| The atmosphere and oceans exhibit many complex motions on a variety of time and length scales. These motions may be categorized as slow or fast depending on their inherent frequencies. Often these different motions interact only weakly and the slow dynamics is naturally constrained to a subspace of phase space, the slow manifold. The fast motions appear more as noise for the reduced system and the slow dynamics determines the long time behaviour. The dynamics is then said to be balanced. Classically, balanced systems are obtained for geophysical fluid models by considering asymptotic limits of two distinct dimensionless parameters: the Rossby number, R, and Froude number, F, which characterize the importance of rotation and stratification, respectively.; We propose an alternative approach. Fundamentally, the greater the separation in their fundamental frequencies, the less interaction we expect between slow and fast motions. We seek, therefore, a dimensionless parameter epsilon that characterizes a time scale separation in the dynamics. In terms of R and F we find e=RFR2+F 2.; We examine both the shallow water model and the Boussinesq model, first at mid-latitudes and then in the tropics. The former situation is completely tractable and we find that a time scale separation is necessary and sufficient for balanced dynamics. Near the equator the weakness of the Coriolis parameter limits our attention to zonal length scales different from the Rossby deformation radius. We find balanced models provided R ≁ F. |
