aurélien, and Pohl, Benjamin
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TREE growth , *SOIL moisture , *BIAS correction (Topology) , *METEOROLOGICAL precipitation , *WATER shortages , *METEOROLOGICAL research , *WEATHER forecasting - Abstract
This paper documents the accuracy of a post-correction method applied to precipitation regionalized by the Weather Research and Forecasting (WRF) Regional Climate Model (RCM) for improving simulated rainfall and feeding impact studies. The WRF simulation covers Burgundy (northeastern France) at a 8-km resolution and over a 20-year long period (1989–2008). Previous results show a strong deficiency of the WRF model for simulating precipitation, especially when convective processes are involved. In order to reduce such biases, a Quantile Mapping (QM) method is applied to WRF-simulated precipitation using the mesoscale atmospheric analyses system SAFRAN («Système d'Analyse Fournissant des Renseignements Adaptés à la Nivologie») that provides precipitation data at an 8 km resolution. Raw and post-corrected model outputs are next used to compute the soil water balance of 30 Douglas-fir and 57 common Beech stands across Burgundy, for which radial growth data are available. Results show that the QM method succeeds at reducing the model's wet biases in spring and summer. Significant improvements are also noted for rainfall seasonality and interannual variability, as well as its spatial distribution. Based on both raw and post-corrected rainfall time series, a Soil Water Deficit Index (SWDI) is next computed as the sum of the daily deviations between the relative extractible water and a critical value of 40% below which the low soil water content induce stomatal regulation. Post-correcting WRF precipitation does not significantly improve the simulation of the SWDI upon the raw (uncorrected) model outputs. Two characteristic years were diagnosed to explain this unexpected lack of improvement. Although the QM method allows producing realistic precipitation amounts, it does not correct the timing errors produced by the climate model, which is yet a major issue to obtain reliable estimators of local-scale bioclimatic conditions for impact studies. A realistic temporality of simulated precipitation is thus required before using any systematic post-correction method for appropriate climate impact assessment over temperate forests. [ABSTRACT FROM AUTHOR]
- Published
- 2017
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317. Theology at Le Bec
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Giles E. M. Gasper, Pohl, Benjamin, and Gathagan, Laura L.
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Philosophy ,Religious studies ,Theology - Published
- 2017
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318. The Role of the Abbots of Le Bec (1034-1281)
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Véronique Gazeau, Centre Michel de Boüard - Centre de recherches archéologiques et historiques anciennes et médiévales (CRAHAM), Centre National de la Recherche Scientifique (CNRS)-Université de Caen Normandie (UNICAEN), Normandie Université (NU)-Normandie Université (NU), Pohl, Benjamin, and Gathagan, Laura L.L.
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Medieval history ,History ,Le Bec ,Abbaye du Bec Normandie ,Ancient history ,[SHS.HIST]Humanities and Social Sciences/History ,Church history ,ComputingMilieux_MISCELLANEOUS - Abstract
International audience
- Published
- 2017
- Full Text
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319. Étude multi-échelles des précipitations et du couvert végétal au Cameroun : Analyses spatiales, tendances temporelles, facteurs climatiques et anthropiques de variabilité du NDVI
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Djoufack , Viviane, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), FALSH Département de Géographie, Université de Yaoundé I [Yaoundé], Université de Bourgogne, Bernard Fontaine(bernard.fontaine@u-bourgogne.fr), Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Université de Yaoundé I, and Pohl, Benjamin
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[SDU.OCEAN]Sciences of the Universe [physics]/Ocean, Atmosphere ,[ SDU.OCEAN ] Sciences of the Universe [physics]/Ocean, Atmosphere ,demography ,utilisation du sol ,[SDU.OCEAN] Sciences of the Universe [physics]/Ocean, Atmosphere ,variability ,couvert végétal ,NDVI ,démographie ,rainfall ,Précipitations ,land use ,intrasaisonnier ,vegetation cover ,variabilité ,dry spells ,séquences sèches ,intraseasonal - Abstract
