Editorial Type:
Article
Article Type:
Research Article

The West African Monsoon Dynamics. Part III: The Quasi-Biweekly Zonal Dipole

Flore Mounier LMD/IPSL, CNRS, Ecole Polytechnique, Palaiseau, France

Search for other papers by Flore Mounier in
Current site
Google Scholar
PubMed Close
,
Serge Janicot LOCEAN/IPSL, IRD, Université Pierre et Marie Curie, Paris, France

Search for other papers by Serge Janicot in
Current site
Google Scholar
PubMed Close
, and
George N. Kiladis Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado

Search for other papers by George N. Kiladis in
Current site
Google Scholar
PubMed Close
Print Publication:
01 May 2008
DOI:
https://doi.org/10.1175/2007JCLI1706.1
Page(s):
1911–1928
Restricted access

Abstract

This paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the African monsoon during northern summer, here identified as the quasi-biweekly zonal dipole (QBZD). The QBZD is primarily characterized by a quasi-stationary zonal dipole of convection whose dimension is larger than the West African monsoon domain, with its two poles centered along the Guinean coast and between 30° and 60°W in the equatorial Atlantic. The QBZD dynamical processes within the Atlantic–Africa domain are examined in some detail. The QBZD has a dipole pattern associated with a Walker-type circulation in the near-equatorial zonal plane. It is controlled both by equatorial atmospheric dynamics through a Kelvin wave–like disturbance propagating eastward between its two poles and by land surface processes over Africa, inducing combined fluctuations in surface temperatures, surface pressure, and low-level zonal winds off the coast of West Africa. When convection is at a minimum over central and West Africa, a lack of cloud cover results in higher net shortwave flux at the surface, which increases surface temperatures and lowers surface pressures. This creates an east–west pressure gradient at the latitude of both the ITCZ (10°N) and the Saharan heat low (20°N), leading to an increase in eastward moisture advection inland. The arrival from the Atlantic of the positive pressure signal associated with a Kelvin wave pattern amplifies the low-level westerly wind component and the moisture advection inland, leading to an increase in convective activity over central and West Africa. Then the opposite phase of the dipole develops. Propagation of the QBZD convective envelope and of the associated 200 high-level velocity potential anomalies is detected from the eastern Pacific to the Indian Ocean. When the effect of the Kelvin wave propagation is removed by filtering, the stationary character of the QBZD is highlighted. The impact of the QBZD in combination with a Kelvin wave is illustrated by a case study of the monsoon onset in 1984.

Corresponding author address: Dr. Serge Janicot, LOCEAN/IPSL, Université Pierre et Marie Curie, Boite 100, 4 Place Jussieu, 75252 Paris CEDEX 05, France. Email: serge.janicot@locean-ipsl.upmc.fr

Abstract

This paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the African monsoon during northern summer, here identified as the quasi-biweekly zonal dipole (QBZD). The QBZD is primarily characterized by a quasi-stationary zonal dipole of convection whose dimension is larger than the West African monsoon domain, with its two poles centered along the Guinean coast and between 30° and 60°W in the equatorial Atlantic. The QBZD dynamical processes within the Atlantic–Africa domain are examined in some detail. The QBZD has a dipole pattern associated with a Walker-type circulation in the near-equatorial zonal plane. It is controlled both by equatorial atmospheric dynamics through a Kelvin wave–like disturbance propagating eastward between its two poles and by land surface processes over Africa, inducing combined fluctuations in surface temperatures, surface pressure, and low-level zonal winds off the coast of West Africa. When convection is at a minimum over central and West Africa, a lack of cloud cover results in higher net shortwave flux at the surface, which increases surface temperatures and lowers surface pressures. This creates an east–west pressure gradient at the latitude of both the ITCZ (10°N) and the Saharan heat low (20°N), leading to an increase in eastward moisture advection inland. The arrival from the Atlantic of the positive pressure signal associated with a Kelvin wave pattern amplifies the low-level westerly wind component and the moisture advection inland, leading to an increase in convective activity over central and West Africa. Then the opposite phase of the dipole develops. Propagation of the QBZD convective envelope and of the associated 200 high-level velocity potential anomalies is detected from the eastern Pacific to the Indian Ocean. When the effect of the Kelvin wave propagation is removed by filtering, the stationary character of the QBZD is highlighted. The impact of the QBZD in combination with a Kelvin wave is illustrated by a case study of the monsoon onset in 1984.

