Editorial Type:
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Article Type:
Research Article

The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation. Part II: The Boreal Autumn

Amin K. Dezfuli Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland

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Sharon E. Nicholson Earth, Ocean and Atmospheric Science, The Florida State University, Tallahassee, Florida

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Print Publication:
01 Jan 2013
DOI:
https://doi.org/10.1175/JCLI-D-11-00686.1
Page(s):
66–84
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Abstract

This paper examines the mechanisms controlling the year-to-year variability of rainfall over western equatorial Africa during the rainy season of October–December. Five regions with distinct behavior are analyzed separately. Only two show strong associations with the ocean and atmospheric features in the global tropics. These two regions, in the east (the eastern Zaire basin) and west (Angolan coast) of the study area, respectively, demonstrate strikingly opposite relationships with the anomalies of sea surface temperatures (SSTs), sea level pressure (SLP), and east–west atmospheric circulation. The wet (dry) conditions in the eastern Zaire basin are associated with El Niño(La Niña)–like phases. The inverse pattern is apparent for the Angolan coast. The other three regions, lying between these two poles of variability, represent a transition zone with a weak linear relationship to the circulation features.

The vital impact of the east–west circulation cells on rainfall variability results in a stronger association with zonal wind than with SSTs or SLP. In addition to the zonal shift, changes in intensity of the zonal cells also play a crucial role. Variability in both magnitude and location of the circulation cells appear to be modulated by the remote forcing from the Pacific via an atmospheric bridge. However, the eastern sector is impacted mainly when synchronous changes occur in the Indian Ocean, and the western sector is impacted mainly when synchronous changes occur in the Atlantic Ocean.

Corresponding author address: Amin K. Dezfuli, Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles Street, 301 Olin Hall, Baltimore, MD 21218. E-mail: dez@jhu.edu

Abstract

This paper examines the mechanisms controlling the year-to-year variability of rainfall over western equatorial Africa during the rainy season of October–December. Five regions with distinct behavior are analyzed separately. Only two show strong associations with the ocean and atmospheric features in the global tropics. These two regions, in the east (the eastern Zaire basin) and west (Angolan coast) of the study area, respectively, demonstrate strikingly opposite relationships with the anomalies of sea surface temperatures (SSTs), sea level pressure (SLP), and east–west atmospheric circulation. The wet (dry) conditions in the eastern Zaire basin are associated with El Niño(La Niña)–like phases. The inverse pattern is apparent for the Angolan coast. The other three regions, lying between these two poles of variability, represent a transition zone with a weak linear relationship to the circulation features.

The vital impact of the east–west circulation cells on rainfall variability results in a stronger association with zonal wind than with SSTs or SLP. In addition to the zonal shift, changes in intensity of the zonal cells also play a crucial role. Variability in both magnitude and location of the circulation cells appear to be modulated by the remote forcing from the Pacific via an atmospheric bridge. However, the eastern sector is impacted mainly when synchronous changes occur in the Indian Ocean, and the western sector is impacted mainly when synchronous changes occur in the Atlantic Ocean.

Corresponding author address: Amin K. Dezfuli, Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N. Charles Street, 301 Olin Hall, Baltimore, MD 21218. E-mail: dez@jhu.edu
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  • Adamec, D., and J. J. O’Brien, 1978: The seasonal upwelling in the Gulf of Guinea due to remote forcing. J. Phys. Oceanogr., 8, 1050–1060.

    • Search Google Scholar
    • Export Citation

  • Alexander, M. A., I. Blade, M. Newman, J. R. Lanzante, N. C. Lau, and J. D. Scott, 2002: The atmospheric bridge: The influence of ENSO teleconnections on air–sea interaction over the global oceans. J. Climate, 15, 2205–2231.

    • Search Google Scholar
    • Export Citation

  • Alexander, M. A., N. C. Lau, and J. D. Scott, 2004: Broadening the atmospheric bridge paradigm: ENSO teleconnections to the tropical west Pacific-Indian Oceans over the seasonal cycle and to the North Pacific in summer. Earth’s Climate: The Ocean-Atmosphere Interaction, Geophys. Monogr., Vol. 147, Amer. Geophys. Union, 85–103.

