Hurricanes: Their Nature and Impacts on Society - Climate Science ...

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HURRICANES: THEIR NATURE AND IMPACT ON SOCIETY
instance, the New Engl<strong>and</strong> Hurricane of 1938 was traveling at over 70 knots
at l<strong>and</strong>fall (Coch 1994). Thus, winds parallel <strong>and</strong> in the direction of the 1938
storm's direction of motion were increased substantially. The 1938 storm is an
example of a hurricane that has intensified over a warm sea <strong>and</strong> then ejected
to a higher latitude.
Occurrence
Intense hurricanes are irregular events that occur with greater <strong>and</strong> lesser
annual frequencies over decadal <strong>and</strong> longer time-scales, at particular locations
on the coast. For example, from 1930 to 1959,25 intense hurricanes struck the
US; from 1960 to 1989 only 15 made l<strong>and</strong>fall. Figure 2.3 shows intense
hurricane tracks for the periods 1947-1969 <strong>and</strong> 1970-1987. In the case of
Florida, decadal variability in hurricane incidence is striking. Figure 2.4 shows
hurricane <strong>and</strong> tropical storm l<strong>and</strong>fall for Florida for the periods 1941-1950
<strong>and</strong> 1971-1980. In the earlier decade 20 storms made l<strong>and</strong>fall (six intense); in
the later decade there were only four (one intense). When intense hurricanes
do occur, they are intensively studied (e.g. Hurricane Frederic, Powell 1982;
Hurricane Hugo, Golden 1990; Hurricane Andrew, Wakimoto <strong>and</strong> Black
1994). Although the hurricane season lasts from June to November, 95% of
Category 3, 4, <strong>and</strong> 5 hurricanes in the Atlantic Basin occur from August to
October (L<strong>and</strong>sea 1993).
Since 1984, William Gray of Colorado State University <strong>and</strong> a team of
researchers have predicted the following season's hurricane activity based on a
number of documented statistical relationships (see Section 4.5.3). His Atlantic
Seasonal Hurricane Variability technique is based on several climatic indices
(Gray 1994). Among the statistical forecasts made using this method are
Hurricane Destruction Potential, Intense <strong>Hurricanes</strong> (reaching at least level 3
at some time in a storm's evolution), <strong>and</strong> Intense Hurricane Days. Gray's
forecasts have received wide attention in the media <strong>and</strong> by user communities
(e.g. Morgenthaler 1994).
It has been suggested by some scientists that the hurricane season may be
forecast with longer than a year lead time with some level of forecast skill
(L<strong>and</strong>sea et al. 1994), including speculation regarding a connection between
decadal variations in the "ocean conveyor belt" <strong>and</strong> sea surface temperatures,
Sahel rainfall, El Nino-Southern Oscillation (ENSO) cycle, <strong>and</strong> Atlantic
hurricanes. Gray (1992) stated that:
...of those 24 hurricane seasons of this century in which major hurricane
l<strong>and</strong>fall occurred [along the US East Coast] in only one season (1992) did
distinctly dry Western Sahel rainfall conditions exist in the same year with
distinctly warm EI Nino sea surface conditions. And, that year <strong>and</strong> storm was
1992 <strong>and</strong> Andrew.