Atlantic Hurricanes Breaking Records

Stormy weather is on the rise in the Atlantic Basin. Over the past decade, the number of tropical cyclones—that is, hurricanes, tropical storms and tropical depressions—has increased dramatically, with 1995–2001 the most active seven-year period on record.

Meteorologists at Environment Canada's Canadian Hurricane Centre in Dartmouth, Nova Scotia, say the trend could spell trouble for coastal regions of Atlantic Canada. Although tropical cyclones often bypass these areas at sea or strike with less intensity because their energy has dissipated over the cooler waters, they frequently cause storm surges, high waves, damaging winds and heavy rainfalls. Measurable sea-level increases in recent years and the gradual sinking of the land—a geological phenomenon known as crustal subsidence—could make the risk of flooding from such storms even greater in the future.

A tropical cyclone is an almost circular storm that has a low surface pressure at the centre of its system, high winds that spiral inwards in a counter-clockwise direction, and is usually accompanied by heavy rain. Wind speed determines whether a cyclone is classified as a tropical depression, tropical storm, or a hurricane—the latter being the most intense, with maximum sustained winds of 118 km/h or higher. Hurricanes have a well-defined surface circulation and an extremely low-pressure area in their centre known as the eye. Tropical storms and hurricanes are called "named storms" because they are given proper names to make it easier for forecasters, the media and the general public to keep track of forecasts, watches and warnings in their area.

The tropical storm season in the Atlantic Basin—which covers the northeastern seaboard of Canada and the United States—usually runs from the beginning of June to the end of November, and peaks around mid-September. That's when the ocean waters that give the storms their energy are at their warmest. Generally speaking, cyclones that affect eastern North America originate in the Gulf of Mexico, the eastern Atlantic Ocean and the Caribbean Sea.

The first tropical storm to hit the basin in 2001 arrived on June 5, and the last on December 4—making it the longest season since 1981. During this time, there were 15 tropical storms, 9 hurricanes, and 4 intense hurricanes—nearly double the previous 30-year average of 9.5, 5.6 and 2.0 respectively. The first hurricane didn't form until September 8, reinforcing the recent trend of later starts. The bulk of the activity occurred in the last three months of the season, with three hurricanes forming in November for the first time on record.

Although 2001 marked the fourth consecutive year of above-average activity in the basin, the number of named storms has been climbing steadily for more than a decade. Over the past century, the average was 8.7 per year, while over the past 50 years it was 9.9. From 1991 to 2000, the number jumped to 11.8—the highest 10-year average on record—with 6.9 hurricanes and 2.8 intense hurricanes per year on average.

The structure of a tropical cyclone. Graphic: NOAA

In Atlantic Canada's "response zone"—the coastal region north of Virginia to the island of Newfoundland—the average annual frequency of named storms has been 3.3 over the past 100 years, 4.2 over the past 50 years, 4.4 over the past decade, and 5.7 over the past seven years. Over the last three years, the zone has seen six named storms per year, many of which have caused flooding in parts of Newfoundland, Prince Edward Island, New Brunswick and Nova Scotia.

In 1999, remnants of Harvey converged with another active storm and dumped 302 mm of rain in a 30-hour period in Oxford, Nova Scotia. In October 2000, Michael made landfall in Newfoundland with peak wind gusts of 172 km/h, and a significant storm with tropical moisture passed east of Cape Breton, Nova Scotia, causing a 1.5-metre storm surge and 12-metre-high waves that flooded northern Prince Edward Island and southern New Brunswick. During a particularly busy 30-day stretch from late August to late September 2001, four tropical storms moved through the waters of the southwestern Grand Banks, each delivering rainfalls in excess of 100 mm on Newfoundland. Gabrielle set a six-hour rainfall record of 90 mm in St. John's, prompting the mayor to call it "the worst storm in 100 years."

There is no doubt that the Atlantic Basin is experiencing a period of increased tropical storm frequency. As with many storms, hurricane activity occurs in cycles that last anywhere from a year to a century. For example, hurricanes were quite active in the basin from the 1930s to 1960s, but were relatively quiet in the 1970s and 1980s.

Average sea-surface temperatures and salinity have been above normal since the 1980s—a situation that primes the ocean for the creation of tropical cyclones. Other climatic conditions have also influenced the formation of hurricanes over the past several seasons, including enhanced monsoons in West Africa, a favourable configuration of the westerly jet stream over Africa, and reduced vertical wind shear (or the change in wind speed and direction with height) over the heart of the hurricane development region.

Although global predictors are sending a mixed signal for the Atlantic Basin this season, hurricane activity is expected to be near normal. Experts say that while tropical Atlantic sea-surface temperatures are neutral in terms of storm development, stratospheric winds are supportive, and the weak-to-moderate El Niño that has been forecast for this fall could act as a suppressant.

Scientists in Canada and the United States are closely monitoring the many factors that influence the creation of tropical cyclones in an effort to improve the forecasting of these events, and the lead time they are able to provide to the public. They are also studying the trends that sea-surface temperatures, wind shear and other factors are undergoing, to determine whether they are being influenced by climate change.

Whatever the underlying causes behind the increase in storm activity in the Atlantic Basin, the message to planners, emergency-response organizations and others in the region is evident: steps must be taken now to prepare for what could be a significant risk of flooding in coming years.

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