Corresponding author address: Dr. Serge Janicot, LOCEAN/IPSL, Université Pierre et Marie Curie, Boite 100, 4 Place Jussieu, 75252 Paris CEDEX 05, France. Email: serge.janicot@locean-ipsl.upmc.fr

Save
Cover Journal of Climate
Journal of Climate

  • Arkin, P. A., 1979: The relationship between fractional coverage of high cloud and rainfall accumulations during GATE over the B-scale array. Mon. Wea. Rev., 107 , 1382–1387.

    • Search Google Scholar
    • Export Citation

  • Cattell, R. B., 1966: The scree test for the number of factors. Multivariate Behav. Res., 1 , 245–276.

  • Chelliah, M., and G. D. Bell, 2004: Tropical multidecadal and interannual climate variability in the NCEP–NCAR reanalysis. J. Climate, 17 , 1777–1803.

    • Search Google Scholar
    • Export Citation

  • Chen, T-C., J. Yoon, K. J. St. Croix, and E. S. Takle, 2001: Suppressing impacts of the Amazonian deforestation by the global circulation change. Bull. Amer. Meteor. Soc., 82 , 2209–2216.

    • Search Google Scholar
    • Export Citation

  • Diedhiou, A., S. Janicot, A. Viltard, P. de Felice, and H. Laurent, 1999: Easterly wave regimes and associated convection over West Africa and tropical Atlantic: Results from the NCEP/NCAR and ECMWF reanalyses. Climate Dyn., 15 , 795–822.

    • Search Google Scholar
    • Export Citation

  • Duvel, J. P., 1989: Convection over tropical Africa and the Atlantic ocean during northern summer. Part I: Interannual and diurnal variations. Mon. Wea. Rev., 117 , 2782–2799.

    • Search Google Scholar
    • Export Citation

  • Duvel, J. P., 1990: Convection over tropical Africa and the Atlantic ocean during northern summer. Part II: Modulation by easterly waves. Mon. Wea. Rev., 118 , 1855–1868.

    • Search Google Scholar
    • Export Citation

  • Folland, C. K., T. N. Palmer, and D. E. Parker, 1986: Sahel rainfall and worldwide sea temperatures 1901–85. Nature, 320 , 602–607.

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

    • Search Google Scholar
    • Export Citation

  • Grodsky, S. A., and J. A. Carton, 2001: Coupled land/atmosphere interactions in the West African monsoon. Geophys. Res. Lett., 28 , 1503–1506.

    • Search Google Scholar
    • Export Citation

  • Grueber, A., and A. F. Krueger, 1984: The status of the NOAA outgoing longwave radiation data set. Bull. Amer. Meteor. Soc., 65 , 958–962.

    • Search Google Scholar
    • Export Citation

  • Hall, N. M. J., G. N. Kiladis, and C. D. Thorncroft, 2006: Three-dimensional structure and dynamics of African easterly waves. Part II: Dynamical modes. J. Atmos. Sci., 63 , 2231–2245.

    • Search Google Scholar
    • Export Citation

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

  • Hodges, K. I., and C. D. Thorncroft, 1997: Distribution and statistics of African mesoscale convective weather systems based on the ISCCP Meteosat imagery. Mon. Wea. Rev., 125 , 2821–2837.

    • Search Google Scholar
    • Export Citation

  • Hodges, K. I., D. W. Chappell, G. J. Robinson, and G. Yang, 2000: An improved algorithm for generating global window brightness temperatures from multiple satellite infrared imagery. J. Atmos. Oceanic Technol., 17 , 1296–1312.

    • Search Google Scholar
    • Export Citation

  • Janicot, S., and B. Sultan, 2001: Intraseasonal modulation of convection in the West African monsoon. Geophys. Res. Lett., 28 , 523–526.

