Extra-tropical cyclone activity changes over the 20th Century

Extratropical cyclones (ETCs) are major producers of mid and high latitude weather, particularly during the colder times of the year, and are responsible for many types of extreme weather, including intense wind, precipitation, and snow storms. A major uncertainty in climate change is how ETCs have changed both globally and regionally over the past 100+ years. Understanding the character and causes of past changes provides important insights into possible future changes. Here we document some significant shifts in the spatial distribution and frequency of ETCs during the late 19th, 20th, and early 21st Centuries in the Northern Hemisphere, based on the newly available 20th Century Reanalysis. Most importantly, trends in ETC activity computed over more than 100 years are in some cases opposite in sign to those computed since 1950. The ratio of the number of high latitude to mid latitude ETCs was much higher in the late 19th/early 20th Centuries. On the surface, this implies a shift in the mean track of ETCs to the south during the latter two-thirds of the 20th Century. These shifts indicate a need to rethink probable weather patterns in a climate change scenario.