Tropical Waves: Hurricane Seedlings From Africa

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 meteorological tropical wave 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, which is why they are called "waves."

The weather out ahead (west) of a tropical wave is typically fair. To the east, convective rainfall is common. 

The Seeds of Atlantic Hurricanes

Tropical waves last from a couple of days to several weeks, with new waves forming every few days. Many tropical waves are generated by the African Easterly Jet (AEJ), an east-to-west oriented wind (much like the jet stream) that flows across Africa into the tropical Atlantic Ocean. The wind near the AEJ moves faster than the surrounding air, causing eddies (small whirlwinds) to develop. This leads to the development of 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. 

The majority of hurricanes form from tropical waves. 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. 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.