Hydroclimate of the western U.S. is influenced by strong interannual variability of atmospheric circulation, much of which is associated with the El Nino-Southern Oscillation (ENSO). Precipitation and temperature anomalies during ENSO often show opposite and spatially coherent warm-dry and cool-wet patterns in the Northwest and California or vice versa. The role of orography in establishing mesoscale ENSO anomalies in the western U.S. is examined based on observed precipitation and temperature data at 1/8 degree spatial resolution and regional climate simulations at 40 km spatial resolution. Results show that during El Nino or La Nina winters, strong precipitation anomalies are found in northern California, along the southern California coast, and in northwest mountains such as the Olympic Mountain, the Cascades, and the Northern Rockies. These spatial features, which are strongly affected by topography, are surprisingly well reproduced by the regional climate simulation.

A spatial feature we investigated further is the positive-negative-positive precipitation anomaly found during El Nino years in the Olympic Mountain, and on the west side and east side of the Cascades in both observation and regional simulation. Observed streamflows of river basins located in those areas are found to be consistent with the precipitation anomalies. The spatial distribution of the precipitation anomalies is investigated by relating flow direction and moisture to the orientation of mountains and orographic precipitation. In the west side of the north-south oriented Cascades Range, the increase in atmospheric moisture is not enough to compensate for the loss of orographic precipitation associated with a change in flow direction towards southwesterly during El Nino years. In California, both increase in atmospheric moisture and shift in wind direction towards southwesterly enhances precipitation along the Sierra, which is oriented northwest to southeast. The spatial signature of the interactions between large-scale circulation and topography may provide useful information for seasonal predictions or climate change detection.