The 2023 Hurricane Season in Review: Part 1

Part 1: Atlantic Impacts

The 2023 hurricane season in the North Atlantic and Northeast/Central Pacific Oceans came to a close on November 30 [1]. While activity is still possible after this point, the probability of such is fairly low. With that caveat in mind, it’s time to look back at some of the memorable moments of the 2023 hurricane season and what we can take away from the season as a whole. In this blog post, we will focus on the Atlantic basin. Make sure to check back soon for an in-depth discussion on the Pacific hurricane season. 

First and foremost, the Atlantic Ocean was at record warm levels throughout the spring and summer of 2023 (Figure 1 and Figure 2) [2, 3]. The equatorial Pacific Ocean was also exceptionally warm, with a moderate to strong El Niño forming by the peak of the summer (Figure 2 and Figure 3) [2, 3]. As such, the major theme of the 2023 season was a tug of war between a record warm Atlantic and a strong El Niño in the tropical Pacific. As we have talked about in our previous blog post here, El Niño conditions in the Pacific work to impart greater vertical wind shear over portions of the tropical Atlantic due to enhanced convective activity being focused over the Pacific [4]. The outflow from the enhanced convection is essentially the driver for much of that shear over the Atlantic (Figure 4) [4]. 

Figure 1: Daily sea surface temperature (°C) averaged over the North Atlantic during 2023 (black line) and for previous years from 1979 to 2022 (grey lines). The average conditions for 1991-2020 are shown by the grey dashed line. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

Figure 2: SST anomalies from NOAA Coral Reef Watch for the beginning of the 2023 hurricane season, 15 June 2023.

Figure 3: Sea surface temperature anomaly (°C) for August 2023, relative to the 1991-2020 reference period. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

Figure 4: Effect of El Niño on tropical Atlantic wind shear. Rising air in the Pacific generates outflow at the upper levels of the atmosphere which enhances vertical wind shear and sinking air over the Atlantic, both of which are unfavorable to the formation and maintenance of tropical cyclones. Graphic borrowed from: https://weather.com/news/climate/news/2023-03-09-la-nina-el-nino-summer-hurricane.

2023 was a unique year because the Atlantic was also anomalously warm in a way that was comparable to the eastern Pacific (Figure 2 and Figure 3) [2, 3]. Therefore, rather than most of the convection being focused over the eastern and central Pacific with a relative dearth over the Atlantic, thunderstorm activity was plentiful over BOTH basins of the deep tropics throughout the season. In other words, the Atlantic basin was able to remain thermodynamically favorable in its own respects despite the negative feedback from the Pacific.

One way that we can diagnose Atlantic season activity is through the accumulated cyclone energy index [5]. The accumulated cyclone energy (ACE) of a storm is a metric used to describe the total amount of energy released by a tropical cyclone over its lifetime [5]. ACE is directly related to the sum of the squares of the maximum sustained winds of the tropical cyclone (measured every 6 hours) and the length of time the tropical cyclone existed [6]. Thus, cyclones that are particularly intense and last for longer periods of time will generate the highest ACE values [6]. The cumulative ACE for the 2023 Atlantic season was 146, compared to the 1991-2020 average of 123 for the Atlantic [7]. Most El Niño years generate below-average Atlantic ACE, yet this year resulted in above-average activity in the Atlantic. Therefore, 2023 was an unusual case that gave us a glimpse into the dynamics of a year when both the Atlantic and Pacific were exceptionally warm.

Despite the above normal activity in the Atlantic, the economic impacts of the season’s tropical cyclones were arguably more tempered in 2023 compared to previous years. Many of the most powerful storms that formed, such as Hurricane Franklin and Hurricane Lee, remained in the open Atlantic and affected few land areas [7]. However, 2023 would not go without generating a major hurricane landfall on the United States. In late August, Hurricane Idalia made landfall in the Big Bend region of Florida as a high-end category 3 storm [8]. Despite causing storm surges of up to 7-12 feet in parts of Florida, Idalia made landfall in a sparsely populated region of the state [8]. As such, industry insured losses were estimated around 2.4 billion USD from the storm, according to Gallagher Re [7]. The only other Atlantic storms that produced losses greater than $100 million in 2023 were Lee ($150 million USD) and Ophelia ($375 million USD) [7]. Consequently, the total damage caused by Atlantic named storms in 2023 was around $4 billion, the lowest since 2015 [7].

A large reason for this dearth in U.S. landfalls compared to previous years was a persistent area of low pressure over the eastern U.S. that resulted in many named storms turning out to sea before affecting land areas [9]. In addition, many of the storms that did form in the Main Development Region became strong quickly, which also made it more likely that they would turn out to sea instead of affecting land further west [9].

While the Atlantic Ocean’s impacts to land were less in 2023 than seen in previous years, the eastern Pacific generated multiple powerful tropical cyclones that affected both the United States and mainland Mexico [7, 10]. Stay tuned for our next blog post covering the 2023 Eastern Pacific hurricane season in depth.

References 

1.   https://www.noaa.gov/news-release/2023-atlantic-hurricane-season-ranks-4th-for-most-named-storms-in-year#:~:text=The%20above%2Dnormal%202023%20Atlantic,in%20a%20year%20since%201950.

2.   https://climate.copernicus.eu/record-high-global-sea-surface-temperatures-continue-august

3.   https://coralreefwatch.noaa.gov/product/5km/index_5km_ssta.php

4.   https://weather.com/news/climate/news/2023-03-09-la-nina-el-nino-summer-hurricane

5.   https://www.earthdata.nasa.gov/topics/atmosphere/weather-events/tropical-cyclones/accumulated-cyclone-energy 

6.   https://www.cpc.ncep.noaa.gov/products/outlooks/Background.html

7.   https://yaleclimateconnections.org/2023/11/the-unusual-2023-atlantic-hurricane-season-ends/#:~:text=Above%2Daverage%20activity%20in%20the,Energy%20(ACE)%20of%20164.

8.   https://www.weather.gov/tae/HurricaneIdalia2023

9.   https://tropical.colostate.edu/Forecast/2023-11.pdf

10. https://www.nhc.noaa.gov/data/tcr/index.php?season=2023&basin=epac