Coupled ocean-atmosphere model experiments of future climate change based on IPCC SRES scenarios

Transient experiments of all the four ( A1, A2, B1, and B2 ) emission scenarios are carried out with CCSR/NIES coupled ocean-atmosphere model for contributing to the IPCC Third Assessment Report. The model consists of CCSR/NIES AGCM ( T21 spectral truncation and 20 vertical levels ), CCSR OGCM ( 2.8‹horizontal grid and 17 levels ), a thermodynamic sea ice model, and a river routing model. Flux adjustment for atmosphere-ocean heat and water exchange is applied to prevent a drift of the modeled climate. A radiation scheme based on the k-distribution, two-stream discrete ordinate method ( DOM ) is adopted for the atmosphere.

Major greenhouse gases of CO₂, CH₄, N₂O, and CFCs ( thirteen species ) are considered individually. Temporal variations of CO₂, CH₄, and N₂O concentrations depend on the emission scenarios, but that of CFCs are common to all the scenarios. Tropospheric ozone data which are kindly provided by UK Met. Office are used. Future increase/decrease of those is given according to the scenario database. Zonally averaged distribution of ozone is supplied as climatology in the stratosphere. Seasonal variations are considered for both the tropospheric and the stratospheric ozone.

Direct and indirect effects of aerosols are included; aerosol scattering is represented explicitly, and effects of aerosols on the effective radius of cloud droplet and on the precipitation efficiency are considered through a relationship between the cloud droplet and the aerosol number concentrations. Major four aerosol species ( sulfate, carbon, dust, and sea salt ) are considered and concentrations of those are calculated offline by an aerosol transport model of CCSR. Emissions of SO₂ by fossil fuel, which are gridded and provided by S.Smith, follow the scenarios. Volcanic and DMS SO₂ emissions are also considered and supplied as climatology with seasonal variations. The carbonaceous aerosols also follow the emission scenarios. Black carbon ( BC ) emissions by fossil fuel, biofuel, and forest fire are calculated from scenario variables which correspond to the three sources. Gridding procedure is the same as that for SO₂ except that the world is divided into four regions. Organic carbon ( OC ) emissions are derived from BC emissions assuming that the ratio of OC/BC is globally uniform. The dust and sea salt aerosols are supplied as climatology with seasonal variations.

In order to investigate the difference of future projections, radiative forcings due to CO₂, other greenhouse gases, direct and indirect effects of sulfate aerosols, and those of carbonaceous aerosols are estimated individually for all the scenario runs. Based on the comparison of these radiative forcings and meteorological fields among the experiments, the range of future climate change which arises from the emission scenario difference will be presented.