Ensemble Historical and Climate Change Simulations

There is a need to estimate both the mean changes in the future climate system as well as the range between the largest and smallest changes. Ensemble climate change simulations can provide insight into quantifying uncertainties of regional climate change projections. Recently, three-dimensional climate models have undergone significant improvements. We will describe climate change ensemble simulations with the DOE supported Parallel Climate Model (PCM) which has relatively high ocean and sea ice resolutions. One of the features of the PCM is that the components have been designed to use massively parallel processor (MPP) and distributed memory (SD) supercomputers. There are now plans to merge the PCM and the NCAR Community Climate System Model (CCSM) model into one modeling system with the only differences being resolution and how they are executed on various computer systems.

Over the last two years, we have conducted several groups of climate change simulations. The results of a 1% CO₂ increase per year simulation have been published. (see Washington et al., 2000 and Weatherly and Washington, 2000). The second set of simulations is historical and 21st century climate change projections. We have closely followed the same greenhouse gas and sulfate aerosol forcings that are used in CCSM.

The PCM yields a realistic simulation of the mean state of the atmosphere, ocean and sea-ice, however, with some biases. The PCM simulates the El Niño, La Niña, North Atlantic and Arctic Oscillation events with roughly the correct amplitude and phase. This is especially important for climate change simulations in which regional aspects are being examined. The major emphasis of the presentation will be to show the regional changes in the climate with special emphasis on sea level pressure, temperature, and precipitation differences. Ensemble simulations from a single climate model will allow for improved estimates of the range differences in predicted climate change.

Washington, W.M., J.W. Weatherly, G.A. Meehl, A.J. Semtner, T.W.Bettge, A.P. Craig, W.G. Strand, J.M. Arblaster, V.B. Wayland, R. James and Y. Zhang, 2000: Parallel Climate Model (PCM) Control and Transient Simulations, Climate Dynamics, in press.

Weatherly, J.W. and W.M. Washington, 2000: The response of the polar regions to increased CO₂ in a global climate model. Journal of Climate, in press.