Signatures of nonlinear dynamics in an idealized atmospheric model

Signatures of nonlinear dynamics are analyzed by studying the phase-space tendencies of a global baroclinic, quasi-geostrophic, three-level (QG3) model with topography. Nonlinear, stochastic, low-order prototypes of the full QG3 model are constructed in the phase space of this model’s empirical orthogonal functions using the empirical model reduction (EMR) approach. The phase-space tendencies of the EMR models closely match the full QG3 model’s tendencies. The component of these tendencies that is not linearly parameterizable is shown to be dominated by the interactions between “resolved” modes, rather than by multiplicative “noise” associated with unresolved modes. The way of defining the leading, resolved modes and the interactions between them plays a key role in understanding the nature of the QG3 model’s dynamics, whether linear or nonlinear, deterministic or stochastic.