Optimal Climate Normals for the North Atlantic Hurricane Activity

Most climatologies use 30-year epochs that are updated at the start of each decade. They will shift from 1981-2010 to 1991-2020 in 2021. North Atlantic hurricane activity has large interdecadal variability that may lead to biases in a 30-year climatology. A previous inactive hurricane period included 1981-1990, while 2011-2020 is a part of the ongoing active era. As a result, the 1991-2020 normals are more active than the 1981-2010 normals, with the median accumulated cyclone energy increasing by similar to 40%. A 50-year epoch would be more likely to capture a full cycle of multidecadal variability, and this study demonstrates that 50-year climatologies have historically been better predictors of the subsequent decade's hurricane activity. This paper argues that the 1971-2020 climatology should, therefore, be the baseline for hurricane activity for the next decade with a possible adjustment for the non-climatic increase in observed short-lived tropical cyclones. Plain Language Summary Climatologies are typically 30-year averages that are updated at the start of a new decade (e.g., in 2021, the 30-year average will be updated from 1981-2010 to 1991-2020). However, known changes in hurricane activity between decades are not typically well represented by a 30-year climatology. For example, 1981-1990 was a part of the last quiet period, but 2011-2020 is a part of the current active era. The 1991-2020 hurricane climatology is, therefore, much more active than 1981-2010. An integrated metric accounting for intensity, duration, and frequency of storms increases by 40% from the 1981-2010 to the 1991-2020 climatology. A 50-year climatology is more likely to include both active and quiet eras, which gives a better picture of normal hurricane activity. This study shows that the 50-year climatology has better predictive skill for seasonal hurricane activity than that of the standard 30-year average. New technology has also led to an increase in the number of short-lived tropical storms. The 50-year average for 1971-2020 with an adjustment for short-lived storms is likely to be the most representative climatology for the next decade. Key Points A 30-year climatology for 1991-2020 is considerably higher than that for 1981-2010 and is likely not representative of the full record Thirty-year North Atlantic climatologies have larger root mean squared hurricane prediction errors than 50-year climatologies The most representative climatology for 2021-2030 is likely 1971-2020 with an adjustment for short-lived named storms

Automated summary

The study demonstrates that 50-year climatologies are better predictors of seasonal hurricane activity compared to standard 30-year averages. The 1971-2020 50-year climatology, adjusted for short-lived storms, is likely to be the most representative climatology for the next decade.