What Is A 'Brown Ocean' And How Did It Turn Ida Into Such A Monster Hurricane?
Ida's path through the Gulf of Mexico couldn't have been planned any better if it was looking to rapidly intensify, then it hit a 'brown ocean'.
Ida bided its time in the Caribbean; throttled by wind shear it skulked impatiently before splashing through a warm eddy in the Gulf of Mexico where it exploded like a hunter loosed on prey.
That deepening to a 150-mph cyclone – a staggering 50 mph gain in less than 24 hours – wasn’t cowed at the coast. Instead, Ida maintained major hurricane muscle for eight hours after landfall – a phenomenon that may be explained by the “brown ocean effect.”
The phrase, which was originated by a University of Georgia research group, is when a tropical cyclone can continue to feed over spongey land covered with enough warm water to mimic the heat energy transmitted by the ocean.
The brown ocean theory has specific required conditions, such as minimal temperature variations in the atmosphere and soil moistened ahead of landfall, including elements such as swamps and wetlands.
Marshall Shepherd, director of the University of Georgia’s atmospheric sciences program, whose researchers worked on papers advancing the brown ocean effect, said he believes the soggy coast and flat terrain near Port Fourchon, LA “definitely contributed” to Ida’s continued strength after landfall.
And that squishy pancaked fringe of Louisiana is similar to South Florida’s river of grass.
“We’re talking swampy conditions, and the water temperature of the marshland in Louisiana is like the Everglades, it’s pretty shallow water so it gets really warm,” said Todd Kimberlain, a senior meteorologist with the South Florida Water Management District. “There was enough water and enough heat energy that it provided Ida with an extra boost even as it was moving inland.”
Where does Ida rank in terms of hurricane strength?
Ida, which scoured southern Louisiana, crushing homes and lives, went on to devastate the East Coast from Maryland to Connecticut with flooding rains that left more than 45 people dead as of Thursday, according to the Associated Press.
It reached Louisiana Sunday on the 16th anniversary of Hurricane Katrina, it tied with 2020’s Laura and the Last Island Hurricane of 1856 as the strongest storm to hit the state, and it made landfall with a central pressure of 930 hPa. That’s the second lowest of any storm to make landfall in Louisiana following Katrina, said Colorado State University hurricane researcher Phil Klotzbach.
“Lower pressure equates to a stronger hurricane,” he said.
Early forecasts from the National Hurricane Center pegged Ida as a rapid intensifier with a bead on Louisiana, but before the devastation it was just a swirl of thunderstorms west of Jamaica.
Nick Shay, professor of ocean sciences at the University of Miami's Rosenstiel School of Marine and Atmospheric Science, was watching closely.
“We said, this looks like a Gulf of Mexico storm and there’s a big warm eddy sitting right there waiting for it,” Shay said. “It was a bullseye.”
Eddies shed periodically from the warm loop current. One of the fastest currents in the world, the clockwise flow moves at about 1.8 mph, digging into the Gulf of Mexico before taking a jagged right to flow through the Florida Straits and north along the east coast of Florida as the Gulf Stream.
The eddy waiting for Ida began to peel off the loop current in July and started moving west in mid-August.
Tropical cyclones form over waters that are generally 78 degrees or warmer. The eddy in the Gulf had a surface temperature of 86 to 88 degrees with heated water that went down about 480 feet, said Shay, who dropped measuring devices from an airplane into the eddy.
The warmer the water, the more evaporation there is into the atmosphere to feed a storm. The water vapor rides high on a cyclone's thunderstorm currents until it cools in the upper atmosphere and turns back into a liquid releasing latent heat that warms the air inside a storm's clouds. That warm air rises, increasing the storm's intensity.
With no cold water to churn up, Ida feasted.
“It had everything going for it,” Shay said. “I couldn’t have coordinated it any better, of course it spelled disaster for the folks along the Gulf Coast.”
The 'brown ocean' concept kept Ida churning over land
Ida made landfall at 12:55 p.m. as a 150-mph hurricane. It maintained Category 4 strength for four hours before sinking to a Cat 3, which it held for another three hours. It wasn’t until 9 p.m. – 8 hours after landfall – that Ida fell to a Cat 2 hurricane with 110 mph winds.
“The most unique aspect of Ida was its ability to maintain Cat 4 intensity well onshore,” said Jeff Weber, an atmospheric scientist at the University Corporation for Atmospheric Research. “A lot of the area near landfall is marsh or bayou, it’s not truly all land, there is a lot of water there that is above 80 degrees.”
That’s what is described in the brown ocean theory – cyclones that maintain or increase strength after landfall because of sufficient soil moisture that is warm enough to simulate ocean evaporation. The term built on the “green ocean” concept, which was used to describe how the Amazon Forest’s moisture contributed to regional and global climate, said UGA’s Shepherd in a 2016 article for Forbes.
Weber compared Ida’s ability to maintain strength to 2005’s Hurricane Wilma. Wilma hit as a 121-mph Category 3 on Oct. 24 near Cape Romano on the southwest coast of Florida. It zipped through the state in 4 hours with winds decreasing during the journey only to 110 mph.
What were Ida's effects on Palm Beach County?
While much of Palm Beach County felt sustained winds of considerably less than Cat 2, there were gusts recorded of 117 mph in Belle Glade, 119 mph west of Boynton Beach and 114 mph in Jonathan Dickinson State Park.
Kimberlain also noted 1992’s Hurricane Andrew, which hit near Homestead as a Cat 5 and maintained major hurricane strength during its entire trek over land into the Gulf of Mexico.
“It wasn’t until it got out of the water conservation areas that it really started to weaken,” Kimberlain said. “This type of phenomenon is rarely observed and there only a few areas where it could really occur; southeast Louisiana and South Florida.”
Water conservation areas are largely flat terrain of Everglades sawgrass marsh with spotty tree islands that help filter water before it reaches Everglades National Park.
Ida is not unique in its determination to conquer terra firma. Most recently, Tropical Storm Claudette was named over land in June after it had breached the coast southwest of New Orleans. A review of 2020’s Sally found that it formed over the Everglades, about 25 miles west of Homestead. In 2016, Tropical Storm Julia was named about 5 miles west of Jacksonville.
“Water is what a storm needs, and it doesn’t have to be ocean water,” Shay said. “I had never thought about the brown ocean concept before, but after seeing Ida maintain for a while, there had to be some impact, and that area is a lot like the Everglades.”
This story was produced in partnership with the Florida Climate Reporting Network, a multi-newsroom initiative formed to cover the impacts of climate change in the state.