Spatial Structure of Ultra‐Low‐Frequency Variability of the Flow In A Simple Atmospheric Circulation Model

Abstract The internally generated variability of the global circulation on time‐scales in excess of one year is investigated using a multilevel baroclinic primitive‐equation model with moderate resolution of dynamical processes. the model includes idealized heating and friction. Maximum variability is found on time‐scales of around a decade. Empirical orthogonal‐function analysis reveals that the principal component of this variability is an alternate splitting and coalescence of the subtropical and mid‐latitude jets. Associated changes in the strength and distribution of eddy activity are found. the character of the ultra‐low‐frequency variability is completely changed for very low resolutions, but seems relatively insensitive to details of model formulation at higher resolution. the possibilities of deriving a low‐order model which includes ultra‐low‐frequency variability are considered.