Every 30 years, the National Oceanographic and Atmospheric Administration revises the baseline of what weather and climate conditions are considered “normal.” The most recent normals for Atlantic hurricane activity will soon be released, and a preview shows a spike in storm frequency and intensity.
During the most recent 30-year period, 1991 to 2020, there has been an uptick in the number of named storms and an increase in the frequency of major hurricanes of category 3 intensity or greater in the Atlantic.
Seven Category 5 storms swirled across Atlantic waters in the past five years.
The newly revised climate normals are not a forecast of upcoming activity, nor are they necessarily illustrative of any one particular climate or meteorological trend. They’re benchmark values.
The National Weather Service calculates new climate normals each decade for all major U.S. cities that have sufficient historical data. A local television meteorologist’s “10 degrees above average,” for instance, relies on this data.
The new hurricane normals are not official yet, though available data shows a rise in storm frequency and intensity, probably related to a combination of climate change, natural variability and improved storm detection.
“The 1991-2020 climate figures for hurricane season will be discussed, finalized and released in May,” said Dennis Feltgen, a meteorologist and public affairs specialist at the National Hurricane Center. The agency plans to coordinate with NOAA’s National Centers for Environmental Information and the Climate Prediction Center before releasing final tallies.
John Bateman, a meteorologist at NOAA’s Satellite and Information Service, said that “right now, what those normals say about Atlantic Basin hurricanes is still being reviewed.”
Between 1961 and 1990, the Atlantic averaged 10 named storms a year and two major hurricanes. Those numbers remained essentially constant in the 1971-2000 climate period.
However, the figures began climbing in the 1981-2010 window, and have escalated significantly since.
Brian McNoldy, an atmospheric scientist, ran the calculations to preview what the National Hurricane Center probably will report for the new normals. He found that an average of more than three major hurricanes have spun up each year since 1990, with about 14 named storms per year.
At first glance, it looks like a concerning spike, especially compared with the 1961-1990 period. But part of that increase, especially the count of named storms, may be the result of technological improvements that allow show more than what may have been missed before the satellite era.
“I would not be too alarmed at the dramatic increase between the 1981-2010 averages and the 1991-2020 averages,” McNoldy wrote in an email. “Keep in mind that both sets have 1991-2010 in common, so we’re really comparing 1981-1990 to 2011-2020.”
McNoldy noted “a gradual increase in technology between the 1980s and the 2010s, especially in scatterometry,” which is a type of satellite-based imaging system that allows meteorologists to detect changes in storm structure and wind speeds.
That may be a key factor in the perceived increase in storm frequency because it’s still an active area of research on whether climate change will significantly change the number of named storms. The science is clearer that storms are expected to be more intense, with greater rainfall and higher wind speeds.
The new normals show an increase in major hurricanes of category 3 intensity or greater.
“We could conceivably see little change in the number of named storms but still an increase in the number of major hurricanes in some future update,” McNoldy said.
He also highlighted other factors, such as the AMO, or the Atlantic Multidecadal Oscillation. Some scientists say this theorized cycle influences ocean water temperatures and atmospheric features, and have linked it to Atlantic hurricane activity. They say that the Atlantic Basin has historically alternated between 25- to 40-year periods of cool and warmer sea surface temperatures and that the trend is closer to the warm side of the spectrum.
Not everyone agrees that the AMO exists, let alone as a driver of storm activity during a particular season.
Michael Mann, a climate scientist at Pennsylvania State University who was one of the earliest ones to theorize the AMO, now thinks it’s bunk.
“The attribution of rising hurricane activity to the ‘AMO’ . . . is simply wrong,” Mann said in an email. “As we have shown in the scientific literature, the warming that is behind the unprecedented recent Atlantic hurricane activity is tied to human-caused greenhouse warming, not natural variability.”
Working with other researchers, Mann issued a seasonal hurricane forecast last year that came the closest to successfully predicting the number of hurricanes and major hurricanes, and it relied mainly on unusually mild waters in a particular part of the tropical Atlantic Ocean.
Natural variability, or expected atmospheric randomness tied to patterns such as El Niño and La Niña, is a factor. McNoldy says this is perhaps the biggest factor in the jump in numbers.
“The 1981-1990 period included several very inactive seasons, while the 2011-2020 period included several very active seasons,” McNoldy wrote. “When one inactive decade drops out of the average and an active one takes its place, of course the average will go up.”
Looking ahead, there’s no guarantee the streak of busy seasons will continue at its current pace. But McNoldy says calculating averages is an important meteorological exercise.
“If the averages are not representative of a modern climate, then both the averages and the anomalies lose their meaning,” McNoldy wrote.
NOAA plans to officially release the updated averages in May, about the same time the agency issues its first long-range hurricane-season outlook.