Due to its shape and location (2°N-13°N - 8°E-16°E; proximity of the Atlantic Ocean), Cameroon is characterized by a panel of cross-regional climate encountered widely in tropical Africa. Over the region, the decrease rainfall during the second half of the last century has been shown to be associated with stronger recurrence of drier periods, specifically in the core of the rainy season. These conditions have favored the degradation of vegetation cover, driven by socioeconomic and demographic constraints. The substantial impacts on human activities and local society highlight the need to better understand how climate and environmental dynamics do interact locally. The aim of this study is to diagnose multi-scale rainfall variability and its relationship with vegetation cover (natural and/or grown), which is directly or indirectly associated to the land-cover and land-use dynamics at these latitudes. Using observed rainfall data (Climatic Research Unit/punctual), the spatial modes of rainfall variability at annual and intraseasonal scales are defined through Principal Component Analysis (PCA) and Agglomerative Hierarchical Clustering (AHC). These regionalizations lead to the discretisation of 5 climatic zones, distinguished from each other, by both the amount of rainfall and seasonality (unimodal / bimodal). New intraseasonal dry spells statistics (number, length, period of occurrence) are produced as well as dates of onset and end of the vegetative seasons by sub-regions. Using unsupervised classification methods (such as ISODATA) in Normalized Difference Vegetation Index (NDVI) data at a 8km spatial resolution, vegetation cover spatiotemporal distribution and typology were produced. Then, based on a concomitant use of statistical and GIS approaches, higher resolutions of NDVI (SPOT-1Km) and Global Land-cover data (GLC 2000), allowed to further evaluate both the pluviometric and anthropogenic factors (demography, land use) influencing vegetation dynamics. Analysis were carried out in Northern Cameroon (6°N-13°N - 11°E-16°E), which is the most sensitive region with regards to climatic and environmental variability, that could lead to important socio-economic thread locally., De par sa géométrie et sa situation géographique (2°N-13°N - 8°E-16°E ; ouverture sur l'océan Atlantique), le Cameroun offre l'avantage de proposer un ensemble représentatif des climats régionaux rencontrés en Afrique tropicale. La diminution des cumuls de précipitations enregistrée dans la région pendant la seconde moitié du XXe siècle, est associée à la récurrence de périodes anormalement sèches, essentiellement au coeur de la saison des pluies. Ces conditions ont amplifié la dégradation du couvert végétal au travers ses contraintes socioéconomiques et démographiques (déforestation, extension des surfaces d'activité). Les conséquences souvent dommageables de la variabilité climatique en général, et des sécheresses en particulier, sur les hommes et leurs activités suscitent l'intérêt de développer des études pour mieux comprendre comment le climat et les pressions naturelles et environnementales interagissent localement. Ainsi, l'objectif de cette thèse est de diagnostiquer la variabilité multiéchelle (saisonnière, interannuelle, intra-saisonnière, synoptique) des précipitations et les relations qu'elle entretient avec le couvert végétal au sens large qui, à ces latitudes, est associé directement ou non, à la dynamique d'occupation et d'utilisation du sol, particulièrement sur la période 1951-2002. A partir de données de précipitations observées (CRU/ponctuelles), les modes spatiaux de la variabilité ont été définis aux échelles annuelles et interannuelles, par Analyses en Composante Principale (ACP) et la Classification Ascendante Hiérarchique (CAH). Ces méthodes de classifications ont permis de discriminer cinq zones climatiques, différentes les unes des autres par l'intensité des cumuls et la saisonnalité (unimodal/bimodal). Pour chaque zone, l'attention a été portée sur les paramètres intrasaisonniers qui modulent la variabilité annuelle telle que, les séquences sèches (nombre, longueur, périodes d'occurrence) et les variations des dates de début et de fin des périodes végétatives. La répartition du couvert végétal dans l'espace et dans le temps (1982-2002) a été étudiée, en utilisant des méthodes de classification non supervisée (ISODATA) sur les données de NDVI (Normalized Difference Vegetation index) à 8km de résolution. Enfin, des méthodes statistiques et de télédétection ont permis d'évaluer l'impact des facteurs pluviométriques et anthropogéniques (croissance démographique et utilisation du sol) sur la dynamique du couvert végétal en utilisant des bases de données à plus fine résolution (NDVI/1Km ; Global Land Cover (GLC 2000/1Km)). Ces dernières investigations ont été menées dans le Nord-Cameroun (6°N-13°N - 11°E-16°E), qui est la région la plus sensible des points de vue climatique, économique et environnemental.
