Seasonal Evolution and Variability Associated with the West African Monsoon System

Guojun Gu Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore County, Baltimore, and Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland

Search for other papers by Guojun Gu in
Current site
Google Scholar
PubMed
Close
and
Robert F. Adler Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland

Search for other papers by Robert F. Adler in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

In this study, the seasonal variations in surface rainfall and associated large-scale processes in the tropical eastern Atlantic and West African region are investigated. The 6-yr (1998–2003) high-quality Tropical Rainfall Measuring Mission (TRMM) rainfall, sea surface temperature (SST), water vapor, and cloud liquid water observations are applied along with the NCEP–NCAR reanalysis wind components and a 4-yr (2000–2003) Quick Scatterometer (Quik SCAT) satellite-observed surface wind product.

Major mean rainfall over West Africa tends to be concentrated in two regions and is observed in two different seasons, manifesting an abrupt shift of the mean rainfall zone during June–July: (i) near the Gulf of Guinea (about 5°N), intense convection and rainfall are seen during April–June and roughly follow the seasonality of SST in the tropical eastern Atlantic, and (ii) along the latitudes of about 10°N over the interior of the West African continent, a second intense rain belt begins to develop in July and remains there during the later summer season. This belt coexists with a northward-moving African easterly jet (AEJ) and its accompanying horizontal and vertical shear zones, the appearance and intensification of an upper-tropospheric tropical easterly jet (TEJ), and a strong low-level westerly flow. Westward-propagating wave signals [i.e., African easterly waves (AEWs)] dominate the synoptic-scale variability during July–September, in contrast to the evident eastward-propagating wave signals during May–June.

The abrupt shift of the mean rainfall zone thus turns out to be a combination of two different physical processes: (i) evident seasonal cycles in the tropical eastern Atlantic Ocean, which modulate convection and rainfall near the Gulf of Guinea by means of SST thermal forcing and SST-related meridional gradient; and (ii) the interaction among the AEJ, TEJ, low-level westerly flow, moist convection, and AEWs during July–September, which modulates rainfall variability in the interior of West Africa, primarily within the ITCZ rain band. Evident seasonality in synoptic-scale wave signals is shown to be a good indication of this seasonal evolution.

Corresponding author address: Dr. Guojun Gu, NASA Goddard Space Flight Center, Code 912, Greenbelt, MD 20771. Email: ggu@agnes.gsfc.nasa.gov

Abstract

In this study, the seasonal variations in surface rainfall and associated large-scale processes in the tropical eastern Atlantic and West African region are investigated. The 6-yr (1998–2003) high-quality Tropical Rainfall Measuring Mission (TRMM) rainfall, sea surface temperature (SST), water vapor, and cloud liquid water observations are applied along with the NCEP–NCAR reanalysis wind components and a 4-yr (2000–2003) Quick Scatterometer (Quik SCAT) satellite-observed surface wind product.

Major mean rainfall over West Africa tends to be concentrated in two regions and is observed in two different seasons, manifesting an abrupt shift of the mean rainfall zone during June–July: (i) near the Gulf of Guinea (about 5°N), intense convection and rainfall are seen during April–June and roughly follow the seasonality of SST in the tropical eastern Atlantic, and (ii) along the latitudes of about 10°N over the interior of the West African continent, a second intense rain belt begins to develop in July and remains there during the later summer season. This belt coexists with a northward-moving African easterly jet (AEJ) and its accompanying horizontal and vertical shear zones, the appearance and intensification of an upper-tropospheric tropical easterly jet (TEJ), and a strong low-level westerly flow. Westward-propagating wave signals [i.e., African easterly waves (AEWs)] dominate the synoptic-scale variability during July–September, in contrast to the evident eastward-propagating wave signals during May–June.

The abrupt shift of the mean rainfall zone thus turns out to be a combination of two different physical processes: (i) evident seasonal cycles in the tropical eastern Atlantic Ocean, which modulate convection and rainfall near the Gulf of Guinea by means of SST thermal forcing and SST-related meridional gradient; and (ii) the interaction among the AEJ, TEJ, low-level westerly flow, moist convection, and AEWs during July–September, which modulates rainfall variability in the interior of West Africa, primarily within the ITCZ rain band. Evident seasonality in synoptic-scale wave signals is shown to be a good indication of this seasonal evolution.

Corresponding author address: Dr. Guojun Gu, NASA Goddard Space Flight Center, Code 912, Greenbelt, MD 20771. Email: ggu@agnes.gsfc.nasa.gov

Save
  • Adler, R. F., G. J. Huffman, D. T. Bolvin, S. Curtis, and E. J. Nelkin, 2000: Tropical rainfall distributions determined using TRMM combined with other satellite and rain gauge information. J. Appl. Meteor, 39 , 20072023.

