The annual cycle of the West African monsoon in a two-dimensional model: mechanisms of the rain-band migration
Peyrillé, Philippe ; Lafore, Jean-Philippe ; Boone, Aaron
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 has been adapted to studying the annual cycle. A suitable forcing methodology for temperature and humidity has been derived which allows the two-dimensional 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 intertropical convergence zone (ITCZ) seasonal displacement, for which the moistening on the northern side of the ITCZ is a 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 moisture to the top of the planetary boundary layer. On a larger scale, the warming of the Saharan heat-low by turbulence and radiation and the cooling/moistening within the ITCZ by convective downdraughts reinforces the monsoon flow. Inertial instability is also at play favouring the acceleration of the monsoon flow and the associated humidification. These mechanisms seem to be 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 downdraughts, horizontal advection and water recycling. Although advection is the first-order process, the water recycling appears as a key element by directly modulating the intensity of rainfall and by allowing the convective downdraughts to feed back onto the WAM.
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