Editorial Type:
Article
Article Type:
Research Article

Analysis of Low-Level Atmospheric Moisture Transport Associated with the West African Monsoon

M. Issa Lélé Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma

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Lance M. Leslie Cooperative Institute for Mesoscale Meteorological Studies, and School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Peter J. Lamb Cooperative Institute for Mesoscale Meteorological Studies, and School of Meteorology, University of Oklahoma, Norman, Oklahoma

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Print Publication:
01 Jun 2015
Collections:
Peter J. Lamb Special Collection: Climate Variability and Its Impacts
DOI:
https://doi.org/10.1175/JCLI-D-14-00746.1
Page(s):
4414–4430
Restricted access

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Abstract

The major objective of this study is to re-evaluate the ocean–land transport of moisture for rainfall in West Africa using 1979–2008 NCEP–NCAR reanalysis data. The vertically integrated atmospheric water vapor flux for the surface–850 hPa is calculated to account for total low-level moisture flux contribution to rainfall over West Africa. Analysis of mean monthly total vapor fluxes shows a progressive penetration of the flux into West Africa from the south and west. During spring (April–June), the northward flux forms a “moisture river” transporting moisture current into the Gulf of Guinea coast. In the peak monsoon season (July–September), the southerly transport weakens, but westerly transport is enhanced and extends to 20°N owing to the strengthening West African jet off the west coast. Mean seasonal values of total water vapor flux components across boundaries indicate that the zonal component is the largest contributor to mean moisture transport into the Sahel, while the meridional transport contributes the most over the Guinea coast. For the wet years of the Sahel rainy season (July–September), active anomalies are displaced farther north compared to the long-term average. This includes the latitude of the intertropical front (ITF), the extent of moisture flux, and the zone of strong moisture flux convergence, with an enhanced westerly flow. For the dry Sahel years, the opposite patterns are observed. Statistically significant positive correlations between the zonal moisture fluxes and Sudan–Sahel rainfall totals are most pronounced when the zonal fluxes lead by 1–4 pentads. However, although weak, they still are statistically significant at lags 3 and 4 for meridional moisture fluxes.

Denotes Open Access content.

Corresponding author address: Dr. M. Issa Lélé, Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, 120 David L. Boren Boulevard, Suite 2100, Norman, OK 73072-7304. E-mail: issalele@ou.edu

This article is included in the In Honor of Peter J. Lamb special collection.

Abstract

The major objective of this study is to re-evaluate the ocean–land transport of moisture for rainfall in West Africa using 1979–2008 NCEP–NCAR reanalysis data. The vertically integrated atmospheric water vapor flux for the surface–850 hPa is calculated to account for total low-level moisture flux contribution to rainfall over West Africa. Analysis of mean monthly total vapor fluxes shows a progressive penetration of the flux into West Africa from the south and west. During spring (April–June), the northward flux forms a “moisture river” transporting moisture current into the Gulf of Guinea coast. In the peak monsoon season (July–September), the southerly transport weakens, but westerly transport is enhanced and extends to 20°N owing to the strengthening West African jet off the west coast. Mean seasonal values of total water vapor flux components across boundaries indicate that the zonal component is the largest contributor to mean moisture transport into the Sahel, while the meridional transport contributes the most over the Guinea coast. For the wet years of the Sahel rainy season (July–September), active anomalies are displaced farther north compared to the long-term average. This includes the latitude of the intertropical front (ITF), the extent of moisture flux, and the zone of strong moisture flux convergence, with an enhanced westerly flow. For the dry Sahel years, the opposite patterns are observed. Statistically significant positive correlations between the zonal moisture fluxes and Sudan–Sahel rainfall totals are most pronounced when the zonal fluxes lead by 1–4 pentads. However, although weak, they still are statistically significant at lags 3 and 4 for meridional moisture fluxes.

Denotes Open Access content.

Corresponding author address: Dr. M. Issa Lélé, Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, 120 David L. Boren Boulevard, Suite 2100, Norman, OK 73072-7304. E-mail: issalele@ou.edu

This article is included in the In Honor of Peter J. Lamb special collection.

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  • Agustí-Panareda, A., and Coauthors, 2010a: The ECMWF re-analysis for the AMMA observational campaign. Quart. J. Roy. Meteor. Soc., 136, 1457–1472, doi:10.1002/qj.662.

    • Search Google Scholar
    • Export Citation

  • Agustí-Panareda, A., A. Beljaars, C. Cardinali, I. Genkova, and C. Thorncroft, 2010b: Impacts of assimilating AMMA soundings on ECMWF analyses and forecasts. Wea. Forecasting, 25, 1142–1160, doi:10.1175/2010WAF2222370.1.

