Wednesday, 17 January 2007: 1:30 PM
Precipitation in the southeast United States during the twentieth century
206A (Henry B. Gonzalez Convention Center)
An analysis of precipitation variability during the 20th century, initially designed to explore the causes of declining water supplies in North Carolina, has been extended to the whole southeast USA. The change in precipitation regime occurring in North Carolina during the second quarter of the century was found to be typical of changes in many of the coastal and mountain regions of the Atlantic Coast states from Virginia to Georgia. Fall and Winter became wetter, Summer became drier. Spring was slightly drier in the middle of the period, then amounts increased slightly. Although creating no marked change in annual total amounts, the trends were not complimentary, the Summer decrease leading to more frequent and larger soil water deficits, while the extra Fall and Winter precipitation commonly fell onto soils already at field capacity, and gave an increase in surface runoff. Regions further inland, including the states of Alabama and Tennessee, had fewer changes in precipitation regime during the century, although a preliminary extension of the study in these states suggests that increasing temperatures encouraged increased evaporation and thus decreasing water availability. Statistically, the variations in most regions were associated most closely with changes in the number of rain events, rather than with the intensity of these events. Synoptically both changes in the strength and position of the offshore subtropical high pressure region and variations in mid-latitude flow characterized by the North Atlantic Oscillation appear to have played a major role in creating the precipitation variations. These two are not independent, but in spring it was the variations in NAO which were closely linked with precipitation variations regionwide, while in summer the position and intensity of the Bermuda high had the major impact, primarily in the coastal states. Refinement of the results for spring and summer, and expansion to the other two seasons, are underway. The potential impacts of global warming on the flow regimes, and thus the implications for the amounts and spatial and temporal distributions of water resources in the region, are being assessed.