Abstract
Previous hydroclimatological analysis of the North American Monsoon (NAM) region of northwest México revealed significant sub‐regions of coherence in seasonal precipitation and streamflow. In this work, interannual variations in regionalized streamflow and rainfall‐runoff relationships are explored. Modulation of the monsoon hydroclimatology by large‐scale forcing mechanisms such as tropical, North Pacific, and Caribbean Sea‐surface temperature perturbations results in nonlinear responses in seasonal runoff that varies by region. Analyses reveal that the El Niño‐Southern Oscillation (ENSO) exerts a modest but statistically significant influence on NAM streamflow. Different sea‐surface temperature and sea‐level pressure indices exhibit markedly different lag‐correlation structures in different NAM streamflow regions and these relationships vary significantly according to the decadal scale sea‐surface temperature (SST) variability in the North Pacific. The structure of correlation patterns between eastern Pacific SST and regionalized NAM streamflow anomalies complicates the application of common ENSO indices as streamflow predictors. The occurrence of ENSO also has a significant impact on regional runoff fractions and on the partitioning of streamflow between the summer and winter seasons. The summer ENSO influence is explained, in part, by changes in the lower tropospheric pressure and wind fields, which result in modest increases in atmospheric moisture over the streamflow regions during La Niña episodes versus El Niño episodes. The results suggest that improved monitoring and prediction of ENSO variability, through careful development of eastern Pacific SST indices in the pre‐monsoon period should provide useful guidance for seasonal forecasts of regional monsoon streamflow. Copyright © 2006 Royal Meteorological Society