    • Search Google Scholar
    • Export Citation

  • Bielli, S., and R. Roca, 2010: Scale decomposition of atmospheric water budget over West Africa during the monsoon 2006 from NCEP/GFS analyses. Climate Dyn., 35, 143–157, doi:10.1007/s00382-009-0597-5.

    • Search Google Scholar
    • Export Citation

  • Bock, O., and M. Nuret, 2009: Verification of nwp model analyses and radiosonde humidity data with GPS precipitable water vapor estimates during AMMA. Wea. Forecasting, 24, 1085–1101, doi:10.1175/2009WAF2222239.1.

    • Search Google Scholar
    • Export Citation

  • Bock, O., and Coauthors, 2011: The large scale water cycle of the West African monsoon. Atmos. Sci. Lett., 12, 51–57, doi:10.1002/asl.288.

    • Search Google Scholar
    • Export Citation

  • Boyd, E., R. J. Cornforth, P. J. Lamb, A. Tarhule, M. I. Lélé, and A. Brouder, 2013: Building resilience to face recurring environmental crisis in African Sahel. Nat. Climate Change,3,631–637, doi:10.1038/nclimate1856.

    • Search Google Scholar
    • Export Citation

  • Cadet, D. L., and O. Nnoli, 1987: Water vapour transport over Africa and the Atlantic Ocean during summer 1979. Quart. J. Roy. Meteor. Soc., 113, 581–602, doi:10.1002/qj.49711347609.

    • Search Google Scholar
    • Export Citation

  • Cook, K. H., 1999: Generation of African easterly jet and its role in determining West African precipitation. J. Climate, 12, 1165–1184, doi:10.1175/1520-0442(1999)012<1165:GOTAEJ>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Couvreux, F., F. Guichard, O. Bock, B. Campistron, J.-P. Lafore, and J.-L. Redelsperger, 2010: Synoptic variability of the monsoon flux over West Africa prior to the onset. Quart. J. Roy. Meteor. Soc., 136, 159–173, doi:10.1002/qj.473.

    • Search Google Scholar
    • Export Citation

  • Druyan, L. M., and R. J. Koster, 1989: Sources of Sahel precipitation for simulated drought and rainy seasons. J. Climate, 2, 1438–1446, doi:10.1175/1520-0442(1989)002<1438:SOSPFS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Eltahir, E. A. B., and R. L. Bras, 1996: Precipitation recycling. Rev. Geophys., 34, 367–379, doi:10.1029/96RG01927.

  • Folland, C. K., T. N. Palmer, and D. E. Parker, 1986: Sahel rainfall and worldwide sea surface temperatures 1901-85. Nature, 320, 602–607, doi:10.1038/320602a0.

    • Search Google Scholar
    • Export Citation

  • Fontaine, B., P. Roucou, and S. Trzaska, 2003: Atmospheric water cycle and moisture fluxes in the West African monsoon: Mean annual cycles and relationship using NCEP/NCAR reanalysis. Geophys. Res. Lett., 30, 1117, doi:10.1029/2002GL015834.

    • Search Google Scholar
    • Export Citation

  • Giannini, A., R. Saravanan, and P. Chang, 2003: Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales. Science, 302, 1027–1030, doi:10.1126/science.1089357.

    • Search Google Scholar
    • Export Citation

  • Gong, C., and E. Eltahir, 1996: Sources of moisture for rainfall in West Africa. Water Resour. Res.,32, 3115–3121, doi:10.1029/96WR01940.

  • Grams, C. M., S. C. Jones, J. H. Marsham, D. J. Parker, J. M. Haywood, and V. Heuveline, 2010: The Atlantic inflow to the Saharan heat low: observations and modelling. Quart. J. Roy. Meteor. Soc., 136, 125–140, doi:10.1002/qj.429.

    • Search Google Scholar
    • Export Citation

  • Gu, G., and R. F. Adler, 2004: Seasonal evolution and variability associated with the West African monsoon system. J. Climate, 17, 3364–3377, doi:10.1175/1520-0442(2004)017<3364:SEAVAW>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Hastenrath, S., 1990: Decadal-scale changes of the circulation in the tropical Atlantic sector associated with Sahel drought. Int. J. Climatol., 10, 459–472, doi:10.1002/joc.3370100504.

    • Search Google Scholar
    • Export Citation

  • Hastenrath, S., 1991: Climate Dynamics of the Tropics. Kluwer Academic, 488 pp.

  • Hastenrath, S., and D. Polzin, 2011: Long-term variations of circulation in the tropical Atlantic sector and Sahel rainfall. Int. J. Climatol., 31, 649–655, doi:10.1002/joc.2116.

    • Search Google Scholar
    • Export Citation

  • Hastenrath, S., and D. Polzin, 2014: Variations of circulation and Sahel rainfall in the twentieth century. Int. J. Climatol., 34, 1693–1698, doi:10.1002/joc.3740.

