ON THE COLD START PROBLEM IN TRANSIENT SIMULATIONS WITH COUPLED ATMOSPHERE-OCEAN MODELS

Finite computer resources force compromises in the design of transient numerical experiments with coupled atmosphere-ocean general circulati on models which, in the case of global warming simulations, normally p reclude a full integration from the undisturbed pre-industrial state. The start of the integration at a later time from a climate state whic h, in contrast to the true climate, is initially in equilibrium then i nduces a cold start error. Using linear response theory a general expr ession for the cold start error is derived. The theory is applied to t he Hamburg CO2 scenario simulations. An attempt to estimate the global -mean-temperature response function of the coupled model from the resp onse of the model to a CO2 doubling was unsuccessful because of the no n-linearity of the system. However, an alternative derivation, based o n the transient simulation itself, yielded a cold start error which ex plained the initial retardation of the Hamburg global warming curve re lative to the IPCC results obtained with a simple box-diffusion-upwell ing model. In the case of the sea level the behaviour of the model is apparently more linear. The cold start error estimations based on a CO 2 doubling experiment and on an experiment with gradually increasing C O2 (scenario A) are very similar and explain about two thirds of the c oupled model retardation relative to the IPCC results.