PHYSICAL-MECHANISMS OF THE 1993 SUMMER FLOODS

The physical mechanisms responsible for the onset and maintenance of t he 1993 summer floods were examined using the localized Eliassen-Palm flux diagnostics and solutions of a single-level primitive equation mo del linearized about a meridionally varying basic state. The unusually long persistent summer pattern is linked with the marked transient ed dy activity in late May and June. The feedback of eddies in the time m ean flow caused a strengthening and eastward extension of the Pacific jet and a strengthening of the jet over North America. Results from th e 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 how maintained by the eddies upstream. Composites from cases similar to that of the 1993 summer exhibit strong low-level sout herly how east of the Rockies and suggest that the low-level jet may b e an important mechanism to sustain the anomalous rainfall. It is conc luded that the effect of the eddies in maintaining a strong upper-leve l zonal flow, the role of the Rockies in sustaining a lee trough, and an associated low-level jet that brings in tropical moisture are essen tial ingredients in developing and maintaining floodlike conditions ov er the central United States.