Hurricane Idalia was one of the most destructive storms of 2023. It underwent rapid intensification in one day – which may have been influenced by warmer fresh water rushing into the Gulf from North Florida rivers.
That's according to a new study led by researchers with the University of South Florida's College of Marine Science.
WUSF talks with one of the lead researchers, Chuanmin Hu. He said their research came almost by accident.
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HU: We actually were studying harmful algal blooms. We were using a boat and a glider to measure ocean properties. Then we encountered this river plume. Then later on, there was a hurricane, Hurricane Idalia. So that hurricane intensified so fast beyond the model predictions.
WUSF: That went from a Category 1 only maybe one day out from landfall in North Florida, right?
Right. It went from Category 1 to Category 4 within 24 hours after passing through that river plume. So that is a surprise. Then we thought this river bloom must have something to do with the hurricane. Then we came up with this study.
You basically found that fresh water coming in, I'd imagine, from rivers in North Florida flowing into the Gulf, is affecting the salinity of the water. Could you explain that?
We have a lot of rivers along the Gulf coast. Mississippi River is the largest one, but at that time, Mississippi River flow was not as high as usual. But river flow from other smaller rivers along the Gulf coast, from the Mississippi Delta to the Florida Panhandle, they had larger than usual river flow. So all contributed to this extensive and the long-lasting river plume extends from the Mobile Bay to the Florida Keys. It's huge, almost the entire East Coast. It lasted for several months.
"The salinity is lower, and this fresher and buoyant water stays on the ocean surface to reduce ocean mixing from the surface to the deep ocean. If the ocean mixing happens, the cold water from the depths is brought to the surface to cool down hurricanes, so the hurricane would not intensify that fast."Chaumin Hu, USF professor of optical oceanography
So what happened was this river plume had fresher water than ocean water. The salinity is lower, and this fresher and buoyant water stays on the ocean surface to reduce ocean mixing from the surface to the deep ocean. If the ocean mixing happens, the cold water from the depths is brought to the surface to cool down hurricanes, so the hurricane would not intensify that fast.
You have lighter water on the top; you have a very dense or heavy water on the bottom. It's very difficult to mix these two types of water.
And is the fresh water cooler or warmer?
It's warm and plus, at that time, we had a very widespread marine heat wave, but across the Gulf now we have a river plume to prevent or to reduce ocean mixing. Then the hurricane continues to get intensified from this warm water.
Professor, it sounds like what you're saying here is that with global warming and heavier rains expected from larger storms, this could maybe fuel more intense storms in the future. It could be like a never-ending spiral?
If all the timing is correct, but say, if you have a similar river plume in the future, and at the same time you have a hurricane, yes, this is likely going to happen again, but that chance is not very often. Typically, you have a river plume, but you don't have a hurricane, or vice versa. You have a hurricane, you don't have a river.
It sounds like we need to be especially careful, maybe for later season storms, when there's been intense rainfall in these areas and the flow into the Gulf is much heavier, so it could intensify maybe later in the hurricane season.
Yes, it's the timing of the river plume, and also the scale of the river plume. If the river plume occurs in, for example, spring, the Gulf is not warm enough yet. We don't have currently in the spring anyway. But later in the year, like in August or September, that's when the Gulf water is the warmest. And now, if you have a large river plume and a hurricane passing by. That is the condition that will fuel the hurricane.
