How strong will that Gulf hurricane get? Hidden hot water, river flow may be key

Miami Herald

3 days ago

Hurricane forecasting has improved dramatically over recent decades, but there's one major phenomenon that can still throw a wrench into even the most accurate forecasts — rapid intensification.
That's what scientists call it when a tropical storm or hurricane's sustained wind speed increases at least 35 mph within 24 hours, and it happens for nearly all Category 4 and 5 storms. Researchers have some idea of the ingredients necessary for a storm to quickly strengthen like that, but it's a less precise guess than many other elements of hurricane forecasting.
However, new research from the University of South Florida continues to chip away at the mysteries surrounding rapid intensification, especially in the Gulf of Mexico, by helping identify previously hidden pockets of hot water that could help feed storms.
One study, published in February in the journal Environmental Research Letters, found that a plume of hot freshwater, floating on top of the denser, slightly cooler saltwater in the Gulf, could have been key to Hurricane Idalia's rapid strengthening in 2023.
Researchers found that shortly before Hurricane Idalia jumped from a Category 1 hurricane to Category 4, it passed over this hot puddle of freshwater, and they determined that this provided the burgeoning hurricane a speed boost.
Chuanmin Hu, a professor of oceanography at the USF College of Marine Science and an author of the study, said the discovery was a coincidence, a matter of the right devices at the right place at the right time. They initially set out to study this plume of freshwater, an annual occurrence in the Gulf fed from various freshwater rivers, including the Mississippi, over red tide concerns.
But the team's slow-moving research gliders turned out to be perfectly placed to catch the incoming hurricane. And they showed, Hu said, that the deep layer of warm freshwater served as a power source for Idalia.
This freshwater plume is a normal thing to see in the Gulf, but the research suggests that in 2023, it was deeper and larger than anything in recent history, potentially because of an intense amount of rain that fell on the Southwest in the months previous.
'Even right now, there's a plume following almost the same path, but the extent is much smaller and the duration may be shorter,' Hu said. 'This extensive long lasting plume [in 2023] was a record in the past 20 years.'
READ MORE: What happens when hurricanes rapidly intensify?
Researchers can easily track the outlines of the annual plume via satellite, but without the gliders, which can sample up to 200 meters below the surface, it's hard to know how deep the plume is. The gliders are funded by the National Oceanic and Atmospheric Administration, and Hu suggested that their continued use could provide forecasters another tool for predicting rapid intensification.
'These are really, really important measurements that help hurricane studies, and I hope they will continue in the future,' Hu said.
Another USF-led study examining the same region of the Gulf also found a connection between unique ocean circulation and rapid intensification. The paper, published in December in the journal Geophysical Research Letters, suggests that in 2022, when Hurricane Ian powered up over waters near the Florida coast, it found waters warmer than usual — both at the sea surface and deeper below.
Yonggang Liu, an associate professor at the University of South Florida and lead author of the study, said that water was hotter than usual because the region hadn't experienced a 'flushing' current like it normally does, so the waters were hotter and more stagnant than usual.
Liu said his team analyzed 27 years of water temperature history from a series of buoys placed in the region and found that the deeper waters were warmer than normal when Ian plowed through. Usually, the sea surface is warmest and waters get cooler the deeper you go.
That means that when Hurricane Ian moved over the Gulf, it churned up ever hotter water, fueling the storm even further, potentially giving it the boost it needed to jump from Category 3 to Category 5 as it closed in on Florida.
'The important part of those subsurface water temperatures is you can't detect it from satellites,' Liu said. 'That should be improved. The buoys are essential in this case.'
Liu's team made a publicly available dashboard to display the data from those buoys. This month, it shows good news, he said. Deeper waters are running only slightly above average, a much lower figure than researchers saw with Hurricane Ian.
That could be because this year, unlike in 2022, the Gulf Loop Current grew wide enough to hit a 'pressure point' that Liu's team pinpointed on the west Florida shelf. When the current grows broad enough to hit that spot, the paper found, it flushes cooler water up and down the west coast of Florida, cutting off a potential food source for would-be storms.
'The entire shelf will be replaced with cooler water from offshore origins,' Liu said. 'That kind of circulation was not seen during the last four years in the summer, only briefly, not persistently. This year is different.'
Liu believes that expanding the buoy network up and down the coast of Florida, or even in geographically similar areas like North Carolina's coast, could provide a helpful tool for forecasters trying to find previously hidden pockets of hot water that could fuel storms passing through the area.
'More observations and monitoring of the subsurface is critical,' he said.