8.2.7 Flux Adjustments and Initialisation

Since the TAR, more climate models have been developed that do not adjust the surface heat, water and momentum fluxes artificially to maintain a stable control climate. As noted by Stouffer and Dixon (1998), the use of such flux adjustments required relatively long integrations of the component models before coupling. In these models, normally the initial conditions for the coupled integrations were obtained from long spin ups of the component models.

In AOGCMs that do not use flux adjustments (see Table 8.1), the initialisation methods tend to be more varied. The oceanic components of many models are initialised using values obtained either directly from an observationally based, gridded data set (Levitus and Boyer, 1994; Levitus and Antonov, 1997; Levitus et al., 1998) or from short ocean-only integrations that used an observational analysis for their initial conditions. The initial atmospheric component data are usually obtained from atmosphere-only integrations using prescribed SSTs.

To obtain initial data for the pre-industrial control integrations discussed in Chapter 10, most AOGCMs use variants of the Stouffer et al. (2004) scheme. In this scheme, the coupled model is initialised as discussed above. The radiative forcing is then set back to pre-industrial conditions. The model is integrated for a few centuries using constant pre-industrial radiative forcing, allowing the coupled system to partially adjust to this forcing. The degree of equilibration in the real pre-industrial climate to the pre-industrial radiative forcing is not known. Therefore, it seems unnecessary to have the pre-industrial control fully equilibrated. After this spin-up integration, the pre-industrial control is started and perturbation integrations can begin. An important next step, once the start of the control integration is determined, is the assessment of the control integration climate drift. Large climate drifts can distort both the natural variability (e.g., Inness et al., 2003) and the climate response to changes in radiative forcing (Spelman and Manabe, 1984).

In earlier IPCC reports, the initialisation methods were quite varied. In some cases, the perturbation integrations were initialised using data from control integrations where the SSTs were near present-day values and not pre-industrial. Given that many climate models now use some variant of the Stouffer et al. (2004) method, this situation has improved.