A 36-yr climatological description of the evaporative sources of warm-season precipitation in the Mississippi River basin

The terrestrial and oceanic sources of moisture that supply warm-season rainfall to the Mississippi River basin and its subbasins are examined over a 36-yr period (1963-98). Using hourly observed precipitation, National Centers for Environmental Prediction (NCEP) reanalyses at 6-h intervals, and a back-trajectory algorithm, the water falling during observed precipitation events is probabilistically traced to its most recent surface evaporative source, terrestrial or oceanic. Maps of these sources generally show dual maxima, one terrestrial and one oceanic, in spring and a dominance of terrestrial sources in summer. Pentad time series averaged over the 36 years show a late-summer maximum of precipitation recycling in all but the Missouri subbasin. During the 36 years analyzed, 32% of warm-season precipitation in the entire Mississippi basin originated as evaporation within the basin (recycled). About 20% of warm-season precipitation was contributed directly by evaporation from the Gulf of Mexico and Caribbean. The Midwest flood year, 1993, represents a positive outlier in terms of July precipitation supplied to the Upper Mississippi directly by evaporation from the Caribbean. The monthly recycling ratios for warm-season precipitation during the drought year, 1988, represent extreme values in the time series but are not identified as outliers. A positive trend in precipitation recycling in the Upper Mississippi and Missouri subbasins and accompanying decrease in Gulf of Mexico/Caribbean-supplied precipitation to those regions are statistically significant but may reflect changes in the observational data stream assimilated by the NCEP model. Perturbation analysis demonstrates that the source fractions and recycling ratios are somewhat sensitive to systematic errors but not to random errors in the model-derived evapotranspiration (ET), arguably the largest source of uncertainty in the back-trajectory approach. Systematic errors in terrestrial ET on the order of 20% introduce errors of about 0.02 in land source fractions (including recycling ratios) that are themselves on the order of 0.10-0.30.