- Published
- 2011
320. Désagrégation dynamique haute résolution spatiale du climat du Centre Est de la France par le modèle climatique régional ARW/WRF
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Thierry Castel, Yiwen Xu, Yves Richard, Benjamin Pohl, Julien Crétat, Denis Thevenin, Cédric Cuccia, Benjamin Bois, Pascal Roucou, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Météo-France [Paris], Météo France, Pohl, Benjamin, Centre de Recherches de Climatologie ( CRC ), and Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS )
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[ SDE.MCG ] Environmental Sciences/Global Changes ,[SDE.MCG] Environmental Sciences/Global Changes ,[SDU.STU.CL] Sciences of the Universe [physics]/Earth Sciences/Climatology ,Précipitation ,Désagrégation dynamique ,[SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology ,[SDE.MCG]Environmental Sciences/Global Changes ,WRF ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology ,Température - Abstract
Ce travail analyse les capacités du modèle climatique régional ARW/WRF à reproduire les principaux traits du climat (températures de surface et précipitations) du Centre Est de la France à des résolutions spatiales fines, et pour les années 1991 et 2003. La simulation utilise pour la désagrégation du signal une descente d'échelle basée sur trois domaines emboîtés de maille variable de 120 km, 30 km et 7,5 km. Les conditions latérales sont imposées toutes les 6 heures par les ré- analyses ERA-Interim. Les températures et les précipitations observées par le réseau de mesures de Météo-France Bourgogne sont utilisées pour l'évaluation. Les simulations reproduisent bien la température (R²~0,95 et pente~0,97) avec une biais froid moyen de -0,73°C. La corrélation entre les précipitations mensuelles simulées et observées est faible avec de très fort biais humides pour le printemps et l'été 2003. Des expériences numériques montrent la forte sensibilité des précipitations à l'occupation du sol et à la physique et microphysique des nuages et des cumulus.
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- 2010
321. Reproductibilité des pluies et de la dynamique atmosphérique en Afrique Australe dans un modèle climatique régional : Approche multiscalaire
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Julien Crétat, Clémence Macron, Benjamin Pohl, Yves Richard, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), and Pohl, Benjamin
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[SDE.MCG] Environmental Sciences/Global Changes ,[ 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 ,Variabilité pluviométrique ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology ,Afrique Australe ,Modélisation climatique régionale - Abstract
30 simulations numériques ne différant que par les conditions initiales de l'atmosphère permettent de documenter la reproductibilité du climat simulé en Afrique australe par un modèle climatique régional. Les simulations portent sur une saison pluvieuse (Décembre-Février 1993-94) proche de la moyenne en termes de pluies. A l'échelle synoptique, les régimes de circulation détectés sont clairement reproductibles d'une expérience à une autre. A des échelles plus fines, le modèle simule correctement les principaux pics pluviométriques et l'alternance entre phases humides et phases sèches. Certains épisodes peu reproductibles sont néanmoins clairement identifiables et correspondent à un contrôle plus faible des situations de large échelle.
