• Baron, C., B. Sultan, M. Balme, B. Sarr, T. Lebel, S. Janicot, and M. Dingkuhn, 2005: From GCM grid cell to agricultural plot: Scale issues affecting modelling of climate impact. Philos. Trans. Roy. Soc. London, 360B , 1463. 20952108.

    • Search Google Scholar
    • Export Citation
  • Betts, A. K., M. Zhao, P. A. Dirmeyer, and A. C. M. Beljaars, 2006: Comparison of ERA40 and NCEP/DOE near-surface data sets with other ISLSCP-II data sets. J. Geophys. Res., 111 .D22S04, doi:10.1029/2006JD007174.

    • Search Google Scholar
    • Export Citation
  • Blackadar, A. K., 1957: Boundary layer wind maxima and their significance for the growth of nocturnal inversions. Bull. Amer. Meteor. Soc., 38 , 283290.

    • Search Google Scholar
    • Export Citation
  • Dai, A., and J. Wang, 1999: Diurnal and semi-diurnal tides in global surface pressure. J. Atmos. Sci., 56 , 38743891.

  • D’Amato, N., and T. Lebel, 1998: On the characteristics of the rainfall events in the Sahel with a view of the analysis of climatic variability. Int. J. Climatol., 18 , 955974.

    • Search Google Scholar
    • Export Citation
  • Dell’Aquila, A., V. Lucarini, P. M. Ruti, and S. Calmanti, 2005: Hayashi spectra of the northern hemisphere mid-latitude atmospheric variability in the NCEP-NCAR and ECMWF reanalyses. Climate Dyn., 25 , 639652.

    • 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 the tropical Atlantic: Results from NCEP/NCAR and ECMWF reanalysis. Climate Dyn., 15 , 795822.

    • Search Google Scholar
    • Export Citation
  • Drobinski, P., B. Sultan, and S. Janicot, 2005: Role of the Hoggar Massif on the West African Monsoon onset. Geophys. Res. Lett., 32 .L01705, doi:10.1029/2004GL020710.

    • 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 , 18551868.

    • Search Google Scholar
    • Export Citation
  • Farge, M., 1992: Wavelet Transform and their application to turbulence. Rev. Fluid Mech., 24 , 395457.

  • Folland, C. K., T. N. Palmer, and D. E. Parker, 1986: Sahel rainfall and worldwide sea temperature 1901–1985. Nature, 320 , 602607.

  • Hastenrath, S., 1995: 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 , 28212837.

    • 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 , 303320.

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

  • 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 , 16311643.

    • Search Google Scholar
    • Export Citation
  • Kutzbach, J. E., 1967: Empirical eigenvectors of sea-level pressure, surface temperature and precipitation complexes over North America. J. Appl. Meteor., 6 , 791802.

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

  • Laing, A. G., and J. M. Fritsch, 1997: The global population of mesoscale convective complexes. Quart. J. Roy. Meteor. Soc., 123 , 389405.

    • Search Google Scholar
    • Export Citation
  • Lamb, P. J., 1978a: Large scale tropical surface circulation patterns associated with Subsaharan weather anomalies. Tellus, 30 , 240251.

    • Search Google Scholar
    • Export Citation
  • Lamb, P. J., 1978b: Case studies of tropical Atlantic surface circulation patterns during recent sub-Saharan weather anomalies: 1967 and 1968. Mon. Wea. Rev., 106 , 482491.

    • 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 , 187202.

    • Search Google Scholar
    • Export Citation
  • Lebel, T., F. Delclaux, L. Le Barbé, and J. Polcher, 2000: From GCM scales to hydrological scales: Rainfall variability in West Africa. Stoch. Environ. Res. Risk Assess., 14 , 275295.

    • 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 , 12751277.

    • 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 , 377406.

    • Search Google Scholar
    • Export Citation
  • Mohr, K. I., 2004: Interannual, monthly, and regional variability in the wet season diurnal cycle of precipitation in sub-Saharan Africa. J. Climate, 17 , 24412453.

    • Search Google Scholar
    • Export Citation
  • Ouergli, A., and P. De Felice, 1997: Wavelet transform technique to study the behavior of the 10–20-day and 25–50-day modes during Indian summer monsoon. Meteor. Atmos. Phys., 63 , 171178.

    • Search Google Scholar
    • Export Citation
  • Parker, D. J., and Coauthors, 2005: The diurnal cycle of the West African monsoon circulation. Quart. J. Roy. Meteor. Soc., 131 , 28392860.

    • Search Google Scholar
    • Export Citation
  • Peyrille, P., and J-P. Lafore, 2007: An idealized two-dimensional framework to study the West African monsoon. Part II: Large-scale advection and the diurnal cycle. J. Atmos. Sci., in press.

    • Search Google Scholar
    • Export Citation
  • Racz, Z., and R. K. Smith, 1999: The dynamics of heat lows. Quart. J. Roy. Meteor. Soc., 125 , 225252.

  • 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 , 317333.

    • Search Google Scholar
    • Export Citation
  • Rotunno, R., 1983: On the linear theory of the land and sea breeze. J. Atmos. Sci., 40 , 19992009.

