ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-26-2569-2008 Large-scale overview of the summer monsoon over West Africa during the AMMA field experiment in 2006 Janicot S. 1 Thorncroft C. D. 2 Ali A. 3 Asencio N. 4 Berry G. 2 Bock O. 5 Bourles B. 6 Caniaux G. 4 Chauvin F. 4 Deme A. 7 Kergoat L. 8 Lafore J.-P. 4 Lavaysse C. 5 Lebel T. 3 Marticorena B. 9 Mounier F. 1 Nedelec P. 10 Redelsperger J.-L. 4 Ravegnani F. 11 Reeves C. E. 12 Roca R. 7 de Rosnay P. 8 Schlager H. 13 Sultan B. 1 Tomasini M. 4 Ulanovsky A. 14 ACMAD forecasters team 15 Institut de Recherches pour le Développement, Laboratoire d'Océanographie et du Climat: Expérimentation et Analyses Numériques, Paris, France University of Albany, Department of Earth and Atmospheric Sciences, Albany, NY, USA Institut de Recherches pour le Développement, Laboratoire d'étude des Transferts en Hydrologie et Environnement, Grenoble, France Meteo-France, Centre National de la Recherche Météorologique, Toulouse, France Université Pierre et Marie Curie, Service d'Aéronomie, Paris, France Inst. de Recherches pour le Développement, Lab. d'Etudes en Géophysique et Océanographie Spatiale, Brest, France Centre National de la Recherche Scientifique, Laboratoire de Météorologie Dynamique, Paris, France Centre National de la Recherche Scientifique, Centre d'Etudes Spatiale de la Biosphère, Toulouse, France Centre National de la Recherche Scientifique, Laboratoire Inter-universitaire des Systèmes Atmosphériques, Créteil, France Centre National de la Recherche Scientifique, Laboratoire d'Aérologie, Toulouse, France Istituto di Scienze dell'Atmosfera e del Clima – Consiglio Nazionale di Ricerca Italiano, Bologna, Italy University of East Anglia, School of Environmental Sciences, Norwich, UK Deutsches Zentrum fuer Luftund Raumfahrt, Institut fuer Physik der Atmosphaere, Oberpfaffenhofen-Wessling, Germany Central Aerological Observatory, Moscow, Russia African Centre of Meteorological Application for Development, Niamey, Niger 05 09 2008 26 9 2569 2595 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/26/2569/2008/angeo-26-2569-2008.html The full text article is available as a PDF file from http://www.ann-geophys.net/26/2569/2008/angeo-26-2569-2008.pdf

The AMMA (African Monsoon Multidisciplinary Analysis) program is dedicated to providing a better understanding of the West African monsoon and its influence on the physical, chemical and biological environment regionally and globally, as well as relating variability of this monsoon system to issues of health, water resources, food security and demography for West African nations. Within this framework, an intensive field campaign took place during the summer of 2006 to better document specific processes and weather systems at various key stages of this monsoon season. This campaign was embedded within a longer observation period that documented the annual cycle of surface and atmospheric conditions between 2005 and 2007. The present paper provides a large and regional scale overview of the 2006 summer monsoon season, that includes consideration of of the convective activity, mean atmospheric circulation and synoptic/intraseasonal weather systems, oceanic and land surface conditions, continental hydrology, dust concentration and ozone distribution. The 2006 African summer monsoon was a near-normal rainy season except for a large-scale rainfall excess north of 15° N. This monsoon season was also characterized by a 10-day delayed onset compared to climatology, with convection becoming developed only after 10 July. This onset delay impacted the continental hydrology, soil moisture and vegetation dynamics as well as dust emission. More details of some less-well-known atmospheric features in the African monsoon at intraseasonal and synoptic scales are provided in order to promote future research in these areas.