    • Search Google Scholar
    • Export Citation

  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437–471, doi:10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Kidson, J. W., 1977: African rainfall and its relation to the upper air circulation. Quart. J. Roy. Meteor. Soc., 103, 441–456, doi:10.1002/qj.49710343705.

    • Search Google Scholar
    • Export Citation

  • Lamb, P. J., 1978a: Case studies of tropical Atlantic surface circulation patterns during recent sub-Saharan weather anomalies: 1967 and 1968. Mon. Wea. Rev., 106, 482–491, doi:10.1175/1520-0493(1978)106<0482:CSOTAS>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Lamb, P. J., 1978b: Large-scale tropical Atlantic surface circulation patterns associated with subsaharan weather anomalies. Tellus, 30, 240–251, doi:10.1111/j.2153-3490.1978.tb00839.x.

    • Search Google Scholar
    • Export Citation

  • Lamb, P. J., 1983: West African water vapor variations between recent contrasting sub-Saharan rainy seasons. Tellus, 35A, 198–212, doi:10.1111/j.1600-0870.1983.tb00197.x.

    • Search Google Scholar
    • Export Citation

  • Lamb, P. J., and R. A. Peppler, 1992: Further case studies of tropical Atlantic surface atmospheric and oceanic patterns associated with sub-Saharan drought. J. Climate, 5, 476–488, doi:10.1175/1520-0442(1992)005<0476:FCSOTA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Le Barbé, L., T. Lebel, and D. Tapsoba, 2002: Rainfall variability in West Africa during the years 1950–90. J. Climate, 15, 187–202, doi:10.1175/1520-0442(2002)015<0187:RVIWAD>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Lélé, I. M., and P. J. Lamb, 2010: Variability of the intertropical front (ITF) and rainfall over the West African Sudan–Sahel zone. J. Climate, 23, 3984–4004, doi:10.1175/2010JCLI3277.1.

    • Search Google Scholar
    • Export Citation

  • Long, M., D. Entekhabi, and S. E. Nicholson, 2000: Interannual variability in rainfall, water vapor flux, and vertical motion over West Africa. J. Climate, 13, 3827–3841, doi:10.1175/1520-0442(2000)013<3827:IVIRWV>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Mera, R., A. G. Laing, and F. Semenzzi, 2014: Moisture variability and multiscale interactions during spring in West Africa. Mon. Wea. Rev., 142, 3178–3198, doi:10.1175/MWR-D-13-00175.1.

    • Search Google Scholar
    • Export Citation

  • Meynadier, R., O. Bock, F. Guichard, A. Boone, P. Roucou, and J.-L. Redelsperger, 2010a: West African monsoon water cycle: 1. A hybrid water budget dataset. J. Geophys. Res., 115, D19106, doi:10.1029/2010JD013917.

    • Search Google Scholar
    • Export Citation

  • Meynadier, R., O. Bock, S. Gervois, F. Guichard, J.-L. Redelsperger, A. Agustí-Panareda, and A. Beljaars, 2010b: West African Monsoon water cycle: 2. Assessment of numerical weather prediction water budgets. J. Geophys. Res., 115, D19107, doi:10.1029/2010JD013919.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S., 1985: Sub-Saharan rainfall 1983-84. J. Climate Appl. Meteor., 24, 1388–1391, doi:10.1175/1520-0450(1985)024<1388:SSR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S., 2008: The intensity, location and structure of the tropical rainbelt over West Africa as factors in interannual variability. Int. J. Climatol., 28, 1775–1785, doi:10.1002/joc.1507.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S., 2013: The West African Sahel: A review of recent studies on the rainfall regime and its interannual variability. ISRN Meteor.,2013, 453521, doi:10.1155/2013/453521.

  • Nicholson, S., and J. P. Grist, 2001: A conceptual model for understanding rainfall variability in the West African Sahel on interannual and interdecadal timescales. Int. J. Climatol., 21, 1733–1757, doi:10.1002/joc.648.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S., and J. P. Grist, 2003: The seasonal evolution of the atmospheric circulation over West Africa and equatorial Africa. J. Climate, 16, 1013–1030, doi:10.1175/1520-0442(2003)016<1013:TSEOTA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S., and P. J. Webster, 2007: A physical basis for the interannual variability of rainfall in the Sahel. Quart. J. Roy. Meteor. Soc., 133, 2065–2084, doi:10.1002/qj.104.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S., B. Some, and B. Kone, 2000: A note on recent rainfall conditions in West Africa, including the rainy season of the 1997 ENSO year. J. Climate, 13, 2628–2640, doi:10.1175/1520-0442(2000)013<2628:AAORRC>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Nieto, R., L. Gimeno, and R. M. Trigo, 2006: A Lagrangian identification of major sources of Sahel moisture. Geophys. Res. Lett., 33, L18707, doi:10.1029/2006GL027232.

