What happens underwater during a hurricane
"We know only a little of what is happening," oceanographer says.
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Hurricanes are one of the most ferocious storms on Earth. They are Mother Nature’s way of trying to keep everything balanced. Ocean water and the air above it exchange heat, which creates storms.
"We call it ‘interaction’ in physics, but it's where they communicate to each other," said Gustavo Goni, Ph.D., an oceanographer with the National Oceanic and Atmospheric Administration. "So, before there was even a hurricane, the ocean was transferring heat and moisture into the atmosphere."
If a favorable environment is maintained, that heat and moisture can organize into storms and eventually develop low-pressure centers to become tropical depressions, tropical storms, or hurricanes. These destructive monsters can produce terrible winds, flooding rains and push walls of water inland. Those are just the effects we can see.
Beneath the surface of the ocean where these storms originate, there are a plethora of changes that happen that sometimes go unseen. While scientists have a basic understanding of what happens underwater during a hurricane, there is still a lot of mystery surrounding these beasts.
"We know only a little of what is happening," Goni said. "Why (do) we know only a little? Because ... to measure the ocean under a hurricane, no doubt ... it's not easy. You don't have a ship there, you know, taking measurements."
Let’s start with what scientists do know about the underwater effects of a hurricane.
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Upwelling
It’s no surprise that hurricanes can produce a lot of wind. All that wind moving along the ocean’s surface creates waves, which begin to churn the water. The churning action produces currents that quickly move the water under a hurricane from one place to another.
"It's doing it so fast, that it is replacing that water that is leaving from one part to another with water that was underneath, creating upwelling," Goni said.
Upwelling develops what Goni described as "internal waves," which break like waves on a beach but behave very differently from the waves most people know.
According to Goni, all of this action mixes up the seawater at a depth of between 165 and 330 feet, Goni said.
"What do you do by doing that? You are either mixing warm waters with colder waters that are underneath or you may be mixing waters with even more even warmer waters that are underneath," Goni said.
The mixing of water can result in temperature changes of a couple of degrees, according to Goni. That may not sound like a lot, but it can play a big role in the lifecycle of a hurricane.
"However, for a hurricane, seeing waters one or two degrees cooler could mean the difference between a Category 1 or a Category 5 or 4," Goni said.
Both the geographic area covered by a storm and its traveling speed are important factors in upwelling, Goni said. If a storm is large enough, the ocean waters in its path could be mixed quite a bit hours before it arrives.
"So by the time the eye of the storm gets there, it would see a different ocean than the ocean that the winds at the front of the hurricane," Goni said.
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Ocean life
Hurricanes can also have a substantial impact on life that lives in water it’s mixing up. Clay Porch, director of NOAA’s Southeast Fisheries Science Center, said those impacts can be both good and bad.
Porch said that as a hurricane approaches, animals feel the wave energy being generated by the storm and take action. Some dive deeper to avoid it, while others hunker down and try to ride it out.
"A lot of them could be transported inshore," Porch said. "We've certainly seen that, where we see saltwater fish and marine mammals, dolphin, etc., carried off into freshwater lakes, which obviously doesn't do them any good."
Porch said the flooding rains over land rush into streams and rivers that drain into the ocean and create a large influx of freshwater into coastal waters, which can result in illnesses for species that rely on a bounty of saltwater to survive.
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"Over the short term, hurricanes wreak havoc on any shallow coastal marine ecosystem," Porch said. "They destroy coral reefs by pounding them with the waves. They ravage mangroves and marshes. They mix up the water column, getting sediment all in the water column, which clogs up things like sponges, etc. They redistribute those bottom sediments, and they can increase pollution by a lot of excess freshwater runoff."
Porch said that in the long term, some species have adapted to benefit from the changes wrought by a hurricane.
"The story isn't all bad because we have to remember that hurricanes are part of the natural ecosystem and have been for thousands of years," Porch said.
Porch said sediment that is stirred up by hurricanes can actually create a more hospitable environment near shorelines for creatures like sponges and corals. In turn, that gives large marine animals like fish and crabs places to find food. Where the agitated sediment eventually settles can aid in the development of new mangroves and marshes.
"Over the long term, even though the short-term impacts can be devastating, the storms and hurricanes can actually make things a little bit better for a lot of animals," Porch said.
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More to learn
While scientists already know a lot about the impacts of hurricanes on oceans, there is still much more that can be learned.
Goni said improved satellites are being used to take measurements before and after storms. He said NOAA is also using drone boats, which he described as "sentinels" to sail into the ocean where more direct measurements can be taken.
"We are accomplishing now what couldn't be done before, that is to observe the atmosphere and the ocean at the same time and in the same place under a hurricane," Goni said.
It’s all meant to help us better understand Mother Nature’s balancing act.
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