With the 2015 hurricane season officially ending Monday, Colorado State University sent out its annual how-we-did press release comparing what it thought would happen during the season to what actually happened.
“The 2015 hurricane season had activity at levels slightly more than predicted,” according to Phil Klotzbach, lead author of the forecast.
“The season had an approximately average number of named storms but most of the hurricanes that formed this year were very short-lived,” Klotzbach said.
Several factors likely combined to make the season quiet. Klotzbach said the strong presence of an El Niño was the primary reason why tropical cyclone activity was limited. El Niño – warmer than normal water in the central and eastern tropical Pacific – suppresses storm activity in the Atlantic. This happens due to increases in vertical wind shear, which is the change in wind direction with height in the atmosphere.
According to the press release, vertical wind shear in the Caribbean was the strongest on record since at least 1979 for June through October. The tropical Atlantic, which began quite cold this year, warmed up significantly over the course of the season, likely aiding in the development of several weak tropical cyclones in the eastern part of the Atlantic basin.
The Colorado State team of Klotzbach and William Gray made their long-range seasonal forecast, which called for a below-average hurricane season, on April 9 and June 1. Two additional updates during the hurricane season were issued on July 1 and August 4. Each of these updates continued the call for a quiet season.
In their first forecast issued April 9, the team called for seven named storms, three hurricanes and one major hurricane. The June 1 and July 1 forecasts called for eight named storms, three hurricanes and one major hurricane. The August 4 update called for eight named storms, two hurricanes and one major hurricane. In fact, there were 11 named storms, four hurricanes and two major hurricanes.
The team bases its annual forecasts on 60 years of historical data and includes factors such as Atlantic sea surface temperatures and sea level pressures, levels of vertical wind shear, El Niño and other factors. While these forecast factors generally work well and explain approximately 50 to 60 percent of the year-to-year hurricane variability in these 60 years of historical data, there still remains 4 to 50 percent of this variability that is not explained.
Hurricane statistics for 2015 show that four hurricanes formed in 2015, which brings the combined 2013 through 2015 total to 12 hurricanes. This is the lowest three-year total since 1992 through 1994. Those years saw 11 hurricanes.
Hurricane Joaquin was the first Category 4 or 5 hurricane to impact the Bahamas during October since 1866.
Other statistics show that Florida has gone without a hurricane since 2005. This is the longest period on record, shattering the previous record of five years from 1980 through 1984. And no major hurricanes made U.S. landfall in 2015. The last major hurricane to make U.S. landfall was Wilma in 2005, so the United States has now gone 10 years without a major hurricane landfall. Since 1878, the United States has never had a 10-year period without a major hurricane landfall.
The Atlantic has seen a large increase in major hurricanes during the recent period of 1995 through 2015 with an average of 3.4 per year. By comparison, the prior 25-year period of 1970 through 1994 saw an average of 1.5 per year. The United States has been fortunate that few major hurricanes have made landfall except in the two very damaging years of 2004 and 2005.
The Atlantic basin has had 27 major hurricanes since Wilma, with no major hurricane landfalls. The 20th-century average is that approximately 30 percent of all major hurricanes forming in the Atlantic make U.S. landfall.
Klotzbach and Gray attribute the upturn in major hurricane activity since 1995 as well as the earlier increase in major hurricane activity from the late 1940s through the mid-1960s to natural multi-decadal variability in the strength of the Atlantic Multidecadal Oscillation or Atlantic Thermohaline Circulation.