The 2021 Atlantic hurricane season ended on November 30 as the third-most active year on record for named storms, trailing only 2020 and 2005. The season produced 21 named tropical or subtropical cyclones - the second season in a row in which the Atlantic named storm list that has 21 names was run out. Among those cyclones, seven became hurricanes, and four further intensified into major hurricanes.

The above-average hurricane season was anticipated by the Seasonal Hurricane Predictions platform run by the Barcelona Supercomputing Center (BSC) and Colorado State University (CSU) in association with AXA XL, which has recently renewed its support for the project. This is the first platform to aggregate forecasts from centres specialising in seasonal Atlantic hurricane forecasting.

The Seasonal Hurricane Predictions platform displays data from 27 different entities located worldwide, including meteorological services (e.g. NOAA, UK Met Office), universities (e.g., Colorado State University, NC State University) and private weather companies (e.g., WeatherBELL, WeatherTiger). The project was launched in 2016, aiming to improve the understanding of hurricane variability among the scientific community and the general public.

The platform collects forecasts on upcoming hurricane activity from the end of March to early August. This year’s predictions were quite accurate, with the average of all forecasts correctly anticipating the number of significant hurricanes (four observed, four predicted) while slightly over-forecasting the total number of hurricanes (seven observed, eight predicted) and under-forecasting the number of storms (21 observed, 18 predicted).

“This year was a notable forecast success for almost all groups issuing predictions. Most groups correctly anticipated an above-average season from as early as March/April,” said Phil Klotzbach, a research scientist in the Department of Atmospheric Science at Colorado State University and co-founder of the website.

Above-average activity for 6th year in a row

The 2021 Atlantic hurricane season was the 6th year in a row to have above-average activity. While the hurricane season officially begins on June 1, this year, tropical storm Ana formed on May 22, before the season's official start.

The season's most devastating storm was Hurricane Ida, which made landfall in the U.S. state of Louisiana on August 29 at category 4 strength. Ida tied with the Last Island Hurricane of 1856 and Hurricane Laura of 2020 as the strongest hurricanes to land in Louisiana on record. Its remnants also caused tremendous flooding in portions of the mid-Atlantic states, including New York City.

Climate conditions at the end of the 2021 Atlantic hurricane season were similar to those of 2020. Both years we had La Niña, a climate phenomenon characterised by cooler than average water temperatures in the eastern and central Pacific that typically increases the Atlantic hurricane activity at the end of the season. However, what was surprising is that October-November 2021 was vastly different from this period in 2020. October-November 2020 was extremely active, with six hurricanes forming, while no storms formed during October-November 2021. This suggests that other climate influences besides La Niña played a critical role in determining levels of hurricane activity.

“That just shows that we still have a lot to learn, and much research remains to be done to fully understand drivers of hurricane activity,” said Louis-Philippe Caron, a research scientist at Ouranos and BSC collaborator as well as co-founder of the website.