USF researchers: Rivers can supercharge Gulf hurricanes

Hurricane Idalia intensified from a Category 1 to a Category 4 storm in less than 24 hours as it barreled toward Florida’s Big Bend in August 2023. 

The phenomenon has become increasingly common – Hurricanes Helene and Milton also suddenly strengthened in the Gulf of Mexico before making devastating landfalls. A recent study led by researchers at the University of South Florida’s St. Petersburg campus found that freshwater from nearby rivers can help supercharge storms. 

While the Gulf experienced record-hot temperatures in 2023, wind conditions were less favorable for development during Idalia’s rapid intensification. Chuanmin Hu, professor of oceanography at USF’s College of Marine Science, and other study authors wondered what external factors contributed to the storm’s rapid intensification.

Hu did not expect to learn that freshwater mixing with the salty gulf created a barrier that trapped ocean heat and helped boost Idalia’s intensity. Storm intensification is typically difficult to predict, and forecasters now have additional data points to consider. 

“We have a lot of river plumes in the gulf and elsewhere,” Hu told the Catalyst. “But it is not observed that often that a river plume could fuel a hurricane to intensify so fast. So, that was a surprise.”

Environmental Research Letters published the study. Hu and his colleagues, including researchers from the College of Marine Science’s Optical Oceanography Lab, its Ocean Technology Group and the University of Miami, initially planned to use data gathered from satellites and underwater gliders to study phytoplankton and dissolved matter in river plumes. 

The authors analyzed a freshwater plume that spanned from Mississippi to Florida’s Panhandle and south to the Florida Keys. Rain-swollen rivers along the Gulf Coast discharged the thick surface layer of low-density, low-salinity water that resisted ocean mixing and cooling. 

“Typically, local small rivers do not contribute, or their contribution is negligible,” Hu said. “What is unusual this time is that several rivers collectively contributed to a very extensive and persistent plume.” 

Timing was also a factor. Hu noted the plume occurred in the summer months, and the Gulf was already unusually warm. 

Satellite imagery shows Hurricane Idalia’s rapid intensification as the storm encountered a massive river plume. Image: NASA Earth Observatory.

Rapid intensification occurs when a hurricane’s sustained winds increase by over 35 mph within 24 hours. Although ocean heat fuels storms, high winds often stir up cooler, deeper water. 

However, Idalia’s winds could not penetrate the freshwater plume enough to sap the storm’s energy. Hu said hurricane models should now incorporate the phenomenon. 

Previous studies have shown a correlation between river plumes and storm intensity in other areas. A research paper from 2007 found that over two-thirds of Category 5 storms between 1960 and 2000 traversed vast plumes discharged from the Amazon and Orinoco Rivers in South America. 

Hu said a projected Category 2 hurricane that encounters an extensive, persistent river plume in the Gulf will likely make landfall as a Category 3 or 4. He also believes people should trust meteorologists, who can frequently update forecasts. 

“The intensity does not jump from Category 1 to Category 4 right away,” Hu added. “It takes 24 hours. So, people still have enough time to respond.” 

He is “not really” concerned with the phenomenon recurring in 2025. “You need to have the right river flow – the right size, the right direction and the right time … when the gulf is already very warm,” Hu said. 

“Then, if you have a hurricane coming by and going across the river plume, it may intensify very fast,” he continued. “The river plume is just another factor to make it worse.” 

College of Marine Science researchers at the University of South Florida St. Petersburg campus also used remotely operated underwater gliders to collect data. Photo by Mark Parker.