Annual cycle of the West African monsoon: Regional circulations and associated water vapour transport

Chris D. Thorncroft, Hanh Nguyen, Chidong Zhang, Philippe Peyrille

Research output: Contribution to journalArticle

117 Scopus citations


Analysis of the annually varying regional circulations and their relationship to surface conditions and water vapour transport in the West African region is presented. The progression of the West African monsoon is described in terms of four key phases: (i) an oceanic phase between November and mid-April when the rain band is broad with peak values just north of the Equator (∼1°N); (ii) a coastal phase between mid-April and the end of June when the rainfall peak is in the coastal region around 4°N (over the ocean); (iii) a transitional phase during the first half of July when the rainfall peak decreases; and (iv) a Sahelian phase between mid-July and September when the rainfall peak is more intense and established in the Sahelian region around 10°N. The annual evolution of the moisture fluxes, associated convergence, and rainfall is strongly impacted by the Atlantic cold tongue (cool water close to the Equator between boreal spring and summer) and the Saharan heat-low. The cold tongue strongly regulates the timing and intensity of the coastal rainfall in spring. The heat-low and its associated shallow meridional circulation strongly affect the profile in moisture flux convergence north of the main rain-band maximum; in particular it is responsible for the establishment of a second peak in column moisture flux convergence there (approximately 8° poleward of the rainfall peak). Particular emphasis is given to the coastal rainfall onset in April. A key aspect of this onset is acceleration of low-level cross-equatorial southerly winds, important for establishing the cold tongue, discouraging convection near the Equator and transporting moisture towards the coast. We argue that the rainfall peak is maintained at the coast, rather than steadily moving inland with the solar insolation, due to persistent warm water in the coastal region together with frictionally induced moisture convergence there.

Original languageEnglish (US)
Pages (from-to)129-147
Number of pages19
JournalQuarterly Journal of the Royal Meteorological Society
Issue number654
StatePublished - Jan 2011


  • Cold tongue
  • Heat-low
  • Moisture flux
  • Onset
  • Rainfall

ASJC Scopus subject areas

  • Atmospheric Science

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