8.2.6 Coupling Advances

In an advance since the TAR, a number of groups have developed software allowing easier coupling of the various components of a climate model (e.g., Valcke et al., 2006). An example, the Ocean Atmosphere Sea Ice Soil (OASIS) coupler, developed at the Centre Europeen de Recherche et de Formation Avancee en Calcul Scientific (CERFACS) (Terray et al., 1998), has been used by many modelling centres to synchronise the different models and for the interpolation of the coupling fields between the atmosphere and ocean grids. The schemes for interpolation between the ocean and the atmosphere grids have been revised. The new schemes ensure both a global and local conservation of the various fluxes at the air-sea interface, and track terrestrial, ocean and sea ice fluxes individually.

Coupling frequency is an important issue, because fluxes are averaged during a coupling interval. Typically, most AOGCMs evaluated here pass fluxes and other variables between the component parts once per day. The K-Profile Parametrization ocean vertical scheme (Large et al., 1994), used in several models, is very sensitive to the wind energy available for mixing. If the models are coupled at a frequency lower than once per ocean time step, nonlinear quantities such as wind mixing power (which depends on the cube of the wind speed) must be accumulated over every time step before passing to the ocean. Improper averaging therefore could lead to too little mixing energy and hence shallower mixed-layer depths, assuming the parametrization is not re-tuned. However, high coupling frequency can bring new technical issues. In the MIROC model, the coupling interval is three hours, and in this case, a poorly resolved internal gravity wave is excited in the ocean so some smoothing is necessary to damp this numerical problem. It should also be noted that the AOGCMs used here have relatively thick top oceanic grid boxes (typically 10 m or more), limiting the sea surface temperature (SST) response to frequent coupling (Bernie et al., 2005).