Description

The West African Disturbance Line (WADL) is a line of convection (similar to a squall line) which forms over West Africa and moves westwards across the continent and into the Atlantic Ocean, sometimes faster than the larger tropical waves (African easterly waves) with which they are associated.

Formation and characteristics

A large, organised system of thunderstorms and squalls that travels west across West Africa, typically in the summer. 

WADL systems primarily form when dry, descending air from the Sahara (known as the Harmattan) converges with moist, southerly monsoon air. This interaction creates a squall-line-like structure of thunderstorms. They appear as long, organised lines of convection, several hundred nautical miles in length and are associated with strong winds and heavy rainfall.

A typical line of convection is about 300 nm long. A WADL forms over West Africa north of the equator and south of 15°N and moves faster than a tropical wave between 20 and 40 kts. They move off the African coast every 4 or 5 days mainly in the summer.

Significance

A key feature of the West African weather and climate system, WADLs are a significant source of rainfall for the region and are a major source of low-level vorticity that contributes to the development of tropical cyclones in the Atlantic Ocean.