    • Search Google Scholar
    • Export Citation

  • Omotosho, J. B., 1990: Onset of thunderstorms and precipitation over northern Nigeria. Int. J. Climatol., 10, 849–860, doi:10.1002/joc.3370100807.

    • Search Google Scholar
    • Export Citation

  • Omotosho, J. B., A. A. Balogun, and K. Ogunjobi, 2000: Predicting monthly and seasonal rainfall, onset and cessation of the rainy season in West Africa using only surface data. Int. J. Climatol., 20, 865–880, doi:10.1002/1097-0088(20000630)20:8<865::AID-JOC505>3.0.CO;2-R.

    • Search Google Scholar
    • Export Citation

  • Peyrillé, P., J. P. Lafore, and J. L. Redelsperger, 2007: An idealized two-dimensional framework to study the West African monsoon. Part I: Validation and key controlling factors. J. Atmos. Sci., 64, 2765–2782, doi:10.1175/JAS3919.1.

    • Search Google Scholar
    • Export Citation

  • Pu, B., and K. H. Cook, 2012: Role of the West African westerly jet in Sahel rainfall variations. J. Climate, 25, 2880–2896, doi:10.1175/JCLI-D-11-00394.1.

    • Search Google Scholar
    • Export Citation

  • Redelsperger, J. L., A. Diongue, A. Diedhiou, J. P. Ceron, M. Diop, J. F. Gueremy, and J. P. Lafore, 2002: Multiscale description of a Sahelian synoptic weather system representative of the West African monsoon. Quart. J. Roy. Meteor. Soc., 128, 1229–1257, doi:10.1256/003590002320373274.

    • Search Google Scholar
    • Export Citation

  • Redelsperger, J. L., C. D. Thorncroft, A. Diedhiou, T. Lebel, D. J. Parker, J. Polcher, 2006: African monsoon multidisciplinary analysis: An international research project and field campaign. Bull. Amer. Meteor. Soc., 87, 1739–1746, doi:10.1175/BAMS-87-12-1739.

    • Search Google Scholar
    • Export Citation

  • Spinks, J., and Y.-L. Lin, 2015: Variability of the subtropical highs, African easterly jet and easterly wave intensities over North Africa and Arabian Peninsula in late summer. Int. J. Climatol., doi:10.1002/joc.4226, in press.

    • Search Google Scholar
    • Export Citation

  • Sultan, B., and S. Janicot, 2003: The West African monsoon dynamics. Part II: The “preonset” and “onset” of the summer monsoon. J. Climate, 16, 3407–3427, doi:10.1175/1520-0442(2003)016<3407:TWAMDP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Thorncroft, C. D., H. Nguyen, C. Zhang, and P. Peyrillé, 2011: Annual cycle of the West African monsoon: Regional circulations and associated water vapour transport. Quart. J. Roy. Meteor. Soc., 137, 129–147, doi:10.1002/qj.728.

    • Search Google Scholar
    • Export Citation

  • Ward, M. N., 1998: Diagnosis and short-lead time prediction of summer rainfall in tropical North Africa at interannual and multidecadal timescales. J. Climate, 11, 3167–3191, doi:10.1175/1520-0442(1998)011<3167:DASLTP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Webster, P. J., 1994: The role of hydrological processes in ocean-atmosphere interactions. Rev. Geophys., 32, 427–476, doi:10.1029/94RG01873.

    • Search Google Scholar
    • Export Citation

  • Xie, P., and P. A. Arkin, 1997: Global precipitation: A 17-year monthly analysis based on gauge observations, satellite estimates, and numerical model outputs. Bull. Amer. Meteor. Soc., 78, 2539–2558, doi:10.1175/1520-0477(1997)078<2539:GPAYMA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation

  • Xie, P., J. E. Janowiak, P. A. Arkin, R. F. Adler, A. Gruber, R. Ferraro, G. J. Huffman, and S. Curtis, 2003: GPCP pentad precipitation analyses: An experimental dataset based on gauge observations and satellite estimates. J. Climate, 16, 2197–2214, doi:10.1175/2769.1.

    • Search Google Scholar
    • Export Citation

  • Xue, Y. K., and Coauthors, 2010: Intercomparison and analyses of the climatology of the West African monsoon in the West African Monsoon Modeling and Evaluation project (WAMME) first model intercomparison experiment. Climate Dyn., 35, 3–27, doi:10.1007/s00382-010-0778-2.

    • Search Google Scholar
    • Export Citation

  • Yin, X., A. Gruber, and P. Arkin, 2004: Comparison of the GPCP and CMAP merged gauge-satellite monthly precipitation products for the period 1979–2001. J. Hydrometeor., 5, 1207–1222, doi:10.1175/JHM-392.1.

    • Search Google Scholar
    • Export Citation