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Figure 2.71a,b. Coupled ocean—atmosphere model results for mean sea-level pressure (Pa) in January 1997 (above) and July 1997 (below) HADCM2-SUL, the model used, which includes sulphate aerosols, has been integrated from pre-industrial times (Mitchell and Johns, 1997).

Current models include topography, a more complete water cycle with a cloud model, and a number of minor components in the atmosphere contributing to the radiation balance. The transfer of energy and momentum between ocean surfaces and the atmosphere has been a difficult process to model, and only since about 1995 has it been possible to formulate coupled ocean—atmosphere models not requiring some artificial correction for the mismatch between oceans and atmosphere to be introduced. This need was not only due to lack of knowledge of the processes involved, but also came from the different numerical treatment of ocean circulation and atmospheric circulation, notably the difference in time steps, and from having to start up the model by having it run for a period of time in order to gain stability (the finding that one could not model ahead from some known state of the atmosphere rests on the fact that input data were never fully consistent with the model structure and restrictions, so that an "artificial" initial state had to be created by running the model for several years, cf. the discussion in Gates et al., 1996). Current models use spatial grids of 30-100 km spacings and up to 30 atmospheric levels plus a similar number of ocean depth levels, with temporal grids of around half an hour for the atmosphere and an hour or more for the ocean part (see e.g. UK Meteorological Office, 1997).

precipitation January 1997

0 to 0.04 0.04 to 0.08 0.08 to 0.12 0.12 to 0.16 0.16 to 0.2 0.2 to 0.24 0.24 to 0.28 0.28 to 0.32 0.32 to 0.36 0.36 to 0.4 I 0.4 to 0.5 ■ 0.5 to 1 J"! ail others

Figure 2.72a,b. Coupled ocean—atmosphere model results for total precipitation (m/y) in January 1997 (above) and July 1997 (below). The HADCM2-SUL model, which includes sulphate aerosols, has been integrated from pre-industrial times (Mitchell and Johns, 1997).

precipitation (m) July 1997

0 to 0.04 0.04 to 0.08 0.08 to 0.12 0.12 to 0.16 0.16 to 0.2 0.2 to 0.24 0.24 to 0.28 0.28 to 0.32 0.32 to 0.36 0.36 to 0.4

Models used near the year 2000 include the greenhouse effects of a number of gases as well as scattering and reflection effects of aerosols, such as

those derived from sulphur dioxide emissions from human activity. Earlier models lumped these together as an effective change in solar radiation fluxes used in the models. Also, the interactions between the biosphere and the atmosphere and oceans are important for both heat and moisture fluxes, albedo and surface roughness experienced by winds. This means that the seasonal variation in standing crops as well as agricultural and forestry practices become important for climate modelling, along with direct anthropogenic interference through emissions of polluting substances to the atmosphere, in addition to natural processes such as volcanic eruptions. The human interference will be further discussed below.

Figure 2.73a,b. Coupled ocean—atmosphere model results for surface solar radiation (W m—2) on a horizontal surface in January 1997 (a: above) and April 1997 (b: below). The HADCM2-SUL model includes sulphate aerosols (Mitchell and Johns, 1997).

Figure 2.70 shows a newer model calculation of ocean salinity, considerably more accurate than the early model shown in Fig. 2.68. Figures 2.71-2.75 show mean sea-level pressure (i.e. pressure adjusted to zero height), precipitation, surface solar radiation, wind speed and temperature from a coupled ocean-atmosphere model including sulphate aerosol effects and greenhouse gases corresponding to 1997 levels. Where seasonal variation is important, the calculation is shown for two or four seasons. Measured data for comparison are contained in Fig. 2.76 for sea-level pressure, with Fig. 2.77

Figure 2.70 shows a newer model calculation of ocean salinity, considerably more accurate than the early model shown in Fig. 2.68. Figures 2.71-2.75 show mean sea-level pressure (i.e. pressure adjusted to zero height), precipitation, surface solar radiation, wind speed and temperature from a coupled ocean-atmosphere model including sulphate aerosol effects and greenhouse gases corresponding to 1997 levels. Where seasonal variation is important, the calculation is shown for two or four seasons. Measured data for comparison are contained in Fig. 2.76 for sea-level pressure, with Fig. 2.77

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