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- 2010
322. La mousson africaine dans un modèle de circulation générale quasi-parfait
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Benjamin Pohl, Hervé Douville, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Centre national de recherches météorologiques (CNRM), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), ANR-06-VULN-0007,IRCAAM,Influence réciproque des climats d'Afrique de l'Ouest, du Sud et du bassin méditerranéen(2006), Pohl, Benjamin, Vulnérabilité : climat et milieux - Influence réciproque des climats d'Afrique de l'Ouest, du Sud et du bassin méditerranéen - - IRCAAM2006 - ANR-06-VULN-0007 - VULN - VALID, Groupe d'étude de l'atmosphère météorologique (CNRM-GAME), Institut national des sciences de l'Univers (INSU - CNRS)-Météo France-Centre National de la Recherche Scientifique (CNRS), Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Groupe d'étude de l'atmosphère météorologique ( CNRM-GAME ), Institut national des sciences de l'Univers ( INSU - CNRS ) -Météo France-Centre National de la Recherche Scientifique ( CNRS ), and ANR-06-VULN-0007,IRCAAM,Influence réciproque des climats d'Afrique de l'Ouest, du Sud et du bassin méditerranéen ( 2006 )
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[ SDE.MCG ] Environmental Sciences/Global Changes ,[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 ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology - Abstract
Le modèle de circulation générale ARPEGE-Climat du CNRM présente des biais secs relativement marqués en Afrique de l'Ouest pendant la mousson, même lorsqu'il est forcé par des termpératures de surface marine observées. Les précipitations sahéliennes restent très inférieures aux observations, en lien avec un flux de mousson de basse troposphère trop faible sur la bande guinéenne et l'océan Atlantique équatorial. On propose ici de guider le modèle ARPEGE-Climat vers les réanalyses ERA-40 en dehors d'une fenêtre comprenant l'Afrique de l'Ouest et le proche Atlantique (technique dite de nudging en point de grille). Ce protocole nous permet de: (i) analyser l'origine régionale ou de large échelle des biais du modèle sur l'Afrique; (ii) séparer l'influence des paramètres locaux et de large échelle dans la variabilité spatio-temporelle du système de mousson, à la fois à des pas de temps interannuels et intrasaisonniers; (iii) réaliser des expériences de sensibilité aux conditions de surface (humidité du sol, températures de surface marine) pour quantifier l'importance des forçages continentaux et océaniques régionaux sur la variabilité de la mousson.
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- 2010
323. Interactions entre le cycle diurne de la convection atmosphérique et l'Oscillation de Madden-Julian en Afrique de l'Est
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Benjamin Pohl, Pierre Camberlin, Pascal Oettli, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), and Pohl, Benjamin
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[SDE.MCG] Environmental Sciences/Global Changes ,[ 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 ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology - Abstract
Interactions entre le cycle intrasaisonnier (MJO) et le cycle diurne de la convection atmosphérique. Comment ces deux cycles sont-ils imbriqués ? Quels sont les effets de la MJO sur le déroulement du cycle diurne (durée, intensité, extension spatiale des évènements convectifs) ?
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- 2007
324. Influence of the Madden-Julian Oscillation on Southern African summer rainfall
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Benjamin Pohl, Nicolas Fauchereau, Yves Richard, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Department of Oceanography, University of Cape Town, Pohl, Benjamin, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Department of Oceanography [Cape Town], and Faculty of Science
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Dynamical climatology ,Atmospheric Science ,010504 meteorology & atmospheric sciences ,Flux ,02 engineering and technology ,Forcing (mathematics) ,subtropical zone ,Atmospheric sciences ,01 natural sciences ,tropical zone ,Déclenchement ,[SDU.STU.CL] Sciences of the Universe [physics]/Earth Sciences/Climatology ,020701 environmental engineering ,Atmospheric convection ,Atmospheric dynamics ,Mécanisme ,Convection atmosphérique ,Madden–Julian oscillation ,Hydroclimatology ,atmospheric precipitation ,[ SDE.MCG ] Environmental Sciences/Global Changes ,[SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology ,Climatology ,Climatologie dynamique ,Outgoing longwave radiation ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology ,Southern Africa ,Geology ,Triggering ,Summer ,[SDE.MCG]Environmental Sciences/Global Changes ,rainfall ,0207 environmental engineering ,mechanism ,Subtropics ,Latitude ,Variation interannuelle ,[SDE.MCG.CG]Environmental Sciences/Global Changes/domain_sde.mcg.cg ,Composite analysis ,[ SDE.MCG.CG ] Environmental Sciences/Global Changes/domain_sde.mcg.cg ,Madden Julian oscillation ,Climate variability ,0105 earth and related environmental sciences ,Interannual variation ,Tropics ,[SDE.MCG] Environmental Sciences/Global Changes ,13. Climate action ,Intraseasonal variation ,Africa - Abstract