  • Rowell, D. P., 2001: Teleconnections between the Tropical Pacific and the Sahel. Quart. J. Roy. Meteor. Soc., 127 , 16831706.

  • Rowell, D. P., C. K. Folland, K. Maskell, and M. N. Ward, 1995: Variability of summer rainfall over Tropical North Africa (1906–1992): Observations and modelling. Quart. J. Roy. Meteor. Soc., 121 , 669704.

    • Search Google Scholar
    • Export Citation
  • Schubert, S. D., H. M. Helfand, C. Wu, and W. Min, 1998: Subseasonal variations in warm-season moisture transport and precipitation over the central and eastern United States. J. Climate, 11 , 25302555.

    • Search Google Scholar
    • Export Citation
  • Simmons, A. J., and J. K. Gibson, 2000: The ERA-40 project plan. ERA-40 project Rep. Series 1, ECMWF, Shinfield Park, Reading, United Kingdom, 63 pp.

  • Sterl, A., 2004: On the (in-)homogeneity of reanalysis products. J. Climate, 17 , 38663873.

  • 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
  • Torrence, C., and G. P. Compo, 1998: A practical guide to wavelet analysis. Bull. Amer. Meteor. Soc., 79 , 6178.

  • 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 , 31673191.

    • Search Google Scholar
    • Export Citation
  • Weare, B. C., and J. S. Nasstrom, 1982: Examples of extended empirical orthogonal function analyses. Mon. Wea. Rev., 110 , 481485.

  • Yang, G-Y., and J. Slingo, 2001: The diurnal cycle in the Tropics. Mon. Wea. Rev., 129 , 784801.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 3 3 3
PDF Downloads 1 1 1

Characterization of the Diurnal Cycle of the West African Monsoon around the Monsoon Onset

View More View Less
  • 1 IRD, LOCEAN/IPSL, Université Pierre et Marie Curie, Paris, France
  • | 2 CNRS, SA/IPSL, Université Pierre et Marie Curie, Paris, France
Restricted access

Abstract

This study investigates the diurnal cycle of the West African monsoon and its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979–2000 NCEP–DOE reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) at 0000, 0600, 1200, and 1800 UTC. This study points out two independent modes describing the space–time variability of the diurnal cycle of low-level wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. This latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date.

The diurnal cycle of dry and deep convection is also investigated through the same method. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection may generate a gravity wave that propagates northward and reaches the Saharan heat low region 12 h later. The diurnal cycle of the dry convection in the Saharan heat low is similar during the preonset and the postonset periods with a peak at night (0000 UTC) consistent with the nocturnal jet intensification. This convection is localized at 15° and 20°N before and after the monsoon onset, respectively. Both during the first rainy season in spring and the monsoon season in summer, the nocturnal jet brings moisture in the boundary layer north of the ITCZ favoring humidification and initiation of new convective cells, helping the northward progression of the ITCZ. At the end of the summer the southward return of the ITCZ is associated with the disappearance of the core of the monsoon jet.

Despite a lot of similarities between the results obtained using NCEP–DOE and ERA-40 reanalyses, giving confidence in the significance of these results, some differences are identified, especially in the diurnal cycle of deep convection, which limit the interpretation of some of these results and highlight discrepancies in the reanalyses.

Corresponding author address: Dr. Benjamin Sultan, LOCEAN/IPSL, UPMC, Boite 100, T45/55, 4 Place Jussieu, 75252 Paris CEDEX 05, France. Email: benjamin.sultan@locean-ipsl.upmc.fr

Abstract

This study investigates the diurnal cycle of the West African monsoon and its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979–2000 NCEP–DOE reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) at 0000, 0600, 1200, and 1800 UTC. This study points out two independent modes describing the space–time variability of the diurnal cycle of low-level wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. This latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date.

The diurnal cycle of dry and deep convection is also investigated through the same method. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection may generate a gravity wave that propagates northward and reaches the Saharan heat low region 12 h later. The diurnal cycle of the dry convection in the Saharan heat low is similar during the preonset and the postonset periods with a peak at night (0000 UTC) consistent with the nocturnal jet intensification. This convection is localized at 15° and 20°N before and after the monsoon onset, respectively. Both during the first rainy season in spring and the monsoon season in summer, the nocturnal jet brings moisture in the boundary layer north of the ITCZ favoring humidification and initiation of new convective cells, helping the northward progression of the ITCZ. At the end of the summer the southward return of the ITCZ is associated with the disappearance of the core of the monsoon jet.

Despite a lot of similarities between the results obtained using NCEP–DOE and ERA-40 reanalyses, giving confidence in the significance of these results, some differences are identified, especially in the diurnal cycle of deep convection, which limit the interpretation of some of these results and highlight discrepancies in the reanalyses.

Corresponding author address: Dr. Benjamin Sultan, LOCEAN/IPSL, UPMC, Boite 100, T45/55, 4 Place Jussieu, 75252 Paris CEDEX 05, France. Email: benjamin.sultan@locean-ipsl.upmc.fr

Save