3.3.7 Coupling between the components - Earth system models

The interactions between the various components of the system play a crucial role in the dynamics of climate. Wind stress, heat and freshwater fluxes at the ocean surface are the main drivers of the ocean circulation (see section 1.3.2). The evaporation at the ocean surface is the largest source of water vapour for the atmosphere, which influences the radiative properties of the air (section 2.1.2) and the atmospheric heat transport (section 2.1.5). Snow falling on ice sheets is an essential element of their mass balance. Many other examples could be cited.

Some of those interactions are quite straightforward to compute from the models state variables, while more sophisticated parameterisations are required for others. For instance, the parameterization of the wind stress and of the heat flux at the atmospheric base (e.g., Eqs. 2.33 and 2.34) can be derived from theories of the atmospheric boundary layer. However, this computation still requires empirical parameters that depend on the characteristics of the surface, introducing some uncertainties into the determination of the flux.

The technical coupling of the various components to obtain a climate- or Earth-system model brings additional difficulties. The numerical codes have generally been developed independently by different groups, using different coding standards, different numerical grids, etc. It is thus necessary to design an interface and to use a coupler, i.e. code specially adapted to represent the exchanges between the various components.

The above presentation includes the main elements of the majority of current Earth-system models. However, the description is far from exhaustive. New components (such as models of the methane cycle, of the nitrogen cycle, more sophisticated representation of atmospheric chemistry, etc) are continuously included in order to obtain a more comprehensive representation of the complex interactions in the system.

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