Analysis of the West African monsoon annual cycle using a two-dimensional model: Some key factors contributing to the rainband displacement

Abstract:

The processes that drive the annual cycle of the West African Monsoon (WAM) is the result of a complex interplay between the ocean, the land and the atmosphere in which several scales interact. Thorncroft et al. (2011) showed it is characterized by several stages, going from oceanic regime to the continental arrival of the monsoon. This study presents an idealized axisymmetric numerical model that retains all the moist physics with the ultimate objective of studying the different regional couplings and processes involved in the WAM. Starting from previous work by Peyrillé and Lafore (2007) in which the same tool has been used to study a steady monsoon regime, the axisymmetric framework is adapted to the study of the annual cycle. In particular the methodology used to determine the suitable atmospheric forcing is described, using a relaxation of temperature and humidity towards a reanalysis. Using that forcing the model represents most of the main features of rainfall annual cycle (meridional displacement of the rainfall maximum) and its dynamical structures (Saharan heat Low, ITCZ).

 As a first step the annual cycle of rainfall and the moisture convergence maximum found in Thorncroft et al. (2011) are analyzed. The processes that lead the ITCZ from an equatorial regime to its northernmost location are examined using temperature and humidity budgets. A relay of dry process in winter- early spring and moist processes in summer helps the monsoon to migrate to the north. In winter/spring the Saharan heat low strengthens through radiation and turbulence and drives moisture to converge ahead of the rainfall maximum. The low layers of the atmosphere are moistened by the turbulent vertical fluxes and by the shallow meridional circulation associated to the SHL and described by previous authors.

Sensitivity experiments are performd to isolate the role of some of the key processes: meridional advection of temperature and humidity, convective downdraft and ground evaporation. The meridional advection by the monsoon flow is a 1^st order process to moisten the continent. Ground evaporation dramatically modulates the extent and intensity of rainfall during the whole seasonal cycle. The convective downdraft helps to moisten the lower atmosphere and decrease the rainfall amount in spring and summer.