Physical Mechanisms of the 1993 Summer Floods

Abstract

The physical mechanisms responsible for the onset and maintenance of the 1993 summer floods were examined using the localized Eliassen-Palm flux diagnostics and solutions of a single-level primitive equation model linearized about a meridionally varying basic state. The unusually long persistent summer pattern is linked with the marked transient eddy activity in late May and June. The feedback of eddies in the time mean flow caused a strengthening and eastward extension of the Pacific jet and a strengthening of the jet over North America. Results from the model suggest that the summer pattern may be interpreted as that of a lee trough forced by the Rocky Mountains in the presence of a strong westerly mean flow maintained by the eddies upstream. Composites from cases similar to that of the 1993 summer exhibit strong low-level southerly flow cast of the Rockies and suggest that the low-level jet may be an important mechanism to sustain the anomalous rainfall. It is concluded that the effect of the eddies in maintaining a strong upper-level zonal flow, the role of the Rockies in sustaining a lee trough, and an associated low-level jet that brings in tropical moisture are essential ingredients in developing and maintaining floodlike conditions over the central United States.