A diagnostic study was conducted to evaluate the applicability of ECMWF's global analysis data to the quantitative analysis of the interannual variability of the water budget of the Mississippi river basin. Vertically integrated vapor flux convergence over the entire basin was compared to monthly precipitation minus evapotranspiration (P-E) from 1985 to 1988. Evapotranspiration was calculated from Thornthwaite's formulation. Comparison with the annual run-off near the river mouth was also made for the years 1985 to 1992.
The absolute values of the monthly vapor flux convergence were found to be less than the values of P-E from September 1986 to the end of 1988. Natural variability was insufficient to explain the discrepancy between P-E and the vapor flux convergence. We believe changes in the 4DDA system were largely responsible for the discrepancy. We also believe the initialization used in the ECMWF analysis weakens the divergent part of the wind field.
With respect to the study conducted with the presently available ECMWF data, the interannual variability of the water budget should not be discussed quantitatively, even for the Mississippi river basin, where many radiosonde stations are in operation.