A synoptic view of trends and decadal variations in autumn precipitation across the United States from 1948 to 2004

Precipitation and streamflow have increased in United States since 1948 with the largest increases generally observed in the Midwest and in the autumn. Autumn precipitation in the central United States also exhibits spatially correlated decadal-scale variability with a period of approximately 12 years. In this study, we identify synoptic circulation patterns from daily atmospheric data conducive for precipitation anomalies in the central United States and link changes in the frequency of these patterns to the observed trends and decadal variations in autumn precipitation over the central United States. A synoptic air-mass classification approach based on classification and regression trees is applied to daily NCAR-NCEP Reanalysis data to identify synoptic circulation patterns associated with changing air-mass characteristics and developing surface weather systems in the central United States. The advantage of the proposed approach is that precipitation data are not used to identify recurring synoptic patterns over North America. The analysis identifies four daily circulation types conducive to spatially correlated precipitation anomalies across the central United States. The frequency of these circulation types is then linked to the observed trends and decadal variations in autumn precipitation over the central United States. Our analysis suggests that the trends and decadal variations in autumn precipitation are related to an increase ( decrease) in the frequency of days with a trough ( ridge) over the western United States. The effect of this change in circulation on the occurrence of intense precipitation is discussed.