Geoscience Reference
In-Depth Information
associated with rainstorms in the humid tropics, but
the sparsity of vegetation in the drier regions allows the
rain to produce flash-floods and considerable surface
erosion.
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 (Figure 10.13).
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 to 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 convection
becomes less effective inland as a result of the sub-
sidence created by the anticyclonic circulation in the
middle troposphere referred to previously (see B.3c, this
chapter).
2 The interior 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 appropriate conditions for convection. Indeed,
Florida probably ranks as the area with the highest
annual number of days with thunderstorms - ninety 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 (Plate 21).
Tropical cyclones contribute 10 to 15 per cent of
the average annual rainfall near the Gulf Coast and in
Florida. According to
Storm Data
reports for 1975 to
1994, hurricanes striking the southern and eastern USA
account for over 40 per cent of the total property damage
and 20 per cent of the crop damage attributed to extreme
weather events in the country. Annually, 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)
(Plate F), but this was far surpassed by the $27 billion
losses caused by Hurricane Andrew over Florida and
Louisiana in August 1992. Winds in excess of 69 ms
-1
(155 mph) led to the destruction of 130,000 homes
(Plate 22). Injuries (deaths) during hurricanes average
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 is interposed between the
temperate maritime type and the arid subtropical desert
climate. The boundary between the temperate maritime
climate of western Europe and that of the Mediterranean
can be delimited on the basis of the seasonality of
rainfall. However, another diagnostic feature is the
relatively sharp increase in solar radiation across a
zone running along northern Spain, southeast France,
northern Italy and to the east of the Adriatic (Figure
10.25). The Mediterranean regime is transitional in a
special way, because it is controlled by the westerlies in
winter and by the subtropical anticyclone in summer.
The seasonal change in position of the subtropical high
and the associated subtropical westerly jet stream in the
upper troposphere are evident in Figure 10.25. The type
region is peculiarly distinctive, extending more than
3000 km into the Eurasian continent. In addition, the
