This year could be a humdinger of a hurricane season

Monday, December 6, 2010

by HENRY HENGEVELD

On Sept. 21, Hurricane Igor swept its way across the eastern shores of Newfoundland, dumping up to 238 millimetres, or more than nine inches, of rain in its wake. Winds as high as 160 kilometres an hour toppled trees, bringing down power lines and leaving much of the Bonavista and Burin peninsulas without power. Many communities were also cut off from the rest of the province because of washed out roads and collapsed bridges.

By the time Igor had wandered off into the North Atlantic, it had left property damage across the province estimated at well in excess of $100 million. The extent of the disaster prompted Prime Minister Harper to promise federal financial assistance to help repair the damage, and to call in the army to assist in the clean-up.

In many ways, Hurricane Igor was just another typical Atlantic tropical storm. It began about two weeks earlier as a low-pressure centre off the west coast of Africa. As it progressed eastward across the hot tropical Atlantic waters, it intensified, reaching hurricane strength by Sept. 11. It rapidly developed into a Category 4 storm (winds in excess of 210 kilometres an hour) by the next day. At one stage it had churned its way up to peak intensities of almost Category 5 strength (winds greater than 250 kilometres an hour). Then, as it moved north over cooler waters, it began to abate, weakening to Category 1 by the time it side-swiped Bermuda and headed on to Newfoundland.

However, in other ways, Hurricane Igor was quite unusual. So were some of the other storms of the 2010 hurricane season. For example:

•    The amount of rain dropped by Igor over parts of Newfoundland was the third highest ever for a tropical storm passing over Canadian territory. Only Hurricane Harvey (1999) and Hurricane Beth (1971) exceeded this magnitude. By comparison, that from Hurricane Hazel in 1954 was only 214 millimetres.

•    Igor's development over the mid-Atlantic from a Category 1 to Category 4 storm was explosive, occurring in less than 12 hours. 

•    By the time it had reached Newfoundland, Igor had expanded to an unusually large diameter of almost 1,400 kilometres.

•    While Igor was migrating across the mid-Atlantic at its peak power, two other major Atlantic hurricanes were circulating near by. To the east was Hurricane Julia, which reached Category 4 wind levels on Sept. 15. Meanwhile, to the west, Hurricane Karl was twisting its way across the Gulf of Mexico as Category 3 storm. This was the first occurrence of two coincident Category 4 storms in the Atlantic since 1926. Julia also became the most easterly Atlantic Category 4 storm since satellite monitoring began some four decades ago. 

•    Igor was the ninth named tropical Atlantic storm of the 2010 season. By the end of September, with two months of the hurricane season still remaining, 14 such storms had already been recorded. Seven of these had reached hurricane intensity and five had reached levels of Category 3 or higher. By comparison, hurricane experts report averages for the entire season, since 1950, of 9.6 named tropical storms per year, 5.9 of which become hurricanes. Of these, 2.3 of these reached Category 3 or higher intensities. 

•    Hurricane Alex, the first Atlantic tropical storm of the season, reached Category 1 level by June 29. This was the first Atlantic hurricane to occur in June since 1995.

So why all the wind over the Atlantic this year? According to the hurricane experts, you can blame it on La Niña. 

On average, winds near the top of the troposphere, where the jet stream is found, are much more intense than those at the surface. This creates a strong vertical increase in wind intensity with altitude, referred to as "wind shear." During La Niña years, tropical surface winds typically become more intense, thus reducing vertical wind shear over low latitudes of the Atlantic Ocean. The reverse happens during El Niño years. Strong wind shear helps to reduce the height to which intense hurricane winds can develop, while weaker wind shear enhances storm development potential. Hence, El Niño years usually have fewer Atlantic tropical storms, while La Niña years have more.

William Gray and Philip Klotzbach, two hurricane experts based at Colorado State University, have been issuing forecasts of hurricane season behaviour for the past 29 years, largely based on expected El Niño/La Niña behaviour and other ocean anomalies.

This year, noting the waning of El Niño conditions and the imminent arrival of La Niña conditions later in 2010, Gray and Klotzbach already predicted in early April that the Atlantic tropical storm season would include about 15 named storms, of which eight would reach hurricane intensity and four would become Category 3 or stronger.

By August, their prediction had been updated to 18 named tropical storms and 10 hurricanes, with five of these reaching Category 3 or greater. Forecasts released by the American National Oceanographic and Atmospheric Administration suggested similar numbers, while the British government forecast office predicted 20 named tropical storms.

It would now appear that these predictions may have been too cautious. With the tropical storms of October and November yet to be officially counted, 2010 could become a humdinger of a hurricane season. BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada