Empirical Mode Reduction and non-Gaussian Signatures of Planetary Low-Frequency Atmospheric Modes.

Details Empirical Mode Reduction and non-Gaussian Signatures of Planetary Low-Frequency Atmospheric Modes. Empirical Mode Reduction and non-Gaussian Signatures of Planetary Low-Frequency Atmospheric Modes.

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmng23b1136k&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #NG23B-1136

Physics

1872 Time Series Analysis (3270, 4277, 4475), 3235 Persistence, Memory, Correlations, Clustering (3265, 7857)

Scientific paper

We demonstrate here a use of Empirical Mode Reduction (EMR) in identifying the relative contributions of resolved and unresolved atmospheric modes to "double-swirls" in mean phase-space tendencies for a global, quasi-geostrophic, three-level (QG3) atmospheric model. In EMR, multiple polynomial regression is used to estimate the nonlinear, deterministic propagator of the dynamics, as well as multi-level additive stochastic forcing, directly from the data set. In this approach, the residual stochastic forcing at a given level is subsequently modeled as a function of the extended state vector involving the variables of all preceding levels. The estimated mean-phase space tendencies of QG3 and EMR agree very well, further confirming the usefulness of our EMR approach. The explicit quadratic form of the EMR model's dynamical operator facilitates estimating the contributions of linear and nonlinear interactions to the resulting tendencies. Purely linear tendencies would be characterized by antisymmetry for reflections through the origin and constant speed along ellipsoids. The characteristic double-swirl pattern for tendencies are indicative of strong nonlinear and non-Gaussian behavior. The EMR approach also allows one to separate the relative contributions, from additive and multiplicative noise, to the full nonlinear tendencies. These contributions are, in turn, related to the attribution of different EOFs to "resolved" and "unresolved" modes. The EOF spectrum of the QG3 model is characterized by higher-order modes having shorter time scales, and somewhat smaller spatial scales, than the leading modes; there is, however, no pronounced time-scale separation. The four leading EOFs are clearly well resolved, since they have the most pronounced deviations from Gaussianity in terms of skewness and kurtosis, and they determine the most interesting dynamical aspects of the QG3 model's low-frequency variability: linear (intraseasonal oscillations) as well as nonlinear (multiple regimes). Our results show that the nonlinear double-swirl features of mean tendencies are mostly due to the resolved nonlinear interactions while the effect of unresolved modes is small. Our study highlights the fact that extra care is needed in assessing the effects of linear and nonlinear deterministic interactions, as well as of additive and multiplicative noise, on the non-Gaussian signatures of planetary low-frequency waves.

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