    • Search Google Scholar
    • Export Citation

  • Janicot, S., S. Trzaska, and I. Poccard, 2001: Summer Sahel-ENSO teleconnection and decadal time scale SST variations. Climate Dyn., 18 , 303–320.

    • Search Google Scholar
    • Export Citation

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

  • Kanamitsu, M., W. Ebisuzaki, J. Woollen, S-K. Yang, J. J. Hnilo, M. Fiorino, and G. L. Potter, 2002: NCEP–DOE AMIP-II reanalysis (R-2). Bull. Amer. Meteor. Soc., 83 , 1631–1643.

    • Search Google Scholar
    • Export Citation

  • Kiladis, G. N., and K. M. Weickmann, 1997: Horizontal structure and seasonality of large-scale circulations associated with submonthly tropical convection. Mon. Wea. Rev., 125 , 1997–2013.

    • Search Google Scholar
    • Export Citation

  • Kiladis, G. N., C. D. Thorncroft, and N. M. J. Hall, 2006: Three-dimensional structure and dynamics of African easterly waves. Part I: Observations. J. Atmos. Sci., 63 , 2212–2230.

    • Search Google Scholar
    • Export Citation

  • Knutson, T. R., and K. M. Weickmann, 1987: 30–60 day atmospheric oscillations: Composite life cycles of convection and circulation anomalies. Mon. Wea. Rev., 115 , 1407–1436.

    • Search Google Scholar
    • Export Citation

  • Laing, A. G., and J. M. Fritsch, 1993: Mesoscale convective complexes in Africa. Mon. Wea. Rev., 121 , 2254–2263.

  • Laing, A. G., and J. M. Fritsch, 1997: The global population of mesoscale convective complexes. Quart. J. Roy. Meteor. Soc., 123 , 389–405.

    • 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.

    • Search Google Scholar
    • Export Citation

  • Lamb, P. J., 1978b: Large-scale tropical surface circulation patterns associated with sub-Saharan weather anomalies. Tellus, 30 , 240–251.

    • Search Google Scholar
    • Export Citation

  • Lamb, P. J., R. A. Peppler, and S. Hastenrath, 1986: Interannual variability in the tropical Atlantic. Nature, 322 , 238–240.

  • Le Barbé, L., T. Lebel, and D. Tapsoba, 2002: Rainfall variability in West Africa during the years 1950–90. J. Climate, 15 , 187–202.

    • Search Google Scholar
    • Export Citation

  • Liebmann, B., and C. A. Smith, 1996: Description of a complete (interpolated) outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc., 77 , 1275–1277.

    • Search Google Scholar
    • Export Citation

  • Mathon, V., and H. Laurent, 2001: Life cycle of Sahelian mesoscale convective cloud systems. Quart. J. Roy. Meteor. Soc., 127 , 377–406.

    • Search Google Scholar
    • Export Citation

  • Mathon, V., H. Laurent, and T. Lebel, 2002: Mesoscale convective system rainfall in the Sahel. J. Appl. Meteor., 41 , 1081–1092.

  • Matthews, A. J., 2004: Intraseasonal variability over tropical Africa during northern summer. J. Climate, 17 , 2427–2440.

  • Mekonnen, A., C. D. Thorncroft, and A. R. Aiyyer, 2006: Analysis of convection and its association with African easterly waves. J. Climate, 19 , 5405–5421.

    • Search Google Scholar
    • Export Citation

  • Mounier, F., 2005: La variabilité intersaisonnière de la mousson d’Afrique de l’Ouest et Centrale. Ph.D. thesis, Université Pierre et Marie Curie, 259 pp.

  • Mounier, F., and S. Janicot, 2004: 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.

    • Search Google Scholar
    • Export Citation

  • Mounier, F., G. N. Kiladis, and S. Janicot, 2007: Analysis of the dominant mode of convectively coupled Kelvin waves in the West African monsoon. J. Climate, 20 , 1487–1503.

    • Search Google Scholar
    • Export Citation

  • Newell, R. E., and J. Kidson, 1984: African mean wind changes between Sahelian wet and dry periods. Int. J. Climatol., 4 , 27–33.

