The West African Monsoon

 The West African Monsoon

Image from brittanica.com

The west african monsoon (WAM) occurs during the northern summer months (May - October) from 9N - 20N latitude. This wet season is the season for  planting on west african farms.

In April-June, when the midday sun is at its highest, thunderstorms are common. The moist air layer at the edge of the advancing monsoonal moist air is generally too thin (~1,000m) to support convection. Convection happens 200-300 km behind the edge, where a ~2000m deep moist layer and hot ground combine to create unstable air. The upper levels of the resulting high-topped cells (or squall lines) tend to extend into the upper-level Easterly Jet, and are driven westward at 40-55 kph.

In July-October, during the main phase of the monsoon, lower clouds (3,500m) dominate. 

During the low-sun season (Dec-Feb), a shallow (~2000m deep) humid layer is overlain by hot, dry, dusty NE winds, which blow in from the Sahara. Upper atmospheric anticyclones can occur at 20N, and the Easterly Jet Stream may occur as far south as 10N. This dry season is the season of harvesting, brushing, burning, and preparing farms.

Complete year-round drought exists in areas north of roughly 20N. The dry season becomes less completely dry the further south one travels. South of roughly 8N, even during the dry season, regular precipitation events occur.