Although the tropical Atlantic has been fairly active this year, the same has not been true for the rest of the world. Although the number of named storms usually gets the most attention, it is only one factor in the determination of tropical activity. The National Oceanic and Atmospheric Administration (NOAA) also use the strength and duration of each tropical storm to calculate what they call the Accumulated Cyclone Energy (ACE) index. The combining of the number, strength and duration of tropical systems into a set mathematical equation can give a much better insight into the differences between tropical seasons rather than just counting the number of storms alone.
During the past four years the global tropical cyclone activity measured by the ACE index has fallen by 50% and currently is ranked as the lowest of the past 33 years. The overall cooling of the Pacific Ocean and changes in wind conditions aloft are the two most likely reasons for the tropics becoming less active in recent years.