Africa's Role in the Atlantic Hurricane Season

One of the "seedlings" of tropical cyclones in the Atlantic Basin is the tropical wave and one of the world's largest deserts plays a significant role.

Tropical waves are not waves in the ocean, but rather in the atmosphere.

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Specifically, a distinct north-south contrast in the atmosphere exists, particularly in the summer months, between deep, hot air over the Sahara Desert and cooler, more humid air over more forested areas of central Africa and the Gulf of Guinea.

A basic tenet of meteorology is that horizontal temperature contrasts lead to vertical contrasts in wind.

This means the north-south temperature contrast we mentioned results in a jet stream moving from east to west across the continent into the tropical Atlantic Ocean, the so-called "African easterly jet." Unlike jet streams affecting the mid-latitudes of the northern hemisphere, this jet stream is located only about 10,000 feet above the ground.

"Similiar to winds blowing over the ocean creating fairly evenly-spaced up-and-down waves in the water, this African easterly jet gets a little unstable every few days," says Hurricane Expert at The Weather Channel and former National Hurricane Center Director, Dr. Rick Knabb.

What results are what we refer to as "tropical waves", discrete areas of horizontal spin in the low to mid-levels of the atmosphere. You can see tropical waves on infrared satellite imagery as clusters of convection originating over north Africa, then pushing westward in the tropical Atlantic Ocean.

Between April and November, a new tropical wave emerges off the west African coast roughly every 2 to 4 days, according to Dr. Knabb, with a yearly average of about 60 to 65 waves. Of course, only a select few of these waves develop into tropical cyclones, as large-scale atmospheric conditions (wind shear, sinking air) can suppress development.

Tropical Waves: Seedlings for Hurricanes

First, let's take a look at a typical tropical wave.

Without a satellite loop at your disposal, you may not know a tropical wave is approaching. Ahead of a mature tropical wave approaching the Caribbean Islands, for instance, the weather is generally fine. Falling surface barometric pressure would be your only clue.

These tropical waves play a very important role in hurricane formation.

Major hurricanes (Category 3 or higher on the Saffir-Simpson hurricane wind scale) are typically responsible for the majority of damage in any hurricane season. It turns out roughly 85 percent of all Atlantic major hurricanes have origins traceable to African easterly waves. Furthermore, about 60 percent of all tropical storms and Category 1 and 2 hurricanes are spawned by these waves.

On the other hand, some tropical waves cross the entire Atlantic Ocean and Caribbean Sea without triggering tropical cyclone development.

Sometimes tropical cyclones develop soon after the parent tropical wave emerges off Africa, near the Cape Verde Islands. These "Cape Verde storms" then take the long voyage for 1 to 2 weeks across the Atlantic Basin, sometimes affecting the Caribbean Sea, Gulf of Mexico and or the eastern U.S. coast, typically in the months of August and September.

Assuming other atmospheric parameters are favorable, such as lack of wind shear or sinking, dry air, these Cape Verde hurricanes cam become intense, given the vast expanse of ocean available.

Hurricane Irma Is a Recent Example

Hurricane Irma in 2017 was one such long-track hurricane that began as a tropical wave off the coast of Africa.

Irma became a tropical storm on August 30 and then underwent a period of rapid intensification that allowed it to reach major hurricane status, Category 3 or higher, on August 31.

Irma then brought tremendous damage as a Category 5 hurricane to portions of the Caribbean. Irma turned northward and made two landfalls in Florida on Sept. 10, 2017, one in the Keys and another one near Marco Island.