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Hurricane season 2021 isn’t over until the end of the month of November and it’s too early to say we’re in the clear for the rest of the year. But in case you haven’t noticed, the tropics around Florida have been extraordinarily quiet for the past several weeks. In the wake of Tropical Storm Fred that moved parallel to the west coast of Florida in August, no systems have formed near or even been close to threatening our area.
Since it’s been so quiet around here lately you might have tuned out what’s been happening in other areas of the tropics. And in those other areas, some pretty remarkable things have been happening with tropical meteorology!
To catch you up on what’s been happening in the water well off our coast, here are the two biggest stories of hurricane season and why they’re important to you.
Story One: New technology sets sail
Everyone is familiar with the work of Hurricane Hunter airplanes that fly into tropical storms and hurricanes. The people on board those planes collect valuable data assessing the strength and organization of the storms, which helps forecast models and meteorologists predict where storms will go and how strong they will get.
But for the first time this year, the planes were joined by a small but mighty fleet of ship drones that were deployed into the ocean with the purpose of studying a hurricane with sensors and video. The ship drones got the most use during Hurricane Sam, one of the biggest and longest-lived hurricanes that formed this year.
“I had chills when I saw those first pictures!” NOAA scientist Greg Foltz told NBC2 News. “Nobody had ever seen something like this from the surface of the ocean in the eyewall of a major hurricane.” You can watch the video taken by the ship drone inside Hurricane Sam here.
Foltz is one of the leaders in this new technology that’s been in the works for the past few years. He’ll be the first to tell you though that despite the incredible video that the ship drones got in the 30 foot plus waves Hurricane Sam produced, these drones have a much more important purpose. That job revolves around trying to better understand the process of hurricane rapid intensification.
“We really need to understand rapid intensification better because it’s likely going to get worse in the future with the ocean warming,” Foltz said.
Rapid intensification is a specific term used to describe a tropical system that sees an increase in maximum sustained winds of at least 35 mph in 24 hours. It doesn’t happen often, but when it does it can mean major issues for any nearby areas of land. As global oceans warm, the concern is that storms that develop will be capable of rapidly intensifying more often. This puts more coastal areas at risk, as the stronger a hurricane is, the greater the impacts can be when they approach the shore.
At the moment you can see how much of the Atlantic basin is running above average when it comes to temperatures during the first week of November.
The hope with this project is that with the ship drones studying the water, they’ll be able to sample closer than ever before, what is happening at the water’s surface when storms rapidly intensify. Eventually, that data will hopefully be able to help give us a better idea of when and where storms may undergo the process of rapid intensification. That’s a goal very important to us in Southwest Florida, an area of the world no stranger to strong hurricanes.
Story Two: The ACE Index
Unless you’ve taken a course in tropical meteorology, you’ve probably never encountered the term “ACE Index” before. Though the index is somewhat complex, the takeaways from the Accumulated Cyclone Energy Index can help to put into perspective how strong one hurricane is compared to another and how active one season is compared to another.
By definition, the ACE Index is the result of “the sum of the squares of the maximum sustained surface wind speed measured every six hours” as per the Climate Prediction Center. Expressed more simply, it’s essentially an index that takes into account how strong and long-lived a tropical storm system is. Weak and brief storms have low ACE index levels. Strong and persistent storms have high ACE index levels.
You can use the 2021 season as a good example of how the ACE index can compare between systems. Subtropical Storm Ana was the first storm of the year. It wasn’t a terribly strong system and it didn’t last for more than 2 days. As a result, Ana’s ACE index was 1. Hurricane Sam though, the same system that the sail drones got such incredible video of, was a big and bad system. It was a named storm for 12 days, and stronger than a Category 3 system for more than a week! As a result, it generated far greater amounts of energy than Ana, ultimately ending with an ACE index of 53.8. This makes it the highest ACE index storm of the year, with the highest value of any storm since Hurricane Irma back in 2017!
What makes the ACE index valuable to you is that it can offer a more apples-to-apples comparison of how hurricane seasons compare. Most people judge a hurricane season by the number of systems that form. But it’s important to remember that over the course of a year, we can sometimes have many systems that spin up briefly and last sometimes less than a day far away from land. Simply counting how many storms form can lead to a somewhat inflated vision of a season, because using this way of judging a season means a weak subtropical system counts as much as a huge hurricane. Using the ACE index on the other hand can give you a better idea of context as to how strong storms really are, not just tallying up how many form during the year.
When you add up the value of all the ACE index levels of a hurricane season, you can also get a better understanding of how energetic an entire season actually is. This year, 18 of the systems that formed had ACE index values of less than 10, leaving three systems: Ida, Larry, and Sam as the main events. Ida had an ACE of 10.8 with Larry coming in at 32.8. The running total for 2021’s ACE index is now 141.2. This is notable because during a typical hurricane season, a whole year normally ends up closer to 122.5. The meaning? This year has been an active and energetic season, and we should consider ourselves lucky that we weren’t in the path of anything; but especially Ida, Larry, and Sam.
You can look at the ACE index values of all the storms so far this year here. Interested in learning more about the tropics? Visit our online NBC2 hurricane guide here.