    • Search Google Scholar
    • Export Citation
  • Adler, R. F., C. Kummerow, D. Bolvin, S. Curtis, and C. Kidd, 2003: Status of TRMM monthly estimates of tropical precipitation. Cloud Systems, Hurricanes, and the Tropical Rainfall Measuring Mission (TRMM): A Tribute to Dr. Joanne Simpson, Meteor. Monogr., No. 51, Amer. Meteor. Soc., 223–234.

    • Search Google Scholar
    • Export Citation
  • Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of North Africa. J. Atmos. Sci, 29 , 7790.

  • Burpee, R. W., 1974: Characteristics of the North African easterly waves during the summers of 1968 and 1969. J. Atmos. Sci, 31 , 15561570.

    • Search Google Scholar
    • Export Citation
  • Eltahir, E. A. B., and C. Gong, 1996: Dynamics of wet and dry years in West Africa. J. Climate, 9 , 10301042.

  • Fontaine, B., and S. Janicot, 1996: Sea surface temperature fields associated with West African rainfall anomaly types. J. Climate, 9 , 29352940.

    • Search Google Scholar
    • Export Citation
  • Goswami, B. N., and D. Sengupta, 2003: A note on the deficiency of NCEP/NCAR reanalysis surface winds over the equatorial Indian Ocean. J. Geophys. Res.,108, 3142, doi;rc10.1029/2002JC001497.

    • Search Google Scholar
    • Export Citation
  • Grist, J. P., and S. E. Nicholson, 2001: A study of the dynamic factors influencing the rainfall variability in the West African Sahel. J. Climate, 14 , 13371359.

    • Search Google Scholar
    • Export Citation
  • Grodsky, S. A., and J. A. Carton, 2001: Coupled land/atmospheric interaction in the West African Monsoon. Geophys. Res. Lett, 28 , 15031506.

    • Search Google Scholar
    • Export Citation
  • Gu, G., and C. Zhang, 2001: A spectrum analysis of synoptic-scale disturbances in the ITCZ. J. Climate, 14 , 27252739.

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

  • Hendon, H. H., and M. L. Salby, 1994: The life cycle of the Madden– Julian oscillation. J. Atmos. Sci, 51 , 22252237.

  • Janicot, S., A. Harzallah, B. Fontaine, and V. Moron, 1998: West African monsoon dynamics and eastern equatorial Atlantic and Pacific SST anomalies (1970–88). J. Climate, 11 , 18741882.

    • Search Google Scholar
    • Export Citation
  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc, 77 , 437471.

  • Laing, A. G., and J. M. Fritsch, 1993: Mesoscale convective complexes in Africa. Mon. Wea. Rev, 121 , 22542263.

  • 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 , 476488.

    • Search Google Scholar
    • Export Citation
  • Lau, K-M., H-T. Wu, and S. Bony, 1997: The role of large-scale atmospheric circulation in the relationship between tropical convection and sea surface temperature. J. Climate, 10 , 381392.

    • Search Google Scholar
    • Export Citation
  • Le Barbe, L., T. Lebel, and D. Tapsoba, 2002: Rainfall variability in West Africa during the years 1950–90. J. Climate, 15 , 187202.

    • Search Google Scholar
    • Export Citation
  • Lebel, T., A. Diedhiou, and H. Laurent, 2003: Seasonal cycle and interannual variability of the Sahelian rainfall at hydrological scales. J. Geophys. Res.,108, 8389, doi:10.1029/2001JD001580.

    • Search Google Scholar
    • Export Citation
  • Li, T., and S. G. H. Philander, 1997: On the seasonal cycle of the equatorial Atlantic ocean. J. Climate, 10 , 813817.

  • Lindzen, R. S., and S. Nigam, 1987: On the role of sea surface temperature gradients in forcing low-level winds and convergence in the Tropics. J. Atmos. Sci, 44 , 24182436.

    • Search Google Scholar
    • Export Citation
  • Liu, W. T., 2002: Progress in scatterometer application. J. Oceanogr, 58 , 121136.

  • Liu, W. T., W. Tang, and P. S. Polito, 1998: NASA Scatterometer provides global ocean-surface wind fields with more structures than numerical weather prediction. Geophys. Res. Lett, 25 , 761764.

    • Search Google Scholar
    • Export Citation
  • Maloney, E. D., and J. T. Kiehl, 2002: MJO-related SST variability over the tropical eastern Pacific during Northern Hemisphere summer. J. Climate, 15 , 675689.

