Hot, humid summers, mild winters, and ample precipitation throughout the
year characterize the climate of the ACE Basin. These patterns result from a
combination of factors including latitude, prevailing pressure and wind
systems, the southern Appalachian Mountains, and the proximity of the Gulf
Stream. Three weather stations, Edisto Island (1956-present), Walterboro (1948-
present), and Yemassee (1896-present), provide data to describe the subtropical
climate of the Basin. A weather station also exists at the ACE Basin National
Estuarine Research Reserve (NERR) field station at Bennett's Point, but data
have only been collected there since 1995 (NERR weather data
).
High solar intensity during the summer months leads to high
temperatures in the ACE Basin. Maximum temperatures in July and August average
approximately 32°C (90°F); morning minimum temperatures average
close to 21°C (70°F)
(Edisto temperatures
)
(Walterboro temperatures
)
(Yemassee temperatures
). Daily high temperatures in the summertime are
moderated along the immediate coast, where sea breezes draw cooler air inland
from the Atlantic Ocean, as can be seen by the comparison of temperatures in
Yemassee and Edisto
.
There is very little year-to-year variability in summer
temperatures as maritime tropical air dominates the region. Winters in the
Basin are cooler than summers because of shorter daylight hours and less
intense solar radiation. With the noon sun approximately 35° above the
horizon
on December 21-22, solar intensity is only half that of the summer
solstice. In addition, the Atlantic subtropical high weakens, limiting the
delivery of mild subtropical air.
Despite these circumstances, the region is warmer than it could be. For example, mean January temperatures are close to 10°C (50°F). Several factors contribute to the relatively mild winter temperatures. First, the warm Gulf Stream along the Atlantic coast supplies energy to the overlying atmosphere. The contrast in temperature between Edisto Island and Yemassee, only 48 kilometers (30 miles) inland, provides evidence that this moderating effect occurs even on a relatively local scale (see the previous figure). Second, the Southern Appalachians limit the flow of air from interior portions of the continent and alter the flow's character when it does move past the range. The mountains are high enough to retard and, in some cases, prevent the intrusion of winter polar air masses. When air does move over the range and sink on the leeward side, it is compressed and warmed.
Of course, average monthly temperatures tell only part of the story. The
long-term temperature record of Yemassee illustrates how mean monthly
temperatures in the Basin have fluctuated from year to year.
Interannual fluctuations are quite large in winter. In January,
for example, the highest mean monthly temperature 17°C (62°F) in
1937 is 12°C (25°F) warmer than the coolest January (3°C or
38°F in 1940). These winter fluctuations result from differences in
upper-air circulation, which redistributes energy across the mid-latitudes.
During some periods, upper-level winds over North America orient themselves in
such a way as to bring a strong influx of cold continental air to the
southeastern United States. During other times, milder flow from the southwest
dominates the circulation pattern. The strong interannual variability in winter
months expresses itself even in the 100-year mean annual temperature record at
Yemassee (Yemassee temperature bar
). The persistent presence of maritime tropical air and
weak circulation in the subtropics limit variability in summer temperatures.
Within the interannual temperature variability, some general trends are noteworthy. These include an increase in temperature during the early part of the twentieth century a gradual decrease during the 1940-1970 period; and an increase during the past 25 years. This pattern is typical of most Northern Hemisphere locations.
The ACE Basin has high
humidity during much of the year because of its proximity to large moisture
sources, the Atlantic Ocean and the Gulf of Mexico, and the wind systems that
carry moisture inland from these water bodies. These winds, typically from the
south or southwest, are often part of a semi-permanent subtropical high
pressure cell in the Atlantic, called the “Bermuda High." The large
quantity of moisture provides an abundant source for precipitation which ranges
from 100-130 centimeters (40 to 50 inches) during most years (Yemassee precipitation bar
)
(Yemassee precipitation quartiles
).
On average, precipitation is plentiful during every month, but the summer
months receive the maximum
(Edisto precipitation
)
(Walterboro precipitation
) figure. Most summer precipitation comes from
thunderstorms that develop as intense surface heat forces warm, moist air
aloft. Precipitation from these thunderstorms can be intense, but is usually of
short duration. Thunderstorms are essential in sustaining water resources
during the summer, when temperature and evaporation rates are high. During some
years, the Bermuda High moves westward, establishing itself over the Southeast.
When this happens, drier air develops, vertical lifting weakens, and clouds and
precipitation are suppressed. Warm season precipitation can also come from
hurricanes (discussed below).
Winter precipitation comes predominantly from mid-latitude cyclones. These are very large storm systems resulting from the convergence of warm moist air from the Gulf of Mexico or the Atlantic Ocean with cooler and drier air from the interior United States and Canada. Usually originating east of the Rocky Mountains and moving eastward with the prevailing winds, mid- latitude cyclones move across the Southeast every five to seven days in the winter. They can bring prolonged periods of drizzle or steady rain. Cold fronts also occur with these mid-latitude lows, and lifting along these fronts often produces thunderstorms. The Nor’easter is an intense form that develops typically in the western Gulf of Mexico or just east of the southern Rocky Mountains. These storms are characterized by extremely low pressure and feed on latent heat from evaporation off the relatively warm Gulf of Mexico and contrasting subtropical and polar air masses. They usually move eastward along the Gulf Coast and then turn northward along the Atlantic Coast, producing strong winds, high waves and storm surges, severe thunderstorms, and heavy precipitation.
While no weather station in the ACE Basin has a sufficient record of wind
data, winds recorded at Charleston can be used to approximate the wind
climatology of the Basin (Wind rose
per year
). The dominant wind direction in Charleston
varies seasonally. Between September and February, winds commonly come from the
north, northeast, or west (See Monthly wind rose
).
By March the predominant directions are west, southwest, and south, a tendency
that persists through August. Average speeds are greatest from December through
May.
Severe thunderstorms produce heavy rainfall, lightning, hail, and, on
occasion, tornadoes. While tornadoes have occurred infrequently in the ACE
Basin and have been less severe than those typical of the Central United
States, they have produced casualties and caused considerable damage. Review
the summary of tornadoes documented in Colleton County. As with the general
U.S. pattern, the majority of tornadoes have struck during the spring and
summer months (Tornadoes
). During these months that the warm land
surface enhances vertical lifting, and subtropical air commonly clashes with
cold polar
air invading from the northern interior part of North America.
Tornadoes most frequently develop between noon and 10 PM, peaking between 4 and
6PM, the time of maximum atmospheric heating and instability.
Hurricanes provide another source of warm-season rain. When they, or their
remnants, strike the Southeast, they can drop several inches of rain over a
short period of time. The storms that
have affected the ACE Basin
developed as low pressure systems in the trade winds west of
Africa, in the Atlantic Ocean, over the Caribbean Sea, or in the Gulf of Mexico
(Hurricanes
). Hurricane season extends from June 1 to
November 30, but peak strike time is from mid-August to mid-September when
ocean temperatures in the source regions are warmest. Hurricane damage results
from winds, flooding, heavy rainfall, and storm surge. Hurricanes also often
spawn
tornadoes. The threat each hazard poses depends on a variety of factors
including the intensity of the storm, the resulting storm surge height, and its
landfall timing with respect to natural tides. While improved forecasting has
reduced human casualties in recent decades, extensive building in coastal areas
has made the human landscape more susceptible to damage. Damage also affects
natural environments by altering the shoreline, killing wildlife, or damaging
habitat.
Authors
G. Carbone, University of South Carolina
M. Brown, Southeast Regional Climate Center
D. Yow, Southeast Regional Climate Center