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

Abstract

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 ac…