Hurricane Idalia cooled waters in parts of the Gulf of Mexico by roughly 1 to 3 degrees Fahrenheit, but experts say this reprieve from the ongoing marine heat wave is already starting to wane.
Sea surface temperatures fell in the eastern gulf in the wake of Idalia, and in the western north Atlantic after Hurricane Franklin.
Hurricanes cool oceans by “upwelling” cold water from below the sea surface. The suction effect of a storm’s low-pressure center thrusts cooler water up to the ocean’s surface, according to the NASA Earth Observatory.
Cold water from raindrops and cloud cover during a storm also contribute to this cooling effect.
Brian McNoldy, a senior research associate at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science, said abnormally hot sea surface temperatures this summer mean the gulf is still very warm, despite the effects of Idalia’s cooling wake.
“That’s kind of crazy, actually, that the cold wake — at its peak — basically just erased the warm anomaly and brought it back to normal,” he said. “Now, it’s getting to be warmer than normal again.”
The lowest water temperatures after a storm occur within days of landfall. McNoldy said the gulf was at its coolest around Sept. 6, when temperatures fell to 77 degrees Fahrenheit.
“That coolest part of the week, it was still plenty warm to support any sort of hurricane that would have come across,” he said. “It’s just getting back to warmer than that again.”
Temperatures in portions of the gulf affected by Idalia have already risen by about 1 degree Fahrenheit in the time since the hurricane made landfall, according to sea surface temperature data from the National Oceanic and Atmospheric Administration.
“Within another few days, you won’t even notice that there was a cool wake there,” McNoldy said.
Surface temperatures tend to heat back up about two weeks after a hurricane leaves the Gulf of Mexico because of its warm, deep water, he said. The Loop Current, which supplies warm water from the Caribbean to the gulf, also plays a role in the returning marine heat.
Gulf temperatures are being watched closely by coral reef researchers, who say heat stress on reef systems this year exceeds anything they’ve seen in Florida’s history.
Liv Williamson, an assistant scientist at the Rosenstiel School of Marine, Atmospheric and Earth Science, said that because of these cooler, post-hurricane temperatures, Florida’s reefs are no longer accumulating heat stress, which causes coral bleaching and deaths.
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But this could quickly change if temperatures rise again, which Williamson warned is likely this early in the year.
“We are not out of the woods yet,” she told the Times in an email. “But very relieved to see cooler temps for now at least.”
It’s not uncommon for hurricanes to cool surface temperatures temporarily before the warm waters return.
In 2005, hurricanes Katrina and Rita decreased water temperatures by more than 7 degrees Fahrenheit in the wake of their paths, which cooled the entire Gulf of Mexico by nearly degrees Fahrenheit, according to NASA data. But water temperatures quickly slid back up and remained high enough to fuel hurricanes through the middle of October.
Jeff Masters, a hurricane scientist formerly with the National Oceanic and Atmospheric Administration, said the cooler period after Idalia and Franklin could weaken any additional storm systems in the gulf for the remainder of the 2023 hurricane season.
“The first is the worst,” he said. “You’re probably not going to be able to get as an intense a hurricane as Idalia in the gulf because it took advantage of the warm water that was there.”
Masters said this is already happening in the Atlantic Ocean — Hurricane Lee has weakened and will decrease more because of Franklin’s cooling wake.
But returning warm waters means this break won’t last long.
“By October, I think your Idalia advantage is going to be completely gone,” Masters said.