The annual cycle of the West African Monsoon in a two-dimensional model:Mechanisms of the rainband migration

Abstract:

The processes that drive the annual cycle of the West African Monsoon (WAM) are analysed using an idealized meridional-vertical numerical model that includes moist physics. Using the work by Peyrillé and Lafore (2007) as a starting point, the framework is adapted to studying the annual cycle. A suitable forcing methodology for temperature and humidity is derived allowing the 2D model to reproduce the main features of the WAM.
A budget analysis of the simulated temperature and humidity variables leads to a picture of the ITCZ seasonal displacement, for which the moistening on the northern side of the ITCZ is key. It is due to the near surface moisture advection by the monsoon flow to the north of the ITCZ, in addition to the turbulent fluxes and shallow convection which transport humidity to the top of the PBL. On a larger scale, the warming of the Saharan Heat Low by turbulence and radiation and the cooling/moistening within the ITCZ by convective downdrafts reinforces the monsoon flow. The mechanism seems at play during the whole seasonal cycle, which is seen as a steady translation of these structures.
Sensitivity experiments show the importance of the low level processes such as downdrafts, horizontal advection and water recycling. Although advection is the 1st order process, the water recycling appears as a key element by directly modulating the intensity of rainfall and by allowing the convective downdraft to feed back onto the WAM.