As Hurricane Ida headed for the Gulf of Mexico, a team of scientists watched closely a giant slowly swirling a pool of warm water directly in its path.
That warm pool, a whirlpool, was a warning sign. It was about 200 miles in diameter. And he was about to give Ida the impetus to power that in less than 24 hours would turn him into a weak hurricane in the dangerous Category 4 storm that crashed against Louisiana on the outskirts of New Orleans on the 29th. August 2021.
Nick Shay, an oceanographer at the Rosenstiel School of Marine and Atmospheric School at the University of Miami, was one of those scientists. Explain how these eddies, which are part of what is known as a loop current, help storms intensify rapidly in monster hurricanes.
How are these eddies formed?
The loop current is a key component of a large turn, a circular current, that rotates clockwise in the North Atlantic Ocean.
Its strength is related to the flow of warm water from the tropics and the Caribbean Sea to the Gulf of Mexico and again through the Florida Strait, between Florida and Cuba. From here, it forms the core of the Gulf Stream, which flows north along the east coast.
In the Gulf, this current can begin to spill large warm eddies when it reaches north approximately from the latitude of Fort Myers, Florida. At any one time, there can be up to three warm eddies in the Gulf. The problem comes when these eddies form during hurricane season. This could mean a disaster for Gulf coastal communities.
Subtropical water has a different temperature and salinity than common Gulf water, so its eddies are easy to identify. They have warm water on the surface and temperatures of 26 degrees Fahrenheit (26 ° C) or more in layers of water that extend to about 400 or 500 feet deep (about 120 to 150 meters).
Since the strong difference in salinity inhibits the mixing and cooling of these layers, warm eddies retain a considerable amount of heat.
When the heat on the ocean surface exceeds 26 ° C (78 F), hurricanes can form and intensify. The whirlpool that passed Ida had surface temperatures above 30 ° C (86 ° F).
How did you know this swirl would be a problem?
We monitor the ocean heat content from space every day and monitor the dynamics of the oceans, especially during the summer months. Keep in mind that warm eddies in the winter can also energize atmospheric frontal systems, such as the “storm of the century” that caused snowstorms all over the South in 1993.
To measure the risk this heat pool posed to Hurricane Ida, we flew over planes over the whirlpool and dropped measuring devices, including those known as consumables.
A dispensable parachute to the surface and releases a probe that goes about 400 to 1,500 meters below the surface. It then returns data on temperature and salinity.
This whirlpool was hot up to about 480 feet (about 150 meters) below the surface. Even if the storm wind caused some mixing with colder water on the surface, that deeper water would not mix to the bottom. The whirlpool would stay warm and continue to provide heat and moisture.
This meant that Ida was about to get a huge supply of fuel.
When the warm water spreads out like this, we begin to see the drop in atmospheric pressure. Moisture transfers, also known as latent heat, from the ocean to the atmosphere are maintained over warm eddies, as eddies do not cool significantly.
As this latent heat release continues, central pressures continue to decrease. Eventually, surface winds will feel the largest horizontal pressure changes through the storm and begin to accelerate.
This is what we saw the day before Hurricane Ida hit land. The storm was beginning to perceive that very warm water in the whirlpool. As the pressure continues to drop, the storms become stronger and better defined.
When I went to sleep at midnight that night, the wind speed was about 105 miles per hour. When I woke up a few hours later and checked the National Hurricane Center update, it was 145 miles per hour and Ida had become a major hurricane.
Is rapid intensification a new development?
We have known about this effect in hurricanes for years, but meteorologists have taken a long time to pay more attention to the higher ocean heat content and its impact on rapid intensification.
In 1995, Hurricane Opal was a minimal tropical storm meandering in the gulf. Unknown to forecasters at the time, a large warm whirlpool was in the middle of the Gulf, moving as fast as Miami rush hour traffic, with warm water up to about 150 feet.
All the meteorologists saw in the satellite data was the surface temperature, so when Opal quickly stepped on its way to the Florida Panhandle, it surprised a lot of people.
Today, meteorologists are watching more closely where the heat ponds are. Not all storms have all the right conditions. Excessive wind shear can break a storm, but when atmospheric conditions and ocean temperatures are extremely favorable, you can get that big change.
Hurricanes Katrina and Rita, both in 2005, had almost the same signature as Ida. They toured a warm whirlpool that had just been prepared to be launched from the Loop Current.
Hurricane Michael in 2018 did not pass a vortex, but passed through the filament of the vortex, like a tail, as it separated from the loop current. Each of these storms intensified rapidly before reaching land.
Of course, these warm eddies are the most common during hurricane season. From time to time you’ll see this happen on the Atlantic coast, but the Gulf of Mexico and the northwestern Caribbean are more contained, so when a storm intensifies there, someone will be hit.
When it intensifies near the coast, as Ida did, it can be disastrous for coastal residents.
What does climate change have to do with it?
We know that global warming is taking place and we know that surface temperatures are warming in the Gulf of Mexico and elsewhere. However, when it comes to rapid intensification, my view is that many of these thermodynamics are local. How big the role global warming plays is still unclear.
This is a fertile area of research. We have been controlling the ocean heat content of the Gulf for over two decades. By comparing the temperature measurements we made during the Ida and other hurricanes with satellite and other atmospheric data, scientists can better understand the role the oceans play in the rapid intensification of storms.
Once we have these profiles, scientists can fine-tune the computer model simulations used in forecasts to provide more detailed and accurate warnings in the future. 
Nick Shay, Professor of Oceanography, University of Miami.
This article is republished from The Conversation under a Creative Commons license. Read the original article.