Lengthening Atlantic Hurricane Seasons with Earlier Storm Formation Dates Including Implications from 2020

Abstract This paper analyzes the formation dates of the n th storm in a sequence for all named North Atlantic tropical cyclones (TCs) and assesses whether the intraseasonal length of the Atlantic hurricane season has changed temporally. The record-breaking 2020 season, with 30 named storms, set records for the earliest third TC formation (Cristobal) and from the sixth TC (Fay) onward. Analysis of season length from 1851 to 2022 identifies only one statistically significant breakpoint detected in the early 1970s, roughly coinciding with the introduction of satellite observations. Since 1970, we also find a trend toward longer North Atlantic hurricane seasons. The 1971–2022 trend is robust and statistically significant, whether assessed as the number of days between the first and last storm or by the distance between intermediate percentiles (e.g., 10th–90th and 15th–85th). These increases are mainly associated with storms forming earlier in the calendar year and are best described as an upward trend rather than a stepwise shift between eras. Although a simple trend fits better than a stepwise model, improvement falls short of significance, so we do not formally reject the stepwise hypothesis. If, following this hypothesis, the 1950–2022 period is segmented into a high-activity era (HAE1; 1950–69), a low-activity era (LAE; 1970–94), and second high-activity era (HAE2; 1995–2022), the median season length of HAE2 is 12 days longer than in the first HAE, but this difference is not statistically significant ( p = 0.58) and could be explained by the substantial difference in the observational network. The median season length in both HAE1 and HAE2 are significantly longer (by 36 and 48 days, respectively) than the intervening LAE.