Revisiting the Causes and Global and Historical Context of the U.S. Midwest Great Flood of 1993

Abstract The 1993 U.S. Midwest summer flood occurred in a year marked by a number of apparently disparate climate extremes including an unusually cold spring Pacific warm pool, a record deep spring Aleutian low, and record wet conditions that spanned the Northern Hemisphere midlatitude land areas during June and July. Here, we provide a dynamical framework that links these extremes and accounts for the uniqueness of the Midwest flooding event. In particular, we show that the deep springtime Aleutian low was part of a wave response forced by unusually strong precipitation/heating anomalies in the equatorial Pacific just west of the date line—heating that was linked to the unusually cold Pacific warm pool juxtaposed to the east with positive SST anomalies tied to a weak but unusually timed El Niño event. The deep springtime Aleutian low in turn produced unusually cold summer North Pacific SSTs and set the stage for the summer’s eddy-driven enhancement of the midlatitude jet and unusually strong hemisphericwide transient (baroclinic) wave activity. The resulting transient vorticity forcing produced two pronounced stationary waves—one in June anchored over northern Eurasia, and another in July anchored over the Pacific/North American region, resulting in record precipitation anomalies over northwestern Eurasia and the northern Great Plains, respectively.