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 Noreaster 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.
G. Carbone, University of South Carolina
M. Brown, Southeast Regional Climate Center
D. Yow, Southeast Regional Climate Center