2021 Season Summary

The 2021 Atlantic hurricane season was quite active, with a total of

21 cyclones attaining tropical depression status,
21 cyclones attaining tropical storm status,
7 cyclones attaining hurricane status, and
4 cyclones attaining major hurricane status.

Before the beginning of the season, I predicted that there would be

17 cyclones attaining tropical depression status,
16 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status,
5 cyclones attaining major hurricane status.

The average numbers of tropical storms, hurricanes and major hurricanes (over the 30 year period 1991-2020) were 14.4, 7.2, and 3.2, respectively. This puts the 2021 season a bit above average overall, with a much higher than usual number of named storms but near average numbers of hurricanes and major hurricanes. Likewise, my predictions underestimated the number of named storms, but were close on hurricanes and major hurricanes. The 21 named storms were the third-most recorded for an Atlantic season at the time, behind 2020 and 2005. The Accumulated Cyclone Energy (ACE) value for the 2021 season (which accounts for duration and intensity of storms as well as number) was roughly 145, once again above normal but not by a huge margin. The exact figure will likely shift slightly in post-season analysis.

A bevy of short-lived, weak systems inflated the total number for the season, especially early on. There were three named storms in June for the first time since 1968. Hurricane Elsa set a record for earliest fifth named storm when it was named on July 1. However, the end of the season was surprisingly quiet. With the exception of meandering Tropical Storm Wanda, which spent a week in early November moving around the north Atlantic, there was no activity from October 4 onward. This is quite unusual: on average, one would expect around 3 named storms and at least 1 hurricane during that time. Even stranger, the entire world experienced a lack of strong cyclones during October and November 2021. To explain this behavior, we'll examine a few factors.

At the beginning of the season, an ongoing La Niña event was on the wane, and indeed it officially ended over the summer. However, equatorial Pacific sea temperature anomalies remained negative (consistent with La Niña) and even began to decrease again by late in the year. The relevant regions of the Pacific and their temperature anomalies throughout the year are illustrated above. Overall, the weak La Niña that occurred was consistent with pre-season predictions. It is correlated with lower wind shear in the Atlantic and thus consistent with the somewhat above normal activity observed. Likewise, the tropical north Atlantic had fairly warm ocean temperatures, which helped tropical cyclones to form and intensify. However, this doesn't explain why the Atlantic shut down in October.

2021 featured a particularly strong west African monsoon; note the high precipitation over Africa around 10° N or so between June and August in the map above. This is signficant because tropical waves, the precursors to most Atlantic tropical cyclones, form over Africa and move westward into the Atlantic. Indeed, in contrast to 2020, a larger proportion of storms originated in the tropical Atlantic (see the map of storm tracks below):

Every year, the opportunity for cyclone formation off of Africa closes in early October, when the wave axis shifts toward the equator and east Atlantic temperatures start to get cooler. The center of activity usually then shifts to the Caribbean, but the Caribbean saw an unusual amount of dry sinking air late in the year, stifling any formation. This is again in stark contrast to 2020, when the west Caribbean saw its most active period in recorded history from October through November. These factors help to indicate why the 2021 season ended earlier than expected.

In my predictions, I put the Caribbean islands and the Gulf of Mexico at highest risk, with the U.S. east coast and central America at lower risk. These predictions were fairly good: Hurricane Elsa, Tropical Storm Fred, Hurricane Grace, and Hurricane Ida impacted the Caribbean Islands, while Ida and Hurricane Nicholas had the greatest Gulf impacts (though Grace also hit the Yucatan and the southwest Gulf). The Caribbean impacts might have been greater still had some of the above storms not tracked directly over the greater Antilles. No landfalls occurred in central America and only the extremely unusual Henri had any significant impact on the U.S. east coast as a tropical cyclone.

Some other notable facts or records from 2021 include:

• Tropical Storm Ana's formation on May 22 made 2021 the seventh consecutive season with a named storm before June 1
• Elsa was the easternmost forming early July hurricane in the tropics since 1933; it also had the highest forward speed of any known Atlantic hurricane in the tropics
• Grace's remnants reformed as a tropical storm in the eastern Pacific and were named Marty
• Henri was the slowest-moving cyclone ever recorded in the U.S. northeast; this led to unusual flooding impacts in the region
• Ida tied 2020's Hurricane Laura for highest winds of a landfalling Louisiana hurricane (150 mph)

Finally, Hurricane Sam was the season's strongest storm, with winds of 155 mph and a central pressure of 929 mb. It was a long-lived and powerful storm, clocking in at over 50 ACE just on its own and holding major hurricane status for 7.75 consecutive days. Despite this, it went out to sea and didn't significantly impact any landmasses.

The 2021 season was quite active, though with many weak systems. The vast majority of the season's damage and loss of life were due to Hurricane Ida, which made landfall in Louisiana as a category 4 hurricane.

Sources: https://www.washingtonpost.com/weather/2021/11/22/earth-hurricane-activity-record-quiet/, https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf