Hurricanes are atmospheric machines that consume heat from the ocean and expel it as extraordinary wind speed. Over the last three days, Irma, Jose, and, to a lesser extent, Katia have been working overtime.
Forecasters use many metrics to describe the severity of a storm—wind speed, sea surface temperature, barometric pressure—but the most dramatic may be the accumulated cyclone energy index (ACE). The calculation is a function of wind speed measurements sampled every six hours. It provides an overall sense of a storm’s power. Tropical Storm Bret, for example, the second named storm of 2017, reached winds of 46 miles per hour and had an ACE of 0.7. Hurricane Harvey hit southeastern Texas as a Category 4 storm on the wind scale, with top winds above 130 mph, and finished with an ACE of 11.1.
Irma, a Category 4 storm expected to make landfall in Florida on Sunday morning, has an ACE of 58.09 so far, according to data from the Tropical Meteorology Project at Colorado State University. The record holder for the Atlantic, according to Phil Klotzbach, research scientist at Colorado State University, is Hurricane Ivan, which scored a 70.4 on the ACE index in September 2004.
Scientists are increasingly comfortable connecting human-driven climate change to the ocean conditions powering these storms.
The ACE can also track 24-hour totals of cyclone activity. Irma and its two siblings, Jose (Category 4), and Katia (now a tropical depression), made Wednesday and Thursday the fourth- and third-most violent dates in the history of ACE Atlantic-basin record-keeping. Sept. 11, 1961, saw four simultaneous major Atlantic cyclones, Betsy, Carla, Debbie, and Esther—all Category 3 or higher.
Friday broke that almost 56-year-old power record by more than 12 percent.
By Saturday morning, Katia’s index score had fallen as the storm weakened, with Irma and Jose holding power-levels similar to those seen on Friday.