Rain-causing mechanisms over Southern Africa (south of 15˚S) involve both tropical and temperate dynamics. Most studies focused on the synoptical timescale, while the intraseasonal (20-120 days) variability has more been neglected to date. This study aims at determining whether the dominant mode of intraseasonal variability in the Tropics, namely the Madden-Julian Oscillation (MJO), has a significant impact on Southern African rainfall and associated atmospheric dynamics. The examination of outgoing longwave radiation (OLR) over Southern Africa shows indeed significant intraseasonal fluctuations at the 30-60 day timescale, i.e. in the pe- riods that are typically reminiscent of the MJO. In order to confirm the implication of the latter, composite analyses are computed, based on the real-time MJO indices defined in Wheeler and Hendon (2004). Strong intraseasonal convective signals are particularly recorded over the region dur- ing its rainy season (November through March). Large-scale organized convective perturbations are seen to propagate eastwards, mainly between 10˚S and 20˚S, and then northwards, over the Rift Valley and the African Great Lakes. They finally reach the MJO-associated equatorial clusters over Tanzania, which complete their circuit towards the East over the Indian Ocean. The corresponding response of the rainfall field, obtained through the analysis of daily rain-gauge records in 7665 stations over Southern Africa, presents the alternation, over the intraseasonal cycle, of a dry and a humid phase, which are both significant. The influence of the MJO on the rainfall field is however not homogeneous spatially. While the southern part of the domain (Western Cape Province and surrounding countries) is very partially influenced, and more closely relates to the mid-latitude dynam- ics, the tropical parts of the domain (Northern Province of South Africa, Namibia, Botswana and Zimbabwe) logically show stronger dependency to the MJO forcing. Rainfall records exhibit there sharp periodicities in the 30-60 day timescale. Moisture flux anomalies, derived from the NCEP-DOE II reanalyses, reveal an in- traseasonal modulation of the mid-tropospheric easterly flow over the Congo basin at 700hPa; these fluctuations are coupled to northerly anomalies that extend from the tropical to the subtropical austral latitudes. They are hypothesized to convey mois- ture from the tropical air masses, and hence to favour wet conditions over the region. During the dry phase, southerly anomalies tend on the contrary to prevail, and are hypothesized to convey dryness from the mid-latitude air masses.
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- 2007
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325. Typology of intraseasonal oscillations based on a Local Mode Analysis
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Benjamin Pohl, Jean-Philippe Duvel, Pierre Camberlin, Pohl, Benjamin, 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 ), 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-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL), 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)
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[ SDE.MCG ] Environmental Sciences/Global Changes ,[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 ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology - Abstract
The Intraseasonal Oscillation (ISO; 20-120 day) is an important component of the variability of the Tropical convection that strongly perturbs the Asian and the Aus- tralian monsoons. Our knowledge of the physical origin of the ISO however remains largely incomplete, partially because of the large variability of its characteristics from one event to another. The aim is to determine how the patterns of the different ISO events may be objectively regrouped in a few types, related for example to season, ENSO or other large scale forcing. Patterns of the different ISO events are extracted using a Local Mode Analysis (LMA, Goulet and Duvel 2000). The LMA is based on complex empirical orthogonal functions computed for successive positions of a moving temporal window. When a maximum of variance percentage is detected, the pattern of the corresponding ISO event is extracted. For the present study, the ISO events are detected by an LMA applied on the 20-120-day band pass filtered OLR field for a large tropical region (0˚-200˚E; 30˚N-30˚S) and for the 1979-2005 period. The 144 events extracted are then objectively clustered into homogeneous types us- ing a Hierarchical Ascending Classification. Our classification in 8 types synthetically describes 50% of the overall diversity of the ISO events. The 8 types depict primarily the seasonality in the ISO modes, with the known north- ward (eastward) propagations over the Indian and Western Pacific basins during the northern summer (winter) seasons. While some similarities are noted from one type to another, important differences confirm however that the ISO may be classified in more than two types (i.e. the summer and winter types). In particular, some modes are very specific to particular months (bogus onset in May over the Bay of Bengal, for example). Also, different types are obtained for each season in relation to interannual variability of the sea surface temperature field related primarily to ENSO. This study also shows different equatorial propagation characteristics (i.e. Madden Julian Oscil- lation) for each type confirming that the MJO is not a phenomenon distinct from the ISO.
