Hurricane Helene made landfall in the Big Bend area of the Florida Gulf Coast as a category 4 storm late in the evening of September 26, 20241. Although slowly weakening as it moved north, Helene caused significant damages and casualties across a large portion of the Southeast U.S., including North Carolina, South Carolina, Georgia, Florida, Tennessee and Virginia and will most likely rank as one of the deadliest storms to hit the U.S. in recent times2.
The Atlantic hurricane season spans from June 1 to November 30 every year. Tropical Storm Helene formed in the Caribbean Sea on Monday September 23, 2024 and then developed into a category 1 hurricane on September 25, 2024. Helene then rapidly transformed from a category 1 hurricane to a category 4 hurricane in less than a day on September 26, 2024, less than a day before landfall1. This rapid intensification is in part due to very high ocean temperatures across the Gulf of Mexico that act as ‘fuel’ for hurricanes3. The storm moved over the Loop Current, a strong flow of warm water that travels from the Caribbean, loops in the eastern Gulf of Mexico like a horseshoe before moving through the Florida Strait up the Atlantic Ocean. Because the Loop Current transports warm waters from the Tropics, storms moving over it often get enough energy from it to rapidly intensify.
Figure a shows the sea surface temperatures (SST) measured by satellites in the Gulf of Mexico on September 23, 2024, right at the time when Tropical Storm Helene formed in the Caribbean Sea. The entire Gulf of Mexico was warm, with surface temperatures above 28°C and above the 2003-2014 climatology (Figure b). After the hurricane’s passage, colder waters from below were brought to the surface by the mixing induced by the hurricane high winds, which is called a ‘cold wake’, as shown by the blue colors (colder than before the passage of the hurricane) in Figure d over the entire eastern part of the gulf and in particular where it intensified to category 4 and on the right side of the hurricane track.
Hurricanes are not only fueled by the heat at the surface of the ocean but also by the heat that is stored at depth, below the surface, that we call ocean heat content. At present satellites cannot directly measure the ocean temperatures below the surface. However, because when water heats up it tends to expand, the high ocean heat content can be observed in sea level data (higher ocean heat content means higher sea level due to water expansion). Figure c shows the sea level observed by altimetry satellites such as Sentinel-6 Michael Freilich in the Gulf of Mexico on September 23, 2024, a few days before Hurricane Helene rapidly intensified. Red colors show locations where sea level is higher (warmer waters at depth) that are particularly present where the hurricane intensified from category 1 to category 2 and then category 3. At the time Hurricane Helene moved over the Gulf of Mexico, not only the surface temperatures were significantly high, a lot of heat was also stored below the surface available to fuel the hurricane to intensify.
The National Oceanic and Atmospheric Administration (NOAA), the U.S. agency in charge of the national weather forecast, uses satellite sea surface temperature and sea level observations such as those available through the NASA Physical Oceanography Distributed Active Archive Center (PO.DAAC), along with other observations, to provide tropical cyclone track and intensity forecasts and issue advisory products.
To further explore these types of data, the NASA State of the Ocean (SOTO) by Worldview web tool allows for easy review and examination. In addition, the jupyter notebook included here allows for replication of these images.