Hurricanes are mighty storms that can roar ashore with winds in excess of 250 km/h. But even the strongest hurricanes are remarkably fragile structures despite their ferocity.
Most strong hurricanes eventually destroy their own eye, often growing a newer and even bigger eye in its place. It’s a sudden process that can force a hurricane to explode in both size and intensity.
Eyewall replacement cycles are one of the great mysteries lurking in the heart of any hurricane. Here’s a look at why some storms shed their eyes like a snake sheds its skin, and why it can spell trouble for folks living near the coast.
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Thunderstorms wrapping around the eye of a hurricane are collectively known as an eyewall. The eyewall acts like the engine that powers these exceptional systems.
Updrafts feeding thunderstorms within the eyewall suck a tremendous amount of air up and away from the surface, leaving behind low air pressure at the centre of the storm. The stronger those thunderstorms get, the larger and more intense the hurricane can grow. A storm’s most energetic winds are found within the eyewall.
This feedback can continue for days if the storm encounters warm waters, ample moisture, and low wind shear. Even when everything seems favourable, though, most powerful hurricanes eventually stumble and struggle to maintain their composure.
Every couple of days, the intense rainbands swirling around a major hurricane can close off and begin to form a secondary eyewall. This ‘second eye’ will eventually choke off the storm’s main eye in a phenomenon called an eyewall replacement cycle.
An eyewall replacement cycle is unmistakable on radar if the storm is close enough to land.
Hurricane Ida began shedding its eye just before striking Louisiana back in August 2021. The radar site out of New Orleans had a clear view of the hurricane’s eye and secondary eyewall just as the centre of the storm pushed ashore.
What happens to the winds during this process? Even the most intense hurricane will lose some strength during an eyewall replacement cycle.
Think about a figure skater spinning on ice. She pulls her arms in to spin faster and reaches outward to slow her rotation. The newer, bigger eye is like the storm reaching its arms out—forcing the winds to slow down.
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But if conditions remain favourable, the newer, bigger eyewall will contract and the storm’s intensity can once again increase, sometimes even beyond its original strength. This is the process through which the most intense hurricanes reached their infamous peak.
This can have huge consequences for folks in harm’s way. A storm can unexpectedly weaken if it starts replacing its eyewall near landfall. But on the other hand, a successful eyewall replacement cycle can force the storm to dramatically grow in size.
Take a look at Hurricane Irma back in 2017. The storm’s wind field was only about 155 km wide when the storm first grew into a hurricane on August 31.
After an eyewall replacement cycle, Irma exploded into a large Category 5 storm with a wind field that stretched more than 450 km wide. The process happened again a few days later, and Irma reached Florida as a major hurricane with an expanse of damaging winds that stretched 775 km from one end to the other.
Eyewall replacement cycles remain unpredictable. While meteorologists know they occur at least once or twice in an intense hurricane’s lifecycle, experts are still researching exactly why and when the process occurs. Further understanding will improve hurricane intensity forecasting in the future.
Header image courtesy of ESA/NASA.
