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Testing the annular mode autocorrelation timescale in simple atmospheric general circulation models (1175)
| Venue: | Wea. Rev |
| Citations: | 17 - 1 self |
BibTeX
@ARTICLE{Gerber75testingthe,
author = {Edwin P. Gerber and Sergey Voronin and Lorenzo and M. Polvani},
title = {Testing the annular mode autocorrelation timescale in simple atmospheric general circulation models},
journal = {Wea. Rev},
year = {1175},
pages = {1523--1536}
}
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Abstract
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 , 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 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, 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, 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 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 for computing the correct response of a model to climate change is explicitly demon-strated by perturbing the pseudospectral model with simple torques. The amplitude of the model’s response to external forcing increases as increases, as suggested by the fluctuation–dissipation theorem. 1.
Keyphrases
annular mode autocorrelation timescale simple atmospheric general circulation model pseudospectral model model variability model response low frequency annular mode autocorrelation time scale correct response extratropical jet vacillates horizontal resolution external forcing increase triangular truncation finite volume low-frequency variability suarez system annular mode autocorrelation function simple torque realistic low-frequency variability different general circulation model high resolution new diagnostic e-folding time scale atmospheric model intrinsic time scale dry primitive equation model vertical resolution common resolution choice held suarez forcings long integration simple procedure fluctuation dissipation theorem
