Testing the annular mode autocorrelation time scale in simple atmospheric general circulation models

A new diagnostic for measuring the ability of atmospheric models to reproduce realistic low-frequency variability is introduced in the context of Held and Suarez's 1994 proposal for comparing the dynamics of different general circulation models. A simple procedure to compute tau, the e-folding time scale of the annular mode autocorrelation function, is presented. This quantity concisely quantifies the strength of low-frequency variability in a model and is easy to compute in practice. The sensitivity of tau to model numerics is then studied for two dry primitive equation models driven with the Held-Suarez forcings: one pseudospectral and the other finite volume. For both models, tau is found to be unrealistically large when the horizontal resolutions are low, such as those that are often used in studies in which long integrations are needed to analyze model variability on low frequencies. More surprising is that it is found that, for the pseudospectral model, tau is particularly sensitive to vertical resolution, especially with a triangular truncation at wavenumber 42 (a very common resolution choice). At sufficiently high resolution, the annular mode autocorrelation time scale tau in both models appears to converge around values of 20-25 days, suggesting the existence of an intrinsic time scale at which the extratropical jet vacillates in the Held and Suarez system. The importance of tau for computing the correct response of a model to climate change is explicitly demonstrated by perturbing the pseudospectral model with simple torques. The amplitude of the model's response to external forcing increases as tau increases, as suggested by the fluctuation-dissipation theorem.