A two-dimensional idealized modeling of the West African Monsoon annual cycle

The West African Monsoon (WAM), as the other monsoons, is characterized at first order by a meridional shift of the rain band. Over West Africa, the rain band is displaced from a mean latitude of 5°N in May to a 10-12°N latitude in July-August, which gives the main rainfall to the Sahelian countries. This is an ocean-land-atmosphere phenomenon in which several timescales and processes are involved. A good understanding of processes that modulate rainfall and its location is a key question for scientist, rendered difficult by the complexity and the number of processes involved in the system.

Although upstream conditions are important for the monsoon dynamics, the relative zonal homogeneity of the atmospheric and surface fields over West Africa makes it possible to study the WAM within a two-dimensional meridional-vertical framework, easier to understand than a full 3D system. Initially developed to study the quasi-steady state regime and diurnal cycle of the WAM, we used here the 2D model of Peyrillé and Lafore (2007) to analyze the annual cycle of the WAM and its mechanism of northward migration.

First the large scale forcing term introduced by Peyrillé and Lafore (2007) is revisited to better account for the effect of departures from the 2D hypothesis in terms of temperature and humidity advections. This improved forcing term is diagnosed from NCEP2 reanalysis at a monthly frequency to be prescribed in the model, allowing realistic simulations of the main characteristics of the complete annual cycle. The model extends from 30°S to 40°N. It is run using prescribed SST over the ocean and with rigid boundary conditions to the north and south, thus isolating the system from mid-latitudes. After 3 years of integration, the model is able to reproduce the key feature of the WAM like the zonal jets, the different low levels flows and the general movement of rainfall over the continent when compared to a climatology.

A budget analysis is then performed to understand the mechanisms associated with the northward movement of the rain band. Humidity budgets reveal that over the Sahel, humidity is advected in the low levels by the monsoon flow from the rain band into the Sahel and turbulent fluxes contribute then to the vertical distribution of humidity within the lower troposphere. Deep convection does not occur in a first time because of the dry air prevailing in the region which causes convection to be shallow for ~10 days. Deep convection only occurs when the atmosphere is sufficiently moist. These humidification processes reduce the high thermal gradient on the southern flank of the Heat Low responsible for the monsoon flux. The northward penetration of the monsoon thus depends on the balance between these two opposite processes.