A team of scientists closely watched Hurricane Ida as it headed towards the Gulf of Mexico. They were watching a large, swirling pool of warm waters directly in front of it.
The warm pool, also known as an eddy or a swarm, was a warning sign. It measured approximately 125 miles (200 km) in size. It was about to give Ida a power boost. In less than 24 hours, it would transform from a weak hurricane into a dangerous Category 4 storm.
Nick Shay was an oceanographer at University of Miami's Rosenstiel School of Marine and Atmospheric Sciences. These eddies are part of the Loop Current that help storms intensify rapidly into hurricane-force winds.
How do these eddies get formed?
The Loop Current, which is a component of large gyres, is a circular current that rotates clockwise in North Atlantic Ocean.
Its strength is due to the flow of warm waters from the tropics, Caribbean Sea, into the Gulf of Mexico. Then it flows again through the Florida Straits between Florida and Cuba. It forms the core for the Gulf Stream that flows north along the Eastern Seaboard.
This current can begin to shed large warm winds in the Gulf when it reaches north of Fort Myers (Florida). There can be up to three warm eddies present in the Gulf at any one time. These eddies can form during hurricane season. This can lead to disaster for the Gulf's coastal communities.
Subtropical water is more salinity and temperature than Gulf water. This makes it easy to identify its eddies. They are warm at the surface, and have temperatures between 78°F (26 C) and 400 feet (about 120 to 150 m) in the water layers.
The strong salinity difference prevents the layers from cooling and mixing, so the warm eddies retain significant heat.
Hurricanes can form when the ocean's surface heat is above 78 F (26 C). Ida was able to pass over an eddy with temperatures above 86 F (30 C).
How did you discover that this eddy would be a problem for you?
Each day, we monitor the ocean heat content from space and keep an eye out for ocean dynamics, particularly during the summer months. Remember that warm eddies can also stimulate atmospheric frontal systems in wintertime, as was the case with the storm of the century that created snowstorms in the Deep South in 1993.
We flew over the heat pool to gauge the danger it posed to Hurricane Ida and dropped measuring devices, also known as expendables, from aircraft.
A parachutist descends to the surface, and then releases a probe that drops to about 1,300-5,000 feet (400- 1,500 meters) below surface. The probe then transmits data about temperature and salinity.
The heat from the eddy was about 480 feet (about 150 meters) below surface. Even though the storm's winds caused some mixing of cooler water at the surface with the deeper water, it wasn't going all the way down. The eddy would remain warm and provide heat and water.
This meant that Ida was soon to have an immense supply of fuel.
Warm water can extend as far as it will go, and we begin to notice an increase in atmospheric pressure. Since the eddies do not cool significantly, the moisture transfers from the ocean to the atmosphere, also known as latent heat are sustained.
The central pressures will continue to fall as the latent heat is released. The storm will eventually cause the surface winds to feel larger horizontal pressure variations and speed up.
This is what we saw on the day Hurricane Ida hit. The storm began to feel the warm water in its eddy. Storms become more defined and stronger as the pressure drops.
The wind speed was 105 miles an hour when I went to sleep at night. Ida was now a major hurricane when I woke up several hours later.
Rapid intensification is a new development?
Although we've known for years about the impact of hurricanes on heat, meteorologists have taken a long time to pay more attention and understand the role of high ocean heat in rapid intensification.
Hurricane Opal, a minor tropical storm that sailed through the Gulf in 1995, was an example of a weakening hurricane. Forecasters didn't know that a large warm eddy existed in the middle of the Gulf. It was moving at about the same speed as Miami traffic during rush hour. The warm water dropped to around 150 meters.
The satellite data only showed the surface temperature. Opal quickly intensified, and eventually hit the Florida Panhandle. This caught many people off guard.
Meteorologists are now more aware of where heat pools are. Every storm is not created equal. However, too much wind shear can cause a storm to become a complete disaster. But, when the atmosphere and ocean temperatures are very favorable, this can make a big difference.
Both Hurricanes Rita and Katrina had the exact same signature as Ida in 2005. They crossed an eddy just beginning to shed its Loop Current sand.
Although Hurricane Michael didn't pass over an eddy in 2018, it did go over the filament of the eddy like a tail while it was separating the Loop Current. Each of these storms intensified rapidly before reaching land.
These warm eddies are more common during hurricane season. This can sometimes happen along the Atlantic Coast too. However, the Gulf of Mexico or the Northwest Caribbean are less affected by storms so it is more likely that someone will be impacted there.
It can cause havoc for the coastal residents if it becomes too close to the coast like Ida did.
What does climate change have do with it?
Global warming is a fact. We also know that the Gulf of Mexico is warming. However, I believe that rapid intensification is a result of local thermodynamics. It is not clear how important global warming plays.
This area is ripe for research. Since more than 20 years, we have been studying the Gulf's ocean heat contents. Scientists can compare the temperature measurements taken during Ida and other hurricanes to satellite data to better understand how the oceans contribute to the rapid intensification and destruction of storms.
Scientists can use these profiles to fine-tune computer simulations for forecasts to give more accurate and detailed warnings in futures.
Nick Shay, University of Miami Professor of Oceanography
This article was republished by The Conversation under Creative Commons. You can read the original article.
