WASHINGTON — As Hurricane Irma barrels dangerously toward Florida, scientists say that a perfect mix of meteorological conditions has conspired over the past week to make the storm unusually large and powerful.
“You need just the right ingredients for a hurricane of this magnitude to last for so long,” said Phil Klotzbach, an atmospheric scientist at Colorado State University. “And Irma has had them all.”
Weather forecasters had already expected this summer to be an active hurricane season in the Atlantic Ocean because of warmer-than-average ocean surface temperatures, which provide fuel for hurricanes, as well as weaker-than-average wind shear, which can help to dissipate storms.
But even in that context, Irma was special.
Irma initially developed near Cape Verde off Africa’s coast on August 30, and a ridge of high pressure kept the storm from wandering off harmlessly into the cooler northern ocean. Instead, it was pushed inexorably westward, with plenty of time to intensify over the warm Atlantic waters.
On its journey west, Irma underwent at least six cycles called “eyewall replacement,” when the eye of the hurricane naturally contracts. While storms can weaken during this process, they also can quickly intensify and expand in size once the cycle is complete. And Irma did not weaken — it just grew and grew.
Ultimately, Irma persisted for three consecutive days in the Atlantic Ocean as a Category 5 storm, a record since scientists have been using satellites to watch storms. It also had maximum sustained winds recorded at more than 185 miles per hour for 37 straight hours — beating the record set by Super Typhoon Haiyan in the Pacific in 2013, according to Dr. Klotzbach.
So far, there has been little to slow down Irma. The hurricane has encountered almost no wind shear to weaken it. And, while Irma passed over and devastated several Caribbean islands, including the Virgin Islands and St. Martin, those land masses were not large enough to make it lose momentum.
On Friday, the National Hurricane Center downgraded Irma slightly to a Category 4 storm as it neared Cuba, with maximum sustained winds of 155 miles per hour. But as the storm passes over unusually warm, deep waters in the Straits of Florida, it’s expected to maintain its intensity or possibly even strengthen before making landfall.
And because Irma now encompasses such a large area — with hurricane-force winds extending out 70 miles from the center — meteorologists say that the storm poses extremely grave risks as it heads toward Florida.
For comparison, Hurricane Andrew, which made landfall in South Florida as a Category 5 storm in 1992 after quickly intensifying near the Bahamas, was a relatively compact storm, with hurricane-force winds extending 25 miles from its center. Hurricane Andrew killed 65 people, destroyed 63,000 homes and brought $26.5 billion in economic losses.
Hurricane Irma is expected to affect a much broader swath of Florida, and its sheer size means it could potentially create a much larger storm surge along the coast, pushing water inland and causing severe flooding. Central and South Florida have grown at a rapid pace since Andrew, putting more people and property in harm’s way.
“It’s flawed to use Andrew as an analogue,” said J. Marshall Shepherd, director of the atmospheric sciences program at the University of Georgia. “You have to look at both the magnitude of the storm and the size. Everything we’re seeing indicates that Irma will be extremely dangerous.”
As for climate change, scientists say they are still working to tease out exactly what role warmer temperatures are playing in recent hurricane activity.
According to Thomas Knutson, a research meteorologist at the National Oceanic and Atmospheric Administration, current climate models suggest that a rise in global temperatures could potentially lead to fewer hurricanes in the Atlantic basin, but those that do form would be more intense, thanks to warmer water near the ocean surface, and bring heavier rainfall because of increased moisture in the air.
But, to date, it has been difficult to detect clear trends against a background of natural variability. Ocean temperatures are certainly rising because of global warming. But over the past century, the Atlantic Ocean has also gone through periods of relatively little hurricane activity, as in the 1960s and ’70s, as well as periods of high activity, as occurred after 1995. Researchers suspect those cycles are linked to fluctuations in wind shear, which can be affected by events like El Niño, as well as by ocean events like the Atlantic Multidecadal Oscillation, a cycle of fluctuations in sea surface temperature that may be affected by ocean currents and possibly air pollution.
“But even if we can’t detect it yet, the models indicate that there should be some effect there,” Dr. Knutson said.