A synoptic assessment of climate change model output: explaining the differences and similarities between the Canadian and Hadley Climate Models

by both. Assessing the impacts of climate change solely from model output of precipitation and surface temperature variables can be confusing especially when the output from two highly regarded GCMs differs significantly and especially when the precipitation and surface temperature output is not augmented with supporting information about flow patterns, storm tracks, and the meteorological features that are generally responsible for these precipitation and temperature patterns. From a forecasting standpoint, these two variables are the most difficult to forecast and hence the most unreliable. Yet, many of the regional assessments as well as the National Assessment of Climate Change that are being conducted under the US Global Change Research Program hinges on examining only these two variables.

Thus, in order to provide a more fundamental understanding of the precipitation and surface temperature changes that are predicted by a model from the Canadian Centre for Climate Modelling (CGCM1) and a model from the United Kingdom Meteorological Office/ Hadley Centre for Climate Prediction and Research (HADCM2), a synoptic assessment of the output from those two models has been completed. The purpose of the synoptic assessment is to understand and to predict subjectively and qualitatively the changes in precipitation and surface temperature based on the synoptic flow patterns. The motivation for comparing the GCMs in this fashion and for constructing a qualitative picture of climate change is based on the fact that 500 hPa height patterns, for example, can be more reliably simulated than precipitation.

For example, while both models show quantitative differences in winter season precipitation along the Pacific Northwest by the last decade of this century (e.g., 2090-2099), both models show a stronger Aleutian Low accompanied by a stronger, more southwardly displaced 200 hPa jet. Other synoptic features in other geographic regions for other seasons compare favorably between the two models. The similarities in the synoptic patterns, rather than the differences in the precipitation and surface temperature patterns lend credibility to the climate change scenarios depicted