On August 25th, Hurricane Harvey barreled through Texas, wreaking havoc and destruction. Now, Hurricane Irma, one of the strongest hurricanes on record, devastated the Caribbean and is threatening south Florida. Tropical Storm Jose trails behind Irma in the mid-atlantic and Hurricane Katia is now threatening eastern Mexico.
Hurricane experts say it not uncommon for the formation of several storms in rapid succession, especially in August, September and October. Dr. Gerry Bell from the NOAA says “This is when 95 percent of hurricanes and major hurricanes form.” However, this season seems especially active. What is going on here?
According to National Geographic, atmospheric conditions were hurricane-friendly and surface sea temperatures were warmer than usual this year. “The Climate Prediction Center stated that multiple conditions, such as a strong west African monsoon, have aligned to make the Caribbean Sea and part of the tropical Atlantic—a storm-spawning area called the “Main Development Region”—particularly well-suited to hurricanes.”
Warming waters and high levels of moisture in the air fuel the growth of storms. For example, Hurricane Irma formed in an area with sea temperatures around 85 degrees Fahrenheit and air humidity levels around 55%. As Hurricane Harvey roared towards Houston, sea-surface waters near Texas rose between 2.7 and 7.2 degrees Fahrenheit above average.
A lack of abrupt wind shifts, called wind shear, also tends to disrupt storm formation. Weak wind shears cause a hurricane to physically tilt, which dampens the force of the winds emanating from it. This combination of conditions made the Atlantic more susceptible to tropical storms and hurricanes.
The U.S. National Oceanic and Atmospheric Administration (NOAA), Colorado State University and the Weather Channel all predicted that there would be more hurricanes in the Atlantic Ocean this year. The NOAA forecasted there would be 14 to 19 named storms and five to nine hurricanes this season, including two to five major ones. In contrast, between 1981 and 2010, the average hurricane season (June to November) featured 12 named storms and only six hurricanes. Since hurricane season began on June 1st, 2017, there have been 12 named storms, four of which strengthened into hurricanes with maximum sustained winds above 73 mph. There are still two months left in the hurricane season.
Climate change is a major contributor to hurricane season behavior. According to Kerry Emanuel, an atmospheric scientist at MIT, Harvey’s record breaking rainfall was almost certainly shaped by rising temperatures and that high intensity (Category 3, 4, or 5) hurricanes will be more common in the future.
Written by Danielle Bayer