As Hurricane Irma winds down its path of devastation over Florida and the Southeast U.S., it's almost hard to believe that the unprecedented Atlantic storm actually began thousands of miles away.
On Aug. 26, the National Hurricane Center began monitoring a tropical wave over the western coast of Africa that would eventually organize into Tropical Storm Irma. Unlike an ocean wave, this type of wave has to do with an elongated area of low pressure undulating in a north-south direction and traveling west off the African coast. When this snake-like phenomenon — more commonly referred to as the African Easterly Jet because of the winds blowing from the east — encounters the right conditions of moisture, warm temperatures, and atmospheric instability, it can lead to the formation of rotating tropical cyclones and, ultimately, a hurricane.
"85 percent of the most intense hurricanes affecting the U.S. and Canada start off as disturbances in the atmosphere over western Africa," Tel Aviv University researcher Colin Price, lead author of a 2015 study on hurricane formation, said in a statement. "We found that the larger the area covered by the disturbances, the higher the chance they would develop into hurricanes only one to two weeks later."
How a hurricane is born
One of the surprising forces behind the birth of most hurricanes that impact North America is the Sahara Desert. The second largest desert in the world after Antarctica, the Sahara's dry air is swept up by the easterly winds passing above to form massive, mushroom-shaped thunderclouds thousands of feet high. As these storms are pushed west, they dip through the atmospheric troughs of the African Easterly Jet, pick up moisture, and can sometimes transition into hurricanes.
According to Price, only 10 percent of the tropical waves that form over Western Africa develop into storms. To figure out what special mechanism is at play for triggering a thunderstorm's transformation into one of the most powerful forces on Earth, his team studied cloud coverage data over West Africa from 2005-2010 during the traditional June-November hurricane season.
The team discovered that thunderstorms with the greatest average coverage of cold-top clouds (those at the highest altitudes) often have the greatest potential to develop into tropical disturbances.
"By looking at each of these storms individually, we found again that the larger the cloud coverage originally in West Africa, the higher the value of the accumulated cyclone energy in a future hurricane," he added. "The conclusion, then, is that the spatial coverage of thunderstorms in West Africa can foretell the intensity of a hurricane a week later."
While the Sahara's climate is a contributor to hurricane formation, it's also something of a check on a storm's intensity. Dust captured from the African Easterly Jet and blown out over the Atlantic can act as a screen against sunlight, reducing surface temperatures by as much as 1 degree Celsius. This, in turn, can negate the sensitive environmental factors needed for hurricane formation.
Unfortunately, a warming globe due to climate change could one day green portions of the southern Sahara and reduce the amount of dust thrown over the Atlantic, leading to an increase in sunlight and warmer Atlantic waters. Model simulations during a 2017 study from researchers at the Department of Meteorology at Stockholm University found that tropical cyclone activity when the Sahara was green, roughly 6,000 years ago, favored increased tropical storm development.
"Our work indicates," the authors conclude, "that if future warming leads to a “regreening” of the Sahel/Sahara region and/or a reduction of dust fluxes over the tropical North Atlantic — as suggested in some recent studies — then the Caribbean, the Gulf of Mexico, and the eastern coast of the United States could become more susceptible to damage from severe tropical cyclones."