Analysis of the West African monsoon using an idealized two-dimensional model

The West African Monsoon (WAM) is studied here with a two-dimensional atmospheric numerical model that allows to analyze the scales and processes involved in the complex system. The aim is not to reproduce real cases but to define idealized experiments to better understand the WAM behavior. Starting from a dry atmosphere at rest, the model reaches a quasi steady state after 15 days with the basic features of the WAM July state (Jets, peak rainfall, low levels fluxes).

Sensitivity tests are performed to identify the main forcing of the rainfall northward penetration. It is shown that the Mediterranean Sea plays a key role in the WAM system by affecting the temperature and humidity budgets of the Saharan Heat Low. The thermal contrast between the desert and the Mediterranean Sea is here fundamental to obtain a northerly Harmattan flux, which helps blocking the monsoon northward shift.

A detailed view of the continental diurnal cycle is also presented. Potential temperature and humidity budgets are performed in the deep convective and heat low areas. The moistening process to sustain deep convection is made through nocturnal advection at low levels and daytime turbulence that redistributes humidity vertically. The same mechanism occurs in the heat low except that the vertical transfers by turbulence help maintain the dryness of the low levels. A possible mechanism of interaction between the deep convective zone and the Saharan heat low is also proposed that involves gravity waves in the upper troposphere.