Atlantic ACE from 1970 - 2013, via wunderground.com
The Atlantic hurricane season has been quiet so far. We've had 11 named storms (storms get names when they strengthen to tropical storms) which is close to the 30-year average. But there's a different meteorological metric that suggests our 2013 Atlantic hurricane season has been the quietest one since 1983.
Accumulated cyclone energy, or ACE, is a measurement of the strength and duration of a tropical system. It's calculated using the maximum wind speed of a storm every six hours. If you add the ACE of every storm together, it illustrates how active the overall hurricane season has been. So far this year our ACE has been 28 - during an average year the value is 110.
The peak of hurricane season falls on September 10th, so we're running out of time for additional tropical systems to develop. Hurricane season officially runs until November 30th. If it were over today, this would be the lowest recorded ACE since 1982 and 1983, when a strong El Nino was in effect. Enhanced wind shear in El Ninos usually suppresses tropical development. But we're currently in a "La Nada" pattern - neither an El Nino or a La Nina, so what gives? The presence of dry air where hurricanes typically form, along with some wind shear that's unfavorable for storms, are the two main factors.
One other noteworthy item - this is the first hurricane season since 1994 that we haven't had a "major hurricane" which is a Category 3 or stronger on the Saffir-Simpson scale.