Tropical Waves: Hurricane Seedlings from Africa

Hawaiian curl
Imagine this in the atmosphere -- that's a tropical wave. M Swiet Productions / Getty Images

When you hear "tropical wave", you probably picture a wave crashing against the shore of a tropical island beach. Now, imagine that wave being invisible and in the upper atmosphere and you've got the gist of a what a tropical wave (in meteorology) is.

Also called an easterly wave, African easterly wave, invest, or tropical disturbance, a tropical wave is generally a slow-moving disturbance that's embedded in the easterly trade winds.

To put that more simply, it's a weak trough of low pressure that develops from an unorganized cluster of thunderstorms. You can spot these troughs on pressure maps and satellite imagery as a kink or inverted "V" shape -- hence the association with, and name of wave.

The weather out ahead (west) of a tropical wave is typically fair. To the east, convective rainfall is common. They're usually found over equatorial West Africa, or further west over the equatorial eastern Atlantic Ocean.

The Seeds of Atlantic Hurricanes

Tropical waves last from a couple of days to several weeks, with new waves forming every few days. One of the ways they're generated is from the African Easterly Jet (AEJ). The AEJ is an east-to-west oriented wind (much like the jet stream that flows across the US) that flows across Africa into the tropical Atlantic Ocean. It blows over the tropical Atlantic as a fast flowing stream of air.

Since the flow near the AEJ goes faster than that further away in the surrounding air, what happens is that eddies begin to develop due to these differences in speed. When this happens, you get an unstable kink or wave in the main AEJ flow pattern -- a tropical wave. (On satellite, these disturbances appear as clusters of thunderstorms and convection originating over North Africa and traveling westward into the tropical Atlantic.) By providing the initial energy and spin needed for a hurricane to develop, tropical waves act like "seedlings" of tropical cyclones.

The more seedlings the AEJ generates, the more chances there are for tropical cyclone development. 

Roughly 1 in 5 Tropical Waves Becomes an Atlantic Tropical Cyclone

Tropical waves are just one way hurricanes form, but they're also how the majority do. In fact, approximately 60% of tropical storms and minor hurricanes (categories 1 or 2), and nearly 85% of major hurricanes (category 3, 4, or 5) originate from easterly waves. 83% of all intense hurricanes originate from tropical or easterly waves from Africa. In contrast, minor hurricanes originate from tropical waves at a rate of only 57%. 

Once a tropical disturbance becomes more organized, it can be called a tropical depression. Eventually, the wave can become a hurricane. Read on to find out how tropical waves grow from  into a full blown hurricane, and what each stage of development is called.


Updated by Tiffany Means