Warren Washington, National Center for Atmospheric Research
Albert Semtner, Naval Postgraduate School
John Weatherly, U.S. Army Cold Regions Research and Engineering Laboratory
Robert Malone, Los Alamos National Laboratory
Tim Barnett and David Pierce, Scripps Institution of Oceanography
John Drake, Oak Ridge National Laboratory
Phil Duffy, Lawrence Livermore National Laboratory
The Program for Climate Model Diagnosis and Intercomparison (PCMDI),
Lawrence Livermore National Laboratory

Research Objectives

The main purpose of this research is to develop and use a new Parallel Climate Model (PCM) for studies of anthropogenically forced climate change simulations. Because it is difficult to separate anthropogenic climate change from natural climate variability, it is necessary to carry out ensembles of simulations in order to statistically find the climate change signal. Results of this research are being used by the Intergovernmental Panel on Climate Change (IPCC) and the U.S. National Assessment of the Potential Consequences of Climate Variability and Change.

Computational Approach

We use a newly developed climate model that has state-of-the-art components for the atmosphere, land, ocean and sea ice. The PCM can execute on distributed and shared memory parallel computer systems.

We have developed an ocean component that uses the finite difference Parallel Ocean Program (POP) with a displaced north pole. The model in its present form runs at 2/3° and 1/3° resolution and yields an extraordinary simulation of the Arctic Ocean, tropical Pacific, and boundary currents, such as the Gulf Steam, with eddies. POP has recently been reformulated with improved data structures and enhanced memory and cache performance in a message passing environment. Many additional model physics options have been added to improve the model's fidelity.

The sea ice model component was developed by Yuxia Zhang of NPS and uses the Hibler ice dynamics with line relaxation for solving the equations. The viscous-plastic constitutive law is applied, and the Parkinson-Washington thermodynamics are included. The grid is transformed such that resolution is roughly linear in distance, thus avoiding the pole convergence problem with a latitude-longitude grid. The spatial resolution is about 27 km, which provides a highly realistic Arctic and Antarctic simulation of ocean and sea ice motion that includes the explicit effects of eddies. Recently, an option for using elastic-viscous-plastic physics has been added using the E. Hunke and J. Dukowicz approach to the solution of the ice dynamics. This latter option is being adapted for use on the IBM SP-3.

The atmospheric model component is the massively parallel version of the NCAR Community Climate Model version 3 (CCM3). This model includes the latest versions of radiation, boundary physics, and precipitation physics and runs at T42 resolution or higher. This model runs efficiently on the Cray T3E, the SGI Origin 2000, and the IBM SP.

This simulation from the DOE-supported Parallel Climate Model (PCM) shows the upper ocean flow of the North Atlantic current system along with the ocean temperature. It demonstrates that a 2/3° horizontal resolution ocean model coupled to an atmosphere model and a sea ice model can simulate more realistic ocean eddies in the Gulf Stream and North Atlantic current systems.

The flux coupler ties the components together and allows the exchange of fluxes and variables. Since the component grids are different, there is an interpolation scheme for passing information between the atmosphere component grid and the ocean/sea ice grid that has been developed by P. Jones of LANL. The interpolation algorithm has been designed to run efficiently on distributed memory architectures. The component interfacing through the flux coupler is sufficiently flexible to handle a large range of resolutions, so that high-resolution models can use the same structure as lower-resolution models.

Accomplishments

We now have a 300-year control experiment that is the baseline for our climate change simulations. This control run shows very little climate drift and has remarkable El Niño and La Niña events that are close to the observed levels of natural variability. Also, these events cause changes in the extratropical planetary wave patterns. After we established a control experiment, we carried out five experiments with a 1%/yr increase in atmospheric CO₂. At the doubling point of roughly 70 years, we stopped the atmospheric CO₂ increase. From the ensemble of 1%/yr runs we can better establish the geographic change in sea surface temperature and other climate variables over the entire globe.

We also have conducted two historical simulations from 1870 to the year 2000 in which the changes in greenhouse gases and sulfate aerosol distributions are provided. We plan to carry out several ensemble experiments to see how much decade-to-decade variability exists and how well the historical simulation agrees with observed changes.

Finally, we are in the process of conducting several ensemble experiments of future climate change from year 2000 to year 2100. These experiments require assumptions of future increases in greenhouse gases and sulfate aerosols.

Significance

Accurate prediction of climate change on decadal and longer time scales is a major scientific objective of the DOE's Environmental Sciences Division. The DOE Climate Change Prediction Program is focused on developing, testing, and applying climate simulation and prediction models that stay at the leading edge of scientific knowledge and computational technology. A unique feature of the program is the establishment of a distributed modeling center involving DOE national laboratories, the National Center for Atmospheric Research, and the non-Federal research community. The intent is to increase dramatically both the accuracy and throughput of computer model-base predictions of future climate system response to the increased concentrations of greenhouse gases.

In the Kyoto Protocol on climate change, there are several climate change scenarios that must be completed for the U.S. National Assessment. These simulations assume emissions of greenhouse gases and sulfate aerosols. Data sets from these simulations are being used by the National Assessment and the IPCC.

Publications

A. J. Semtner, "Ocean and climate modeling on advanced parallel computers: Progress and prospects," Communications of the ACM (in press).

W. M. Washington, J. W. Weatherly, G. A. Meehl, A. J. Semtner, T. W. Bettge, A. P. Craig, W. G. Strand, J. Arblaster, V. B. Wayland, R. James, and Y. Zhang, "Parallel Climate Model (PCM) control and 1%/year CO2 simulations with a 2/3° ocean model and a 27 km dynamical sea ice model," Climate Dynamics (submitted).

http://www.cgd.ucar.edu/pcm