Extratropical transitions in the North Atlantic with special reference to Hurricane Igor

Masson, Athena (2013) Extratropical transitions in the North Atlantic with special reference to Hurricane Igor. Masters thesis, Memorial University of Newfoundland.

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Abstract

There is a tendency to equate the word "hurricane" with the tropical regions of the world. Few recognize the danger and risks that occur when a tropical cyclone reaches colder subtropical waters and undergoes extratropical transition. Atlantic Canada, particularly the island of Newfoundland, is most at risk from extratropical transitions. The circumstances, location and dynamics of extratropical transitions in the North Atlantic have not been extensively studied. Consequently, forecasters continue to call approaching storms "hurricanes," when most are extratropical cyclones by the time they reach Atlantic Canada. -- Extratropical transitions in the North Atlantic between 1991 and 2011 were analyzed to determine if the frequency, magnitude and intensity of potential shifts can be calculated for the purpose of more accurate forecasting and the benefit of public awareness, safety management, and preparedness. Between 1991 and 2011, 324 tropical cyclones formed, and 121 of these underwent extratropical transition, a mean of 5.76 per year. Extratropical transitions occurred more frequently in the middle of the Hurricane Season, with the peak transition month being September when 43.3% of cyclones transitioned. The largest percentage of cyclones began extratropical transition between 30 and 39.9°N, and 50.4% of cyclones completed their transition between 40 to 49.9°N. Of the 121 storms, 49.6% weakened after completing extratropical transition; 21.5% had little or no re-intensification after transition; and 29.2% re-intensified. Identifying if a cyclone will re-intensify after transition is a necessity. Cyclones have emerged from transition stronger than the tropical state, bringing widespread disaster to areas in the storms' path. -- Newfoundland, in particular, has suffered devastating impacts from extratropical transition, notably Igor in 2010. Igor impacted Newfoundland as a Category 1 hybrid system which was still undergoing extratropical transition. Twenty-seven cyclones directly impacted Newfoundland between 1991 and 2011, 8% of the total for the entire North Atlantic. A third of cyclones qualified as Cape Verde Cyclones, which are cyclones that form in the deep tropics close to the Cape Verde Islands off the west coast of Africa. Cape Verde Cyclones usually have enough time to build up strength as they cross the Atlantic and frequently reach "major hurricane" status. Igor was an example of a classic Cape Verde Cyclone. Flooding was a severe problem, destroying property and roads and isolating communities. Total damages were estimated to be at least $110 million CAD with some values reaching as high as $200 miIlion CAD. Fire and Emergency Services - Newfoundland and Labrador, the government of Canada, climatologists and meteorologists will benefit from a deeper understanding of extratropical transitions. Better forecasts could warn a given population of when and where a transition could take place and how best to prepare for the consequences.

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