Severe storms affect many coastal regions during the winter and, to a lesser extent, in summer. Regional differences exist, with oceanic extratropical cyclones affecting the coast of California to Alaska and continental extratropical cyclones moving from the interior of the continental United States to the east coast. Tropical cyclones, including hurricanes, may affect almost any part of the coast during summer and fall. Particularly important effects are caused by cold air outbreaks along the east coast and by rapidly intensifying storms over the Gulf Stream.
As we have seen in the previous sections many of the
meteorological phenomena that we associate with the coast
are a consequence of synoptic-scale forcing; for example,
the formation of coastally-trapped Kelvin waves [ Dorman,
1985; Overland and Bond, 1993] and low level frontogenesis
caused by large gradients in surface temperature [ Bane and Osgood,
1989; Riordan and Lin, 1992; Holt and Raman, 1992].
The response time of the coastal currents are sufficiently short
that changes in the shelf circulation can be driven by
rapidly moving atmospheric events [ Lee et al., 1989]. The
response time of the inner- and middle-shelf regions in these conditions
is between 3 and 12 hours. Land-falling storms are directly
modified in the coastal region where changes in the bottom
boundary conditions are dramatic. Although hurricanes weaken
over land because of the absence of latent heating at the
surface, orographic barriers can affect the dynamics of the
storm through blocking and actually enhance precipitation
and winds in coastal areas. Increased friction over the
land decreases the surface winds, which cause an expansion
of the radius of the eye wall. This can cause an increase
in the vertical wind shear, which may explain the occurrence
of tornadoes in these storms [ Pielke, 1990]. The
extensive damage caused by Hurricane Hugo and Hurricane
Andrew along the eastern seaboard of the United States prompted
detailed analyses of these storms and the disaster-preparedness
of coastal communities. The National Research Council's
Committee on Natural Disasters summarized the meteorology;
aircraft reconnaissance; mesoscale variations in storm structure;
forecast performance, precipitation; flooding; and
emergency services response to Hurricane Hugo [ Chung,
1994]. Mayfield and Avila [1993] described the development
of Hurricane Andrew, which made landfall near Homestead
Air Force Base in Florida with an estimated central pressure of
922 mb and maximum sustained surface winds of 62 m s
.
This was the most expensive natural disaster in U.S. History,
costing an estimated $20--$25 billion. Rodriguez et al.
[1994] discussed the impact of Hurricane Hugo on the shelf and
coastal zone of Puerto Rico. In the five years since Hurricane
Hugo struck Puerto Rico the coastal ecosystem has been slow to
recover where severe erosion of berms occurred and where shallow
coral was totally destroyed. Fitzgerald et al. [1994]
investigated the shoreline damage caused by a storm along the
Massachusetts coast in 1991. This storm occurred about six
weeks after Hurricane Bob had significantly decreased the
beach buffer zone. Erosion and structural damage were
controlled primarily by the beach and dune morphology,
sediment composition, exposure to waves and wind, and
the presence or absence of coastal structures. As might
be expected, the most widespread damage occurred in areas where
the storm buffer zones were narrow or nonexistent. A severe
storm in March 1989, one of the three highest energy storms
between 1942 and 1989, caused major damage and beach erosion
along the mid-Atlantic coast. A hindcast analysis by Dolan
et al. [1990] showed that this storm generated deep-water waves
of 1.5 m or more for 115 hours. The severity of recent storm
systems and enormous property loss have prompted some insurance
companies to suggest a connection between climate change and
severe weather [ Wilson, 1994]. Davis et al.
[1993] have shown that the frequency of Atlantic coast storms
decreased from the mid-1960s through the mid-1970s and increased
through to 1984, but the frequency of potentially damaging
storms has increased since 1965. These so-called ``nor'easters''
are low pressure systems that rarely acquire the strength of the
smallest hurricane, but can cause significant damage along
stretches of coast up to 1500 km [ Davis and Dolan, 1993].
These storms often form along baroclinic zones inland, and
although they generate strong winds, most damage and loss of life is
caused by the waves and storm surge associated with the long
fetch and duration of the northeasterly wind over the sea.
The formation of intense storms along coastlines or ice edges at
high latitudes have been documented by Businger and Reed
[1989]. These appear to form over water just beyond the
ice edge, where the large vertical temperature gradient between
the water and the air leads to strong low level baroclinicity.
Emanuel and Rotunno [1989] and Businger [1991] have
shown that some polar lows can attain the intensity and structure
commonly associated with hurricanes.