CSIRO-Mk2 GCM Model Information

Background Information

The CSIRO Atmospheric Research Mark 2b climate model (Hirst et al., 1996, 1999) has recently been used for a number of more sophisticated climate change simulations. These start from 1880 to avoid the "cold start problem".

This version of the CSIRO model includes the Gent-McWilliams mixing scheme in the ocean and shows greatly reduced climate drift relative to earlier versions (e.g. Dix and Hunt, 1998). The drift in global mean surface temperature in the new control run is about -0.02 ºC/century. Note that the model uses flux correction.

The model atmosphere has 9 levels in the vertical and horizontal resolution of spectral R21 (approximately 5.6 by 3.2 degrees). The ocean model has the same horizontal resolution with 21 levels.

The equilibrium sensitivity to doubled CO2 of a mixed layer ocean version of the model is 4.3 degrees. This is at the high end of the range of model sensitivities (e.g. IPCC 1995, Table 6.3). In the basic greenhouse gas experiment the model combines the effect of all radiatively active trace gases into an "equivalent" CO2 concentration. Observed concentrations are used from 1880 to 1990 and the IS92a projections into the future. This gives close to a 1%/year compounding increase of equivalent CO2.

Another model experiment includes the negative radiative forcing from atmospheric sulphate aerosol. The direct aerosol forcing is included via a perturbation of the surface albedo, similarly to the Hadley Centre experiments described by Mitchell et al (1995) and Mitchell and Johns (1997) . The sulphate concentrations are the same as used in the Hadley Centre experiments. However the chosen aerosol optical properties are different, giving a present day forcing due to anthropogenic sulphate of about -0.4 W/m^2. This can be compared to the 1880-1990 greenhouse gas forcing of about 2 W/m^2.

The magnitude of the 20th century warming in the model including aerosol matches the observed reasonably well. However there are a number of forcings missing from the model, including solar variability, sulphate indirect effect and the effect of soot.

Global-Mean Temperature, Precipitation and CO2 Changes (w.r.t. 1961 - 1990) for the CSIRO-Mk2 Integrations

Notes

The climate sensitivity of CSIRO-Mk2 is about 4.3ºC (Watterson et al.,1997).

* The CO2 concentration has been estimated assuming an IS92a mix of greenhouse gases and concentration / forcing relationships reported in the IPCC Second Assessment Report.

References and other reading

Dix, M. R., and Hunt, B. G. 1998. Transient climatic change to 3 x CO2 conditions. Global and Planetary Change, 18 (1-2): 15-36.

Hirst, A. C., Gordon, H. B., and O'Farrell, S. P. 1996. Global warming in a coupled climate model including oceanic eddy-induced advection. Geophysical Research Letters, 23 : 3361-3364.

Hirst A.C., O'Farrell S.P. and Gordon H.B., 1999: Comparison of a coupled ocean-atmosphere model with and without oceanic eddy-induced advection. 1. Ocean spin-up and control integrations. J. Climate, submitted.

Mitchell J.F.B., Johns T.C., Gregory J.M. and Tett S. 1995. Climate response to increasing levels of greenhouse gases and sulphate aerosols. Nature, 376, 501-504.

Mitchell J.F.B. and Johns T.C. , 1997. On modification of global warming by sulphate aerosols, J. Climate, 10, 245-267.

Watterson,I.G., O'Farrell,S.P. and Dix,M.R. 1997 Energy and water transport in climates simulated by a general circulation model that includes dynamic sea ice. Journal of Geophys. Res., 102: 11,027-11,037.

Further Details

http://www.cmar.csiro.au/