Saturday, December 14, 2013

2013 Season Summary

The 2013 Atlantic hurricane season was below average, with

15 cyclones attaining tropical depression status,
14 cyclones attaining tropical storm status,
2 cyclones attaining hurricane status, and
0 cyclones attaining major hurricane status.

Before the beginning of the season I predicted that there would be

18 cyclones attaining tropical depression status,
16 cyclones attaining tropical storm status,
9 cyclones attaining hurricane status, and
4 cyclones attaining major hurricane status.

My predictions were well above the actual number of tropical cyclones in all categories, but particularly in the "hurricanes" and "major hurricanes" categories.

The ENSO was roughly neutral this year, with neither a significant El Nino nor a notable La Nina event occurring. Due to the lack of an El Nino, upper-level winds were favorable for tropical cyclone formation. In addition, ocean temperatures throughout the tropical Atlantic were warm (though they were relatively cold in the Gulf of Mexico for parts of the season). However, other, unforeseen conditions overwhelmed these favorable factors. During early August, an exceptionally large volume of dust moved off of the Saharan desert into the eastern Atlantic. This dust blocked some of the sunlight that would otherwise each the ocean surface, cooling water temperatures and inhibiting thunderstorm formation. This event prevented cyclone formation through the first half of August, a normally active period.

In addition, cyclones were plagued by large areas of stable air throughout the season, particularly in the central Atlantic. To survive, tropical cyclones require continued updrafts of warm and humid air to fuel convective growth. This is known as unstable air. Stable air, on the other hand, can be deadly to tropical cyclones. Dorian and Humberto in particular both met their swift demise due to stable air masses. Finally, wind shear was anomalously strong given the neutral ENSO, particularly over the Gulf of Mexico. For example, Tropical Storm Karen moved into the Gulf as a tropical storm in early October. Typically, such a storm would present a great danger to some landmass, and certainly make landfall, but Karen did neither, as shear destroyed the system before it reached the U.S. coastline.

Some notable facts and statistics concerning the 2013 season are:

  • The name Ingrid was retired after the season's conclusion due to the damage Hurricane Ingrid caused in Mexico
  • Tropical Storm Andrea was the only cyclone of the season to make landfall in the U.S., and it did so with 65 mph winds
  • No cyclones made landfall at hurricane intensity during the season
  • The only hurricanes during the season where Humberto and Ingrid; the last time only two hurricanes formed was 1982
  • There were no major hurricanes for the first time since 1994
  • The season's strongest storm was Hurricane Humberto, which had maximum winds of 85 mph and a minimum pressure of 980 mb; this is the first time no cyclone reached category 2 intensity since 1968
  • The ACE of the season was 33.4, the lowest since 1994
  • An unnamed subtropical storm formed in the Atlantic basin on December 4, unnamed because it was not recognized as such at the time, but only during post-season analysis; it was the first December storm since 2007

Overall, the 2013 season was unexpectedly quiet, and caused little damage in comparison to other seasons in recent years.