Influence of the Arctic oscillation on central United States summer rainfall

Effects of the Arctic oscillation (AO) on summer rainfall variability in the central United States are examined in order to improve understanding and prediction of the interannual variation in the summer rainfall. Major results show persistent AO effects that resulted in less summer rainfall in the central United States during the positive phase of the AO and more rainfall during the negative AO phase. These effects are most prominent at the interannual time scale. The key physical processes connecting the AO with the rainfall variations are shown in changes in latitudinal location of the midlatitude upper tropospheric westerly jet over North America, the transverse circulation around the jet, and the low-level moisture flux divergence in the central United States. Diagnostic analyses show that the change of the jet stream location may have resulted from the AO-induced eddy heat and momentum forcing on the mean zonal flow in the upper troposphere. The eddy-mean flow interactions caused a northward shift of the jet in the positive phase of the AO. The associated anomalies of downward motion and moisture divergence in the lower troposphere over the central United States suppressed rainfall development. A set of reversed anomalies developed in the negative phase of the AO, encouraging summer rainfall.