A Decade of Bad Atlantic Hurricanes

Atlantic hurricanes from 1995 to 2005 arose much more frequently and ferociously than normal. Over the decade, 45 hurricanes in the North Atlantic Ocean, Caribbean Sea and Gulf of Mexico reached category 3 or higher on the Saffir-Simpson scale, generating winds of at least 115 miles per hour (100 knots or 185 kilometres per hour).

That's an average of four major hurricanes a year, double the long-term average of two per year.

The decade wasn't entirely unusual though. A similar peak in hurricane frequency occurred during the 1950s and early 1960s. The following years, however had a relative lull in Atlantic hurricane activity.

A compilation of hurricane characteristics by US National Oceanic and Atmospheric Administration (NOAA) scientists finds that some facets of these storms are highly individual and variable, while other aspects have remained constant throughout the years.

The strength of the latest storms is one thing that stands apart from the longer-term pattern. While the proportion of storms reaching category 5 intensity has held at 16% over the years, a greater proportion after 1994 became category 4 storms, with winds of 131 to 155 mph.

By far the strongest of the lot was Hurricane Wilma in 2005. Her sea level pressure of 882 hPa broke the previous record low of 888 hPa set by Gilbert in 1988. Wilma dropped to that pressure at an unprecedented intensification rate of 97 hPa a day and at one point equalled a daily rate of 240 hPa. In comparison, the average rate of pressure decline in building hurricanes during that decade was 25 hPa per day.

Wilma also generated the fastest winds, accelerating by 110 mph daily to peak at 185 mph. On average for the 45 storms, winds maximized at 139 mph, which is three mph higher than the long-term average.

At their peak intensity of wind and pressure, the 1995 to 2005 major hurricanes were highly individual in the direction they moved. Their travel speeds ranged from nearly stationary to 25 mph. They also varied widely in size, with a storm's radius of 34-knot winds extending from 265 to 58 miles (427 to 93 km).

Despite that variability, the timing of hurricane season from 1995 to 2005 was consistent with the pattern observed since 1950. Storm activity typically builds during August to peak in mid-September. During the recent decade, about three-quarters of the hurricanes formed between Aug 23 and Sep 25.

Their typical location at maximum intensity was also fairly unchanged since 1950. Hurricanes on average get most wicked in a region east of the Bahamas, located 435 miles (700 km) northwest of Puerto Rico and 620 miles (1000 km) southeast of Miami, Florida. In reality though, the storms are scattered throughout the region while at their worst. But several of the most powerful, the category 5 hurricanes, congregated in the Caribbean west of Jamaica.

Not all the storms reached land as major hurricanes, and those that did so were seldom near their peak intensity. By the time she hit shore in Cozumel, Mexico on Sep 22 2005, Wilma had calmed down to devastating 150 mph winds and 927 hPa pressure. Altogether from 1995 to 2005, hurricanes with at least 115 mile-an-hour winds made landfall 35 times, including 11 landings in the United States.

The spate of bad hurricanes culminated with 2005 standing out as an exceptionally active and disastrous year. Seven major storms made 13 landfalls. That year saw four category 5 storms, more than in the entire previous decade.

Reference

Raymond M. Zehr and John A. Knaff. 2007. Atlantic Major Hurricanes, 1995–2005—Characteristics Based on Best-Track, Aircraft, and IR Images. Journal of Climate. 20(24): 5865-5888.

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