  • Nguyen, T. T. H., 2007: Analyses des perturbations synoptiques et de la modulation diurne des systèmes convectifs sur l’Afrique centrale. Ph.D. thesis, Université Pierre et Marie Curie, 254 pp.

  • Nicholson, S. E., 1978: Climatic variations in the Sahel and other African regions during the past five centuries. J. Arid Environ., 1 , 3–24.

    • Search Google Scholar
    • Export Citation

  • North, G. R., T. L. Bell, R. F. Cahalan, and F. J. Moeng, 1982: Sampling errors in the estimation of empirical orthogonal functions. Mon. Wea. Rev., 110 , 699–706.

    • Search Google Scholar
    • Export Citation

  • Reed, R. J., D. C. Norquist, and E. E. Recker, 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE. Mon. Wea. Rev., 105 , 317–333.

    • Search Google Scholar
    • Export Citation

  • Richman, M. B., 1986: Rotation of principal components. Int. J. Climatol., 6 , 293–335.

  • Rossow, W. B., A. Walker, and M. Roiter, 1997: International Satellite Cloud Climatology Project (ISCCP) description of reduced resolution radiance data. WMO Tech. Doc. 58, World Meteorological Organization, 163 pp.

  • Roundy, P. E., and W. M. Frank, 2004: A climatology of waves in the equatorial region. J. Atmos. Sci., 61 , 2105–2132.

  • Rowell, D. P., 2001: Teleconnections between the tropical Pacific and the Sahel. Quart. J. Roy. Meteor. Soc., 127 , 1683–1706.

  • Rowell, D. P., C. K. Folland, K. Maskell, and M. N. Ward, 1995: Variability of summer rainfall over tropical North Africa (1906–92): Observations and modelling. Quart. J. Roy. Meteor. Soc., 121 , 669–704.

    • Search Google Scholar
    • Export Citation

  • Straub, K. H., and G. N. Kiladis, 2002: Observations of a convectively coupled Kelvin wave in the eastern Pacific ITCZ. J. Atmos. Sci., 59 , 30–53.

    • Search Google Scholar
    • Export Citation

  • Sultan, B., and S. Janicot, 2000: Abrupt shift of the ITCZ over West Africa and intraseasonal variability. Geophys. Res. Lett., 27 , 3353–3356.

    • 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.

    • 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 , 3389–3406.

    • Search Google Scholar
    • Export Citation

  • Thompson Jr., R. M., S. W. Payne, E. E. Recker, and R. J. Reed, 1979: Structure and properties of synoptic-scale wave disturbances in the intertropical convergence zone of the eastern Atlantic. J. Atmos. Sci., 36 , 53–72.

    • Search Google Scholar
    • Export Citation

  • Thorncroft, C. D., and B. J. Hoskins, 1994a: An idealized study of African easterly waves. Part I: A linear view. Quart. J. Roy. Meteor. Soc., 120 , 953–982.

    • Search Google Scholar
    • Export Citation

  • Thorncroft, C. D., and B. J. Hoskins, 1994b: An idealized study of African easterly waves. Part II: A nonlinear view. Quart. J. Roy. Meteor. Soc., 120 , 983–1015.

    • Search Google Scholar
    • Export Citation

  • Torrence, C., and G. P. Compo, 1998: A practical guide to wavelet analysis. Bull. Amer. Meteor. Soc., 79 , 61–78.

  • Wang, H., and R. Fu, 2007: The influence of Amazon rainfall on the Atlantic ITCZ through convectively coupled Kelvin waves. J. Climate, 20 , 1188–1201.

    • 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.

    • Search Google Scholar
    • Export Citation

  • Wheeler, M., G. N. Kiladis, and P. J. Webster, 2000: Large scale dynamical fields associated with convectively coupled equatorial waves. J. Atmos. Sci., 57 , 613–640.

    • Search Google Scholar
    • Export Citation

  • Yang, G-Y., and J. Slingo, 2001: The diurnal cycle in the tropics. Mon. Wea. Rev., 129 , 784–801.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 399 91 11
PDF Downloads 248 72 5