  • Ashok, K., Z. Guan, N. H. Saji, and T. Yamagata, 2004: Individual and combined influences of ENSO and the Indian Ocean dipole on the Indian summer monsoon. J. Climate, 17, 3141–3155.

    • Search Google Scholar
    • Export Citation

  • Balas, N., S. E. Nicholson, and D. Klotter, 2007: The relationship of rainfall variability in west central Africa to sea-surface temperature fluctuations. Int. J. Climatol., 27, 1335–1349.

    • Search Google Scholar
    • Export Citation

  • Behera, S. K., J.-J. Luo, S. Masson, P. Delecluse, S. Gualdi, A. Navarra, and T. Yamagata, 2005: Paramount impact of the Indian Ocean dipole on the East African short rains: A CGCM study. J. Climate, 18, 4514–4530.

    • Search Google Scholar
    • Export Citation

  • Camberlin, P., 1995: June-September rainfall in north-eastern Africa and atmospheric signals over the tropics - A zonal perspective. Int. J. Climatol., 15, 773–783.

    • Search Google Scholar
    • Export Citation

  • Camberlin, P., 1996: Intraseasonal variations of June-September rainfall and upper-air circulation over Kenya. Theor. Appl. Climatol., 54, 107–115.

    • Search Google Scholar
    • Export Citation

  • Clark, C. O., P. J. Webster, and J. E. Cole, 2003: Interdecadal variability of the relationship between the Indian Ocean zonal mode and East African coastal rainfall anomalies. J. Climate, 16, 548–554.

    • Search Google Scholar
    • Export Citation

  • Dezfuli, A. K., 2011: Spatio-temporal variability of seasonal rainfall in western equatorial Africa. Theor. Appl. Climatol., 104 (1–2), 57–69.

    • Search Google Scholar
    • Export Citation

  • Flohn, H., 1956: Zur Kenntnis der äquatorialen Westwindzone über Angola. Miscelanea Geofisica, Luanda, 25–28.

  • Gong, X., and M. B. Richman, 1995: On the application of cluster analysis to growing season precipitation data in North America east of the Rockies. J. Climate, 8, 897–931.

    • Search Google Scholar
    • Export Citation

  • Hastenrath, S., 1973: Zur Kenntnis der Windverhältnisse über Angola. Bonner Meteor. Abh., 17, 353–360.

  • Hastenrath, S., D. Polzin, and C. Mutai, 2010: Diagnosing the droughts and floods in equatorial East Africa during boreal autumn 2005–08. J. Climate, 23, 813–817.

    • Search Google Scholar
    • Export Citation

  • Jackson, B., S. E. Nicholson, and D. Klotter, 2009: Mesoscale convective systems over western equatorial Africa and their relationship to large-scale circulation. Mon. Wea. Rev., 137, 1272–1294.

    • Search Google Scholar
    • Export Citation

  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437–471.

  • Kistler, R., and Coauthors, 2001: The NCEP–NCAR 50-Year Reanalysis: Monthly means CD-ROM and documentation. Bull. Amer. Meteor. Soc., 82, 247–267.

    • Search Google Scholar
    • Export Citation

  • Klein, S. A., B. J. Soden, and N.-C. Lau, 1999: Remote sea surface temperature variations during ENSO: Evidence for a tropical atmospheric bridge. J. Climate, 12, 917–932.

    • Search Google Scholar
    • Export Citation

  • Mapande, A., and C. J. C. Reason, 2005: Interannual rainfall variability over western Tanzania. Int. J. Climatol., 25, 1355–1368.

  • Meeuwis, J. M., and J. R. E. Lutjeharms, 1990: Surface thermal characteristic of the Angola–Benguela front. S. Afr. J. Mar. Sci., 9, 261–279.

    • Search Google Scholar
    • Export Citation

  • Mohr, K. L., and E. J. Zipser, 1996: Mesoscale convective systems defined by their 85-GHz ice scattering signature: Size and intensity comparison over tropical oceans and continents. Mon. Wea. Rev., 124, 2417–2437.

    • Search Google Scholar
    • Export Citation

  • Moore, D. W., P. Hisard, J. McCreary, J. Merle, J. O’Brien, J. Picaut, J. Verstraete, and C. Wunsch, 1978: Equatorial adjustments in the eastern Atlantic. Geophys. Res. Lett., 5, 637–640.

