Atlantic-Pacific influence on western U.S. hydroclimate and water resources

The El Niño Southern Oscillation (ENSO) in the tropical Pacific Ocean is an important driver of winter precipitation variability over western North America as a whole, but ENSO exhibits a weak and inconsistent relationship with precipitation in several critically important headwaters including the upper Colorado River Basin. We present interactions between North Atlantic sea surface temperatures (SSTs) and ENSO that influence western U.S. precipitation, accounting for substantial variability in areas where ENSO alone yields limited guidance. Specifically, we performed a statistical analysis on hemispheric SSTs and western U.S. winter precipitation in a century of observations and a 10,000-year perpetual current-climate simulation. In both frameworks, the leading coupled pattern is ENSO, and the second pattern links an Atlantic Quadpole Mode (AQM) of SST variability to precipitation anomalies over most of the western U.S., including the transition zone where ENSO provides little predictability. The AQM SST anomalies are expansive in latitude, but its primary mechanism appears to involve a strengthening/shifting of the intertropical convergence zone (ITCZ) over northern South America and the tropical Atlantic. The ENSO pattern accounts for a larger fraction of the total covariance between SSTs and precipitation (65% versus 12% for the AQM pattern), but the percent anomalies of precipitation associated with ENSO and the AQM are comparable in magnitude, meaning 20% or larger over much of the western U.S. The interaction between ENSO and AQM influences precipitation across the western U.S., with cold AQM generally reducing precipitation irrespective of ENSO whereas warm AQM increases the amount of precipitation and the area of influence of ENSO; knowledge of these interactions can increase predictability of western U.S. precipitation.