Atlantic Tropical Cyclone Days

This indicator shows changes in annual Atlantic tropical cyclone activity near land.

Atlantic tropical cyclone activity has increased since a minimum in the 1970s. In addition, the duration or proportion of time that storms spend at major hurricane strength has increased. A warming Atlantic Ocean is expected to lead to further increases in storm intensity in the coming decades, which would pose greater risks to coastal communities. Tropical cyclone activity changes from year to year and decade to decade due to both natural and human-induced factors. For example, storms tend to be more active during La Niña years and less active during El Niño years. Natural changes in the Atlantic Ocean circulation can also contribute to decades with warmer and cooler ocean temperatures, thus altering the amount of fuel for these storms. In addition, increasing greenhouse gas concentrations and a reduction in small particles in the atmosphere (called aerosols) have contributed to higher sea surface temperatures and more storms since the 1970s. See NOAA GFDL's Global Warming and Hurricanes for more information on climate change factors affecting tropical cyclone activity.

Tropical cyclone days are calculated by counting the number of active storms on each day of the year and then summing these totals for the year. The storms are broken into three groups based on their intensity. Named storm days count all tropical cyclones with winds of at least 39 mph. Hurricane days count storms with winds of at least 74 mph, and major hurricane days count storms of at least 111 mph. These counts are cumulative; that is, a storm at major hurricane intensity is included in the counts for named storms, hurricanes, and major hurricanes. For example, a storm that spends three days as a tropical storm, two days as a Category 1–2 hurricane, and one day as a major hurricane (Category 3–5) is counted as six named storm days, three hurricane days, and one major hurricane day. Taken together, these counts represent an integrated metric of the strength, duration, and frequency of tropical cyclones during a given year. Major hurricanes are by far the most destructive tropical cyclones.

These counts are based on the best estimates of the strength and location of storms provided by forecasters at NOAA’s National Hurricane Center after each hurricane season, which is officially June–November. This indicator includes data from 1950 onward, which is when routine aircraft reconnaissance began. The data also comprise tracks west of 60°W only, which are more likely to impact coastal communities. We also have more complete historical data for this region.

The Atlantic was generally active before 1969, inactive from 1970 to 1994, and then active again from 1995 to present. The inactive period was associated with cooler ocean temperatures in the Atlantic. This cooling was caused by a combination of natural and human-induced factors: changes in the Atlantic Ocean circulation and emissions of aerosols from human activities, which reflected more sunlight back to space and countered the effects of greenhouse gas–associated warming.

Following the Clean Air Act of 1970, the emission of aerosols decreased over time and more sunlight reached the oceans, which accelerated the warming of the sea surface due to greenhouse gas emissions. The Atlantic Ocean’s circulation also changed naturally around 1995 and contributed to the warmer sea surface temperatures.

Multiple studies predict that the average number of tropical cyclones each season will remain steady or even decrease with climate change (see NCA4 Our Changing Climate: Changes in Severe Storms). However, warming sea surface temperatures due to increasing greenhouse gas concentrations are expected to make those storms more intense, which would increase the proportion of storms that become major hurricanes. We are beginning to detect these changes in the observational record, including in basins other than the Atlantic.