Winter precipitation on the US Pacific Coast and El Nino Southern oscillation events

Winter precipitation increases 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 Nino events are related to greater than average precipitation south of 43degreesN along the US Pacific coast. The largest negative height anomalies during an El Nino year are centred over the North Pacific Ocean west of Oregon. La Nina events display all asymmetric pattern relative to El Nino, with below-normal rainfall along the California coast south of 41 degreesN and above-normal precipitation along coastal Oregon and Washington. La Nina years have negative 700 hPa geopotential height anomalies over the Gulf of Alaska. These characteristics delimit the region between 41 and 43 degreesN as the transition zone defining El Nino-southern oscillation influence along the US Pacific coast. Copyright (C) 2004 Royal Meteorological Society.