Regional climate change in a topographically complex region

A regional climate model (RCM) was applied to estimate the impact of increasing carbon dioxide concentration on the future climate of California. The first experiment with the RCM consisted of two cases with 280 ppm CO ₂ (preindustrial) and 560 ppm CO₂ (doubled preindustrial) as initial conditions. The RCM was run for a period of five years, with 3 ensemble members for each case. The results indicated statistically significant temperature increases in all months of the year over the entire state. The precipitation results indicated increases in the northern half of the state and decreases in the southern, but these differences were not statistically significant. In the mountainous areas of the state there were large statistically significant decreases in snow accumulation. The two cases above were extended to fifteen years and the results were subdivided and analyzed for the ten hydrologic regions of the state as defined by the California Department of Water Resources. Model results indicated increased temperature in all regions of the state in all months. Results for precipitation indicated decreases in the winter through spring months for almost all regions. For the mountainous regions, snow accumulation was dramatically reduced. The output of the RCM for the two fifteen year cases, 280 and 560 ppm CO₂, was used to calculate wind stress curl anomaly along the coast of California. Wind stress curl is used to estimate changes in coastal upwelling due to changes in climate. Two additional cases, twenty years in length, were run with the RCM with time-varying CO₂ concentrations. The RCM results for wind stress curl indicate the intensity of the peak upwelling season (July–August) will increase, and that in the region north of Pt. Conception the upwelling season is lengthened. Lastly, the two IPCC cases were analyzed for three geographic regions of California. Results for the RCM indicated statistically significant increases in temperature for all regions, with the greatest warming occurring in the high elevation Mountain region. The precipitation results indicate some increases and decreases, but they are not statistically significant. Snow accumulation is greatly reduced in Mountain region, and the differences are statistically significant.