 References Adler, R. F., Huffman, G. J., Chang, A., Ferraro, R., Xie, P., Janowiak, J., Rudolf, B., Schneider, U., Curtis, S., Bolvin, D., Gruber, A., Susskind, J., Arkin, P., and Nelkin, E.: The Version 2 Global Precipitation Climatology Project (GPCP) Monthly Precipitation Analysis (1979–Present), J. Hydrometeor., 4, 1147–1167, 2003. Andreae, M. O., Chapuis, A., Cros, B., Fontan, J., Helas, G., Justice, C., Kaufman, Y. J., Minga, A., and Nganga, D.: Ozone and Aitken nuclei over equatorial Africa – Airborne observations during Decafe-88, J. Geophys. Res.-Atmos., 97, 6137–6148, 1992. Avila, L. A. and Pasch, R. J.: Atlantic tropical systems of 1991, Mon. Weather Rev., 120, 2688–2696, 1992. Berry, G., Thorncroft, C., and Hewson, T.: African easterly waves during 2004 – Analysis using objective techniques, Mon. Weather Rev., 135, 1251–1267, 2007. Bevis, M., Businger, S., Herring, T. A., Rocken, C., Anthes, R. A., and Ware, R. H.: GPS Meteorology: Remote Sensing of the Atmospheric Water Vapor Using the Global Positioning System, J. Geophys. Res., 97, 15 787–15 801, 1992. Bock, O., Guichard, F., Janicot, S., Lafore, J. P., Bouin, M. N., and Sultan, B.: Multiscale analysis of precipitable water vapor over Africa from GPS data and ECMWF analyses, Geophys. Res. Lett., 34, L09705, doi:10.1029/2006GL028039, 2007. Bourles, B., Brandt, P., Caniaux, G., Dengler, M., Gouriou, Y., Key, E., Lumpkin, R., Marin, F., Molinari, R. L., and Schmid, C.: African Monsoon Multidisciplinary Analysis (AMMA): Special measurements in the tropical Atlantic, CLIVAR Newsletter Exchanges, 41, 7–9, 2007. Cros, B., Fontan, J., Minga, A., Helas, G., Nganga, D., Delmas, R., Chapuis, A., Benech, B., Druilhet, A., and Andreae, M. O.: Vertical profiles of ozone between 0 and 400 meters in and above the African equatorial forest, J. Geophys. Res.-Atmos., 97, 12 877–12 887, 1992. D'Almeida, G. A.: A model for Saharan dust transport, J. Climate Appl. Meteorol., 25, 903–916, 1986. Delmas, R., Lacaux, J. P., Menaut, J. C., Abbadie, L., Leroux, X., Helas, G., and Lobert, J.: Nitrogen compound emission from biomass burning in tropical African savanna Fos/Decafe-1991 Experiment (Lamto, Ivory Coast), J. Atmos. Chem., 22, 175–193, 1995. Delmas, R., Druilhet, A., Cros, B., Durand, P., Delon, C., Lacaux, J. P., Brustet, J. M., Serca, D., Affre, C., Guenther, A., Greenberg, J., Baugh, W., Harley, P., Klinger, L., Ginoux, P., Brasseur, G., Zimmerman, P. R., Gregoire, J. M., Janodet, E., Tournier, A., Perros, P., Mrion, T., Gaudichet, A., Cachier, H., Ruellan, S., Masclet, P., Cautenet, S., Poulet, D., Biona, C. B., Nganga, D., Tathy, J. P., Minga, A., Loemba-Ndembi, J., and Ceccato, P.: Experiment for regional sources and sinks of oxidants (EXPRESSO): An overview, J. Geophys. Res.-Atmos., 104, 30 609–30 624, 1999. Diedhiou, A., Janicot, S., Viltard, A., de Felice, P., and Laurent, H.: Easterly wave regimes and associated convection over West Africa and the tropical Atlantic~: Results from NCEP/NCAR and ECMWF reanalyses, Clim. Dynam., 15, 795–822, 1999. Fontan, J., Druilhet, A., Benech, B., Lyra, R., and Cros, B.: The Decafe Experiments – Overview and meteorology, J. Geophys. Res.-Atmos., 97, 6123–6136, 1992. Grist, J. P. and Nicholson, S. E.: A study of the dynamical factors influencing the rainfall variability in the West African Sahel, J. Climate, 14, 1337–1359, 2001. Gruhier, C., de Rosnay, P., Kerr, Y., Mougin, E., Ceschia, E., Calvet, J. C., and Richaume, P.: Evaluation of AMSR-E soil moisture product based on ground measurements over temperate and semi-arid regions, Geophys. Res. Lett., 35, L10405, doi:10.1029/2008GL033330, 2008. Guichard, F., Kergoat, L., Mougin, E., Timouk, F., Baup, F., Hiernaux, P., and Lavenu, L.: Surface thermodynamics and radiative budget in the Sahelian Gourma: seasonal and diurnal cycles, J. Hydrol., accepted, 2008. Jonquieres, I., Marenco, A., Maalej, A., and Rohrer, F.: Study of ozone formation and transatlantic transport from biomass burning emissions over West Africa during the airborne tropospheric ozone campaigns TROPOZ I and TROPOZ II, J. Geophys. Res.