While the Atlantic Basin experienced a very active tropical season in 2017, global activity was actually below-normal for the year by one type of measurement thanks to quiet seasons in the northern Pacific Ocean and throughout the Southern Hemisphere.
The global “accumulated cyclone energy” as we close out the year is 78% of normal year-to-date and there are currently no named tropical storms around the world.
Accumulated Cyclone Energy (ACE)
One of the best ways to measure tropical activity is with “Accumulated Cyclone Energy” (ACE) which takes into account the number, strength and longevity of tropical systems. ACE is a commonly-used metric for assessing tropical activity because it is not dependent on exact numbers of named storms or hurricanes, but rather is based on both the intensity and longevity of all tropical storms and hurricanes (so a long-lived tropical storm could contribute as much ACE as a short-lived storm that reached hurricane intensity).
The global ACE measure as we close out 2017 is 608.025 which is 78% of the normal year-to-date value and is lower than the level at this same time of year in 2016. While the 2017 ACE measure in the North Atlantic is more than double the normal year-to-date amount, it is significantly below-normal in the North Pacific Ocean and less than half of normal for the Southern Hemisphere.