Low-Frequency Aspects of the Large-Scale Circulation and West Coast United States Temperature/Precipitation Fluctuations in a Simplified General Circulation Model

Abstract

Behavior of regional precipitation and temperature over the West Coast of the United States was examined in a long perpetual winter simulation from a simplified global general circulation model. The model, a simplified version of the U.S. National Weather Service global operational forecast model, was run over a series of 568 winters, complete with geopotential, precipitation, and near-surface temperature. In spite of the fixed climatologica boundary conditions, the simulated winter-mean precipitation and temperature anomalies have a fairly realistic low-frequency regional variability. Both synoptic-scale events and seasonal average behavior are produced quite realistically by the model. Like observations, the regional surface variations can be related to the large-scale low-frequency circulation. Four regional temperature/precipitation extreme?namely, warm/dry, cool/wet, cool/dry, and warm/wet?can be identified from the simulated winter-mean time series over the West Coast. Associated with these four regional extremes, model Northern Hemisphere 500-rnb height composites exhibit distinct planetary-scale circulation patterns. An empirical orthogonal function analysis further reveals that the first and third modes of the 500-mb height anomalies am primary contributors to these four regional extremes. The first mode largely governs the regional temperature variation, whereas the third mode largely determines the precipitation variation.