Present and future surface climate in the western USA as simulated by 15 global climate models

Abstract

We analyze results of 15 global climate simulations contributed to the Coupled Model Intercomparison Project (CMIP). Focusing on the western USA, we consider both present climate simulations and predicted responses to increasing atmospheric CO₂. The models vary in their ability to predict the present climate. In the western USA, a few models produce a seasonal cycle for spatially averaged temperature and/or precipitation in good agreement with observational data. Other models tend to over-predict precipitation in the winter or exaggerate the amplitude of the seasonal cycle of temperature. The models also differ in their ability to reproduce the spatial patterns of temperature and precipitation in the USA. Considering the monthly mean precipitation responses to doubled atmospheric CO₂, averaged over the western USA, we find some models predict increases while others predict decreases. The predicted temperature response, on the other hand, is invariably positive over this region; however, for each month, the range of values given by the different models is large compared to the mean model response. We look for possible relationships between the models’ temperature and precipitation responses to doubled CO₂ concentration and their ability to simulate some aspects of the present climate. We find that these relationships are weak, at best. The precipitation response over the western USA in DJF and the precipitation response over the mid- and tropical latitudes seem to be correlated with the RMS error in simulated present-day precipitation, also calculated over the mid- and tropical latitudes. However, considering only the responses of the models with the smallest RMS errors does not provide a different estimate of the precipitation response to a doubled CO₂ concentration, because even among the most accurate models, the range of model responses is so large. For temperature, we find that models that have smaller RMS errors in present-climate temperature in the north eastern Pacific region predict a higher temperature response in the western USA than the models with larger errors. A similar relation exists between the temperature response over Europe in DJF and the RMS error calculated over the Northern Atlantic.