Construction of the linear response operator of an atmospheric general circulation model to small external forcing

- In this paper we present a method for constructing an approximate response operator of the average state of an atmospheric general circulation model to small external forcing. The method is based on Kraichnan’s fluctuation-dissipation relationship under the assumption of ‘quasiregularity’ of the systems studied. Its advantage is that only the statistical characteristics of an unperturbed system are needed to calculate the approximate response operator. This method is implemented for CCM0, an atmospheric general circulation model of the National Center for Atmospheric Research, USA. It is shown with the aid of a number of numerical experiments that the constructed approximate response operator allows us to reproduce both the amplitude and the spatial structure of the linear part of the CCM0 model response with high accuracy. Thus, it approximates the real response operator of the model with good accuracy. Therefore using this method we can solve the problem of determining the model sensitivity to small external forcing and the problem of reconstructing the forcing that generates a given response.