Hurricane Ida, which hit the Louisiana coast in winds of 250 mph on Sunday, gained strength faster than most storms. Due to climate change, amplification is more common because hurricanes absorb more energy from the warmer seawater than before.
But in a summer with extreme weather, Ida’s intensification was extreme.
According to the National Hurricane Center forecast bulletins, the storm’s maximum sustained winds were about 85 miles per hour on Saturday morning, making it a Category 1 hurricane. Less than 24 hours later, they were 65 mph stronger, which put Ida close to a Category 5.
The storm was stronger than the hurricane center’s forecast, which had demanded maximum wind speeds of 140 miles per hour. The definition of hurricane center for rapid intensification is an increase in wind speed of at least 35 miles per hour in 24 hours. That’s how much Ida strengthened in just six hours overnight.
Climate change is part of the cause. Researchers have found that the frequency of rapidly intensifying hurricanes in the Atlantic has increased over the past four decades as ocean temperatures have risen, in large part because warmer water provides more energy to power those storms. In the 1980s, the chance that a hurricane would increase rapidly was about a 1 percent chance. Now there is a 5 percent chance.
However, experts studying hurricane behavior said other factors played a role in Ida, including seasonal warming of the Gulf of Mexico, the amount of moisture in the atmosphere, and the presence or absence of winds that affect the structure of a storm can.
Right now the Golf is extremely warm because it has been accumulating heat all summer. It is this seasonal warming that also occurs in the Atlantic that makes mid-August through October the most active part of the hurricane season each year.
But it’s not just the gulf’s surface temperature that matters, said Joshua Wadler, a researcher at the University of Miami and the National Oceanic and Atmospheric Administration. Hurricanes actually cool the ocean as they cross it because they stir the water up to around 50 feet and mix in colder water from below.
In this case, Ida was traveling over water, which was much warmer at that depth. Probes sent into the water by hurricane fighter planes on Saturday showed that the temperature after being mixed by Ida was around 30 degrees Celsius, or 86 degrees Fahrenheit, said Dr. Wadler.
“That’s at the very high end of the ocean surface temperatures hurricanes ever see,” he said.
The storm’s path happened to be over this warm water in what scientists call a vortex, said Chris Slocum, a NOAA researcher.
“Ida found the perfect path across the gulf where the warmest water is,” he said, and that gave the storm a lot of energy to extract. “You could say it’s a worst-case scenario.”
Dr. Slocum compared the situation to that of Katrina in 2005, who crossed a cooler column of water as it approached Louisiana and weakened from Category 5 to Category 3. Ida did not encounter any cooler water.
“This continues the upward trend,” he said. “The only thing that will stop the intensification process is the landing,” he said.
Every year there are vortices in the Gulf that form when part of a loop flow breaks off, said Dr. Wadler. And while it’s extremely difficult to relate any particular one to climate change, it is “as profound as we’ve seen in a long time,” he said.
While ocean temperatures are most important, two other factors affect how strong and how quickly a storm intensifies, said Dr. Slocum.
Humidity affects the thunderstorms that make up a tropical cyclone. The more humid the air, the longer these storms will survive and last. And the way these thunderstorms interact with each other, especially at the eye of the storm, can affect whether it intensifies or weakens.
Wind shear – changes in wind speed and direction with altitude in the atmosphere – can also affect the structure of a hurricane. If the wind shear is too strong, “you can tear apart a storm,” said Dr. Slocum.
The forecasters at the hurricane center had been watching the wind shear closely. It had been a factor when the storm hit the Gulf on Friday, giving Ida an asymmetrical structure. However, on Saturday the shear broke up and the storm took on a more regular spiral shape.
The impact on wind speed can be compared to what happens to figure skaters during a spin. Skaters who keep their arms in a tight, precise position will rotate faster. But when one of their arms is outstretched, they rotate much slower.
It can be difficult to predict whether a hurricane will intensify quickly, said Dr. Slocum.
“It’s kind of a Goldilocks problem,” he said. “If any of these ingredients are a little wrong, we won’t see them.”