Spatial Associations of Midtropospheric Circulation and Upper Mississippi River Basin Hydrology

Abstract

This paper identifies the associations that exist between the summer midtropospheric circulation and the hydrology of the Upper Mississippi River Basin (UMRB). The major objectives consist of the identification of spatial associations between midtropospheric height anomalies and UMRB precipitation frequency and maximum stream discharge, and the calculation of the occurrence probabilities of precipitation frequency and maximum stream discharge for the various midtropospheric anomaly patterns. A data matrix containing a spatial record of mean monthly values of 700 mb heights, monthly precipitation frequencies, and monthly maximum stream discharges is subjected to a principal component analysis in order to obtain the dominant spatial associations of midtropospheric circulation and UMRB hydrology. A catalog of the associations is assembled and used to calculate the occurrence probabilities of various categories of maximum stream discharge and precipitation event frequency for different midtropospheric circulation patterns. Variability of precipitation frequency and maximum stream discharge within the UMRB is associated with midtropospheric height anomalies of opposing sign positioned to the west and east of the basin. The probability of months experiencing above-average conditions of precipitation frequency and maximum discharge is greatest for months with an anomalous southerly or southwesterly flow over the basin. Below-average values of precipitation frequency and maximum discharge have the highest probability of occurring in months with anomalous northerly or northwesterly components of air flow.