Atmospheric water vapor fluxes and budgets during wet and dry episodes over the central United States

We analyze monthly and daily time series of atmospheric vapor fluxes, vapor flux convergence (VFC), precipitation (P) and river discharge (D) to diagnose wet and dry episodes over the central United States for a period spanning 1973 to 1995. In agreement with several other studies we find that the largest monthly mean vapor flux convergence anomalies occur below 700 hPa.

We identify wet or dry episode as periods when April through July anomalies of the area averaged precipitation and river discharge are more than one standard deviation positive or negative. According to this criteria there were four wet (1973, 1979, 1993 and 1995) and three dry (1976, 1987, 1988) episodes during 1973 - 1995 period. For each of these events we analyze the three-dimensional structure of the water vapor flux anomalies over the area.

During the strong dry episode of 1988 negative anomalies of the vapor flux from the Gulf of Mexico prevailed through the warm season. Atmospheric vapor transport for the 1993 strong wet episode was the opposite of 1988. However, for the weaker wet and dry events vapor flux anomalies of both signs occur and vary in consecutive months. This suggests that there is a variety of conditions that give rise to moderate droughts and floods in the Central United States including modulation of the mean transports and eddy terms in the atmospheric water budget.