-Atmos., 103, 19 059–19 073, 1998. Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woolen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetma, A., Reynolds, R., Jenne, R., and Joseph, D.: The NCEP/NCAR 40-year reanalysis project, B. Am. Meteorol. Soc., 77, 437–471, 1996. Lamb, P. J.: Large-scale tropical surface circulation patterns associated with Subsaharan weather anomalies, Tellus, 30, 240–251, 1978a. Lamb, P. J.: Case studies of tropical Atlantic surface circulation patterns during recent sub-Saharan weather anomalies: 1967 and 1968, Mon. Weather Rev., 106, 482–491, 1978b. Lavaysse C., Diedhiou, A., Laurent, H., and Lebel, T.: African Easterly Waves and convective activity in wet and dry sequences of the West African Monsoon, Clim. Dynam., 27, 319–332, 2006. Liebmann, B. and Smith, C. A.: Description of a complete (interpolated) outgoing long-wave radiation dataset, B. Am. Meteorol. Soc., 77, 1275–1277, 1996. Mathon, V., Laurent H., and Lebel, T.: Mesoscale convective system rainfall in the Sahel, J. Appl. Meteor., 41, 1081–1092, 2002. Matthews, M.: Intraseasonal variability over tropical Africa during northern summer, J. Climate, 17, 2427–2440, 2004. Mari, C. H., Cailley, G., Corre, L., Saunois, M., Attié, J. L., Thouret, V., and Stohl, A.: Tracing biomass burning plumes from the Southern Hemisphere during the AMMA 2006 wet season experiment, Atmos. Chem. Phys., 8, 3951–3961, 2008. Marticorena, B. and Bergametti, G.: Two-years simulations of seasonal and interannual changes of the Saharan dust emissions, Geophys. Res. Lett., 23, 1921–1924, 1996. Mekkonen, A., Thorncroft, C. D., Aiyyer, A. R., and Kiladis, G. N.: Convectively coupled Kelvin waves over tropical Africa during the boreal summer: Structure and variability, J. Climate, in press, 2008. Morel, C. and Sénési, S.: A climatology of mesoscale convective systems over Europe using satellite infrared imagery. I: Methodology, Q. J. Roy. Meteorol. Soc., 128, 1953–1992, 2002. Moulin, S., Kergoat, L., Viovy, N., and Dedieu, G.: Global-Scale of Vegetation Phenology Using NOAA/AVHRR Satellite Measurements, J. Climate, 10, 1154–1170, 1997. Mounier, F. and Janicot, S.: Evidence of two independent modes of convection at intraseasonal timescale in the West African summer monsoon, Geophys. Res. Lett., 31, L16116, doi:10.1029/2004GL020665, 2004. Mounier, F. and Janicot, S.: 40-day periodicity mode of variability in the West and Central African monsoon, AMS Conference on Climate Variability and Change, San Antonio, TX, 14–18 January 2007. Mounier, F., Kiladis, G., and Janicot, S.: Analysis of the dominant mode of convectively coupled Kelvin waves in the West African monsoon, J. Climate, 20, 1487–1503, 2007. Mounier, F., Janicot, S., and Kiladis, G.: The West African monsoon dynamics. Part III: The quasi-biweekly zonal dipole, J. Climate, 21, 1911–1928, 2008. Nganga, D., Minga, A., Cros, B., Biona, B., Fishman, J., and Grant, W. B.: The vertical distribution of ozone measured at Brazzaville, Congo, during TRACE A, J. Geophys. Res.