    • Search Google Scholar
    • Export Citation
  • Matthews, A. J., 2004: Intraseasonal variability over tropical Africa during northern summer. J. Climate, 17 , 24272440.

  • Merle, J., 1980: Seasonal heat budget in the equatorial Atlantic Ocean. J. Phys. Oceanogr, 10 , 464469.

  • Mitchell, T. P., and J. M. Wallace, 1992: The annual cycle in equatorial convection and sea surface temperature. J. Climate, 5 , 11401156.

    • Search Google Scholar
    • Export Citation
  • Neelin, J. D., and I. M. Held, 1987: Modeling tropical convergence based on the moist static energy budget. Mon. Wea. Rev, 115 , 312.

  • Nicholson, S. E., 2000: Land surface processes and Sahel climate. Rev. Geophys, 38 , 117139.

  • Nicholson, S. E., and J. P. Grist, 2003: The seasonal evolution of the atmospheric circulation over West Africa and equatorial Africa. J. Climate, 16 , 10131030.

    • Search Google Scholar
    • Export Citation
  • Nicholson, S. E., and Coauthors, 2003: Validation of TRMM and other rainfall estimates with a high-density gauge dataset for West Africa. Part II: Validation of TRMM rainfall products. J. Appl. Meteor, 42 , 13551368.

    • Search Google Scholar
    • Export Citation
  • Opoku-Ankomah, Y., and I. Cordery, 1994: Atlantic sea surface temperatures and rainfall variability in Ghana. J. Climate, 7 , 551558.

  • Redelsperger, J-L., A. Diongue, A. Diedhiou, J-P. Ceron, M. Diop, J-F. Gueremy, and J-P. Lafore, 2002: Multi-scale description of a Sahelian synoptic weather system representative of the West African Monsoon. Quart. J. Roy. Meteor. Soc, 128 , 12291257.

    • Search Google Scholar
    • Export Citation
  • Reynolds, R. W., and T. M. Smith, 1994: Improved global sea surface temperature analyses using optimum interpolation. J. Climate, 7 , 929948.

    • Search Google Scholar
    • Export Citation
  • Rowell, D. P., and J. R. Milford, 1993: On the generation of African squall lines. J. Climate, 6 , 11811193.

  • Smith, S. R., D. M. Lehler, and K. V. Verzone, 2001: Quantifying uncertainties in NCEP reanalyses using high-quality research vessel observations. J. Climate, 14 , 40624072.

    • Search Google Scholar
    • Export Citation
  • Sultan, B., and S. Janicot, 2000: Abrupt shift of the ITCZ over West Africa and intra-seasonal variability. Geophys. Res. Lett, 27 , 33533356.

    • 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 , 34073427.

    • Search Google Scholar
    • Export Citation
  • Sultan, B., S. Janicot, and A. Diedhiou, 2003: The West African monsoon dynamics. Part I: Documentation of intraseasonal variability. J. Climate, 16 , 33893406.

    • Search Google Scholar
    • Export Citation
  • Tai, K-S., and Y. Ogura, 1987: An observational study of easterly waves over the eastern Pacific in the northern summer using FGGE data. J. Atmos. Sci, 44 , 339361.

    • Search Google Scholar
    • Export Citation
  • Thorncroft, C. D., and M. Blackburn, 1999: Maintenance of the African easterly jet. Quart. J. Roy. Meteor. Soc, 125 , 763786.

  • Thorncroft, C. D., and Coauthors, 2003: The JET2000 project: Aircraft observations of the African easterly jet and African easterly waves. Bull. Amer. Meteor. Soc, 84 , 337351.

    • Search Google Scholar
    • Export Citation
  • Weingartner, T. J., and R. H. Weisberg, 1991: A description of the annual cycle in sea surface temperature and upper ocean heat in the equatorial Atlantic. J. Phys. Oceanogr, 21 , 8396.

    • Search Google Scholar
    • Export Citation
  • Wentz, F. J., 1997: A well-calibrated ocean algorithm for special sensor microwave/imager. J. Geophys. Res, 102 , 87038718.

  • Wentz, F. J., C. Gentemann, D. Smith, and D. Chelton, 2000: Satellite measurements of sea surface temperature through clouds. Science, 288 , 847850.

    • Search Google Scholar
    • Export Citation
  • Wheeler, M., and G. N. Kiladis, 1999: Convectively coupled equatorial waves: Analysis of clouds and temperature in the wavenumber–frequency domain. J. Atmos. Sci, 56 , 374399.

    • Search Google Scholar
    • Export Citation
  • Zhang, C., 1993: Large-scale variability of atmospheric deep convection in relation to sea surface temperature in the Tropics. J. Climate, 6 , 18981912.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 1000 303 21
PDF Downloads 720 230 11