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- 2007
326. The Madden-Julian Oscillation (MJO) impacts on Southern African summer rainfall and Tropical-Temperate Interactions
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Benjamin Pohl, Nicolas Fauchereau, Yves Richard, Mathieu Rouault, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Department of Oceanography [Cape Town], University of Cape Town, Pohl, Benjamin, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), and Department of Oceanography
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[ SDE.MCG ] Environmental Sciences/Global Changes ,[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 ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology - Abstract
Composite maps of outgoing longwave radiation (OLR) anomalies over the Madden–Julian oscillation (MJO) cycle show marked intraseasonal fluctuations over southern Africa (south of 15°S). Large-scale convective clusters are seen to propagate eastward and then northward over the continent, mainly between 10° and 20°S. The corresponding response of the rainfall field presents the alternation, over the cycle, of dry and humid phases, which are both significant. Moisture flux anomalies indicate an intraseasonal modulation of the midtropospheric easterly flow over the Congo basin at 700 hPa; these fluctuations are coupled to meridional flux anomalies that extend from the tropical to the subtropical austral latitudes, and favor occurrences of wet or dry conditions over the domain. Though statistically significant, the influence of the MJO on southern Africa is however not homogeneous spatially, and only the tropical areas exhibit sharp periodicities in the 30–60-day period range.
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- 2006
327. Effets des émissions anthropiques de gaz à effet de serre sur le climat du bassin de l'océan Indien
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Benjamin Pohl, Pierre Camberlin, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), and Pohl, Benjamin
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[SDE.MCG] Environmental Sciences/Global Changes ,petite saison des pluies d'Afrique de l'Est ,[ SDE.MCG ] Environmental Sciences/Global Changes ,[SDU.STU.CL] Sciences of the Universe [physics]/Earth Sciences/Climatology ,températures de surface de la mer ,[SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology ,[SDE.MCG]Environmental Sciences/Global Changes ,téléconnexions ,Simulation numérique du climat ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology ,variabilité interannuelle - Abstract
La simulation numérique du climat par le modèle couplé de circulation générale ARPEGE-OPA est utilisée ici pour prédire les effets d'un doublement du CO2 atmosphérique en l'an 2100 sur le climat du bassin de l'Océan Indien. Les températures de surface de la mer, les précipitations et les champs moyens de vents à 850 et 200 hPa sont étudiés au pas de temps trimestriel, sur les deux normales climatiques 1968-1997 et 2070-2099. Pour la fin du XX° siècle, le modèle a tendance à surestimer légèrement les TSM et plus fortement les précipitations, résultat d'un cycle hydrologique simulé trop développé. L'extraction d'indices pluviométriques calculés sur des régions homogènes montre que les téléconnexions entre ces deux paramètres sont également trop fortes. Pour la fin du XXI° siècle, un réchauffement de l'océan superficiel d'environ 2°C est prédit, tandis que les précipitations évoluent de manière contrastée selon la région considérée. Certaines évolutions non négligeables affectent aussi les téléconnexions concernant l'Afrique de l'Est durant le trimestre octobre-décembre ; une modification à long terme de la circulation zonale sur l'Océan Indien peut être évoquée afin d'expliquer ces changements.
