Geoscience Reference
In-Depth Information
surpassed by the $27 billion losses caused by
Hurricane Andrew over southern Florida and
Louisiana in August 1992, when winds destroyed
130,000 homes, and the $81 billion losses
attributed to Hurricane Katrina in August 2005.
The 6m storm surge led to the failure of the levees
at New Orleans and to the flooding of 80 per-
cent of the city with widespread destruction of
property and the loss of 1836 lives. In contrast,
injuries (deaths) during hurricanes average only
250 (21) per year, as a result of storm warnings
and the evacuation of endangered communities.
Winter precipitation along much of the eastern
seaboard of the United States is dominated by an
apparent oscillation between depression tracks
following the Ohio valley (continental lows) and
the southeast Atlantic coast (Gulf lows), only one
of which is normally dominant during a single
winter. The former track brings below-average
winter rainfall and snowfall, but above-average
temperatures, to the mid-Atlantic region, whereas
the reverse conditions are associated with systems
following the southeast coast track.
The region of the Mississippi lowlands and the
southern Appalachians to the west and north is
not simply transitional to the 'interior type', at
least in terms of rainfall regime (see
Figure 10.20
).
The profile shows a winter-spring maximum and
a secondary summer maximum. The cool season
peak is related to westerly depressions moving
northeastward from the Gulf Coast area, and it is
significant that the wettest month is commonly
March, when the mean jet stream is farthest south.
The summer rains are associated with convection
in humid air from the Gulf, although this convec-
tion becomes less effective inland as a result of the
subsidence created by the anticyclonic circulation
in the middle troposphere referred to previously
(see B.3, this chapter).
2
The southeastern United States
The climate of the subtropical southeastern
United States has no exact counterpart in Asia,
which is affected by the summer and winter
monsoon systems (discussed in Chapter 11).
Seasonal wind changes are experienced in Florida,
which is within the westerlies in winter and lies on
the northern margin of the tropical easterlies in
summer. The summer season rainfall maximum
(see
Figure 10.20
for Jacksonville) is a result of this
changeover. In June, the upper flow over the
Florida peninsula changes from northwesterly to
southerly as a trough moves westward and
becomes established in the Gulf of Mexico. This
deep, moist, southerly airflow provides appro-
priate conditions for convection. Indeed, Florida
probably ranks as the area with the highest annual
number of days with thunderstorms - 90 or more,
on average, in the vicinity of Tampa. These
often occur in late afternoon, although two factors
apart from diurnal heating are thought to be
important. One is the effect of sea breezes
converging from both sides of the peninsula, and
the other is the northward penetration of
disturbances in the easterlies (see Chapter 11).
The latter may of course affect the area at any
time of day. The westerlies resume control in
September to October, although Florida remains
under the easterlies during September, when
Atlantic tropical cyclones are most frequent.
Tropical cyclones contribute around 15
percent of the average annual rainfall in the coastal
Carolinas and 10-14 percent along the central
Gulf Coast and in Florida. According to
Storm
Data
reports for 1975-1994, hurricanes striking
the southern and eastern USA account for over 40
percent of the total property damage and 20
percent of the crop damage attributed to extreme
weather events in the country. Annual losses from
hurricanes in the United States averaged $5.5
billion in the 1990s, with comparable national
losses due to floods ($5.3 billion annually). The
single most costly natural disaster up to 1989 was
Hurricane Hugo ($9 billion), but this was far
3 The Mediterranean
The characteristic west coast climate of the
subtropics is the Mediterranean type with hot,
dry summers and mild, relatively wet winters. It
