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

Shyh-Chin Chen Scripps Institution of Oceanography, University of California. San Diego, La Jolla, California

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Daniel R. Cayan Scripps Institution of Oceanography, University of California. San Diego, La Jolla, California

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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.

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.

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