    • Search Google Scholar
    • Export Citation

  • Nguyen, H., and J. P. Duvel, 2008: Synoptic wave perturbations and convective systems over equatorial Africa. J. Climate, 21, 6372–6388.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., 1986: The spatial coherence of African rainfall anomalies: Interhemispheric teleconnections. J. Climate Appl. Meteor., 25, 1365–1381.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., 1996: A review of climate dynamics and climate variability in eastern Africa. The Limnology, Climatology, and Paleoclimatology of the East African Lakes, T. C. Johnson and E. O. Odada, Eds., Gordon and Breach, 25–56.

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

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., and J. P. Grist, 2003: The seasonal evolution of the atmospheric circulation over West Africa and equatorial Africa. J. Climate, 16, 1013–1030.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., 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. E., and A. K. Dezfuli, 2013: The relationship of rainfall variability in western equatorial Africa to the tropical oceans and atmospheric circulation. Part I: The boreal spring. J. Climate,26, 45–65.

  • Okoola, R. E., 1999a: A diagnostic study of the eastern Africa monsoon circulation during the Northern Hemisphere spring season. Int. J. Climatol., 19, 143–168l.

    • Search Google Scholar
    • Export Citation

  • Okoola, R. E., 1999b: Midtropospheric circulation patterns associated with extreme dry and wet episodes over equatorial eastern Africa during the Northern Hemisphere spring. J. Appl. Meteor., 38, 1161–1169.

    • Search Google Scholar
    • Export Citation

  • Okumura, Y., and S.-P. Xie, 2006: Some overlooked features of tropical Atlantic climate leading to a new Niño-like phenomenon. J. Climate, 19, 5859–5874.

    • Search Google Scholar
    • Export Citation

  • Pohl, B., and P. Camberlin, 2006: Influence of the Madden-Julian oscillation on East Africa rainfall. II: March-May season extremes and interannual variability. Quart. J. Roy. Meteor. Soc., 132, 25 401–2559.

    • Search Google Scholar
    • Export Citation

  • Rao, A. R., and V. V. Srinivas, 2006: Regionalization of watersheds by hybrid-cluster analysis. J. Hydrol., 318, 37–56.

  • Segele, Z. T., P. J. Lamb, and L. A. Leslie, 2009: Large-scale atmospheric circulation and global sea surface temperature associations with Horn of Africa June-September rainfall. Int. J. Climatol., 29, 1075–1100.

    • Search Google Scholar
    • Export Citation

  • Smith, T. M., R. W. Reynolds, T. C. Peterson, and J. Lawrimore, 2008: Improvements to NOAA’s historical merged land–ocean surface temperature analysis (1880–2006). J. Climate, 21, 2283–2296.

    • Search Google Scholar
    • Export Citation

  • Todd, M. C., and R. Washington, 2004: Climate variability in central equatorial Africa: influence from the Atlantic sector. Geophys. Res. Lett., 31, L23202, doi:10.1029/2004GL020975.

    • Search Google Scholar
    • Export Citation

  • Unal, Y., T. Kindap, and M. Karaca, 2003: Redefining the climate zones of Turkey using cluster analysis. Int. J. Climatol., 23, 1045–1055.

    • Search Google Scholar
    • Export Citation

  • Vigaud, N., Y. Richard, M. Rouault, and N. Fauchereau, 2007: Water vapour transport from the tropical Atlantic and summer rainfall in tropical southern Africa. Climate Dyn., 28, 113–123.

    • Search Google Scholar
    • Export Citation

  • Vondou, D. A., A. Nzeukou, A. Lenouo, and F. M. Kamga, 2010: Seasonal variations in the diurnal patterns of convection in Cameroon-Nigeria and their neighboring areas. Atmos. Sci. Lett., 11, 290–330.

    • Search Google Scholar
    • Export Citation

  • Webster, P. J., A. M. Moore, J. P. Loschnigg, and R. R. Leben, 1999: Coupled ocean-atmosphere dynamics in the Indian Ocean during 1997-98. Nature, 401, 356–360.

    • Search Google Scholar
    • Export Citation
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