Abstract
Winter precipitation increases as much as seven times from south to north along the US Pacific coast. For 1948–95, nine coastal grid points are used to identify spatial and temporal winter precipitation variations and to assess atmospheric circulation influences on precipitation variability. A strong negative correlation to the northwest of each precipitation grid point is revealed by one‐point correlation maps of precipitation at a given coastal grid point and 700 hPa geopotential heights at 288 grid points. The strength of the correlation decreases from north to south, and the northernmost precipitation grid point displays a correlation pattern nearly opposite that of the southernmost precipitation grid point. El Niño events are related to greater than average precipitation south of 43°N along the US Pacific coast. The largest negative height anomalies during an El Niño year are centred over the North Pacific Ocean west of Oregon. La Niña events display an asymmetric pattern relative to El Niño, with below‐normal rainfall along the California coast south of 41°N and above‐normal precipitation along coastal Oregon and Washington. La Niña years have negative 700 hPa geopotential height anomalies over the Gulf of Alaska. These characteristics delimit the region between 41 and 43°N as the transition zone defining El Niño–southern oscillation influences along the US Pacific coast. Copyright © 2004 Royal Meteorological Society