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- 2005
328. Intraseasonal oscillations of the East African long rains and their connection with MJO activity over the Indian Ocean
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Pierre Camberlin, Benjamin Pohl, Okoola, R. E., Pohl, Benjamin, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Department of Mateorology, University of Nairobi ( UoN ), Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), and University of Nairobi (UoN)
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[ SDE.MCG ] Environmental Sciences/Global Changes ,[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 ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology - Abstract
Our understanding of the East African long rains (March-May) variability remains relatively poor. Interannual variations are quite small compared to intraseasonal vari- ations. An analysis of pentad rainfall and OLR data shows organised variations in the range of 20-75 days, though quite irregular from year to year. However, rainfall and OLR variations are strongly consistent over the highland region only. For this region, NCEP-DOE II reanalysis data are used to detect atmospheric patterns associated to wet events. Significant zonal wind anomalies, of opposite sign at 850 and 200 hPa, are found locally over East Africa. Anomalous low-level westerlies (upper-level easter- lies) are observed during wet events. Years of weakened or enhanced correlations be- tween rainfall and zonal wind tend to occur simultaneously at 850 and 200 hPa. Zonal cross-sections show that these anomalies are neither isolated in time nor in space : the wind anomalies often tend to propagate eastward, especially over the Indian Ocean. They are suggested to be associated with Madden-Julian Oscillations (MJO). To confirm this hypothesis, a MJO signal is extracted based on an EOF analysis of pentad velocity potential anomalies along the tropics, for the MAM season. The first two principal components depict MJO activity over the Indian Ocean and the Pacific Ocean, respectively, and are in quadrature. Based on Wheeler and Hendon (2004), eight different phases of the MJO signal are identified and their association with at- mospheric dynamics over the East African region is investigated. It is found that sig- nificant zonal wind anomalies occur over East Africa in conjunction with the eastward propagation of a MJO over the global tropics. These anomalies display an opposite signal in the upper and lower levels, a pattern reminiscent of that associated to wet events over the East African Highlands. However, the maximum wind, ascent and rainfall anomalies over this region occur when the MJO-induced convective activity has already settled over the central Indian Ocean. It is also found that rainfall and circulation anomalies near the East African coastal area do not follow this pattern. In particular, anomalous ascending motion occurs well before the development of deep convection over the highlands, and is restricted to the mid-troposphere. The resulting rainfall is considered to be of stratiform origin, hence the absence of a clear OLR signal for wet events along the coast.
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- 2005
329. Influence de l'Oscillation de Madden-Julian sur la variabilité intrasaisonnière des pluies en Afrique de l'Est (Kenya-Tanzanie)
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Benjamin Pohl, Pierre Camberlin, Pohl, Benjamin, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Centre de Recherches de Climatologie (CRC), and Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS)
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[ SDE.MCG ] Environmental Sciences/Global Changes ,[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 ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology - Abstract
Les incidences de l'Oscillation de Madden-Julian, mode de variabilité atmosphérique intrasaisonnier dominant dans la bande tropicale, sur la répartition des pluies d'Afrique de l'Est durant le trimestre Mars-Avril-Mai, sont étudiées sur la période 1979-2002. L'analyse révèle une modulation significative des précipitations. Cependant, de fortes hétérogénéités existent du point de vue des pluies entre les Hautes terres et la côte kenyane, couplées à des mécanismes pluviogènes de nature différente.
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- 2005
330. Variabilité climatique et incidences en région des savanes arbustives au sud de la République du Congo : l'exemple de la vallée du Niari
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M'Bayi, Romuald and Pohl, Benjamin
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[SDE.MCG] Environmental Sciences/Global Changes ,[SDU.STU.CL] Sciences of the Universe [physics]/Earth Sciences/Climatology ,[SHS.GEO] Humanities and Social Sciences/Geography - Abstract