-Atmos., 101, 24 095–24 103, 1996. Nicholson, S. E.: Land surface processes and Sahel climate, Rev. Geophys., 38,~117–139, 2000. Njoku, E.: Updated daily AMSR-E/Aqua daily L3 surface soil moisture, interpretive parameters and QC EASE-Grids, March to June 2004, Boulder, CO, USA: National Snow and Ice Data Center, Digital Media, 2004. Okomura, Y. and Xie, S. P.: Interaction of the Atlantic equatorial cold tongue and the African monsoon, J. Climate, 17, 3589–3602, 2004. Parker, D., Fink, A., Janicot, S., Ngamini, J. B., Douglas, M., Afiesimama, E., Agusti-Panareda, A., Beljaars, A., Dide, F., Diedhiou, A., Lebel, T., Polcher, J., Redelsperger, J. L., Thorncroft, C. D., and Wilson, G. A.: The AMMA radiosonde program and its implications for the future of atmospheric monitoring over Africa, B. Am. Meteorol. Soc., 89, 1015–1027, 2008. Pierrehumbert, R. T. and Roca, R.: Evidence for control of Atlantic subtropical humidity by large scale advection, Geophys. Res. Lett., 25, 4537–4540, 1998. Prospero, J. M., Ginoux, P., Torres, O., Nicholson, S. E., and Gill, T. E.: Environmental characterization of global sources of atmospheric soil dust identified with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product, Rev. Geophys., 40(1), 1–31, 2002. Redelsperger, J. L., Parson, D. B., and Guichard, F.: Recovery processes and factors limiting cloud-top height following the arrival of a dry intrusion observed during TOGA-COARE, J. Atmos. Sci., 59, 2438–2457, 2002. Redelsperger, J.-L., Thorncroft, C., Diedhiou, A., Lebel, T., Parker, D., and Polcher, J.: African Monsoon Multidisciplinary Analysis (AMMA): An International Research Project and Field Campaign, B. Am. Meteorol. Soc., 87, 1739–1746, 2006. Reed, R. J., Norquist, D. C., and Recker, E. E.: The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Weather Rev., 105, 17–33, 1977. Roca, R., Lafore, J. P., Piriou, C., and Redelsperger, J. L.: Extra-tropical dry air intrusions into the West African Monsoon mid-troposphere: an important factor for the convective activity over Sahel, J. Atmos. Sci., 62, 390–407, 2005a. Roca, R., Brogniez, H., and Picon, L.: Variability of dry air in the mid-troposphere over the last 20 years: Satellite and trajectory climatologies. AMMA First International Conference, Dakar, Senegal, 28 November–4 December 2005b. Roca, R. and Deme, A.: Dry intrusions in the African monsoon mid-troposphere during the AMMA experiment. Proceedings on the "Celebrating the Monsoon" Conference, Bangalore, India, in press, 2008. Samain, O., Kergoat, L., Hiernaux, P., Guichard, F., Mougin, E., Timouk, F., and Lavenu, F.: Variability of Sahelian albedo from in situ and MODIS data, J. Geophys. Res., 113, D14119 doi:10.1029/2007JD009174, 2008. Sauvage, B., Thouret, V., Cammas, J. P., Brioude, J., Nedelec, P., and Mari, C.: Meridional ozone gradients in the African upper troposphere, Geophys. Res. Lett., 34(3), L03817, doi:10.1029/2006GL028542, 2007. Sauvage, B., Thouret, V., Cammas, J.-P., Gheusi, F., Athier, G., and Nédélec, P.: Tropospheric ozone over Equatorial Africa: regional aspects from the MOZAIC data, Atmos. Chem. Phys., 5, 311–335, 2005. Sultan, B. and Janicot, S.: The West African monsoon dynamics. Part II: The "pre-onset" and the "onset" of the summer monsoon, J. Climate, 16, 3407–3427, 2003. Taylor, C., Kergoat, L., and de Rosnay, P.: Land Surface – Atmosphere Interactions During the AMMA SOP, Clivar Exchanges NewsLetter, AMMA special issue, 41, 20–21, 2007. Thorncroft, C. D. and Blackburn, M.: On the maintenance of the African Easterly Jet, Q. J. Roy. Meteorol. Soc., 125, 763–786, 1999. Thorncroft, C. D. and Hodges, K.: African easterly waves and its relationship to Atlantic tropical cyclone activity, J. Climate, 14, 1166–1179, 2000. Tomasini, M. , Lafore, J. P., Piriou, C., Roca, R., Ramage, R., Laurent, H., Morel, C., and Senesi, S.: Atlas on a climatology of West African Mesoscale Convective Systems, AMMA European Deliverable, 2006. Torrence, C. and Compo, G. P.: A practical guide to Wavelet Analysis, B. Am. Meteorol. Soc., 79, 61–78, 1998.