A study on dry periods in Atlanta, Georgia

Precipitation data for Atlanta, Georgia from the years 1879 to 1999 were used to determine the distribution of precipitation departures, with emphasis on the frequency of deficits, for the period from October of one year to June of the next (Oct-Jun). Sea surface temperature (SST) anomalies for the tropical Pacific were used to determine if periods of precipitation deficit were related to cold (La Nina) phases of the Southern Oscillation (SO).

Monthly precipitation departures from the arithmetic mean were summed to obtain the departures for each Oct-Jun period. A frequency analysis of the normally distributed precipitation departures showed that on the average, Atlanta has an excess of precipitation during Oct-Jun. Precipitation deficits on the order of 5-10 inches, as occurred during Oct 1998 to Jun 1999, when only 68 percent of normal precipitation was received, have a probability of occurrence of 0.15 with a return period of 7 years.

A comparison of all precipitation deficient Oct-Jun periods with SO phases revealed no preference for the dry periods to be associated with La Nina phases. Nor was there any obvious relationship between strengths of the phases and magnitudes of the precipitation deficits. It was found that La Nina phases were associated with only 39 percent of the dry Oct-Jun periods, while the warm El Nino phases were associated with 35 percent, and the remaining 26 percent with neutral years.

As a check, several years of precipitation data for Macon, Georgia were analyzed. The results were similar.