The Many Eccentricities of Hurricane Season 2010 (So Far)
Most Americans may not realize it, but the 2010 Atlantic hurricane season has been a doozy, with 17 named storms so far. Remarkably, the U.S. has gotten through most of the season largely unscathed, with not a single landfalling hurricane. Two storms — Earl and Hermine — did bring hurricane force winds onshore, however.
Forecasters at the National Oceanic and Atmospheric Administration (NOAA), as well as experts in academia and the private sector, had called for a much above average season, a forecast that was based mainly on two factors: record warm sea surface temperatures throughout the region where hurricanes tend to develop, and the presence of a building La Nina event in the tropical Pacific. In fact, water temperatures in some areas of the Atlantic have been so warm that they have been contributing to the death of coral reefs in the Caribbean, according to recent reports.
Since tropical cyclones — the general term given to tropical storms and hurricanes — require warm waters of about 80°F to form, the presence of a large area of record warm water was a clear indication that more storms, and perhaps more major hurricanes of Category Three strength or greater on the Saffir-Simpson scale, would form.
Atlantic hurricane activity this season, compared to average and the official NOAA outlook.
Credit: Climate Central/Russell Freedman.
This has indeed been the case, with five major hurricanes so far this season, compared to two by this date in an average season. “We really thought it would be very active,” said John Cangialosi, a hurricane specialist at NOAA’s National Hurricane Center in Miami. “That has turned out to be true.”
While the Atlantic was unusually hot, parts of the Pacific Ocean were cooler-than-average, due to a natural climate cycle known as La Nina. La Nina, which is characterized by an area of below average sea surface temperatures in the equatorial Pacific Ocean, can be a boon for hurricanes because it tends to reduce wind shear over the tropical Atlantic. Wind shear, or winds that differ in speed or direction with height, can tear brewing storms apart.
This season has been remarkable in many ways. The lack of a U.S. landfall is statistically rare given the large number of storms that have occurred, according to researchers Brian McNoldy and Phil Klotzbach of Colorado State University. They found that in seasons that are this active, the U.S. has a whopping 95 percent chance of being hit by a hurricane, and about a 21 percent chance of being struck by a major hurricane.
**October 25 Update: Hurricane Richard, which struck Belize on October 24 before dissipating over Mexico, has pushed this hurricane season into even rarer territory, McNoldy said. "In the last 110 years, there has never been a season with 10 hurricanes and 0 US hurricane landfalls," McNoldy wrote via email.**
The season has also featured storms of unusual size. Hurricane Igor, which affected Bermuda and caused significant damage in Newfoundland, Canada, was one of the largest hurricanes on record, whereas Hurricane Paula, which struck Cuba, was one of the smallest. For a time, Igor’s tropical storm force wind field stretched almost 500 miles out from the center of the storm -- about the distance between Boston and Washington, D.C. This compares to Paula’s tropical storm force wind field of only 60 miles.
“Paula was a very small storm, very tiny,” Cangialosi said. He noted that it is difficult to obtain reliable statistics on the size of a storm’s wind field, due in part to sparse measurements.
The size of a storm’s wind field has implications for the storm surge it can create when it comes ashore. “In terms of storm surge and wave action, size is one of the critical measurements,” he said, noting that Paula created a very small storm surge when it made landfall in Cuba.
Comparison between the wind fields of Hurricane Igor and Hurricane Paula. Credit: Climate Central/Russell Freedman.
The locations of some of the major hurricanes were also unusual. According to meteorologist Jeff Masters of WeatherUnderground, a private weather forecasting company, Hurricane Julia, which reached Category Four status, was the strongest Atlantic hurricane ever observed so far east. Masters wrote:
When one considers that earlier this year, Hurricane Earl became the fourth strongest hurricane so far north, it appears that this year's record SSTs [sea surface temperatures] have significantly expanded the area over which major hurricanes can exist over the Atlantic.
Also, this year marked only the second time in recorded history that two Category Four or stronger storms occurred in the Atlantic at the same time, Masters noted.
But why the lack of a U.S. landfall this year? According to Cangialosi, the U.S. has been protected from hurricanes by a persistent upper air steering pattern that has featured a "trough" of low pressure, so named because it appears on weather maps as a dip in the jet stream, along the east coast. This pattern caused numerous Atlantic hurricanes to recurve out into the open ocean rather than striking land.
“In all these cases troughing over the western Atlantic caused these storms to recurve over the open sea,” Cangialosi said. Storms that formed farther west did not recurve out into the open Atlantic, but instead headed for Central America and Mexico, which have been particularly hard hit this year.
The persistent steering pattern of storms away from the U.S. points to a limitation of seasonal hurricane forecasts: they give no indication of which locations are more vulnerable to a landfalling storm. Cangialosi said there just is not enough forecasting skill in predicting average weather patterns, combined with the locations where storms will form, so far into the future. “We’re nowhere near doing that,” he said. “We have enough trouble doing that as a five-day forecast.”