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The Second Generation Coupled Global Climate Model

Canadian Centre for Climate Modelling and Analysis

The second version of the Canadian Centre for Climate Modelling and Analysis (CCCma) Coupled Global Climate Model (CGCM2), is based on the earlier The First Generation Coupled Global Climate Model , but with some improvements aimed at addressing shortcomings identified in the first version. In particular, the ocean mixing parameterization has been changed from horizontal/vertical diffusion scheme to the isopycnal/eddy stirring parameterization of Gent and McWilliams (1990), and sea-ice dynamics has been included following Flato and Hibler (1992). In addition, some technical modifications were made in the ocean spinup and flux adjustment procedure. A description of CGCM2 and a comparison, relative to CGCM1, of its response to increasing greenhouse-gas forcing can be found in Flato and Boer (2000). CGCM2 has been used to produce ensemble climate change projections using the older IS92a forcing scenario, as well as the newer IPCC SRES A2 and B2 scenarios. CGCM2 results were used in the recent Intergovernmental Panel on Climate Change, Third Assessment Report (2001) and in the ongoing Arctic Climate Impact Assessment. Some selected results are briefly summarized here.

Surface air temperature change graph

Figure 1: Global annual average surface temperature change, relative to 1900-1929 average as produced by the Fisrt and Second Generation Coupled Global Climate Models for various forcing scenarios.

Figure 1 shows the global, annual mean surface air temperature change simulated by CGCM1 and CGCM2, when forced with the widely-used "IS92a" (1% per year increase) scenario. The two models agree rather closely in terms of their global mean response. Also shown in the figure are time series obtained using the newer IPCC SRES "A2" and "B2"forcing scenarios.

Surface air temperature change colourmap

Figure 2: Annual mean surface air temperature change, 1971-1990 to 2041-2060 as projected by the First Generation Coupled Global Climate Model (upper panel) and Second Generation Coupled Global Climate Model (lower panel).

Figure 2 shows the annual mean surface air temperature change between 1971-1990 and 2041-2060 obtained with the two models and the "IS92a"forcing scenario. Here one can see differences which result largely from the change in ocean mixing parameterization. Most notable is the north/south asymmetry in warming apparent in the CGCM1 result (qualitatively similar to many such models), and the more symmetrical pattern produced by CGCM2. This difference arises from a change in the depth and vigor of vertical mixing in the Southern Ocean.

CGCM2 has also been used to perform simulations of the climate during the Last Glacial Maximum (LGM) some 18,000 years ago (Kim et al. (2002a;b)).

Acknowledgements

The development of CGCM2 was a team effort involving G.M. Flato and G.J. Boer along with W.G. Lee and S. Tinis.

References:

Flato, G.M. and G.J. Boer, 2001: Warming Asymmetry in Climate Change Simulations. Geophys. Res. Lett., 28, 195-198.

Flato, G.M. and Hibler, W.D. III, 1992: Modelling Pack Ice as a Cavitating Fluid. J. Phys. Oceanogr., 22, 626-651.

Gent, P.R. and J.C. McWilliams, 1990: Isopycnal Mixing in Ocean Circulation Models. J. Phys. Oceanogr., 20, 150-155.

IPCC, 2001: Climate Change 2001: The Scientific Basis. J.T. Houghton et al. (eds.), Cambridge University Press, 881pp.

Kim, S.-J., G.M. Flato, G.J. Boer and N.A. McFarlane, 2002a: A coupled climate model simulation of the Last Glacial Maximum, Part 1: transient multi-decadal response. Climate Dynamics, 19, 515-537.

Kim, S.-J., G.M. Flato, G.J. Boer, 2002b: A coupled climate model simulation of the Last Glacial Maximum, Part 2: approach to equilibrium Climate Dynamics, in press.

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