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

Using hourly observed precipitation data, National Centers for Environmental 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 States during the drought year of 1988 and the flood year of 1993. These results are compared to calculations using a bulk-transport approach and monthly 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 recycling ratio was considerably lower than in other months, while the source of moisture from the western Gulf of Mexico and Caribbean Sea increased enormously. 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 back-trajectory approach represents an improvement over the bulk recycling approach by rejecting the assumption of uniform distributions of rainfall in space and time, instead focusing specifically on the sources and transport of moisture contributing to observed rainfall events.