La variabilité climatique dans la vallée du Niari, au sud du Congo, est étudiée à partir des précipitations et des températures, auxquelles est associée l'étude des principaux processus du bilan hydrique. À partir d'une classification automatique hiérarchique ascendante, trois domaines climatiques, correspondant à différentes topographies, sont mis en évidence, permettant ainsi de mieux caractériser la climatologie de la vallée du Niari. Une influence déterminante du relief sur les conditions climatiques d'échelle locale et notamment sur l'organisation spatiale des précipitations est montrée. Cette climatologie permet de préciser une forte linéarité entre les cumuls pluviométriques, les températures et le forçage topographique au pas de temps annuel. Cette thèse étudie aussi la modélisation des principaux termes du bilan hydrique. Ils mettent en évidence une complexité du domaine d'étude, précisant le caractère déficitaire de l'humidité atmosphérique et des réservoirs-sol sur une période variant, selon les domaines, de quatre à six mois. La sécheresse édaphique est montrée principalement par le processus du déstockage et de la déficience hydrique des réservoirs-sol. La caractérisation hydrique est aussi mise en évidence à partir de la réserve et de l'évapotranspiration réelle qui ont permis de déterminer et de préciser le noyau de faible pluviosité, notamment dans les « Basses terres ». Une étude de la synergie entre les mécanismes de la végétation, des paramètres climatique et ceux du bilan de l'eau, est réalisée. Les données de végétations sont issues des relevés d'inventaires des espèces herbacées et arbustives constituées à partir de la bibliographie d'une part, et d'autre part des observations satellitaires provenant des capteurs AVHRR-NOAA. Si le premier jeu de données établit quelques corrélations significatives entre les associations végétales caractérisées par les espèces dominantes, les spectres biologiques et la pluviométrie, le NDVI, quant à lui, permet du suivi du cycle phénologique de la végétation. L'intégration de l'indice de végétation du NDVU montre ainsi la dynamique des biomes savanicoles et forestiers et une variabilité mensuelle spatiotemporelle liée fortement aux données climatiques et aux processus du bilan hydrique. L'évolution temporelle des paramètres climatiques, du bilan hydrique et du signal spectral de la végétation met en évidence une relation synchrone, le maximum de variance étant obtenu après un mois de décalage. Toutefois, le couplage à partir du modèle de corrélation de Pearson, entre la réflectance de la végétation, la réserve hydrique et l'indice de déstockage des réservoirs-sol, ne permet pas d'estimer la productivité végétative des biomasses savanicoles de certaines régions des « Basses et moyennes terres ». L'étude de l'évolution annuelle et saisonnière des corrélations entre les paramètres climatiques et quelques indicateurs océano-atmosphériques a permis de déterminer le forçage qu'exercent les paramètres d'échelles régionale et globale sur la vallée du Niari. Les corrélations significatives montrent que la climatologie de la vallée du Niari reste sensible aux températures de surface de la mer de l'Atlantique sud, aux composantes thermiques et barométriques de l'ENSO et au vent troposphérique liée à l'OQB. Les liens significatifs n'apparaissent plus et sont altérés au pas de temps mensuel. Cependant, les indicateurs océano-atmophériques montrent de bonnes capacités à générer des anomalies climatiques significatives dans la vallée du Niari. Les origines paléoenvironnementales des savanes arbustives étant connues, l'objectif, à terme, est d'essayer de prévoir les réactions spontanées et cultivées aux modifications futures d'origines anthropique et climatique. En outre, l'objectif est de suggérer des approches scientifiques qui permettent une meilleure gestion des écosystèmes et des espaces agricoles en vue d'un développement durable de la vallée du Niari.
- Published
- 2004
331. On the recent warming of the Agulhas Current
- Author
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Mathieu Rouault, Benjamin Pohl, Pierrick Penven, Department of Oceanography, University of Cape Town, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Laboratoire de physique des océans ( LPO ), Institut de Recherche pour le Développement ( IRD ) -Institut Français de Recherche pour l'Exploitation de la Mer ( IFREMER ) -Université de Brest ( UBO ) -Centre National de la Recherche Scientifique ( CNRS ), Department of Oceanography [Cape Town], Laboratoire de physique des océans (LPO), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS), Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), and Pohl, Benjamin
- Subjects
[SDE.MCG] Environmental Sciences/Global Changes ,[ 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 ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology - Abstract
The Agulhas Current is an energetic current driven by the wind field over the Indian Ocean. It has a profound effect on the climate and the coastal ecosystem of South Africa and plays a key role in the global ocean circulation. The current carries warm and salty water from the tropics polewards and controls the exchange of heat and salt between the Indian and Atlantic Oceans. Since the 1980s, the sea surface temperature of the Agulhas Current system has increased significantly. This is due to an increase of its transport in response to an augmentation in wind stress curl in the South Indian Ocean. This causes an intensification of the Agulhas Current system and leads to an increased flux of salt and heat into the Atlantic Ocean. There is also an augmentation in the transfer of energy from the Agulhas Current to the atmosphere due to increased evaporation. These observed changes could have far-reaching consequences over and above their potential regional impacts on ecosystems and climate.
332. L'Oscillation de Madden-Julian et la variabilité pluviométrique régionale en Afrique Subsaharienne
- Author
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Benjamin Pohl, 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 ,[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.
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