Contrasting evaporative moisture sources during the drought of 1988 and the flood of 1993

Using hourly observed precipitation data, National Centers for Environmenta l Prediction reanalyses at 6 hourly intervals, and a quasi-isentropic back- trajectory algorithm, we have examined the transport and surface sources of moisture supplying rainfall during the spring and summer over the United S tates during the drought year of 1988 and the flood year of 1993. These res ults are compared to calculations using a bulk-transport approach and month ly mean data. We find that about 41% of precipitation over the Mississippi River basin originated as evaporation from the same basin during April-July 1988 and 33 % during 1993. During the July peak of the 1993 flood the recy cling ratio was considerably lower than in other months, while the source o f moisture from the western Gulf of Mexico and Caribbean Sea increased enor mously. By contrast, at the June climax of the 1988 drought, the recycling ratio reaches a maximum. We also find a small source region in the eastern subtropical Pacific Ocean during both years, which decreases significantly with the onset of the Mexican monsoon around the beginning of July. The bac k-trajectory approach represents an improvement over the bulk recycling app roach by rejecting the assumption of uniform distributions of rainfall in s pace and time, instead focusing specifically on the sources and transport o f moisture contributing to observed rainfall events.